Competitive and Sustainable Growth (GROWTH) Programme

SAMARISSustainable and Advanced MAterials for Road InfraStructure

DELIVERABLE D30

GUIDANCE FOR THE OPTIMAL ASSESSMENT OF HIGH-WAY STRUCTURES

Document number: SAM-GE-DE30 Version: 6 Date: 20.1.2006

SAMARIS SAM-GE-DE30

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Name and signature

Date

Drafted: Aleš Žnidarič

Reviewed:

Verified:

Validated:

Approved by SAMARIS Management Group:

ABSTRACT

TABLE OF CONTENTS

ABSTRACT........................................................................................................... III

EXECUTIVE SUMMARY.....................................................................................V

FOREWORD AND ACKNOWLEDGEMENTS.................................................XI

1. INTRODUCTION..........................................................................................1

2. CONDITION ASSESSMENT........................................................................22.1 Condition Assessment in general...................................................................22.2 Objectives of Condition Assessment..............................................................22.3 Short critical review of CA system in Europe...............................................32.4 Hazard issues in Condition Assessment........................................................42.5 Advice on good practice of Condition Assessment........................................5

2.5.1 Catalogue of defects.........................................................................................52.5.2 Training of inspectors.......................................................................................72.5.3 Health issues of inspectors................................................................................82.5.4 Suitable equipment for inspectors.....................................................................82.5.5 Range of available investigation.......................................................................92.5.6 Safe and long-term data storage (written documentation, computers, software)112.5.7 Quality Control of Condition Assessment.......................................................12

2.6 Integration of condition assessment into the remaining load capacity of the structure.......................................................................................................13

3. SAFETY ASSESSMENT..............................................................................163.1 General about safety assessment.................................................................163.2 Load testing..................................................................................................18

3.2.1 Classification of load tests..............................................................................183.2.2 Load testing methodology..............................................................................213.2.3 Bridge assessment based on load tests............................................................23

3.3 Site-Specific Load Assessment.....................................................................343.3.1 Site-specific assessment of dead loading.........................................................343.3.2 Proposal for Assessment Load Model which can be adjusted to reflect truck

numbers and distribution of truck weights.......................................................353.3.3 Allowance for Dynamic Amplification...........................................................373.3.4 Recommendations for site-specific assessment of load in individual cases......39

4. REFERENCES............................................................................................. 40

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ABSTRACT

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EXECUTIVE SUMMARYMaintenance of concrete structures is a heavy burden for society, both, in financial terms and due to its risk of causing major and longer-term disturbance of traffic. SAMARIS project has addressed this problem in two ways:

through improved maintenance of highway structures using radically enhanced re-pair methods on one side and

by proposing methods and procedures for improved maintenance of highway structures; there special attention was given to the New Member States and other Central European countries where the condition of the highway structures is fall-ing behind the situation in the Western European countries.

This report, produced by the members of work package 15, is focusing on optimised bridge management through improved bridge inspection, more accurate static and dynamic traffic load modelling and by applying calibrated structural models and real site data through higher levels of assessment.

As the topic is too wide, the authors had no ambition to prepare specification or guidelines. This document should be rather seen as a guiding document, which will, rather than strictly applying the design rules, asses bridges with optimised tools that take advantage from bridge inspection, site measurements and even load test. Several examples are included to illustrate the benefits of the procedures proposed.

The report is divided into two major chapters: on condition and on safety assessment. The first one gives and overview of the existing procedures and addresses the most important is-sues associated with efficient bridge inspection. The second one is focussing on optimisation of safety assessment through evaluation of realistic carrying capacity and live (traffic) load effects. In both cases, significant improvements of the present knowledge were obtained.

Condition assessmentCondition assessment (CA) of highway structures provides the owner or responsible authorit-ies for the maintenance of highway structures with the appraisal of the present situation of the structures. Assessment gives data about the intensity and extent of observed defects on the structures, the cause for these defects and possible deterioration processes and the impact of such findings on the safety and service life of the structures. These data are the basis for the estimation of possible intervention and for a rough estimation of costs for possible remedial work.

The assessment is not always an easy task. Deterioration processes may have several causes and it is difficult to find simple explanations and understanding of the problems. Usually, for each defects several types of possible remedial options can be proposed which depend on the technological possibilities as well as on the requirements for the users' safety, service life of the structure, operational requirements during repair (lanes closed, closing of the structure, weather conditions, etc.) and available funds.

The main objectives of condition assessment are to: detect possible deterioration processes,

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reveal condition of the structures and their elements, individually and as part of the entire highway structures’ stock,

rank the structures for urgent repair and maintenance strategies, optimise the maintenance budget allocation.

Further issues discussed in the report are: catalogue of defects, training of inspectors, health issues of inspectors, suitable equipment for in-

spectors, range of available investiga-

tion, safe and long-term data stor-

age (written documentation, computers, software) and

quality control of condition assessment.

One of the main deliverables of this part of the projects is the Internet based cata-logue of defects, built around examples from all over the Europe, to characterise the widest spectrum of damage types. The important feature of this catalogue is that any registered user can upload new photo-documented examples of damages.

SAFETY ASSESSMENTFive levels for the assessment of highway structures, with Level 1 being the simplest and Level 5 the most complex and accurate, are recommended. Means for carrying out assessments at Levels 1, 2 and 3, are now generally available. Levels 4 and 5 involve struc-tural reliability calculations and are currently only used by experts. In general, the safety as-sessment begins at level 1 and passes to a higher level only if the bridge fails the assessment in the current level.

Load testing

When applying the standard safety evaluation methods, bridges often fail to pass the assess-ment calculation despite carrying normal traffic satisfactorily. One reason for this is because the normal methods for calculating the bridge resistance tend to be conservative as they do not take into account some reserve capacity that structures usually have. Consequently, the applied bridge model does not perfectly match with the real bridge itself. Load testing can be used to identify such sources of additional strength and to quantify the hidden reserve strength of individual bridges. The objective of load testing is to optimise bridge assessment which results in less severe and expensive rehabilitation measures on deteriorated structures. However, as the execution of a load test is costly, its use is recommended only when the be-nefits from the data gathered in the test are higher than the costs of the execution of the load test. The appropriate bridges are those for which structural idealisation is very difficult and

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those with a lack of documentation (drawings, calculations…). Only the bridges that fail the assessment by calculation should be considered as candidates for load testing.

In the document, the load tests are classified in 3 categories, according to the load level im-posed on the bridge during the test.

Proof load tests apply a high percentage of the design loading and are intended to obtain the carrying capacity on bridges (for example on old structures with absolutely no reliable data) where this information cannot be obtained in a simpler, cheaper and safer way. Despite exper-ienced personnel required for such procedure, due to very high level of loading, damages on the structure cannot be excluded.

Diagnostic load tests are the most traditional way of load testing and serve to verify and ad-just the predictions of an analytical model. As with proof load testing, the bridge is closed to normal traffic and the applied load is at a level similar to the serviceability conditions or nor-mal use of the bridge. As a consequence, extrapolation of the analytical models up-dated via diagnostic testing to the assessment of bridge performance at the ultimate limit states requires combination of test results and traditional analytical methods.

The novel concept of soft load testing was introduced through the development and imple-mentation of the new generation of bridge weigh-in-motion systems. These systems weigh moving vehicles at normal speed through instrumented bridges and can, in addition to the “normal” traffic data (axle loads, gross weight, speed, vehicle class, etc…) measure important structural parameters of the measured bridges, such as real influence lines and load distribu-tion factors. The method can be efficiently applied on a large number of bridges without in-terrupting the traffic. However, due to even lower level of loading than during diagnostic load testing, special care is needed when applying the results.

Further issues discussed in the report are: limitations of load testing, load testing methodology, bridge assessment based on

load tests, preliminary theoretical assess-

ment, selection of load tests, load test planning, execution of the load test, results and reporting, bridge assessment based on load tests and in the appendix C, main specifications on diagnostic load tests used in several

European countries.

Site-Specific Load AssessmentLoading represents the second part of the safety assessment equation. As for carrying capa-city, it is beneficial if information about realistic loading is available and can be applied in an efficient way.

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The main focus of the work done was on improving the reliability of methods for determining traffic loading on bridges. This is being done by determining the actual traffic loading from existing and newly collected weigh-in-motion (WIM) data.

Site-specific assessment of dead loading

Dead loading is defined as the gravity loading due to the structure and other items perman-ently attached to it. This may be divided into dead loading and superimposed dead loading. Bridges are unusual in that, especially for longer bridges, a high proportion of the total load-ing is due to the dead and superimposed dead load. Discussed in the report are also:

material properties and their measuring on site and probability-based assessment.

Assessment Live Load Model

Traffic loading on bridges varies considerably between regions and between sites within re-gions. For example, it has been found that the mean characteristic load effect from three sites in Slovenia are about 20% less that the corresponding values from three sites in the Nether-lands. Traffic load can be assessed at a site in one of two ways:

Monte Carlo simulation or calibrated notional load model.

In the Monte Carlo simulation approach, the procedure is quite elaborate, but it makes it pos-sible to simulate data representing years or decades of traffic from which such rare events can be identified. Statistical techniques are used to find the 100-year characteristic value from the database of load effects. It is good practice to identify the maximum-per-day load effects for several days of data. This data is generally consistent with one of the Extreme Value distribu-tions, Gumbel or Weibull. By fitting to such a distribution, it is possible to extrapolate to de-termine the characteristic value.

A calibrated notional load model is considerably easier to implement than a Monte Carlo simulation approach which requires specialist expertise. A simplified model was developed which aims to reproduce similar critical loading events from knowledge of the site-specific traffic characteristics without having to perform a full Monte Carlo simulation. The investiga-tion has been limited to the case of mid-span moment and end shear in simply supported bridges with spans ranging from 15 to 35 meters. The bridges were assumed to have two traffic lanes, one in each direction.

The model is different for the two load effects. For bending moment, the 3 rd axle of the 1000-year truck is placed at the center while the 3rd axle of the 1-week truck is placed αML from the end of the bridge. For shear, the most critical location for the 1000 year truck is when the 5 th

axle has just entered the bridge. At this moment in time, the third axle of the 1-week truck is placed αSL from the end. Will a non-technical reader understand the 1000-year truck and a few lines above a 100-year characteristic value?

The optimal values for αM and αS are 0.63 and 0.42[This value does not correspond with the figure where alpha 3 is more than 0.5L—Joan], respectively.

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Allowance for Dynamic Amplification

Dynamic modelling of bridges in the past has shown that dynamic amplifications of traffic loading due to the passing heavy vehicles are generally lower than those prescribed in differ-ent codes and standards. The most rigorous reductions were shown for the heaviest loading events, which occur due to several heavy vehicles on the bridge. Applying such over-conser-vative Dynamic Amplification Factor (DAF) is beneficial for the design of a bridge, as it may during the lifetime of the structure provide additional safety needed to withstand higher loads and reduced carrying capacity due to deterioration or any other reason. However, using such conservative DAF estimates in the bridge assessment stage may result in unrealistically high loadings and, consequently, unnecessary measures that the bridge is submitted to.

SAMARIS project has investigated realistic values of DAF for bridge assessment purposes. The new generation of bridge weigh-in-motion system, which is using instrumented bridges from the road network to weigh heavy vehicles, was upgraded to measure the dynamic re-sponse of the structure under random traffic conditions. The objective of the experiments was to establish the dependency between the DAF and the total static weight of the loading event (any combination of heavy vehicles on the bridge) and to see how repair of the uneven pave-ment would influence the DAF.

The 2-week measurements before and after resurfacing of the pavement on the test bridge captured 10 700 loading events for which dynamic amplification factors were calculated. The analysis of the results gave the following answers:

a) The DAF decreased drastically as a function of increasing weight of the loading events.

b) Resurfacing of the pavement decreased the average DAF factors of the heaviest loading events for another 50%.

c) There was no obvious correlation between the dynamic amplification factor and velocity of the heaviest multiple-vehicle events.

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It should be noted that this was the first test site where such extensive experiments were per -formed and that more experiments, supported by numerical modelling of extreme events, are needed before conclusions will be sufficiently reliable for, for example, updating the bridge assessment codes. Nevertheless, as the bridge selected was extremely susceptible to traffic vi-bration, it can be concluded that the real DAF values are much lower than those prescribed in the design codes. Before SAMARIS, this was only demonstrated with analytical simulations.

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FOREWORD AND ACKNOWLEDGEMENTSThis technical report is prepared on the basis of Contract No. GRD2-2000-30228-M between the European Community represented by the Commission of the European Communities and the Danish Road Institute acting as coordinator of the SAMARIS Consortium. Its goal is to provide an overview on the available practise in optimised bridge assessment and give to the end users through the examples the opportunity to learn how optimised assessment can provide from selecting over-conservative decisions related to bridge management.

This SAMARIS report was produced by the Work Package WP15 team of contractors: Slovenian National Building and Civil Engineering Institute (ZAG), Slovenia, Road and Bridge Research Institute (IBDiM), Poland, University College Dublin (UCD), Ireland, Technical University of Catalunya (UPC), Spain and Trinity College Dublin (TCD), Ireland,

with the assistance of the following subcontractors: Vienna University of Technology, Institute for Structural Concrete, Austria, Institute for Transport Sciences Ltd. (KTH); Hungary, Czech Road Administration, Czech Republic, Polish Road Administration, Poland, Norwegian Public Roads Administration, Norway and Directorate of the Republic of Slovenia for Roads, Slovenia.

The same time has also prepared the deliverable D19 State of the art report on assessment of structures in selected EEA and CE countries, available on http://samaris.zag.si/, which provided an overview on the assessment of structures in selected CE (Central European) and EEA (European Economic Area) countries. This was done through a questionnaire that was sent to six of these countries and through extensive survey of existing policies and literature on

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1. INTRODUCTIONAlthough this report is devoted to the different methods developed worldwide for the struc-tural safety assessment of bridges, it is good to clarify from the beginning the differences between condition assessment and structural safety assessment, because in some cases, dur-ing the development of the report, some mixing of both concepts is possible to occur. This is due to the fact that in some cases the available methods themselves do not clearly differenti-ate between the two concepts.

THE PREVIOUS PARAGRAPH SHOULD BE DELETED

The condition assessment is the process where, starting from the results of the inspection, the final objective is to determine the functional capability and the physical condition of bridge components, including the extent of deterioration and other defects. The condition as-sessment can be either qualitative, in the form of definition of classes, or quantitative, in the form of a so-called “condition rating”, a value that indicates the global state of conservation of the bridges and their ranking according to its value.

The structural safety assessment is the process where, starting from the actual resistance of the structure (up-dated with the results of inspection and testing) and the actual loading, the remaining safety (measured in terms of partial safety factors, reliability index, probability of failure or similar) is derived. This report presents a literature review of the methods applied in different countries or proposed by several International Bodies.

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2. CONDITION ASSESSMENT

2.1 Condition Assessment in general

Condition assessment (CA) of highway structures provides the owner or responsible authorit-ies for the maintenance of highway structures with the appraisal of the present situation of the structures. Assessment gives data about the intensity and extent of observed defects on the structures, the cause for these defects and possible deterioration processes and the impact of such findings on the safety and service life of the structures. These data are the basis for the estimation of possible intervention and for a rough estimation of costs for possible remedial work.

The assessment is not always an easy task. Sometimes deterioration processes have several causes and it is difficult from time to time to find the basic causes and simple explanations and understanding of the problems. Only when the problems are well defined and understand-able can a reliable treatment of defects be efficiently proposed and executed. For each de-fects, usually several types of possible remedial options can be proposed which depend on the technological possibilities as well as on the requirements for the users' safety, service life of the structure, operational requirements during repair (lanes closed, closing of the structure, weather conditions, etc.) and available funds.

2.2 Objectives of Condition Assessment

The main objectives of condition assessment are: Detect possible deterioration processes, Indication of the condition of the structure and its elements as well as the highway struc-

ture stock, Ranking the structure for urgent repair and maintenance strategies, Optimisation of urgent maintenance budget allocation.

These tasks are fundamental in assessing the safety and serviceability of structures. It is es-sential that every maintenance inspection be carried out in a consistent and reliable manner. To get valuable data for the evaluation of condition assessment and to use in further analysis results of condition assessment properly and effectively, a few basic demands should be ful-filled, such as: Structures must be inspected regularly at properly determined time intervals from the be-

ginning of their life time, i.e., since the structure is put into service or after every major repair work is carried out.

Different levels of inspections must be executed with adequately trained and educated personnel and with suitable equipment.

Some defects may have origins in the construction phase of the structure. Therefore, knowing the history of possible difficulties, which might have happened during the con-struction phase, can be very helpful for condition assessment.

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Catalogue of definitions and descriptions of defects, deterioration processes and their pos-sible causes should be available.

Also available should be well-defined methods for quantification of defects with respect to: their extent and intensity, their possible impact on the users’ safety and on durability of structural elements.

Experience and knowledge gained through permanent execution of inspections, evaluation of condition assessment and further analysis of the results of condition assessment have an im-pact on the development of new procedures for condition assessment or on improving exist-ing ones.

2.3 Short critical review of CA system in Europe

One of the previous SAMARIS documents – Deliverable D19. State of the art report on as-sessment of structures in selected EEA and CE countries – has presented detailed description of condition systems used around Europe. Existing procedures use a condition rating as a measure of a condition state of a structure or its element. The main difference among various procedures is the method used for quantification of the condition rating. Condition rating is an effective means of quantifying in a relative way the general deterioration of a structure. Methods have been developed for bridge management purposes to identify the most damaged structures for further in-depth inspection and examination and to establish preliminary priorit-ies for further rehabilitation. Similar methods can also be adopted for other highway struc-tures. Condition assessment should be based on a simple scoring for the inspected members or for the whole structure. The evaluation of any deterioration should take into account all types of defect revealed during an inspection, whose character, severity and extent might have a substantial impact on the safety and durability of the structural member or structural com-ponent.

