A Seminar Report OnDURABILITY OF STEEL BRIDGES BY