Therefore, the evaluation of every damage type should account for: The type of damage and its affect on the safety and/or durability of the affected structural

member. Maximum intensity of any defects of the inspected members. Influence of the affected structural member on the safety and durability of the whole

structure (e.g. bridge) or structural component (e.g. bridge superstructure). Extent and expected propagation of the damage on the observed members within a com-

ponent and its influence on durability.

A review of methods for condition assessment of bridges used in Europe and the United States showed that there are basically two approaches to the evaluation of the condition of the whole structure based on the condition assessment of its elements:1. The first one is based on a cumulative condition rating, where the most severe damage on

each element is summed for each span of the superstructure, each part of the substructure, the carriageway and accessoriesbridge equipment (or utilities?) [I don't know what is meant by accessories in a bridge context – we need a different word – Eugene]. The final result is the condition rating for the structure, which can be used for a preliminary priorit-

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isation of the structure. Condition assessment systems based on this approach are for ex-ample used in Slovenia (see Figure 2.1).

Figure 2.1. Software implementation of the Slovene condition assessment system

2. The second method uses the highest condition rating of the bridge components as the rep-resentative condition rating for the structure itself. Result of this approach is number of structures in each deterioration class, but direct comparison of condition between struc-tures is not possible. This procedure is used in Poland, where in 2005 new guidelines for structural assessment were published. They have introduced a new classification of de-fects but the principle of condition rating was preserved.

2.4 Hazard issues in Condition Assessment

The problem of safety can be divided into three general concerns:

1. safety of the traffic and the road users,

2. personal safety of inspectors and

3. safety of the structure.

As the inspections carry an element of risk both to the personnel undertaking the inspection and the users of the structure, inspectors are required to comply with safety requirements con-tained in the various national codes. Procedures to reduce risk also include detailed risk as-sessments that are undertaken before an inspection is carried out. A common rule is that in-vestigation activity might not in any way impede the traffic, although it might cause tempor-ary disruption on condition that road users are noticeably warned about the situation.

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2.5 Advice on good practice of Condition Assessment

Through the analysis of the past projects, such as BRIME or COST 345, and the results of the SAMARIS questionnaire, presented in SAMARIS deliverable D19 (), one may summarise some directions on how to perform and maintain the quality of the Condition Assessment pro-cess. The main factors are described below.

2.5.1 Catalogue of defects

Many countries have generated catalogues of damages to be used in every day practice. They allow for quick and certain descriptions of damage and, which is more important, for adroit identification of potential origins of damage. A good catalogue will allow for anticipation of further damage progression and its consequences for the future structure condition. The cata-logues should be routinely updated and revised. The best way to keep it modernized is to de-velop an appropriate electronic (Internet-based) tool. On the other hand, many road adminis-trations do use sophisticated tools (like electronic notebooks) during on-site data collection. That solution allows for easy regular updating of the catalogue (database) in the mainframe computer, similar to an antivirus software, or online connection to the database through the Internet.

The SAMARIS project has developed such an Internet based catalogue of defects, built around examples from all over the Europe, to characterise the widest spectrum of damage types (see Figures 2.2 to 2.4). At the moment, 20 main types of damage, with an additional 47 sub-types, are available. The important feature of this catalogue is that it can be updated with new examples by any registered user. A detailed description of the SAMARIS damage cata-logue is given in the Appendix X.

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Figure 2.2. Defects software – opening page

Figure 2.3. Defects software – types of defects

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Figure 2.4. Defects software – example of available information about a specific defect

2.5.2 Training of inspectors

The weakest link in the bridge assessment chain is the human factor. The methods and pro-cedures of structure examination are specified and approved, as are the tools for the job. Nev-ertheless, human intelligence, training and experience are often needed to decide exactly where further tests and investigations must be done and what should be measured. Thus, qual-ification of the inspectors plays a key role in the process of bridge management and the road authorities should apply procedures to ensure that they are fit and accurate. This concerns not only professional qualification but also personal skills and responsibility.

Unfortunately, in most European countries bridge inspectors are employed for bridge inspec-tion work only. They do not deal with bridge management problems and are not involved in new projects but are expected to know how their decisions influence the bridge management process.

Still, in most countries the inspectors are expected to be structural (bridge) engineers with good experience. Additionally, they must attend special trainings to gain new knowledge about deterioration and structural safety of structures and about technologies and procedures for assessment of damages. Some countries have introduced special systems for certification of the inspectors. Training courses, accomplished with the official certificate, which enforces the bearer to execute inspections, are practiced in Czech Republic, Poland and Spain. In Po-land the Certificate is a formal requirement for bidders, who want to work for highway ad-ministrations. In Slovenia there is currently no training system. Until year 2000 all bridge in-spectors were recruited from a single, highly specialised state owned company, which ensured qualification of its workers. Others rely on strong supervising system.

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Such activities must be encouraged as many bridge owners, including some major owners of highway infrastructure, do not understand the necessity and importance of the quality of the work that the condition assessment inspections do on a regular basis.

The basis of future inspectors qualification development should be constant training and edu-cation. The increasing value and sophistication level of engineering structures requires mod-ern knowledge and constant experience progress from maintenance inspectors. Herein some other reasons of education necessity are presented:

new, modern materials – as well as construction as repairing ones, new types of structures (new points of weakness), new, more precious and more efficient techniques of investigation, increase of load level due to traffic development, environmental changes, which became more aggressive, sudden increase of defects – in some countries structures were built more or less

the same time (due to war losses or time of prosperity for example) which means that at the same time more serious, not familiar yet, harms would appear.

2.5.3 Health issues of inspectors

The good physical condition of the inspectors is a crucial as it affects both; the quality of the work and above all, the safety of inspectors. They always work on fresh air, often on a high structure or an inspection platform which requires special physical capabilities, including fit-ness and proper weight. The inspection process must not suffer by any indolence of the in-spector, who would omit an important part of bridge because not being physically capable to reach it. The inspector must be able to locate, recognize and assess the damage, so any oph-thalmic disorder, including color blindness, are not acceptable.

In conclusion, the bridge owner should attaché importance to the physical condition of the in-spectors as its lack might affect the structural condition assessment.

2.5.4 Suitable equipment for inspectors

The other important factor in condition assessment procedures is the inspectors’ equipment. It refers to both, to the equipment that allows for reaching the least accessible bridge compon-ents and the measurement tools.

In the first case, the key issues are the inspectors’ safety and quality of the inspections. In the first case, the inspector can neglect certain important bridge areas of or inspect them indiffer-ently. It is very likely that the worker, who is under stress due to safety deficiency, will com-mit mistakes, which will clearly have a negative impact on the quality of the work.

The quality and availability of investigation tools and equipment is also extremely important. Lacking it will tailor the range of investigation to the areas, which the inspector can cover and will thus not provide the owner with the clear insight into the condition of the inspected bridges. Clearly, all equipment must work properly and should posses and appropriate certi-ficate.

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2.5.5 Range of available investigation

To obtain sufficient information to select the most appropriate maintenance strategy, visual inspections are often supplemented by testing. There is a wide range of test methods avail-able. They include sampling to obtain a measure of the material properties; use of non-de-structive methods for detecting hidden detects, monitoring to determine the change in the con-dition of a structure over time and load testing.

During a bridge inspection, the structure is examined according to the inspection standards and guidelines. Inspections are performed at different levels, from most common regular in-spections, which are generally only visual, to more detailed inspections, that are done at longer intervals or on special request. During these inspections the defects are localised on element level and are assessed according to various attributes (e. g. degree of deterioration, influence on structural and traffic safety, durability, importance of the structure etc.). Then, the potential maintenance measures can be identified based on the remaining service life of the structure the whole-life costs.

The tests used are generally selected by the inspector and are based on previous experience and engineering judgement. The selection of test methods should also take into account re-commendations from the various organisations that specialise in structural testing. The in-spector may use different types of investigation to get the structure condition. In general the methods could be divided into semi destructive and non destructive methods.

During structural testing, some destructive interventions are normally needed to extract the specimens, which are later tested in the laboratory. Selection of test locations and type of test-ing varies from one case to another. Generally, the selection of the number and exact position of the test locations depends on:

Type of the structure. Test locations are selected on different elements of the struc-ture, such as columns, beams, slabs, abutments on bridges; portal structure and in-ner lining in tunnels; entire wall on the retaining structures.

Accessibility. The engineer should ensure that all parts of the structure, as spe-cified in the specifications, are tested.

Number of typical areas of each type of deterioration type. The extent and intensity of deteriorated locations.

A few locations in a good condition should also be selected as a reference. At each location several tests may be carried out to provide information on condition and to enable the future performance to be predicted and, if necessary, determine the most appropriate rehabilitation work. Tests may be carried out on-site (destructive or non-destructive tests – NDT) or in the laboratory on the samples taken from the structure.

Destructive testsOn concrete structures the destructive interventions are carried with the aim of: Obtaining cores for physical, chemical tests on the concrete, on-site or in the laboratory.

Physical tests include concrete strength (compressive, tensile, splitting tensile), static mod-ulus of elasticity, Poisson’s ratio, density and freeze-thaw resistance. Chemical tests in-clude measurement of chloride profiles, carbonation depth and chemical composition, and petrography investigation for determination of cement content; alkali-carbonate reaction and alkali-silica reaction. Cores can also be used to obtain other information such as thick-

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ness of the structural elements, either where the dimensions are unknown or for checking the compliance. They can also be used for calibrating NDT methods. Cores are also used to determine tensile strength using the pull-off method or to determine the bond strength between two concrete layers. The depth of cracks and the variation in their width with depth can also be investigated.

Determining in-situ concrete stresses by drilling cores or cutting slots in the concrete. Inspecting the conditions inside post-tensioning ducts using an endoscope inserted through

holes drilled into the ducts. This can be used to determine whether any voids are present and the condition of exposed tendons and the state of the tendon anchorage.

Undertaking a more detailed examination at locations where damage has occurred by re-moval of the cover concrete. This may be done to determine the cross-section of the rein-forcement or the condition of a length of a post-tensioned tendon. Access is also required for the assessment of stresses in tendons. Such measurements can be made using a special device on the tendons and measuring the strain released as the wires are cut. These invest-igations are also carried out to obtain specimens of the reinforcement (ordinary or prestressed) for investigation the chemical composition or to determine their mechanical properties.

Applying certain NDT methods. Some techniques require minor damage to be caused to the structure. Quite often the measurement technology requires direct tool access to struc-tural part while it is covered with other material (pavement for example)

On steel structures destructive intervention is carried out with the aim of: Obtaining samples for investigation of their mechanical characteristics, chemical

composition and susceptibility of material to brittle failure. Drilling small holes into the steel for measurement of principal stresses. Obtaining specimens for the investigation of welds.

On masonry structures destructive intervention is carried out to obtain specimens for the in-vestigation of:

Mechanical properties (compressive, bending, tensile and, splitting tensile strength) of the masonry units and the mortar.

Composition of the fill using trial pits. Durability of the masonry units (freeze thaw resistance, absorption, saturation

coefficient). In-situ measurements of the compressive and shear strength of the masonry. Strength of the anchors.

Non destructive testsNon destructive methods play an important role in the investigation of individual structures but to ensure that they are applied in a uniform manner guidelines are necessary.

If special defects occur (defects where cause and extent cannot be determined during a visual inspection), the bridge inspector may wish to undertake additional investigations. During the course of such investigations the preference is to use non-destructive or semi-destructive test-ing methods.

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As a result of the application of non-destructive methods the investigated defects have to be assessed by taking into account the existing assessment guidelines. Furthermore, based on the detailed analysis of defects, possible rehabilitation measures, costs of these measures and the expected deterioration behaviour, with and without measures, should be described.

Beside the application in frame of inspection of structures non-destructive testing can be used to update the inventory data, e. g. material properties or geometrical data.

Table 2.1. Capability of NDT-techniques for detecting defects in concrete structures

Method Capability of defect detection

Cracks in concrete. scaling corro-

sionbuild-in objects

Faults, honey-comb.

thick-ness

voidsin ducts

reinf.fracture

Radar – + – + o o – –

Thermography o1) + – – o1) – – –

Radiography o – – o o – – –

Impact Echo o + – o o + o –

Ultrasonic o + – o o + – –

Potential map. – – + – – – – –

Magnetic flux – – – – – – – +1) in combination with water+ good o medium – poor – danes

Table 2.2. Capability of NDT-techniques for detecting defects in steel structures

MethodCapability of defect detection

surfacecracks

internal cracks

fatiguecracks

internalvoids

poros.and slag

thick-ness

blister-ing

corro-sion

Radiography – o 2) o 2) + + o o +Magnetic part. + – – + – – – –

Eddy Current o – – - o o – –

Ultrasonic o + – o o + – –2) if beam is parallel to the cracks+ good o medium – poor

2.5.6 Safe and long-term data storage (written documentation, computers, software)

Since the technology race has started at the end of the twentieth century the problems of data recording and storage have increased. The main difficulty is the 100 years required lifetime of the bridge compared to the lifetime of the data storage system which can change every dec-ade. Before the computer era the results of investigation have been stored on paper or micro-films, which allowed for free entry for any future user. Disadvantage of that solution is the

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flimsiness and volume of the paper documents. Many files have been lost due to war con-flicts, fire, floods, reorganisations etc. On the other hand, digital means are easy to store and copy, but require sophisticated appliances to be readable. The other problem is the archive data, which must be digitalised to comply with the modern requirements.

Only a few asset management systems store the results of tests which enable changes over time to be monitored. This type of information can help predicting the future deteriorations and select the optimum maintenance time on the basis of whole-life costing. At present there is a lack of data and a lack of experience but this is likely to improve with time.

2.5.7 Quality Control of Condition Assessment

In order that the inspectors can provide reliable results from the visual inspections, they must have a basic knowledge of bridge design and construction, and should be familiar with the clear criteria for inspection and the detailed catalogue of defects and should attend the appro-priate training courses. However, this is still not enough to ensure reliable inspections. To en-sure that the results define the actual condition of the structure properly, it is as important to ensure that the inspections have been performed in accordance with the required procedures and to verify the results.

When a large number of structures is inspected, a survey of inspections should be carried out as a part of a quality assurance procedure. This ensures that the results of visual inspections indeed correspond to the condition of the structures inspected. Surveys should be done by in-dependent inspection teams, which inspect a certain percentage of the total number of bridges and, then, compare the results.

The selection of bridges to be checked should account for the different types of structure, dif-ferent materials, different inspectors and different locations. It is also important to check both structures that are in a good condition and others where the appointed inspections have found significant defects.

Making this kind of survey means that some bridges are inspected twice and results from two different inspectors are available. As results from the two inspectors will be to some extent different, it is necessary to define criteria for validation of the results. Usually, only the im-portant results are compared, while the “minor defects” are allowed to be different. Therefore, the following criteria for verification of the inspection results can be recommended:

Location of the bridge should be undoubtedly allocated. Inspection should be carried on the basis of past inspection results. It should be checked that: all important defects have been properly detected and located, all important defects have been described as prescribed in the Manuals. quantities required for repair works for all the important defects have been properly

estimated. where a risk to users or to vehicles has been identified, a report has been produced. defects affecting the load capacity and durability are detected and properly defined. Inspection records should meet manuals regulation and/or owners requirements. Final opinion regarding structure condition and possible actions should be clearly

stated.

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2.6 Integration of condition assessment into the remaining load capacity of the structure

One of the key points in the safety assessment of an existing structure is how to take into ac-count the actual state of the structure including the influence of deterioration and damages in its remaining capacity. One possibility is to use a so-called condition factor related to the con-dition of the member or the condition rating (if available). As described in the SAMARIS de-liverable D19, the concept of condition factor is used by different Agencies and Owners. However, in many cases its numerical value is based only on engineering judgement. Slove-nia and the USA are the only countries identified where the condition rating is directly used to estimate the remaining structural capacity of a deteriorated structure.

The LRFR (Load-Resistance Factor Rating) Manual in the USA [4] uses the following gen-eral expression in determining the load effect (axial force, flexure or shear) capacity:

For the Strength Limit States: C = φc φs φ Rn (φc φs ≥ 0.85)

For the Service Limit States: C = fR

RF= rating factorC = capacityfR = allowable stress specified in the LRFD CodeRn = nominal member resistance (as-inspected)DC= dead-load effect due to structural componentsDW= dead-load effect due to wearing surface and utilitiesP = permanent loads other than dead loadsLL= live-load effectIM= dynamic load allowanceγDC = LRFD load factor for structural componentsγDW = LRFD load factor for wearing surfaces and utilitiesγp = LRFD load factor for permanent loads other than dead loadsγL = Evaluation live-load factorφc = condition factorφs = system factorφ = LRFD resistance factor

The condition factor specifies the estimated reduction to account for the increased uncertainty in the resistance of deteriorated members. Once the member experiences deterioration and be-gins to degrade, the uncertainties and resistance variability are greatly increased. The factor accounts for the increases in uncertainty and anticipated future accelerated loss, but not for the observed change in the actual physical dimensions of the members. The specified ap-proach is to take the as-inspected member information and apply it in finding the member res-istance and then apply the condition factor to decrease the deteriorated resistance for reasons

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previously noted. The condition factor is linked to the condition rating as defined in the NBI (National Bridge Inventory) through the relations presented in tables 2.3 and 2.4.

Table 2.3. Condition factor

Structural condition of a member φc

Good or satisfactory 1.00Fair 0.95Poor 0.85

Table 2.4. Relation between member condition and condition rating

Superstructure conditions rating (NBI) Equivalent member structural condition6 or higher Good or satisfactory

5 Fair4 or lower Poor

In Slovenia, during the last 10 years, a method based on the same principles has been used [33]. In a similar way as in the USA evaluation code, the effective carrying capacity of deteri-orated concrete bridge members is determined by multiplying the design resistance with the capacity reduction factor. The main difference is that this reduction factor is directly related to the condition rating of the bridge in the following way:

Φ = BR × e -α βc V

where:

BR is the bias of carrying capacity, i.e., the ratio of existing and designed mean resistance of the critical member section.

α is the deterioration factor, accounting for:

the general condition of the observed member,

the anticipated further reduction of the carrying capacity until next inspection,

α is related to the deterioration class or to the condition value of the inspected part of the structure RC, which is obtained as the sum of all damage values associated with each detected damage on the structure of part of the structure of concern (see D.19). The re-lation between the condition value and the deterioration factor is presented in Table 2.5.

V is the coefficient of variation of the member resistance recognising also the reliance of the testing data,

Βc is the target value of the safety index, accounting also for the redundancy of the struc-ture.

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Table 2.5. Relation between condition value and deterioration factor

Class Inspected condition Necessary intervention Condition

value Rc

Deterioration factor R

1 Very good No maintenance/repair work required <5 0.32 Good Regular maintenance work needed 3 to 10 0.43 Satisfactory Intensified maintenance/repair work needed

within 6 years 7 to 15 0.54 Tolerable Substantial repair work needed within 3 years 12 to 25 0.65 Inadequate Immediate posting and repair required 22 to 35 0.76 Critical Immediate closing, then repair (strengthening)

required >30 0.8

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3. SAFETY ASSESSMENT

3.1 General about safety assessment

Add 1 page general description about safety, explaining loading and capacity.

Safety factors, see example.

Five-level approachFive levels for the assessment of highway structures are recommended varying from simple but conservative to complex but more accurate. These levels of assessment, numbered 1 to 5 with Level 1 being the simplest and Level 5 the most soph-isticated, are explained below. Means for carrying out assessments at Levels 1, 2 and 3, are now generally available. Levels 4 and 5 involve structural reliability calculations and are currently only used by experts. In general, the safety assessment will begin at level 1 and will pass to a higher level only if the bridge fails the assessment in the current level.

The earlier SAMARIS report D19. State of the art report on assessment of structures in selec-ted EEA and CE countries – has presented a detailed description of the safety assessment ap-proach around Europe, USA and Canada. Applied procedures are mainly based on the exist-ing design codes (levels 1 and 2) and only a few countries have specific codes for bridge as-sessment.

Level 1 assessment

This is the simplest level of assessment, giving a conservative estimate of load capacity. At this stage, only simple analysis methods are necessary and partial safety factors from the design or assessment (if available) standards are used to give a conservative estimate of load capacity.

Level 2 assessment

This level involves the use of more refined analysis and better structural idealisation. The more refined analysis may include grillage analysis or possibly finite element analysis whenever it is considered that these may improve the result. Non-linear and plastic methods of analysis (e.g. yield line or orthotropic grillages) may also be used.

This level also includes the determination of characteristic strengths for materials and the cor-responding partial safety factors based on existing available data. This may be in the form of existing mill test certificates or recent tests on another similar structure. If any new tests are to be carried out on the structure being assessed then this should be considered as a Level 3 assessment.

Level 3 assessment

Level 1 and Level 2 assessments make use of assessment live loadings given in the standards or estimated as applicable generally to the network. Level 3 assessment includes the option to determine and use structure-specific loading (see chapters 2.2 and 2.3). For many bridges,

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particularly where located on a lightly trafficked road, the use of bridge-specific traffic load-ing can be quite beneficial.

Level 3 may also make use of material testing to determine better estimates of characteristic strength or yield stress. In this case, again partial safety factors from the design or assessment Standard are used. Furthermore, in Level 3, consideration should be given to the use of load testing in the form of soft load tests and diagnostic load tests (see chapter 2.1). It should be noted however, that pending further research, proof load tests are not recommended, unless performed by experienced people who use proper diagnostic equipment that prevents from unnecessary damage on the structure.

Level 4 assessment

Level 1 to 3 assessments are based on code implicit levels of safety, incorporated in the nom-inal values of loads and resistance parameters and the corresponding partial safety factors. The corresponding reliability is related by implication to past satisfactory performance of the bridge stock through calibrations where these have been carried out.

Any calibration involves an element of averaging which makes the results acceptable for the bulk of the structures of the type concerned. Nevertheless, the resulting rules may be over-conservative for a particular structure which may be significantly different in some way from the norm used in the calibration. Level 4 assessments can take account of any additional safety characteristic to that structure and amend the assessment criteria accordingly.

Any changes to the criteria used in this level may be determined through rigorous reliability analysis, or by judgemental changes to the partial safety factors.

In the decisions based on Level 4 assessments, care should be taken not to double count bridge-specific benefits which have already been taken into account. For instance, if methods that account for system reserves, such as the yield line method, have already been used in Levels 2 or 3, the same or similar reserves should not be applied again in Level 4 to further optimise the structural model.[I don't understand this point and am not sure that it is written correctly – Eugene. I tried to rewrite. Is this clearer? – Aleš]

Level 4 assessment may be particularly beneficial in the following circumstances:

1. The bridge assessment criteria have been primarily devised for longitudinal effects on main deck members. All other elements such as cantilever slabs, cross beams, pier heads etc. may be examined in Level 4 for determining element specific target reliabil-ity.

2. The whole life reliability of a structure, in the absence of any significant deterioration, increases from the day it is constructed to the end of its functional life. This effect has not been taken into account in the present criteria.

3. The failure of a bridge carrying a minor road over a very small watercourse will obvi-ously have much lower consequences than the failure of a major bridge.

Level 5 assessment

Level 5 assessment involves reliability analysis of particular structures or types of structure. Such analyses require statistical data for all the variables defined in the loading and resistance equations. The techniques for determining the probability of failure from such data are now available and can be undertaken relatively easily in modest time frames.

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Level 5 assessment provides greater flexibility but the results are very sensitive to the statist-ical parameters and the methods of structural analysis used. At present, therefore, Level 5 as-sessment should not be used in conjunction with prescribed target reliability, as there is no guarantee of achieving consistency in different assessments. However, Level 5 may be used if the target reliability is determined specifically by the same Assessing Organisation for a class of identical structures or structural elements, e.g. pier cross-heads, taking the reliability of the structures as designed in respect of the assessment load, as the target reliability.

Level 5 assessment requires specialist knowledge and expertise.

In the case that a bridge has failed to pass all explained 5 levels or when the available data and documentation of the bridge does not allow the building of a reliable theoretical model of behaviour, then proof load testing may be used for accurate evaluation of load-carrying capa-city (see chapter 3.2). [But we already said not to use it in Level 3 – is this not a contradic-tion? I have added explanation there- Aleš]

3.2 Load testing

When applying standard calculation methods in the safety evaluation of bridges, there are many cases where bridges that seem to carry normal traffic satisfactorily, fail to pass the as-sessment calculation. One reason for this is because the normal methods for calculating the bridge resistance tend to be conservative and in many cases do not take into account some re-serve capacity that comes from additional sources of strength (composite action between slab and girders in bridges that were designed as non-composite, rigid or semi-rigid connections that were designed as flexible,…). In summary, the available model of the bridge does not perfectly match with the real bridge itself. Load testing can be used to identify such sources of additional strength not considered in the theoretical model, and to quantify the hidden re-serve strength of individual bridges. The objective is to optimise bridge assessment to find re-serves in load carrying capacity. Savings in optimised assessment and, consequently, in less severe rehabilitation measures on deteriorated structures can be tremendous. This will be il-lustrated in chapter 3.2.3. However, because the execution of a load test is costly, its use will be only recommended when the benefits from the data gathered in the test, implemented in the safety assessment process and deriving in the corresponding maintenance policies, are higher than the costs of the execution of the load test. Bearing in mind this economical issue, the bridges that are to some extent the best candidates for a load test are those for which structural idealisation is particularly difficult, and bridges with a lack of documentation (drawings, calculations…). Only the bridges that fail the assessment by calculation should be considered as candidates for load testing.

3.2.1 Classification of load tests

In the present document, load tests are classified in 3 categories, according to the load level imposed on the bridge during the test: soft load test, diagnostic load test and proof load test.

Soft load testsThis type of test corresponds to the lowest level of load application. The test is aimed to sup-plement and check the assumptions and simplifications made in the theoretical assessment. In other words, it provides information to optimise the structural model used for safety assess-

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ment of a particular bridge which should answer if a bridge is safe or not under specific (usu-ally normal) traffic conditions.

The novel concept of “soft load testing” was introduced through the development and imple-mentation of the new generation of bridge weigh-in-motion systems (B-WIM). These systems weigh moving vehicles at normal speed through instrumented bridges and can, in addition to the “normal” traffic data (axle spacing and loads, gross weight, speed, vehicle class, etc…) provide information about structural behaviour of the bridge that the sensors are attached to.

In the last few years, advanced bridge B-WIM systems [ ] were upgraded to evaluate struc-tural data (influence lines, statistical load distribution and impact factors) from signals of nor-mal traffic. This means no need for pre-weighed vehicles to load the bridge statically, as it is done during a traditional diagnostic load test. Furthermore, this means no interference with traffic during measurements as the bridge need not be closed to traffic, which is very import-ant when the load test is used to asses an existing structure. The only pre-weighed vehicle ne-cessary is the one that is used to calibrate the bridge WIM system to correlate the measured strains with the actual vehicle loading (axle loads). Also, due to the lower load levels, there is no risk of overloading and potentially damaging the structure, which is one of the main con-cerns with other load tests.

It will be shown in the next chapter that it can be very efficient in acquiring the realistic influ-ence lines (of bending moments especially), which can, in old bridges particularly, differ con-siderably from the theoretical ones (for example, due to restricted movements of the expan-sion joints or because the design details at the time of assessment are not known). Further im-provements of the structural model can be achieved if real dynamic amplification factors (DAF) and load distribution to different structural members are known (measured). These can be also obtained during a soft load test.

As it will be shown in the Appendix XXX, using results of soft load testing can result in de-claring a completely good bridge safe, while when not doing it, due to lack of information about its behaviour under the load, the bridge would have to be posted or some over-conser-vative measures would be prescribed. Such decisions have obvious severe financial con-sequences.

If a diagnostic load test has been carried out in the bridge before its opening, then soft loading tests can be repeated later at different points in time to check if the results of the initial test are still valid.

Limitations of soft load testing

1. Soft load testing is not intended to predict the ultimate state behaviour of a bridge but rather to optimise its structural model used for safety assessment.

2. The validity of bridge assessment is often short-term (from a single specific event to a few years or a decade) and so should be the validity of the soft load test. In critical cases, on bridges with low structural safety, it is recommended to repeat it in regular terms.

3. If the traffic loading, due to the traffic density of overloading, can exceed the one dur-ing the soft load test for more than 50%, it is recommended to extend the measure-ments to capture such vehicles or to perform normal diagnostic load test.

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4. At this moment, the soft load testing procedure has only been tested and used on bridges shorter than 40 m.

Diagnostic load testsIn this case, the test uses pre-weighed vehicles and is also aimed to supplement and check the assumptions and simplifications made in the theoretical assessment. This type of test is ex-actly located in the Levels II and III of the assessment process as they are looking for a better structural idealisation and appraisal of material properties.

Diagnostic tests (Figure 3.5) serve to verify and adjust the predictions of an analytical model. Contrary to the soft load test, the bridge is closed to normal traffic and the applied load is at a level similar to the serviceability conditions or normal use of the bridge. As a consequence, extrapolation of the analytical models up-dated via diagnostic testing to the assessment of bridge performance at the ultimate limit states is not feasible. There is no safe basis to extra-polate the results of tests carried out with fairly low levels of loading, unless the materials and their interconnections can be determined with a degree of certainty and some pattern of load carrying behaviour at advanced levels of loading has been established.

Normally, diagnostic tests are classified according to the variation with position/time of the load applied to the bridge. Therefore, they are divided into: static (the load, a vehicle or a weight, is applied at fixed points), pseudo-static (a vehicle moves across the bridge at a crawl speed) and dynamic (the vehicle moves at normal road speed over the undisturbed pavement or over

an obstacle placed on the pavement, provoking an impact. In other cases the vehicle sud-denly stops on the structure after reaching a high speed to record the dynamic response of the bridge).

Appendix A gives an example of a dynamic diagnostic load test in a prestressed concrete bridge.

One of the main objectives of this type of test is to estimate correctly the traffic load distribu-tion between the main load carrying members. The tests provide useful information when structural models, including grillage or finite element methods, can not accurately predict be-haviour due to uncertainties in member properties, boundary conditions and influence of sec-ondary members. In the design of the test, it is important to enassure the placement of the test load at various positions to determine the response in all critical bridge members.

The load level achieved in the test must be representative of serviceability conditions. It is re-commended to achieve a load level corresponding to a 5 year return period. In practice, this means to raise the test load up to approximately 60 % of the characteristic live load present in the design code. It is recommended to never go beyond 70 % of the design load. However, this recommendation may vary from country to country (see appendix C).

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Figure 3.5. Diagnostic load test with pre-weighed vehicles, static (left) and dynamic

with one vehicle (right)

Some countries, like Estonia, Slovenia, Spain, Latvia, …. are still obliged to perform dia-gnostic load test on every larger bridge after construction or major rehabilitation (strengthen-ing).

Proof load testsIn this case, the load test is used to verify component and system performance under a known external load and is aimed to provide an alternative assessment methodology to the theoretical assessment. From this point of view it is located even beyond Level V of the assessment pro-cess (one could talk about a Level VI), since the use of such tests, due to the risks of collapse or of damaging essential elements of the structure, must be restricted to bridges that have failed to pass the most advanced theoretical assessment and are therefore condemned to be closed to traffic or demolished.

In this test, the bridge is loaded with a high percentage of the design loading to prove that its behaviour is in compliance with the design. One of the main concerns when executing a proof load test is the level of damage and risk due to the high load applied. The load is applied in -crementally and the most important decision is when the loading increase must stop in order to not permanently damage the bridge or even cause a failure. The way to control the risk is by appropriate monitoring during the test.

The use of this type of test may also be recommended in the case of bridges with high re-dundancy levels and where an accurate theoretical model of behaviour or an accurate defini-tion of geometry and material properties is not possible due to a lack of information (no drawings). This can be, for instance, the case for old masonry arch bridges.

3.2.2 Load testing methodology

Inspection and condition stateThis comprises the collection of all available information on the bridge. It is necessary to know the condition of the bridge to see if it is feasible to carry out a load test. If the bridge or some parts are very heavily deteriorated, the execution of a load test may be too risky. Based on the results of this inspection the type of test to be carried out may be also decided. The ac-

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curate appraisal of the state of condition of the bridge is of main importance in the case that a proof load test is foreseen. The effect of corrosion in reinforcing bars or in the section loss of flanges and webs of steel girders should be documented. The deterioration of the parapet, shoulder or any other secondary element that may provide additional resistance to the bridge has to be investigated.

Preliminary theoretical assessment At least a theoretical model of the bridge is necessary during the preparation and execution of the diagnostic or proof test. Based on this model a preliminary assessment will help to decide on the type of load tests to carry out, the type of sensors to use and their location within the bridge. Preliminary theoretical assessment is not required before the soft load test as the load-ing there is simply the normal running traffic.

Selection of load testsAs stated above, load tests have two basic purposes. If the structural model needs to be optim-ised, then soft or diagnostic load testing should be considered. The soft load testing may often be sufficient to acquire the necessary structural parameters, but the traditional diagnostic load tests will still be necessary to test longer and more complex structures.

Proof load tests are more or less reserved for bridges with a lack of documentation (drawings, calculations…) and any other information about their carrying capacity.

Selection between diagnostic and proof load test is based on the results of the preliminary theoretical assessment. In the case that a proof load test is selected, before the planning and execution, the target proof load has to be calculated since the type and placement of load as well as the instrumentation and data acquisition setup depend on the target proof load level (see 3.2.3).

Load test planningThis involves the definition of the test equipment: loading means and the monitoring during the tests (type of sensors, location into the bridge, special safety issues…). Also the methodo-logy of loading (increase of load, load steps, definition of the load intensity chart) and the loading locations should be defined. A protocol and criteria to decide on the advancing or stopping of the loading based on the available results is also necessary to be defined in this phase. In general, the test should stop when some evidence of non-linear behaviour becomes clear.

Other issues to be considered in the planning stage are: infrastructure (accessibility…), per-missions (closure of traffic, work at night…)

Execution of the load testIn the case of a static test, the maximum target load should be applied in several increments while observing structural response. Also in this case, the measurement of the temperature during the period of execution is necessary to correct the results of the test according to the temperature variations.

Before a new loading scheme is applied, it is mandatory to remove from the deck all loads corresponding to the previous loading scenario. This is not necessary in the case of simply supported bridges.

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Load-deflection response at critical locations should be measured during the loading increase to determine the onset of non-linear behaviour. Once any non-linearity is observed, the bridge should be unloaded immediately and the deflection recovery recorded.

In the case of proof load tests, the following equipment is recommended: load cells for the measurement of test loads, strain gauges for measurement of strain and curvature, displace-ment transducers, inclinometer, vibration measuring equipment, thermometer and anemo-meter and equipment for acoustic emission (AE) analysis to observe the development of cracks and material degradation. The load is applied in accordance with a loading scheme and held for a certain time period

The personnel in charge of the execution should have a proved qualification and experience in the execution of similar tests.

Results and reportingLoads must be moved to different positions to check all load path components. Upon execu-tion of a proof load test and load removal, the bridge should be inspected to see that no dam-age has occurred.

3.2.3 Bridge assessment based on load tests

The possibility to integrate the results of a load test into the load rating assessment framework is different depending on the type of test carried out.

Procedure for soft load testsThe main purpose of soft load testing is to validate or update the structural model and to get a realistic estimate of its safety, for example, expressed by the RF formula (see chapter …). Due to its ease of application it is particularly recommended for:

1. Old bridges, with no drawings and no information about the design and construction details and about behaviour under loading.

2. Posted bridges, to check if the posting (limiting of the traffic loading) is justifiable or it can be released or removed.

3. Providing input data for efficient management of heavy vehicles with special permits.

Being a method under development, it has not been verified yet on longer structures and should therefore be used only on bridges where the governing traffic loading effect is one vehicle per lane. This practically means that the upper length of the bridge to apply soft load testing is around 30 m, with up to 3 spans. This, in general, still includes more than 80% of all bridges. In Slovenia, for example, only 3.5% of all brides from the state road network ex-ceed these criteria. If motorways are included, the percentage raises to around 16%.

Although it can be combined with more sophisticated analytical calculations, its main advant-age is to combine it with simple and quick calculation methods which allow for efficient eval-uation of a great number of under-rated bridges. [Ales – the point made about diagnostic load testing is very relevant here and should be repeated: "extrapolation of the analytical models updated via testing to the assessment of bridge performance at the ultimate limit states is not feasible. There is no safe basis to extrapolate the results of tests carried out with fairly low levels of loading, unless the materials and their interconnections can be determined with a de-

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gree of certainty and some pattern of load carrying behaviour at advanced levels of loading has been established". – Eugene ] I tried to explain non feasibility for the ultimate limit states conclusions above

I WOULD LIKE COMMENTS FROM OTHERS PLEASE!

I think that if we use soft load testing with appropriate time constraints and for normal traffic (for example, should we post a bridge or not? or, can we do something with this bridge which RF is 0.9 and looks OK?), it should be perfectly safe in a sense that if we use parameters measured as a result of the heaviest (usually overloaded) vehicles then even heavier vehicles should not change the structural behaviour considerably. At least not more than it would be well covered by the safety factors. The question is how to deal with structures:

that are really heavy deteriorated and where additional loading may indeed exceed the linear behaviour of the structure. The question is which structure is sufficiently deteriorated that this could happen under normal, even overloaded traffic. We plan experimental validations in ARCHES, but we should conclude something here.

that have secondary elements, like hand rails, that may influence the behaviour un-der higher loading. I cannot imagine which elements would cause such non-linear behaviour at realistic traffic loads, but if you know examples, we should list them.

where much higher overloading that the measured one is expected (i.e. > 20% heavier). I am confident that 3-axle trucks over 32 tons or 5-axle semis over 48-tons or 4-axle rigid trucks over 40-tonnes are quite unlikely to appear in Slovenia, but what, if they are (and load effects from these vehicles are multiplied at least by safety factor 1.4). Can we assume that it is very likely that vehicles at 80% of the expected maximum weight will be caught during the measurements anyway?

I know that some of my assumptions above are heretic if we talk strictly modelling of ex-treme load modelling. But I constantly keep in my mind hundreds of cases where safety ac-cording to traditional assessment methods is (maybe just slightly) below 1, while soft load testing in combination with other field testing and better analysis would easily raise this num-ber well above 1.

The general procedure is as follows:

1. First, limited or full bridge weigh-in-motion measurements are performed. At least one pre-weighed truck is used during the calibration procedure to calibrate the real (measured) influence lines. The measurements last for at least 24 hours or until at least 100 relevant heavy vehicles in each lane are recorded. This provides a sufficiently high number of results for reliable calculation of influence lines and of load distribu-tion factors.

2. If the ambition of the soft load test is to provide information about real traffic loading (potentially, for developing the site-specific traffic load model, see chapter ____) and/or to evaluate the realistic value of DAF, then the measurements should last as long as possible. At the moment, the available experience indicates minimum meas-urements of 2 weeks or 5 000 (10 000?) heavy vehicles in each lane. If possible, ac-quiring continuous data or at least data from1 week in each of the 4 seasons should be done.

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3. Influence lines of the bridge are calculated from the heaviest 5-10 vehicle runs (that cause the highest strains at the points of the measurements).

4. The measured influence lines are compared to the theoretical, generated influence lines. Figure 3.6 shows such a comparison in ILG software developed for this pur-pose, but any other procedure can be used. The ILG software first uses simple finite element modelling to generate influence lines for bridges with up to 3 spans. By vary-ing the boundary conditions and relative stiffness of the individual spans, a close match between the measured and the updated (calibrated) influence lines is obtained.

Figure 3.6. Comparison of theoretical and measured influence lines

5. Global load effect of the bridge, obtained from the linear model, is then distributed between the individual structural members. These are either beams/girders or longit-udinal sections of the slab that are covered by a strain sensor during the WIM meas-urements (usually 0.8 to 1.2 meters apart). Statistically evaluated measured load distri-bution factors are applied to each such section to calculate corresponding bending mo-ments under traffic. Section modules of the entire sections, including the pavement, side-walks and similar, are applied to convert the measured strains into the bending moments. This is essential in order not to underestimate the load effect based on the measured strains according to the formula:

M = E × × W,

where M is the calculated bending moment, E is the Young’s modulus, W is the sec-tion modulus and the measured strain. Therefore, accounting for more effective cross sections than they really are increases the loading and thus adds the necessary conservatism into the loading part of the structural model. Ales – I am very con-cerned about a lot of this. Things like footpath, hand rails and pavement strengths can-not be relied upon at the ultimate limit state. The distribution of load could change

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pretty dramatically as loading approaches design levels if such elements fail. It could be very dangerous to recommend this for general use. Eugene]

I rewrote the paragraph, please comments from all! Consider the following:

1. We have no ambition to assess the ultimate limit states. We are talking about the normal (measured) traffic + e.g. 20% or 30%. Which in most real life cases is far from the design levels. What you see on figure 3.1 is around 75% of the design traffic load (unfactored).

I agree that exceptions exist and I would rather list them than not to propose some-thing which can be very useful and economically efficient for majority of bridges.

2. I agree that if we can expect dramatic events we must be careful and should list some exceptions when we cannot apply this method. Can you give some examples when this may happen?

[I agree with the concern from Eugene. In fact, to make extrapolation to the ULS can be dangerous. One example is for instance in the case of slab over beam bridges where the non-composite action is not considered in the analysis as no connection is provided. In some cases, the friction between the slab and the girders can provided higher stiffness and strength than predicted by the models, but as you know, friction forces have a maximum value and when reached, then the force keeps constant and the movement is free. This could be the case when applying a heavier load that could cause a shear force higher than the friction force in the joint. The response of the bridge will be completely different for the range of loads up or below the load provok-ing the sliding in the interface. In one case the composite action can be considered and in the other it can not. What happen if this change in behaviour happens for a load 20 or 30 % higher than the normal (measured traffic)? Nobody can assure that this will not happen at least a complete knowledge of the friction behaviour (friction forces,….) is know. Of course, this can be obtained by additional measurement. – Joan ]

Let me repeat again that the procedure above is used only to assess the real bending moments caused by measured traffic. Joan however presented some useful examples which could be listed as exceptions where such analysis is inappropriate or too risky.

6.

7. Dynamic amplification factor, either measured or theoretical, is applied to the traffic load model.

8. Results are used to calculate RF (see chapter XXX) for any traffic load model. As the inputs are the calibrated influence lines, it is easy to apply any combination of vehicles. The most common are the really heavy trucks (i.e. rating or reference vehicles) and any special heavy transports.

Figures 3.3 and 3.4 demonstrate how this procedure was implemented as a part of the safety assessment procedure used in Slovenia (see SAMARIS D19 and appendix XX).

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Figure 3.7. Input parameters for RF calculation obtained from soft load testing

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Figure 3.8. Calculation of rating factors RF

The main benefit of such automated process is that once all soft load test parameters are available, the RF factors for any traffic loading can be calculated in a matter of seconds.

In the case from Figure 3.6, which is explained in detail in appendix XXX, the measured bending moment reached only XX% of the theoretical values.

Procedure for diagnostic testsIn the case of diagnostic load tests, the integration is achieved through the updating and im-provement of the structural model of the bridge from the test results.

Static tests

As long as the bridge exhibits linear behaviour, the test can be used to validate and up-date analytical model and bridge load capacity. There are two ways to incorporate the results of the static tests in the assessment process:

1. By up-dating the structural model and calculation of the new bridge capacity (reliabil-ity index, load factor) based on the new model. The idea is to change the bridge prop-erties (area, inertia, modulus of elasticity…) in such a way that the theoretical model matches as well as possible the results of the load test.

To this end, an acceptable match is considered to have been reached when the differ-ences between the site-measured maximum deflections and the analytical values are within the following limits:

+/- 10% for prestressed concrete and metallic bridges

+/- 15 % for reinforced concrete and composite bridges

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Once the model is up-dated, the assessment calculations are carried out using the re-vised model and it can be used in the recalculation of the bridge safety (reliability in-dex, load factor…)

2. Direct calculation of the load capacity from the test results. In this case, it is assumed that the bridge assessment is carried out using the partial safety factor format and the load capacity is the value for which the rating live load should be multiplied to reach the failure limit state. According to AASHTO [4], the proposed equation is:

LCT is the load capacity based on the result of the load test

LCC is the load capacity based on calculations and before incorporating the results of the load test

Ka can be positive or negative depending on the results of the load tests and can be calculated as:

εT is the maximum member strain measured during the load test

εC is the calculated strain due to the test vehicle at its position on the bridge which produced εT. It should be calculated using a section factor (area, inertia…) which most closely approximates the member´ s actual resistance during the test.

Kb a factor that between 0 and 1.0 that takes into account the possibility that the bridge has adequate reserve capacity beyond the rating load level and also the load level (compared to the rating load) that the bridge has faced during the test. The factor indicates the level of test benefit that is expected at the rating load level. A value equal to 1.0 means that the test measurements can be directly ex-trapolated to performance at higher loads corresponding to the rating levels. If the relationship between the unfactored test vehicle effect (T) and the unfactored gross rating load effect (W) is less than 0.4, it is recommended to take Kb = 0. If this relationship is higher than 0.7, then a value of 1.0 is recommended if the be-haviour of the member during the load test can be extrapolated for a load level of 1.33 W, if not, the value is 0.5. An example?

Dynamic tests

The principal results of a dynamic test are the natural frequencies, mode shapes and, damping and dynamic allowance[I don't agree that you can get reliable dynamic allowances using lim-ited tests – there are too many variables that a limited test could easily miss. We know from experience (eg, Mura River bridge, Hrastnik bridge) how much the dynamic factors vary. Eu-gene]. [In my opinion, the dynamic allowance is a result that may be obtained from a dy-namic test. The other question where I agree with you is about the reliability of the obtained value depending on the type of tests performed. However, as you show in the example on

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Hrastnik bridge, the values obtained are based on the execution of different dynamic tests. We should give the reader the opportunity to experimentally obtain the dynamic allowance, but giving him information how the dynamic tests should be carried out to get reliable values for the assessment. I suppose you agree with me that the consequences of an overestimated dy-namic allowance at the design stage are very different from those in the case of an assess-ment. Therefore, I propose the following expression: “The principal results of a dynamic test are the natural frequencies, mode shapes, damping and dynamic allowance. They are obtained from a series of time records of displacements and/or accelerations in at different points of the bridge. In the case of the assessment of the Ultimate Limit States, to obtain reliable es-timates of the dynamic allowance at high load levels, the standard procedure of a single vehicle crossing the bridge is debatable and should not be used, as overestimation is very feasible. Therefore, a procedure similar to the one described in appendix B is proposed. In fact, it is there explained how the DAF decreases as a function of increasing weight of the loading events. ] They are obtained from a series of time records of displacements and/or ac-celerations in at different points of the bridge.

The dynamic model and the dynamic response of the bridge are updated mainly through the calculation of the actual impact factor and damping ratio. In fact, despite many theoretical and empirical formulations and equations being available in the literature for the estimation of the impact factor and damping coefficient of a bridge (based on the span length, natural frequency, structural configuration…), their accuracy is in many cases not sufficient due to the high number of parameters that influence the dynamic response and that can be hardly considered in a theoretical or parametric study (deck irregularities, vehicle suspension proper-ties, bridge approach…). The measured dynamic response of the specific bridge under study can provide an approximate exact estimate of the dynamic amplification and damping to be used in the assessment. The type of test dynamic test to be carried out will depend on the limit state to check. For serviceability and fatigue limit states, the use of single vehicles at dif-ferent speeds can be adequate. However, in the case of ultimate, it must be taken into account that the DAF and damping may depend on the load level in the bridge. As a consequence, a methodology as presented in appendix B is proposed. [I am strongly opposed to saying that this is exact. Neither theoretical nor measured dynamic factors are accurate at this point in time. It requires a combination of theory and measurement and we have some distance to go before we will have reliable figures. Eugene] Several parameters influence dynamics, particu-larly vehicle speed and especially for short bridges, road surface profile. [Joan: Eugene, I see your point. I think that with the added text in blue above things become clearer].Damping is another [it is not "key" and I'm not even sure it is important. Eugene] key parameter in a dy-namic model to correctly predict the maximum peak response and the number of cycles of vi-bration. These parameters are also of great interest in the assessment of user’s comfort and fa-tigue. [In my opinion, damping is necessary in the assessment of serviceability (maximum ac-celeration) and fatigue (number of cycles of load) limit states, and therefore we should give an indication how to obtain it. Of course, damping has a negligible effect in the ULS because no dynamics is considered in this case----Joan] Therefore, it is of major importance to have a good estimate of the real damping through a dynamic test.

With the respect of Eugen Bruehwiler’s idea of FAD = 0, Colin Caprani’s PhD which show-ing a few % FAD and my practical experience, incl. Hrastnik experiment, which confirms this, why cannot we be more pragmatic and leave the “estimate”. It is surely not exact, but we do not need it to be. Even if we miss FAD for 100% (which is quite unlikely in the case of multiple vehicles on the bridge and measurements that last long enough, as prescribed above),

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if we in a very special case get for example 16% instead estimated 8%, this increases the factored live load for around 5% and the total loading, incl. the dead load, for even much less. I am not talking about the fatigue problems here.

I agree that there is more work to be done and we will try to do some in ARCHES, but I would hesitate to just say that we have to do more research. We are providing guidance for the end-users and I think we have enough knowledge, experience and results to be more con-crete. We can (should) be conservative and express need for more research, but we need some clear instructions on how to deal with realistic FAD. An idea was for example not to allow FAD less than 1.1 unless for example we have measured at least 100 multiple events, or something similar, that would prove the opposite…,

[ I agree with Ale´s comment - Joan]

In addition, the execution of a dynamic load test is of a great help when assessing the remain-ing fatigue life. A dynamic test can provide data concerning the range of stress in the most prone fatigue members as well as the number of stress cycles. For bridges where fatigue is a potential problem, there is particular potential for the use of soft load testing.

The comparison of experimental and theoretical natural frequencies and mode shapes can be used to up-date the distribution of stiffness and mass within the bridge. It can be also used to detect defects and damages related to the stiffness and mass characteristics of the bridge. I However, in the last case, it is fundamental important (I would keep fundamental, temperat-ure can easily change them for 10%) to bear in mind that environmental conditions (changing temperature, thaw-freeze cycles) may influence the changes in natural frequencies and mode shapes during the year.

In order to get a good correlation between site data and the model, the bridge frequencies, stiffness, span, mass and damping must be accurately matched in the analytical model. The recommended order for adjusting parameters is:

i) damping (based on measured values),

ii) mass of the structure,

iii) structure/element stiffness,

iv) effective span.

The natural frequencies of the bridge are the most critical parameters required for obtaining a good match with site data and this is normally the first step in the updating process. It should be noted that the fundamental modes contribute to the majority of the structure’s displace-ment/strain and as such, the first step involves matching the displacement/strain traces. Then the model is further optimised in order to match the higher order modes (at least 5) with the site data. Careful extraction of self-weight and member stiffness from the detailed site exam-inations will assist in predicting modal frequencies close to the values observed. There is a considerable influence of the transverse location of the vehicle – dynamic amplification is highly dependent on the lane in which the vehicle travels.

Matching natural frequencies, mass, stiffness, damping etc. of the structure will be an iterat-ive process. An acceptable match is considered to have been reached when the differences between the site-measured peak accelerations and displacements and the analytical values are within the following limits:

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Deflections/strains: ± 5%

Acceleration: ± 20%

Alternatively, one may consider that a good match is obtained when the difference between theoretical and experimental frequencies for the 5 lowest vibration modes is less than 5 % for prestressed concrete and metal bridges and 8 % for reinforced concrete and composite bridges.

An example of diagnostic dynamic test is presented in appendix YYY.

Procedure for proof load testsNormally, proof load testing is used to check the ability of the bridge to carry a specified im-posed traffic load (design live load in the Code, maximum allowable legal load…). The two important parameters when designing and executing a proof load test are the target proof load level and the load level at which the test should stop to minimise the risks to bridge integrity and users safety. The last is normally decided based on the test results according to the re-sponse to incremental loading.

Calculation of the target proof load level

The proof load level should be sufficiently high to ensure the desired level of safety if the bridge passes the test. A larger load than the live load the bridge is expected to carry is placed on the bridge. This accounts for uncertainties, in particular the possibility of bridge overloads during normal operation and the impact factor, because proof load tests are normally executed in a static way.

The target proof load (LT) is calculated in the following way:

LR is the checking load (or load effect due to the rating vehicle), i.e, the load level that can be guaranteed to safely cross (with a specified reliability level) the bridge after the execution of the load test (assuming the bridge has pass the test to a load level LT)

I is the dynamic amplification corresponding to the checking load (rating vehicle) XpA is the target live load factor. Its value depends on: the target reliability level assumed in the bridge assessment, the deck response to live loads (if one lane load controls response or not), the existence of fracture critical details, the bridge redundancy, the bridge condition, bridge condition based on in-depth inspection, existence of hidden details in the structure and number of trucks crossing the bridge.

The procedure to calculate the value of XpA consists of a reliability-based calibration process where a uniform value of the target reliability is sought for the different bridges within a pre-defined span-length range and subject to the abovementioned influencing variables.

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In the case of the USA (LRFR AASHTO Manual [4]), the proposed values, calibrated for a target reliability of 2.5 (1 year), are as follows:

Xp = 1.4

Σ% represents the adjustments to the basic value of Xp corresponding to the influencing vari-ables. The values in table 3.1 are recommended in the AASHTO Manual [4].

Table3.1. Adjustments to Xp

Consideration AdjustmentOne-lane load controls the response + 15 %Non-redundant structure + 10 %Fracture-Critical details present + 10 %Bridges in poor condition + 10 %In-depth inspection performed - 5 %Ratable, existing Rating Factor > 1.0 - 5 %Average daily truck traffic < 1000 -10 %Average daily truck traffic < 100 - 15 %

If the value of the target proof load LT is achieved during the proof load, then the bridge may be certified as able to carry the load LR with the specified target reliability level. If signs of distress are observed prior to reaching the target proof load and the tests must be stopped at a lower load Lp , then the certified load level based on the result of the proof load test is 0.88 Lp. In order to get relevant figures for other levels of reliability, a calibration procedure should be undertaken.

Obtaining the maximum load test

The maximum load that the bridge will be able to bear without any distress is something that can not be theoretically calculated and therefore is not available before the execution of the test. The load level at which the load should stop will be obtained from the following up of the test execution and the comparison with results from the analytical model with the aim to avoid accidental overload or excessive deformations. In slab-girder bridges it is recommen-ded to compare the experimental results with the results from two theoretical models, one that takes into account the composite action between slab and girders and the other that neglects this effect.

As a general rule, the tests must stop when any sign of non-linearity appears in the experi-mental response.

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It is of special interest to place a sensing and monitoring system that could predict abnormal increases of cracking, debonding or crack growth. The Acoustic Emission (AE) technique can be of application to this end.

3.3 Site-Specific Load Assessment

Loading represents the second part of the safety assessment equation. As for carrying capa-city, it is beneficial if information about realistic loading is available and can be applied in an efficient way.

3.3.1 Site-specific assessment of dead loading

Dead loading is defined as the gravity loading due to the structure and other items perman-ently attached to it. This may be divided into dead loading and superimposed dead loading.

Permanent dead load: This is the gravity load of all structural elements also termed Self Weight. It is calculated as the product of volume and material density. It is recommended, to avoid excessive stresses especially in prestressed concrete, that the self weight be calculated as accurately as possible rather than rounding it up.

Superimposed dead load: This is the gravity load of non-structural parts of the bridge. Items, such as the parapets, are long term but might be changed during the lifetime of the structure. Because of this uncertainty, superimposed dead load tends to be given a higher factor of safety than dead load. The road pavement is another significant superimposed dead load and it is common for road pavements to get increasingly thicker over a number of years as each new surface is laid on top of the one before it.

Bridges are unusual in that, especially for longer bridges (can we put a figure here), a high proportion of the total loading is due to the dead and superimposed dead load.

Material PropertiesAt the first stage of assessment, during the initial desk study standard material properties are assumed.

Reinforced concrete can be assumed to have density in the order of 23.6 kN/m3. It is rarely less than 23.6kN/m3, which is the minimum density recommended in most codes of practice, but varies with the density of the aggregate and the amount of reinforcement. National/Inter-national references should be consulted to establish typical weights for plain and reinforced concrete, solid concrete slabs, hollow clay block slabs, concrete products, finishes, light-weight concretes and heavy concrete.

Structural steel can be assumed to have a density in the order of 77.0 kN/m3.

National/International references should be consulted to establish typical densities for fin-ishes, asphalt and other applied waterproofing layers, pavement lights, steelwork, reinforce-ment, road surfacing etc.

Measuring material properties on siteIn higher levels of assessment as discussed in chapter 3.1, standard material properties cannot be assumed, and so, testing should be carried out on site to determine the material properties

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needed to accurately establish the dead- and superimposed dead load. Documents such as the Concrete Bridge Design Group Guide to Testing and Monitoring the Durability of Concrete Structures should be consulted for best practice in this regard.

Probability-based AssessmentIn probability-based analysis of highway bridge structures the dead- and superimposed dead load are assumed to be time invariant and are commonly assumed to be closely approximated by the normal distribution (because the individual permanent loadings are additive), typically with a mean equal to the nominal load and, and a coefficient of variation chosen to reflect the uncertainty of the load. The Model Code prepared by the Joint Committee on Structural Safety should be consulted for further information.

3.3.2 Proposal for Assessment Live Load Model which can be adjusted to reflect truck numbers and distribution of truck weights

Traffic loading on bridges varies considerably between regions and between sites within re-gions. It has been found that the mean characteristic load effect from three sites in Slovenia are about 20% less that the corresponding values from three sites in the Netherlands. This dif-ference is due to the numbers and weights of trucks at the sites. The variation between partic-ular sites can be far greater.

Traffic load can be assessed at a site in one of two ways: Monte Carlo simulation Calibrated notional load model

In the Monte Carlo simulation approach, the procedure is quite elaborate. Truck weight data is collected at the site for an extended period (many weeks) representing seasonal fluctuations and periods when heavy loading might be expected. Statistical distributions are fitted to the histograms of weight data. It is common to use Bi-modal or Tri-modal Gaussian distributions for this but it has been shown that this is not always accurate. It is better to simulate directly from the histogram for trucks of up to about 45 tonnes and to fit a Gaussian distribution to the tail above this level (Getachew & OBrien 2005). In addition to vehicle weights, the gaps between vehicles is are very important and truck gap statistics are required for which WIM data is needed with time stamps to the nearest 0.01 seconds (OBrien & Caprani 2005).

Monte Carlo simulation is used to "sample" the fitted weight distribution to generate trucks with "typical" weights. Hence, truck crossing and meeting events are simulated and, using the influence line for a particular load effect (bending moment or shear force) a database of load effects can be found. It is of course possible to directly calculate the load effects due to recor-ded truck weight data or measure load effect data directly. While this clearly has advantages, it is generally limited by the size of the database available. The loading events that govern are extremely rare involving the simultaneous occurrence of very heavy trucks. Monte Carlo sim-ulation makes it possible to simulate data representing years or decades of traffic from which such rare events can be identified.

Statistical techniques are used to find the 1000-year characteristic value from the database of load effects. [If you are talking about 1000 year return period, then you should delete charac-teristic. If you are talking about characteristic value, then should be the characteristic value

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corresponding to the maximum in 100 years. I suppose you are not talking about a character-istic value for the maximum in 1000 year, no ?----Joan] It is good practice to identify the maximum-per-day load effects for several days of (simulated) data. This maximum-per-day data is generally consistent with one of the Extreme Value distributions, Gumbel or Weibull. By fitting to such a distribution, it is possible to extrapolate to determine the characteristic value. Recent research has shown that the accuracy of this approach can be much improved by separating the load effects by type (single truck crossings, 2-truck meetings, etc.), fitting to each type separately and combining the resulting probabilities (Caprani 2005).

Calibrated Notional Load Model

A calibrated notional load model is considerably easier to implement than a Monte Carlo sim-ulation approach which requires specialist expertise. One of the simplest concepts for a site-specific calibration of a notional model is Turkstra's Rule (Nowak 1993) where the 1000-year truck is placed in the critical lane and a second heavy truck is placed in an adjacent lane. Other approaches involve calibration of more sophisticated notional load models. Many au-thors (e.g., Nowak (1993), Agarwal & Cheung (1986), Cooper (1997)) have proposed adjust-ments to notional code models as a simplified method of allowing for site-specific variations. In particular, Moses (2001) proposes an adjustment of the HS20 model for bridge evaluation in the United States and a factored HL93 model is specified in the AASHTO (2003) specific-ation.

In this work, the site traffic dependence of extrapolated load effects is investigated. A simpli-fied model is developed which aims to reproduce similar critical loading events from know-ledge of the site-specific traffic characteristics without having to perform a full Monte Carlo simulation. The investigation has been limited to the case of mid-span moment and end shear in simply supported bridges with spans ranging from 15 to 35 meters. The bridges are as-sumed to have two traffic lanes, one in each direction. It should be noted that loading on bridges where there are two same-direction lanes is generally greater as there may be correla-tion between the weights of trucks in adjacent lanes.

The model is different for the two load effects. For bending moment, the 3 rd axle of the 1000-year truck is placed at the center while the 3rd axle of the 1-week truck is placed αML from the end as illustrated in Figure 3.9. For shear, the most critical location for the 1000 year truck is when the 5th axle has just entered the bridge. At this moment in time, the third axle of the 1-week truck is placed αSL from the end.

Figure 3.9. Description of the simplified model

The optimal values for αM and αS are 0.63 and 0.42[This value does not correspond with the figure where alpha 3 is more than 0.5L—Joan], respectively.

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3.3.3 Allowance for Dynamic Amplification

There is considerable variation in dynamic amplification between runs of given vehicles at specified speeds and even greater variation when speed, vehicle etc. are changing. Theoret-ical studies are generally not good at predicting dynamic amplification. Models vary greatly in their complexity. As might be expected, the very simple models are generally the least ac-curate. Anything short of full 3-dimensional models give significantly different results to simple 2-dimensional models. Articulation is important – articulated tractor/semi-trailers be-have quite differently to rigid 2- and 3-axle trucks. Suspension type is important. It is well known that air suspensions are considerably more "road friendly" than steel leaf spring sus-pensions and should be modelled differently. There are also modelling issues with suspen-sions and it has been shown that the Coulomb damping usually assumed is a simplification. Road profile is also quite very important. It has been shown that it is not just road roughness that affects the dynamic amplification in a bridge – the location of the particular bumps that make up the profile is very important and IRI is at best a very crude indication of the level of dynamics that can be expected. This is further complicated that trucks will not follow the ex-act same track each time they travel over a bridge and will therefore experience a slightly different profile which can have a significant effect. Finally, there is a dearth of knowledge on truck dynamic properties. Some spring stiffnesses, damping coefficients etc. are available for some trucks (and some good truck models exist for particular trucks) but there is little known about the model properties that should be used for the truck population at large.

As a result of all these issues, theoretical truck/bridge models are not reliable indicators of dynamic amplification. Unfortunately, field measurements are not good indicators either Here of course I cannot agree. I think that the method we are proposing (and other available methods that are used for fatigue evaluation, for example) is a big step towards very realistic assessment of bridge dynamics. After all, they incorporate in the results all unknowns and variations that are so difficult to model. The issue remains how to combine experiments and modelling to forecast the dynamic amplifications during the extreme events. And how the results relate to the accuracy of the WIM system. It probably will not be always as high as in Hrastnik. It has been demonstrated in this project by both theory (Mura River bridge – see Appendix A) and field measurements (Hrastnik bridge) that there is a tendency for dynamic amplification to decrease as static bending moment increases. Thus extreme loading events which may include several heavy trucks meeting simultaneously on a bridge have a lesser dynamic amplification than less significant crossings of single trucks or meetings of two light trucks. If this trend is extrapolated to the 1000-year extreme, the dynamic amplification is quite small. In the case of the Mura River bridge, it was shown to be only 5.8% in a case where dynamic amplification for more common loading events was as high as **%.50% and above.

Similarly, experiments on Hrastnik bridge (Appendix B), which from the structural point of view was similar to the Mura bridge form appendix A, showed even higher differences. While during a run of a single 3-axle rigid truck the measured DAF were as high as 105%, the values dropped below 10% in cases with at least 2 heavy vehicles on the bridge. The av-erage DAF value for the heaviest measured events (multiple-presence events with total weight of both vehicles above 380 kN, which equals to on full 5-axle semi-trailer) was only 5.0% on a bad and 3.5% on a smooth pavement. These results have not even been extrapol -ated to the 1000-year extreme.

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We could propose something like this for the procedure on assessment of FAD.

The experimental DAF should be applied in the following way:

1. If no data or modelling are available, then values of FAD and associated safety factor from the design codes are taken (level 1 assessment)

2. If measurements are available:

a. If done with 1 or 2 vehicles, this can be relevant for fatigue assessment but not for assessment of DAF.

b. If the proposed procedure with WIM measurements is applied, then:

i. Accuracy of the bridge WIM system for gross weights must be in class B+ or better according to the COST 323 WIM specifications.

ii. Calculate the average value of FAD for the relevant heavy multiple presence events (We have to define what a relevant MP event is. I pro-pose to take into account only those that meet in the critical part of the bridge: maybe we could relate it to the α value from the previous chapter?). Only events with total weight of the vehicles heavier than 380 kN (400, 500, 600??) should be taken into account. Here, lower we go, higher FAD we will obtain, which means more conservatism in lack of sufficient data.

iii. If number of these events is lower than X (this depends on ii), and the averaged FAD is lower than 1.1 (which should probably be always if sufficiently high number of events is taken into account), then FAD 1.1 should be applied; if number of events is greater than X, the calculated value of FAD can be used.

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iv. If one wants to further optimise the FAD value then the results of measurements should be combined with numerical modelling.

v. In case of measurements it is allowed to reduce the safety factor for traffic loading for 10-20%.

We should further state that measurements or modelling have limited life time as any consid-erable change of pavement can completely change it.

We could say that results of modelling and experiments done in SAMARIS show that this can be an appropriate procedure but more research is needed to optimise factor X, α, influence of WIM accuracy and others.

Is it possible to get some confirmations about this from Colin’s thesis?

3.3.4 Guidelines Recommendations for site-specific assessment of load in individual cases (all)

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4. REFERENCES 1. AASHTO (1989): Guide Specifications for Strength Evaluation of Existing Steel and Con-

crete Bridges. Washington, D.C.

2. AASHTO (1990): Guide Specifications for Fatigue Evaluation of Existing Steel Bridges. Washington, D.C.

3. AASHTO (2001): Manual for condition evaluation of bridges. Second Edition. Washington,D.C.

4. AASHTO (2003): Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges (LRFR-1)

5. BA16/97: The Assessment of Highway Bridges and Structures (Incorporating Amendment No. 1 dated November 1997 and Amendment No.2 dated November 2001). Design Manual for Roads and Bridges. Vol. 3, Section 4, Part 4. HMSO

6. BA38/93: Assessment of the fatigue life of corroded or damaged reinforcing bars. Design Manual for roads and bridges. Vol. 3, Section 4, Part 5. HMSO, London.

7. BA51/95: The assessment of concrete structures affected by steel corrosion. Design Manual for roads and bridges. Vol. 3, Section 4, Part 13. HMSO, London.

8. BA52/94: The assessment of concrete structures affected by alkali-silica reaction. Design Manual for roads and bridges. Vol. 3, Section 4, Part 10. HMSO, London.

9. BA54/94: Load testing for bridge assessment. Design Manual for roads and bridges. Vol. 3, Section 4, Part 8. HMSO, London.

10. BA55/00: The assessment of bridge substructures and foundations, retaining walls and buried structures. Design Manual for roads and bridges. Vol. 3, Section 4, Part 9. HMSO, London.

11. BD21/01: “The Assessment of Highway Bridges and Structures”. Design Manual for Roads and Bridges. Vol. 3, Section 4, Part 3. HMSO, London

12. BD44/95: “The assessment of concrete highway bridges and structures”. Design Manual for Roads and Bridges, Vol. 3, Section 4, Part 14. HMSO, London.

13. BD56/96: The Assessment of Steel Highway Bridges and Structures. Design Manual for roads and bridges. Vol. 3, Section 4, Part 11. HMSO, London

14. BD61/96: The Assessment of Composite Highway Bridges and Structures. Design Manual for roads and bridges. Vol. 3, Section 4, Part 16. HMSO, London.

15. BRIME: Bridge Management in Europe. Deliverable D1 (2000): “Review of current proced-ures for assessing load carrying capacity”

16. BRIME: Bridge Management in Europe. Deliverable D2 (2001): “Review of current proced-ures for assessment of structural condition and classification of defects”

17. BRIME: Bridge Management in Europe. Deliverable D10 (2001): “Guidelines for assessing load carrying capacity”

18. CAN/CSA-S6-2000 (2000): “Canadian Highway Bridge Design Code”. Toronto

19. COST 345 (2004): Procedures required for assessing highway structures. Report of Working Groups 2 and 3: “Inspection and Condition Assessment“, available on http://cost345.zag.si/

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20. COST 345 (2004): Procedures required for assessing highway structures. Report of Working Groups 4 and 5: “Safety and Serviceability“, Ljubljana and Dublin“, available on http://cost345.zag.si/

21. CSN 73 6220 (1996): “Zatizitelnost a Evidence Mostu Pozemnich Komunikaci” (Loading ca-pacity and register of the road bridges”. Cesky Normalizacni Institut. Praha

22. DP.-T.16 M (1990): “Instruction for current bridge structures examination located on inter-urban public roads”, General Directorate of Public Roads, Warsaw

23. DP.-T.17 M (1990): “Instruction for basic bridge structures examination located on interurban public roads”, General Directorate of Public Roads, Warsaw

24. DP.-T.18 M (1990): “Instruction for detailed bridge structures examination located on inter-urban public roads.”, General Directorate of Public Roads, Warsaw

25. ISO 2394:1998: “Basis for Design of Structures – General Principles on Reliability of Struc-tures.”. International Organization for Standardization. Geneva

26. ISO/CD 13822:2001: “Basis for Design of Structures – Assessment of existing structures”. In-ternational Organization for Standardization. Geneva

27. Joint Committee on Structural Safety JCSS (2001): “Probabilistic Assessment of Existing Structures”, RILEM Publications S.A.R.L, Cachan

28. Madaj, A., Wołowicki, W. (2001): “Budowa i utrzymanie mostów/Bridge construction and maintenance”, WKL Warsaw

29. Moses, F.; Verma, P.: “Load Capacity Evaluation of Existing Bridges”, National Cooperative Highway Research Program (NCHRP). Report N. 301, 1987.

30. PIARC (1999): “Reliability-based assessment of highway bridges”. Committee C11. World Road Association. Paris

31. PIARC (2004): “Asset Management in Relation to Bridge Management” Committee C11. World Road Association. Paris

32. SIA 462 (1994): “Evaluation de la sécurité structurale des ouvrages existants”. Société Suisse des Ingénieurs et des Architectes. Zürich

33. Žnidarič, A., Moses, F. (1997): Structural Safety of Existing Road Bridges", 7th International Cocnference on Structural Safety and Reliability ICOSSAR '97, Kyoto, 1843-1850.

34. Žnidarič, A. (2002): “Recommendations for updating of procedures used for calculation of structural safety and posting of bridges”, ZAG, Ljubljana, in Slovene

35. Žnidarič, J., Terčelj, S., Marolt, J. and Žnidarič, A. (1990): “Methods for reliability assess-ment of bridges”, ZRMK Ljubljana, in Slovene

36. Žnidarič, J.; Žnidarič, A. (1994): “Evaluation of the carrying capacity of existing bridges”, Fi -nal Report , ZAG Ljubljana

37. Žnidarič, J.; Žnidarič, A.; Peruš, I. (1995): “Assessment of safety and remaining service life of existing structures”, Final Report, ZAG Ljubljana, in Slovene

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Figures

Figure 2.1. Software implementation of the Slovene condition assessment system4

Figure 2.2. Defects software – opening page........................................................6

Figure 2.3. Defects software – types of defects.....................................................6

Figure 2.4. Defects software – example of available information about a specific defect.................................................................................................. 7

Figure 3.1. Diagnostic load test with pre-weighed vehicles.................................21

Figure 3.2. Comparison of theoretical and measured influence lines...................25

Figure 3.3. Input parameters for RF calculation obtained from soft load testing. 27

Figure 3.4. Calculation of rating factors RF........................................................28

i

Tables

Table 3.1 Adjustments to Xp.............................................................................33

Table C1: The reasons and background to the different policies on load testingxxii

Table C2: List of obligatory codes dealing with test loading..........................xxiv

Table C3: Field of application of load testing in various countries..................xxv

Table C4: Static load quantity.......................................................................xxvii

Table C5: Method of dynamic loading...........................................................xxix

Table C6: Range of investigations...................................................................xxx

Table C7: Measurement methods...................................................................xxxi

Table C8: Analysis of results and assessment criteria...................................xxxiii

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SAMARIS

Appendix A - Mura River Bridge

A1 Objective?

The Mura River Bridge in Slovenia is an example of a diagnostic load test of the dynamic type. The Dynamic Amplification Factor for the bridge was required but full details were not available for the dimensions or material properties. A diagnostic load test was used to determ-ine the values which gave a best fit between field measurements and a Finite Element model. I cannot see here a clear objective why such procedure was selected. Can we give here an in-struction why and when to use such procedure to improve the assessment?

Experimental tests are an important source of information on dynamic amplification factors. They offer a realistic measure of how bridges respond due to vehicle excitation. However, the dynamic amplification factors obtained from tests are site specific and in general yield very little information regarding the contribution various bridge and vehicle characteristics make towards the dynamic response. Therefore, a test program must involve a large number of dif-ferent bridges and vehicles if it is going to yield general information regarding a bridge's dy-namic amplification factors.

The Mura River Bridge (Figure A1) in north-eastern Slovenia is a 32 m long, simply suppor-ted structure that forms part of a larger structure. It has two lanes with bi-directional traffic flow.

The bridge is of beam and slab construction. The slab is constructed from concrete and a layer of asphalt pavement surfacing is present on top of the concrete. Supporting the slab is five concrete longitudinal beams. Five concrete diaphragm beams are also present in the transverse direction; two of these beams are immediately above the bridge supports. No in-formation was available on either the depth of the asphalt pavement or the nature of the steel reinforcement or prestress tendons.

Since the depth of both the asphalt pavement and concrete slab were unknown, and were not determined from on-site tests, they had to be estimated. The sum total of the depth of the bridge slab and the pavement is 0.34 m. It is reasonable to assume that the depth of the pave-ment is between 50 mm and 100 mm; therefore the depth of the slab is between 0.29 m and 0.24 m.

Two pre-weighed vehicles were used in the test, a two-axle and a three-axle truck with steel suspensions. The axle masses were determined at a local static weigh station and the axle spa-cings were determined by tape measure.

A variety of instrumentation was used in the test. The bridge was instrumented with 12 strain transducers placed on the underside of the bridge beams – see locations in Figure A2. Axle detectors were placed on the road surface to determine vehicle velocity and the number of axles of the vehicle as it crossed the bridge. Photocells were placed on each vehicle and, in conjunction with reflective strips on the bridge, gave more precise information regarding vehicle velocity.

iii

(a) Two test vehicles on the bridge

(b) Underside of bridge showing longitudinal beams and diaphragms

Figure A1. Mura River Bridge

The axle detectors used in the experiment were thick walled rubber tubes that are attached to the road surface – Figure A3. The method of mounting was firstly to use steel clamps to fix the tube at the roadside and then to cover the whole tube with an asphalt-based tape. A set of two tubes is placed in each vehicle lane, a short distance before the entrance of the bridge. As a vehicle enters the bridge, each of its axles crosses both tubes. The time elapsed as an axle travels between each of the tubes is recorded and, since the distance between the tubes is known, the velocity of the vehicle can easily be determined.

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Figure A2. Plan of position of strain transducers, all dimensions are in metres

Figure A3. Axle detector installation

A2 Finite Element Model

The finite element program MSC/NASTRAN for Windows was used to model the bridge and vehicles. A transient analysis takes into account the complex interaction that occurs between the vehicle and the bridge. The slab is modelled with plate elements while the longitudinal and transverse beams are modelled as offset beam elements.

The depth of the main section of the slab was chosen as 0.25 m. As stated earlier the depth of the slab was unknown and was not determined from the site investigation. From the measure-ments collected it was concluded that the depth of the slab and the asphalt surface combined was 0.34 m. If an estimation of 0.9 m is made for the depth of the asphalt this results in a slab

v

depth of 0.25 m. The accurate estimation of the bridge slab is important and will be discussed in greater detail when examining the bridge’s natural frequencies. The depth of the bridge slab was the only unknown geometric property.

The footpath is modelled using thicker and offset PLATE elements. The thickness of the foot-path elements is 0.51 m and the thickness of the footpath and downstand elements is 0.6 m. These elements are then offset until the required geometry is achieved.

Figure A4 shows the model with the offset beam elements in place.

Figure A4. Bridge model with PLATE and longitudinal BEAM elements

The bridge is of concrete construction, but no information was available of either the Youngs Modulus or Poisson’s Ratio of the concrete (I feel this “no information was available “ awk-ward if we are giving an example for the users. Can’t we simply “lie” that such parameters were obtained from the design or measurements?). In the absence of information regarding the bridge material, several assumptions were made. Since the beams were precast and the slab was poured in situ, the concrete used in each case will probably differ. This was not taken into account in the NASTRAN model and the entire bridge was assumed to be con-structed from the same concrete. The preliminary material chosen was defined as a typical bridge concrete with a value of Youngs modulus of 31010 N/m2 and a Poissons Ratio of 0.15.

It is important to note that this is the preliminary bridge model design; the model has yet to be validated and modified to match the experimental results. Again, would the end-users not ex-pect more definite results, not a model that still needs to be validated and modified? I would avoid such excuses, if possible. However, since the majority of the bridges geometric inform-ation is known, the only parameters that may be modified are the bridge slab depth and the bridge construction material properties.

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A3 Modal analysis of bridge model

From the modal analysis carried out on the experimental data, it was found that the first nat-ural frequency of the bridge was 3.576 Hz. An eigenvalue analysis was carried out on the NASTRAN model defined above. The first natural frequency of the finite element model was 2.80 Hz. Clearly the natural frequency of the bridge model is too low, thus bridge parameters must be adjusted to achieve the actual frequency. Since the depth of the bridge slab was the only unknown geometric property, it was adjusted to determine how the frequency changed due to varying slab thickness. The effect of including the asphalt pavement on the bridge nat-ural frequency was also investigated. Table A1 shows the variation of bridge first natural fre-quency resulting from various slab thicknesses. In each case the offsets of both the longitud-inal and transverse beams were adjusted.

Table A1.Variation of the bridge models first natural frequency with respect to slab thickness

Slab thickness (m) Bridge first natural frequency (Hz)

0.20 2.812

0.25 2.802

0.29 2.796

It is clear from the table that the depth of the bridge slab does not significantly affect the first natural frequency of the bridge. This is due to the fact that the even though the stiffness of the bridge is increasing, the mass per metre of the bridge is also increasing. Thus, the relative in-crease in the two parameters tends to cancel each other out. This is only the case because the asphalt pavement is omitted from the model.

The model of the bridge with the asphalt pavement included is investigated. Although the as-phalt does not structurally contribute to the bridge, it does add mass. The density of the as-phalt was assumed to be 2360 kg/m3 and the overall depth of the bridge slab plus the asphalt was 0.34 m. Table A2 shows the variation of bridge first natural frequency with the depth of the slab with asphalt included.

Table A2. Variation of the bridge models first natural frequency with respect to slab depth with asphalt included

Slab thick-ness (m)

Asphalt thickness (m)

Asphalt mass per area (kg/m2)

Bridge first natural frequency (Hz)

0.2 0.14 330.4 2.55

0.25 0.09 212.4 2.64

0.29 0.05 118 2.71

vii

As can be seen from the table, including the asphalt road pavement in the model does not res-ult in significantly large variation in the bridges first natural frequency with respect to slab depth. In addition, the inclusion of the asphalt does not increase the bridges natural frequency to the desired value of 3.58 Hz. Therefore, a slab dept of 0.25 m was assumed for the model and the asphalt pavement was omitted.

Since the rest of the bridge geometry is known, the only remaining model parameter that can be adjusted is the Young’s Modulus of the bridge material. Adjusting the Young’s Modulus to a value of 4.81010 N/m2, results in a first natural frequency of 3.544 Hz. This was con-sidered a reasonable approximation of the experimental result. Figure A5 shows the first four mode shapes and corresponding frequencies for the finite element model.

(a) – 3.544 Hz (b) – 4.656 Hz

(c) – 13.380 Hz

(d) – 13.920 Hz

Figure 5 – First four mode shapes of the bridge model

As can be seen from the figure the first mode shape is in bending while the second mode is torsional. The third and fourth mode is both bending and torsion. These frequencies can then be compared to the higher modes of bridge vibration determined from the frequency analysis to ascertain if the assumptions concerning slab depth and modulus of elasticity are correct. The experimental first and second frequencies of 3.576 Hz and 4.60 Hz compare well to the NASTRAN values of 3.544 Hz and 4.656 Hz. Likewise, the higher experimental frequencies of 12 Hz and 13.02 Hz are a reasonable match to the NASTRAN frequencies of 13.380 Hz

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SAMARIS

and 13.920 Hz. Thus it is clear that, in the frequency domain, the NASTRAN bridge accur-ately models the dynamic characteristics of the experimental bridge.

The experimental damping value for the bridge was found to be in the range, 2% to 4%. Therefore, a value of 3% damping was chosen for the model.

The present example shows how the results of a dynamic test have been used to update the model parameters of the Mura bridge. The results from the test were used to obtain reliable values of the unknown parameters of the bridge (thickness of the upper slab, material proper-ties) that are of essential importance to derive an accurate assessment of the bridge capacity. It is shown how the test can be used in the case of lack of information or drawings in the bridge under assessment.

I propose to explain how such analysis could be beneficial for the bridge managers. Also, what is the final result?

ix

Appendix B HRASTNIK EXPERIMENT

B1 Introduction

Dynamic modelling of bridges in the past has shown that dynamic amplifications of traffic loading due to the passing heavy vehicles are generally lower than those prescribed in differ-ent codes and standards. The most rigorous reductions were shown for the heaviest loading events, which occur due to several heavy vehicles on the bridge. Applying such over-conser-vative Dynamic Amplification Factor (DAF) is beneficial for the design of a bridge, as it may provide additional structural safety needed in the future to withstand higher loads and reduced carrying capacity due to deterioration or any other reason. However, using such conservative DAF estimates in the bridge assessment stage may result in unrealistically high loadings and, consequently, unnecessary measures that the bridge is submitted to.

Among others, the SAMARIS project has been investigating realistic values of DAF for bridge assessment purposes. The new generation of bridge weigh-in-motion system, which is using instrumented bridges from the road network to weigh heavy vehicles, was upgraded to measure the dynamic response of the structure under random traffic conditions. The objective of the experiments was to establish the dependency between the DAF and the total static weight of the loading event (any combination of heavy vehicles on the bridge) and to see how repair of the uneven pavement would influence the DAF.

B2 Site description

To achieve the objective, a typical bridge with high dynamic response and very uneven pave-ment was selected. Furthermore, the bridge over the Sava river in Hrastnik, Slovenia, was planed for resurfacing in year 2004, which fulfilled the second part of the study, how even-ness of the pavement influences bridge dynamics.

The bridge has five 30.5 meters long simply supported spans. Three of them are over the Sava river and one of them over the railway tracks. The area under the last span was clear and was therefore used to mount the bridge WIM instrumentation.

Figure B1. Hrastnik bridge - side and top views

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Apart from its great susceptibility to vibration, one of the main reasons that this bridge was selected for SAMARIS measurements was its extremely bad surface with ruts in the pave-ment reaching 10 cm (Figure B2) and the intention from the Slovene Road Administration that the surface would be repaved in 2004.

Figure B2. Hrastnik Bridge – Unevenness and rutting before repaving of the pavement

B2.1 Measurements of longitudinal evenness with ZAG-VP longitudinal profilometer

B2.1.1 Measuring procedure

ZAG-VP (ZAG longitudinal profilometer) was used to perform the measurements. It is one of the numerous similar devices used around Europe and was verified within the European FIL-TER project []. It measures the pavement longitudinal profile using an accelerometer and an angular-variable differential transformer. It is a high speed monitoring device and has no sig-nificant impact on the results when moving at speeds from 30 to 120 km/h. The measuring equipment itself is mounted on a standard car and is composed of the following main parts:

an accelerometer, an angular-variable differential transformer, odometer and laptop computer inside the vehicle.

The working principle of the device is shown in Figure B3. The accelerometer is fixed in the vertical direction to the sprung mass of the car. A referential, electric signal, which defines the absolute displacement of the sprung mass as a function of the horizontal distance travelled, is obtained by double integration and filtering of the measured accelerations (the simple GMR inertial reference). An angular-variable differential transformer is fixed on the rear axle of the vehicle and is used to measure the angle of the oscillating arm of the rear wheel. The data from this transformer are used to define the vertical distance from the road pavement.

The measurements were performed over the entire Hrastnik bridge and extended for around 100 m further in the direction of Hrastnik. They were repeated six times (six runs), three times in each driving lane. Run number 1 was performed in wheel-path, run number 2 closer to the road axis and run number 3 close to the road edge.

xi

Figure B3. Working principle of the ZAG-VP longitudinal profilometer

B2.1.2 Results

The results were evaluated in a standard way as calculated deviations from a 4-m long straightedge. The measured profiles in Figure B4 show that the surface after the repaving im-proved considerably. While before the repair values exceeded 10 mm on the normal road and even 15 mm over the expansion joints, after maintenance works the only deviations above 4 mm (5.5 mm) were found at the expansion joint at the end of the bridge.

-20

-15

-10

-5

0

5

10

15

20

0 5 10 15 20 25 30 35

Une

vene

ss (m

m)

Right 1 Left 1Right 2 Left 2Right 3 Left 3Right Left

-20

-15

-10

-5

0

5

10

15

20

0 5 10 15 20 25 30 35From the beginning of the bridge (m)

Une

vene

ss (m

m)

Right 1 Left 1Right 2 Left 2Right 3 Left 3Right Left

Figure B4. Hrastnik Bridge – road profile before (above) and after resurfacing (below)

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SAMARIS

B3 WIM measurements

Obtaining reliable traffic data sample that represents the actual traffic flow requires WIM (weigh-in-motion) measurements. Although less accurate than static weighing, WIM systems are the only means to weigh practically all vehicles that pass the measuring device. The Si-WIM weigh-in-motion system [reference to WIM] was used because: it is a bridge WIM system that is attached to the bridge and can measure some additional

bridge characteristics essential for improving bridge assessment. it was for the purpose of SAMARIS upgraded with a module that can calculate DAF for

each individual loading event (any number of heavy vehicles on the bridge).

The first span on the left bank of the Sava river was selected. The cross-section of the simply supported span is composed of four prefabricated post-tensioned I beams and prefabricated slab on the top of them. The span is 30.5 meters long and slightly skewed.

Two pneumatic tubes spaced 5 meters from each other were placed over the road at the mid-span to registered axles of passing vehicles. Two strain transducers were attached to the bot-tom flange at the midspan of each beam (B1 to B4). Apart from this common SiWIM instru-mentation, several additional sensors were attached to the beams: four strain transducers (channels 13 to 16) were mounted at the quarter-span of each beam four calibrated inductive strain transducers (channels 17 to 20) were mounted in parallel

with the SiWIM transducers and two inductive displacement transducers (channels 23 and 24) were mounted to the inner

two beams to measure deflection at the midspan.

The first two-week measurements were done before resurfacing of the pavement in April 2004 and were repeated in July 2004 when the repair works were completed. Then, the instru-mentation was similar, with the exception of strain transducers at the quarter-span which were moved to a new position to detect individual axles of the vehicles.

Ch13

Ch06

Ch05 Ch20

Ch08

Ch07

Ch10

Ch09

Ch19

Ch18

Ch17

Ch14

Ch15

Ch16

Ch23

Ch24

B1

B2

B3

B4

Figure B5. Hrastnik Bridge – SiWIM instrumentation for April 2004 measurements

xiii

B4 Results

The SiWIM system was calibrated with two statically weighed heavy vehicles (Figure B6): a three axle rigid truck with steel suspension and a five axle semi-trailer with air suspension.

To provide good calibration of the SiWIM system and to study influences of different types of heavy vehicles on the bridge, more than one hundred runs with calibration vehicles were recorded. While typical DAF from the 5 axle semi-trailer was in the range of a few % (FigureB8), the one from the 3 axle rigid truck was much higher and often exceeded the static value for up to 40% (Figure B9). The calibration was extended to record around 40 multiple-pres-ence events, with both calibration vehicles on the measured span at the same time and at dif-ferent positions on the bridge. This was necessary to verify the accuracy of calculation of multiple-vehicle events, which represent the extreme loading cases and were therefore essen-tial for the conclusions of this research.

Figure B6. Hrastnik Bridge – Calibration trucks on a static scale

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Time (s)

Stra

in (V

)

Static

Dynamic

Figure B7. Hrastnik Bridge – Dynamics of strain signals from the 5-axle and the 3-axle

calibration vehicles, following each other

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SAMARIS

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

0 1 2 3 4 5 6

Stra

in (V

)

Static

Dynamic

Figure B8. Hrastnik Bridge – low dynamic amplification due to a 5-axle semi-trailer

-4,0-3,5-3,0-2,5-2,0-1,5-1,0-0,50,00,51,0

0 0,5 1 1,5 2 2,5 3

Stra

in (V

)

Static

Dynamic

Figure B9. Hrastnik Bridge – Extreme dynamic amplification due to a 3-axle truck

-4,0-3,5-3,0-2,5-2,0-1,5-1,0-0,50,00,51,0

0 1 2 3 4 5 6 7 8 9

Stra

in (V

)

Static

Dynamic

Figure B10. Hrastnik Bridge – Strain signals of a multiple truck event

As the SiWIM system calculates weights using an algorithm that minimises the difference between the measured (dynamic) and the calculated static signal, a procedure was developed to automatically calculate the DAF according to the equation XXX of the entire traffic flow, including all the extreme events during the measured period. Figure B11, above presents the DAF factors obtained as a function of loading, expressed as the maxim static strain that was measured on the structure (an average strain of a typical 5-axle semi-trailer reached 17,8V).

Each dot in the graph represents one loading event with at least one vehicle heavier than 3.5 tonnes. These were divided into single vehicle events in both lanes (yellow and green circles), events with meeting one vehicle above and one below 3.5 tonnes (blue squares) and events

xv

with 2 vehicles exceeding 3.5 tonnes (red diamonds). These values are compared to the values (flat lines) taken from the Slovene (DAF = 1.17) and Swiss bridge design codes (DAF = 1.70).

The clear conclusion of the measurements was that while the DAF caused by single vehicles, especially the lighter ones that obvious initiated resonance effects, exceeded the 1.7 factor of the Swiss code, the value of the DAF approached 1 as the (multiple) truck events were getting heavier. The DAF value of the heaviest measured event was only 1.03.

Resurfacing of the pavement considerably improved the results (Figure B11, below). TableB1 compares the average DAF factors of a) all loading events, b) loading events where total weight of all vehicles was more than 220 kN, corresponding to one fully loaded 3-axle truck and c) loading events where total weight of all vehicles was more than 380 kN, corresponding to one fully loaded semi-trailer. These were further divided into traffic in lanes 1 and 2, to combination of one heavy and one light vehicle and combination of two heavy vehicles.

0,91,01,11,21,31,41,51,61,71,81,92,02,1

0 4 8 12 16 20 24 28 32

Strain (V)

DA

F

One vehicle - Lane 2One vehicle - Lane 1MP with a light vehicileMP of heavy vehiclesSlovene Bridge design code

Sem

i-tra

iler 4

0 to

ns

0,91,01,11,21,31,41,51,61,71,81,92,02,1

0 4 8 12 16 20 24 28 32

Strain (V)

DA

F

One vehicle - Lane 1One vehicle - Lane 2MP with a light vehicileMP of heavy vehiclesSlovene bridge design code

Sem

i-tra

iler 4

0 to

ns

Figure B11. Hrastnik Bridge – Measured DAFs of loading events before (above) and after resurfacing of the pavement (below)

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SAMARIS

Table B1. Decrease of DAF factors after resurfacing of the pavement

Includ-ingEvents

Vehicles in

lane 1

Vehicles in

lane 2+ another

heavy+ another

light

Before resurfacing

All 1.137 1.111 1.099 1.122> 220 kN 1.071 1.051 1.077 1.081> 380 kN 1.063 1.040 1.050 1.056

After resurfacing

All 1.112 1.045 1.070 1.083> 220 kN 1.077 1.023 1.067 1.060> 380 kN 1.049 1.020 1.036 1.025

Difference After vs. Before

All 81.9% 40.1% 71.2% 67.7%> 220 kN 108.5% 44.8% 87.0% 74.1%> 380 kN 77.8% 50.3% 72.0% 44.6%

The clear conclusions from comparison of results are:2. DAF values in the lane 1 decreased for only for a few percents. The main reason was the

sudden change of the slope of the pavement (a bump) at the first expansion joint, which was not repaired and thus remained the main source of dynamic excitation, despite the new pavement.

3. In the lane 2 the DAF values dropped to less than 50% of the values before the resurfa-cing of the pavement. The average DAF value of all loading events were:

for all events as low as 1.045 (4.5%), with standard deviation 0.058, for all loading events heavier than 220 kN 1.023 (2.3%), with standard deviation

0.038, and for all events heavier than 380 kN only 2.0% , with standard deviation 0.035.

4. The values for the multiple-presence events are between the values for lanes 1 and 2, which can be explained by very uneven approach in lane 1, because of which vehicles from this lane contributed most to the DAF values. It can be expected that with 2 equally smooth lanes the DAF values of multiple-presence events would be lower that the values from the individual lanes and individual vehicles.

5. Despite very uneven approach in lane 1 and a bridge which, by experience, is among the most susceptible to vibrations, the average DAF value of all multiple presence events with 2 heavy vehicles (over 380 kN total weight) before resurfacing was only 1.050, with standard deviation 0.041, and dropped after it to 1.036, with standard deviation 0.029. The prescribed value for such bridge in the Slovene bridge design code is 1.156.

It can be also noted that there for the heaviest multiple presence events there was no obvious correlation between velocity and dynamic amplification factor (Figure B12). For the single truck events the highest values of DAF can be observed at speeds between 40 and 50 km/h, with the exception of some lighter and faster vehicles in the lane 1 before resurfacing of the pavement.

xvii

0,91,01,11,21,31,41,51,61,71,81,92,02,1

10 20 30 40 50 60 70 80 90

Speed (km/h)

DA

F

One vehicle - Lane 1One vehicle - Lane 2MP with a light vehicileMP of heavy vehicles

0,91,01,11,21,31,41,51,61,71,81,92,02,1

10 20 30 40 50 60 70 80 90

Speed (km/h)

DA

F

One vehicle - Lane 1One vehicle - Lane 2MP with a light vehicileMP of heavy vehicles

Figure B12. Hrastnik Bridge – DAF as a function of velocity before (above) and after re-surfacing of the pavement (below)

B5 Conclusion

Bridge WIM measurements proved themselves as an appropriate tool for measuring loads of the entire traffic flow and also, when applying its unique feature of comparing dynamic and static signals of the passing vehicles, for estimating the dynamic amplification factors for each individual loading event. The 2-week measurements before and after resurfacing of the pavement on the Hrastnik bridge captured 10 770 loading events, 370 of which were multiple presence events with another light and 166 with another heavy vehicle. For all these events the dynamic amplification factors were calculated. The analysis of the results gave the fol-lowing answers:

The DAF decreased radically as a function of increasing weight of the loading events. The average DAF value of all multiple presence events with 2 heavy vehicles (over 380 kN total weight) was as low as 1.050 before resurfacing of the pavement and 1.035 after it.

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Resurfacing of the pavement decreased the average value of DAF factors of all loading events for around 50%.

There was no obvious correlation between the dynamic amplification factor and velocity of the multiple-vehicle events.

Hrastnik experiment was the first test site where such extensive experiments were performed and where the results confirmed the conclusions of many up to now only theoretical model-ling of DAF under the extreme traffic loading. On the other hand, before conclusions of this research can be applied for, for example, updating of the bridge design and assessment codes, more measurements on whole spectra of different bridges is needed. These will have to be supported by numerical modelling of the extreme events which cannot be captured during the limited duration of measurements. Nevertheless, as for the SAMARIS experiment we have selected a bridge that was very susceptible to traffic vibration and has extremely uneven pave-ment, it can be concluded, that the real DAF in general are much lower than prescribed in the bridge design code. If performed, such measurements can optimise assessment of the existing bridges, because:

the measured DAF values will be likely much lower than those prescribed in the design codes and consequently,

because of the measured rather than estimated DAF values it is reasonable to re-duce the safety factor for the traffic loading, as discussed in chapter 3.1.

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Appendix C Diagnostic load tests. Main specifications in several European countries

The origin of many diagnostic load tests comes from the necessity of investigating the ability of the bridge structure to carry the designed loads before entering into service. Diagnostic load testing has been performed in many countries for a long time and is connected with the tradition that obliged the designer to stand underneath the structure while the test was being carried out.

There are different policies on load testing in different countries now. The reasons and back-ground to the different policies in different countries are presented in Table C1.

Table C6: The reasons and background to the different policies on load testing

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Country Load tests? Why (Yes/No)

Austria Usually not

It's not a common method in Austria as we are not convinced of the benefits of load tests. Destructive load tests are sometimes per-formed in cases where a structure has to be replaced for research purposes and to gather additional knowledge on the structural be-haviour and the materials.

Czech Republic Yes

Loading test is carried out, when project prescribed it or investor asked it. Usually, when bridge span is great and/or construction of bridge is uncommon.

Denmark Usually not Refined calculation methods and inspection regularly is sufficient.

Germany Gener-ally not

The determination of the load carrying capacity only by load test-ing may deliver additional damages of the existing structures. For bridges in the federal highway network load testing is permitted only in very special cases in a frame of structural assessment.

Italy Yes

One of the main advantages (of static load testing) is the possibility of globally evaluating the behaviour of the structure, or of a part of it, by loading it with loads close to the design ones

Netherlands No

We do trust the inspection results in combination with (re)calcula-tion of the strength. Our codes ask for checking both Service Limit State (SLS) and Ultimate Limit State (ULS). Load testing is im-possible for ULS.

Norway No

Load test are only performed in exceptional cases and usually only in relation to R&D. As far as I know, three load tests were per-formed in Norway during the last five years: an aluminium bridge (design verification and R&D) three span fly over to collapse (demolished due to new road

building, R&D) composite bridge (to verify a new measurement system, R&D.If performed for assessments purpose it is so infrequent that no formal guidelines exist and would be handled as an experimental investigation.

Poland Yes

Tradition to test bridge before put into service.From time to time they do reveal deficiencies (mainly excessive support displacement).Possibility to develop scientific and engineering knowledge about real bridge behaviour under loads.

Slovenia Yes Static and dynamic load testing is carried out on structures with spans exceeding 15 meters to confirm that: behaviour of the structure is in accordance with design assump-

tions,

xxi

Country Load tests? Why (Yes/No)

quality of the work is in accordance with design requirements, structure is able to carry loads at design load level, structure is made in accordance with the project.

Since 2004 soft load tests have been applied on around 30 bridges.

Spain Yes

Load test are always performed, after finishing construction of bridges, before they are “opened to public use”.Reason is “tradition” as a method of checking construction of new structures.

SwitzerlandIn special cases

Until the 1980’s load tests were performed for all mayor highway bridges.In the Eastern part of Switzerland they were usually executed by EMPA (Swiss Federal Laboratories for Materials Testing and Research) and in the Western part by EPFL (Swiss Federal Institute of Technology Lausanne, Prof. Renaud Favre).By the changes of core competences at EMPA and the retirement of Prof. Favre load testing has lost its major promoters. The owners still require it in special cases.

United Kingdom Yes

To provide information on structural behaviour as an aid to the as-sessment of load carrying capacity. On national roads the loads are limited to that actually carried on a day to day basis. Some local authorities have previously allowed the full unfactored design load to be applied to their structures

Table C7: List of obligatory codes dealing with test loading

Country Code number and title Field of applicationAustria No codes.

Czech Republic

EN 1317-1 Road restraint systems – Part 1: Terminology and general criteria for test methodsEN 1317-2 Road restraint systems – Part 2: Performance classes, impact test acceptance criteria and test methods for safety barriersEN 1824 Road markings materials – Road trialsENV 1993-2 Design of steel structures – Part 2: Steel bridgesCSN 73 6209 Loading tests of bridges

New bridges are load tested, when span is long and construction is not of a common type

Denmark No codes and standards.

Germany No codes, only regulations for the authorities of the federal highway network.

Experimental assessment of load

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SAMARIS

Country Code number and title Field of applicationDAfStb-Richtlinie "Belastungsversuche an Betonbauwerken"; Guideline for load testing on structural engineering (except bridges); Guideline for bridges is planned

carrying capacity of existing structures

Italy Italian codes: D.M.LLPP 09/01/96 (concrete and steel structures) and D.M.LLPP 04/05/90 (bridges)

Norway No codes. Only mentioned in general terms as a possible investigation procedure.

Poland

PN-89/S-10050 Bridges. Steel structures. Requirements and testing.PN-/S-10040 Reinforced, prestressed stayed and concrete bridges. Specifications and technical testing.

According to the title.

Slovenia JUS U.M1.046(1984), Testing of bridges with test load.

Bridges and other structures with spans exceeding 10 or 15 m.

Spain“Recomendaciones para la realización de pruebas de carga de recepción en puentes de carretera” (1999).

According to title a code for making load testing for opening of new structures is provided

Switzerland

The code SIA 160(1970) Standard for Load Assumptions, Commissioning and Monitoring of Structures required load tests for railway bridges with L > 10 m and highway bridges with L > 20 m. Owners and authorities partly adopted these requirements for their own directives. The proceeding code SIA 160(1989) Actions on Structures does not cover commissioning anymore and therefore does not deal with load testing.In 1987 load testing was regulated by a recommendation SIA 169(1987) Preservation of Engineering Structures. In 1997 this recommendation has been replaced by SIA 469(1997) Preservation of Construction Works with a broader view, not covering load testing anymore.If not otherwise stated, all subsequent details refer to SIA 169(1987), expiring July 31, 1997.(SIA means: Swiss Society of Engineers and Architects, the standardization body of Switzerland in the construction field.)

United Kingdom

No requirement to carry out load testing of bridges. BD 21: The assessment of highway bridges and

Only permitted to check structural behaviour or to verify

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Country Code number and title Field of applicationstructures (Highways Agency 1997). This document permits the use of supplementary load testing as an aid to assessment. BA 54: The use of load testing for bridge assessment (Highways Agency 1994). This document provides guidelines for load testing (supplementary and proving load tests) and gives the limitations imposed on its use. Proof load testing for assessment is not permitted.Guidelines for the supplementary load testing of bridges (ICE 1998) provides additional guidelines.

method of analysis.

Load testing of new bridges is rarely carried out.

Table C8: Field of application of load testing in various countries

Country Structure type Loading necessity

Czech Republic

Static loadingbridge works in general, turntables, travelling platforms, wagon weighbridges, discharge chutes, car dumpersbridge structures of unusual statical systems, extraordinary spans, works produced when applying new technologies or new materials, bridge reconstruction, and provisional structuresDynamic loadingReliability checking by comparison of the dynamical behaviour changes (eg of self-excited frequencies, modes of vibration attenuation).

Germany

Static and dynamic loading in general only for research; see the finished research project "EXTRA" (experimental analysis of load carrying capacity) of the universities Bremen, Dresden, Weimar, Leipzig.

ItalyBefore opening to traffic.During service life to check the structural performances.

Netherlands None.Norway R&D.

Poland

Static loadingPrototype structuresTypical road bridges spansSuperstructure renovated or strengthened bridges Others according to investor or user demandDynamic loading

AllL >20 mAllAll

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SAMARIS

Country Structure type Loading necessity

Others according to investor or user demand All

Slovenia

Routine load testing, static and dynamic, is carried out on all new, strengthened or rehabilitated road and railway bridges and on pedestrian footbridges, if required by the designer or the owner. Load testing is carried out for reinforced and prestressed concrete, steel and composite structures.Special load testing are carried out on all structures without span limitation, which do not meet the design requirements with respect to the quality of the material, dimensions, quality of the foundations, connections (steel structures), etc. and if structure failed to satisfy the load testing requirements after routine load testing had been repeated.Exceptional load testingStatic load testing for all structures without span limitation, if transport of the exceptional load will impose internal forces greater than due to design load. Results of the load testing are valid only for the particular exceptional transport.

L 15 mL 10 m

SpainStatic loading is used as an acceptance test of new structures.Dynamic loading is required where vibrations SLS is expected.

L 10 m

Switzerland Important, complex or extraordinary structures, especially highway bridges with spans L > 20 m.As an acceptance test or previous to heavy loading.As a part of principal or special inspections.A Guideline of the Swiss National Road Office Project and Execution of Structures of the National Highway Network (edition 1999) says in clause 9.2:The advisability of a load test is evaluated by the Canton (i.e. the state authority) from case to case. It may be performed during commissioning of new bridge or after an extensive rehabilitation.The advisability of a load test may be ascertained in the following cases: complex structures, for which modelling of the serviceabil-

ity limit state is difficult. innovative structures, having been treated with a new design

concept. structures that react sensitively to deformations and for

which the knowledge of the effective stiffness is important. before and after strengthening of a structure, when the struc-

tural system is changed considerably or when the strengthen-

Recommended in SIA 169(1987)Recommended in SIA 169(1987)Option in SIA 169(1987)

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Country Structure type Loading necessity

ing participates much to the load bearing capacity. when severe difficulties in execution have occurred.

United Kingdom

Static loadingLoad testing only permitted as an aid to assessment – see above.Dynamic loadingNot required and rarely used, but only for research purposes.

Magnitude of load limited to site-specific situation.

L – span

C1 Test loading

The bridge structure is loaded with static and (or) dynamic loads during the investigation. The highway bridges are usually statically loaded with vehicles (lorries). The main parameter of the load test is the magnitude of the load. As a rule the load quantity is chosen in relation to characteristic loading (OC) or calculated ones (OO). In table C4 are shown some of the load levels as required in different countries.

Table C9: Static load quantity

Country Load quantity - L Remarks

Czech Republic

For highway bridges and footbridges: road vehicles, with the exception of dozers, tanks filled with water, with concentrated loads and suchlike.For the static action due to the test loading it holds

UN = k UVs

where k the efficiencyN test loadingVS a characteristic value of the short-term vertical mov-

able live loading

Germany

According to "EXTRA" the limit of test load is defined by deformation criterions, e.g. strain limiting of concrete and reinforcement steel, limiting of crack width, limiting of nonlinear deflection.

deformation criterions have to be check in real time during the test

Italy Usually 80% of design loads.Netherlands No testing.

Norway For R&D, a measurable loading (typically equal to unfactored axle load i.e. excluding UDL)

Poland Test loading should cause the following internal forces level:Steel structures: 0,75 OO L 1,05 OO

Min time:deflection increment during 15 min

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SAMARIS

Country Load quantity - L RemarksReinforced, prestressed and concrete structures:

1,00 OC Lintervals 2%

Slovenia

Routine load testing0,5 OO L 1,0 OO

Special load testing1,0 OO L 1,1 OO

Exceptional load testing (for spans (S) up to 100 m)1,1 OO L (1,3 – S/1000) OO; (S in m)

Spain Load is limited to 60% of representative value of nominal load, provided 70% of maximum stresses is not reached.

Switzerland Up to the representative values of traffic loads of the relevant action code SIA 160. SIA 169(1987)

United Kingdom

Load testing permitted as an aid to assessment only – see above. Applied load cannot exceed the normal day-to-day load experienced by the structure.

Proof load testing not recommended because of magnitude of loading required and because of fears of hidden damage.

The highway bridges are usually dynamically loaded with vehicles (lorries). The main para-meter of the dynamic test loading is the vehicle speed. The secondary parameters are vehicle suspension and road profile.

Table C10:Method of dynamic loading

Country Method of loading Remarks

Czech Republic

Vibration actuator with variable frequency, impulse rocket engines, unloading of the bridge by a sudden loosing of the load and similar;

an amenable road or railway vehicle or a platoon moving over the bridge

in the case of the road bridges, the crossing of artifi-cial surface irregularities

for the footbridges, individuals or groups of people

GermanyRarely carried out;Heavy goods vehicles run over the bridge with different speed.

Italy Traffic, artificial loading (vibrodine).Netherlands No testing.

NorwayHeavy truck traveling at different constant speeds, across the bridge, with and without a plank (neoprene) approximately 5 cm thick.

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Country Method of loading Remarks

Poland Vehicle passages (1 or 2) with varied speed (10 km/h, 30 km/h, 50 km/h, up to maximum appropriate speed).

The artificial discontinuity (the obstacle 10 cm high) is put on the pavement to gain deck vibration

Slovenia

Moving truck (usually one) with varied speed 10, 20, 30, 40 km/h,… up to maximum appropriate speed. In special cases dropping of weight. Excitation with vibrators or people for pedestrian footbridges.

Spain One or two heavy trucks moving at different constant speeds from 5 km/h to 60 km/h.

Switzerland

Generally:rotating masses, hydraulic jacks, impacts.For bridges:Heavy truck with different constant speeds, circulating on the bridge deck and passing a board 45 mm thick.

SIA 169(1987)Reference:Cantieni, R.: Dynamische Belastungsversuche an Strassenbrücken in der Schweiz – 60 Jahre Erfahrung der EMPA (Dynamic load tests with highway bridges in Switzerland – 60 years of experience at EMPA), Report Nr. 116/1, July 1983, EMPA Section 116, Dübendorf CH, 78 pp.

UKNot carried out except for research purposes. Methods used have included dropping weight, moving mass, and pedestrian excitation of footbridges.

C2 The investigation range and measurement methods applied.

Table C11:Range of investigations

Country The scope of investigation and measurements Remarks

Czech Republic

Measurement of:a) vertical displacement in the point of the largest anticipated

deflection of the spans,b) settlement of supports and pushing in bearings.The observation and reading of:a) temperature of both the air and structure (continuously),b) relative strains in the exposed localities in the bridge

construction,c) deflection, displacements and rotations of other important

parts of the bridge structure,d) setting – down of the foundations,e) horizontal transverse deformations of the compression

xxviii

SAMARIS

Country The scope of investigation and measurements Remarksflanges of the bridges of an open cross section,

f) Initiation of cracks and their growth.

Germany

Measurement of: applied load, reaction load, linear and non-linear deflection, strain/stress, crack width, temperature.

Italy Mainly accelerations and displacements.Netherlands No testing.

Norway

As testing is related to R&D, measured parameters vary in relation to the objective of the test. Most common are (in this order): deflection, temperature, strain, acceleration, velocity, cable forces, wind speed.

Poland

Visual examination before load testing Measurements under the test loading Deflection (basic determination) Support displacement (basic determination) Stresses (extra determination) Visual examination during the load testing Visual examination after the load testing

Slovenia

Visual examination before load testing.Measurements during every test loading: vertical deflection of every span, displacement of the support, stresses (for prestressed, steel and composite structures at

location of maximum values), visual examination during the load test and as extra demands: horizontal and vertical displacement of

the bearings, crack width, temperature and inclination.Visual examination after the load test.

SpainVisual examination before load testingMeasurement of deflections during load testing

Switzerland

Elastic and residual deflection (before, between and after two loadings)Strains, rotational angles, bearing forces and other measurementsSecondary effects like temperature, sun exposure, windNatural frequencies, damping ratio, dynamic increment

mandatory

optionalmandatoryfor dynamic tests

United Kingdom

Visual inspection before and after load testing Measurements under the test loading Applied load Deflection Strain Visual examination during load test

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Country The scope of investigation and measurements Remarks Visual examination after the load test

Table C12:Measurement methods

Country The range of investigation and measurements Remarks

Czech Republic

Measurements cover: Applied load (by means of load cells) Deflection (LVDTs, dialgauges) Strains (vibrating wire strain gauges, resistance gauges) Temperature Geodesic methods Photogrammetry methods

Germany

Applied load: load cellsDeflection: Inductive mechanic gauges, dial gauges,

inclinometer, accelerometer,laser technique

Strain: vibrating wire strain gauges,resistance gauges

Temperature: PT100 (resistance)Italy Continuous data acquisition (time history) or discontinuouslyNetherlands No testing.

Norway

Measurements (adapted to objective of the test): Applied load (truck must be weighted beforehand at calib-

rated weight station) Deflection (LVDT’s, dial gauges, surveying equipment,

laser) Strain (Electric resistance wire strain gauges, vib. wire), Inclination (electronic inclinometer), Cracks (crack meter), Temperature, Accelerometers.

Poland

Displacements, Deflection Inductive mechanic gauges (S & D) dial indicators (S) Geodesic methods – leveling (S) Laser methods (S & D) Photogrammetry methods (S & D) Strains Electric resistance wire strain gauges (S & D)

No remarks about measurement methods in Polish Codes

Slovenia Measurements: Applied load (trucks must be weighted beforehand on a cal-

ibrated weighing station), Deflection (LVDT’s, dial gauges, surveying equipment), Strain (Electric resistance wire strain gauges), Inclination (electronic inclinometer), Cracks (crack meter), Temperature,

xxx

SAMARIS

Country The range of investigation and measurements Remarks Laser (D).

Spain

Deflections: gauges, surveying equipment, laser methods

Switzerland No indications in codes

United Kingdom

Measurements include: Applied load (weigh pads or load cells) Deflection (LVDTs, dial gauges, surveying equipment) Strain(DEMEC, vibrating wire strain gauges, resistance

gauges) Temperature

Supplementary load tests require comprehensive instrumentation to determine structural behaviour.

S – Static load measurements D – Dynamic load measurements

C3 Results analysis and structure assessment criteria

Table C13:Analysis of results and assessment criteria

Country Analysis of results and assessment criteria.

Czech Republic

Static load After static testing, total components of the particular total, Stot, permanent, Sr, and elastic actions, Se, are calculated from the values obtained through the measuring process. At the same time, it reads:

Stot = Sr + Se

Furthermore, the recalculation of the effects Scal is performed in dependence on an actual magnitude + of the applied load.The following notation holds:

Stot .... total actionsSr ...... permanent effectsSe ...... elastic effectsScal .... the action values determined in a theoretical manner

The bridge structure is serviceable if the conditions are fulfilled at once, as follows:a) β < ≤ α (see Table 1 below)b) ≤ α1

c) the width of cracks weakening the reinforced and prestressed concrete does not exceed, at the test loading, the proportionate part of the limit value given in Table 2

Table 1. Values of coefficients α, α1, βCon-

structionunits of prestressed concrete, composite

steel-concrete, prestressed concrete-concretereinforced concrete, and compos-

ite, reinforced c.-concrete 1.05 1.1

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Country Analysis of results and assessment criteria.α1 0.2 0.25β 0.7 0.6

Table 2. Limit crack width (dimensions in mm)

Superstructures Environmental class (according to CSN 73 6206)Limit crack width

reinforced concrete

1 (dry), 2.3 (moist)individually in accordance with aggressivity, max 0.1

partially prestressed1

2.34.5

additionally prestressed 0.1prestressed in advance 0.0restrictedly and fully

prestressed of any sort 0.0

Dynamic load When carrying dynamic tests out, the following data are specified:a) natural modes of vibration and belonging frequencies of an unloaded bridge

construction,b) time courses and modes of the constrained structure vibration,c) logarithmic damping decrement Θ of an unloaded bridge structure,d) for the test application of load, eg dynamic rates Smax – Sm, resonance curves,

amplitudes, velocity and acceleration of oscillation or the like, are evaluated,e) dynamic measured coefficient δobs in compliance with:

δobs = where:Smax the largest value of dynamic response owing to the test loadingSm the largest value of stoical response due to the same test load with value

Smax being determinedGermany Results from load test are used to improve the applied structural model for assessment.Italy Results from load test are used to check conditions (structural response)

Norway Compare measured and calculated response, compare to accepted bridge performance values, explain differences.

Poland

Static load MeasuredElastic deflection calculated measurements Permanent deflection in correlation to absolute deflection 20 % reinforced concrete structures 10 % prestressed concrete structuresDynamic load Deflection static deflection or quasi static (v=0 or 10 km/h) multiplied by dynamic fac-tor (also calculated are free vibration frequency and dumping decrement).

Slovenia Static load Measured elastic deflection calculated value

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SAMARIS

Country Analysis of results and assessment criteria. Permanent deflection in correlation to the value under the loading:

25% reinforced concrete structures, 20% prestressed concrete structures, 15% steel and composite structures.

Max. measured crack width must be equal or less than design value;Maximal measured deflection values do not impact on the functionality or aesthetics of the bridge structure.Dynamic load Measured dynamic coefficient should be less or equal to the design value. Periods of free vibration should be in the frame of theoretical values. Vibrations should not impose uncomfortable feelings to the users.

If requirements at static loading are not met and permanent deflections are 25% or more above the allowable values, load testing should be repeated. After repeated load testing the residual deflections must not be greater than: 12.5% for reinforced concrete structures, 10% for prestressed concrete structures, 7,5% for steel and composite structures.

If after the repeated load testing these values are exceeded, the structure is re-analyzed and appropriate measures undertaken.

Spain

Static loading test: Measured elastic deflection <= 110% calculated deflection (prestressed concrete struc-

tures) Measured elastic deflection <= 110% calculated deflection (steel structures) Measured elastic deflection <= 115% calculated deflection (reinforced concrete struc-

tures) Permanent deflection <= 15% calculated deflection (prestressed concrete structures) Permanent deflection <= 10% calculated deflection (steel structures) Permanent deflection <= 20% calculated deflection (reinforced concrete structures)

Switzerland Test report, comparing measured and calculated deflections, explaining differences, evaluating the dynamic behaviour and drawing conclusions.

United Kingdom

No particular criteria. Analysis must be carried out as part of the load test and the applied load limited to ensure that that no permanent deformation or damage occurs.Results from load test used to re-assess the structure using a more accurate structural model.

xxxiii

Appendix D - Validation of Simplified Notional Load Model

In this work, a simplified model is developed which aims to reproduce similar critical loading events from knowledge of the site-specific traffic characteristics without having to perform a full Monte Carlo simulation.

The new method is validated using WIM data from four different sites. From these, load ef-fects corresponding to different return periods are calculated. The results of the Monte Carlo simulation are then compared to the results obtained from the simplified model.

The first three data sets were recorded on three different highways in the Netherlands while the fourth was recorded on a highway in France. The sites are referred to here as Site 1, Site 2, Site 3 and Site 4, respectively. In all cases, the highways are dual carriageway with three to four traffic lanes in each direction. The data was collected on the outermost (slower) lanes in each direction. Data from the slow lanes of multi-lane carriageways will have a significantly greater proportion of trucks than the same traffic on a 2-lane road. It is therefore conservative to use such data for simulations of bridges with only two opposing lanes of traffic. However, where possible, more appropriate WIM data should be used for simulations of 2-lane bridges. Only vehicles weighing at least 3.5 tonnes (i.e., only trucks) were registered. The measure-ment locations and periods are given in Table 1. The data were recorded continuously for dif-ferent periods as can be seen in the table.

Table D1: Measurement locations and periods

Denotation Highway Site location Measurement period Site 1 R04 Amsterdam Oct. 6–19, 2003 Site 2 R12 Utrecht Oct. 6–19, 2003 Site 3 R16 Dordrecht Oct. 6–19, 2003 Site 4 A1 near Ressons Sep. 9–14, 1996

Analysis of the daily maximum load effects obtained from full Monte Carlo simulations re-vealed that five-axle trucks, because of their lengths and weights, were dominant in the crit-ical loading scenarios. The simplified model is therefore formulated with pairs of heavy five-axle trucks placed at critical locations on the bridge. Many simplified models were con-sidered. Many models, including Turkstra's Rule, gave inconsistent results for different sites, spans and load effects. While Turkstra's Rule is accurate in particular cases, it gave significant inaccuracies in others. The simplified model developed here was found to be the most effect-ive and consistent. In this model, the second truck is assumed to be in a different location to the first, not quite at the most critical point.

The results obtained from the full simulations and the simplified model are compared. The 1000 year load effects are determined from the distributions of the daily maximum load ef-fects for each bridge length obtained from the full simulations. For the simplified model, single optimal values for αM and αS were sought that produced equivalent characteristic load effect values to the full simulations. According to this investigation, the optimal values for αM

xxxiv

SAMARIS

and αS are 0.63 and 0.42, respectively. Finally, the 5% and 95% confidence limits for the ex-trapolated values obtained from the full simulations are calculated. The 1000 year load effects obtained from the full simulations together with their 5% and 95% confidence limits and the corresponding values calculated from the simplified model are shown in Table 2 (for mid-span moment) and in Table 3 (for end shear). The relative differences between the results from the two approaches are also given in the tables. As can be seen, the absolute differences observed are between 0.9% and 10.9% for the mid-span moment and between 0.3% and 13.6% for end shear. These differences are small relative to the differences between sites evident in the table. The simplified model gives reasonably good estimates of the character-istic values in all cases. It is clearly possible to get a low value of αM and αS for a particular site. However, it is highly significant to find values which are consistent across different sites with completely different traffic and a wide range of spans.

Table D2: Comparison of the 1000 year mid-span moment (M ) in kNm obtained from the full simulations (FS) and the corresponding values obtained from the simplified model (SM) with αM=0.63. The 5 % (M0.05) and 95 %(M0.95) confid-ence limits of the extrapolated values are also given in the table

Site L [m]

FS SM Diff. [%]M0.05 M M0.95 M

1

15 20 25 30 35

3554 5217 6992 9302

10549

3573 5242 7021 9347

10592

3591 5266 7051 9388 10633

3787 5601 7578 9592

11649

-5.7-6.4-7.3-2.5-9.1

2

15 20 25 30 35

3852 5685 7647 9829

12457

3874 5714 7681 9873

12523

3895 5741 7717 9916 12583

3959 5796 7875

10035 12163

-2.1-1.4-2.5-1.63.0

3

15 20 25 30 35

4187 6052 8529

10258 12573

4214 6085 8578

10305 12635

4242 6118 8624 10353 12696

3947 5715 7762 9845

11974

6.86.510.54.75.5

4

15 20 25 30 35

2154 3581 5167 6289 8281

2162 3598 5194 6311 8330

2169 3616 5222 6333 8382

2420 3567 4871 6177 7511

-10.70.96.62.210.9

MaxMin

10.90.9

xxxv

Table D3: Comparison of the 1000 year end shear (S) in kN obtained from the full sim-ulations (FS) and the corresponding values obtained from the simplified model (SM) with αS=0.42. The 5 % (S0.05) and 95 % (S0.95) confidence limits of the extrapolated values are also given in the table.

Site L [m]

FS SM Diff. [%]S0.05 S S0.95 S

1

15 20 25 30 35

1117 1245 1293 1336 1382

1124

1253

1299

1342

1390

1131 1259 1306 1348 1397

1137

1192

1222

1402

1416

-1.15.16.3-4.3-1.8

2

15 20 25 30 35

1247 1326 1392 1432 1496

1255

1333

1400

1439

1504

1263 1339 1407 1446 1511

1191

1247

1269

1473

1479

5.46.9

10.3-2.31.6

3

15 20 25 30 35

1226 1389 1411 1485 1496

1233

1397

1417

1491

1505

1240 1405 1423 1498 1513

1172

1234

1248

1447

1453

5.213.313.63.13.5

4

15 20 25 30 35

650 756 821 880 924

652 758 824 883 926

654 760 826 885 929

707 738 755 917 923

-7.82.89.1-3.70.3

MaxMin

13.60.3

It should also be mentioned that when αM = 0.63, not all axles are on the bridge for spans of 25 meters and less. For αS = 0.42, all axles of the lighter truck are involved in the critical

xxxvi

SAMARIS

loading cases for all spans with the exception of the 15 meter span bridge where only the last four axles are involved.

xxxvii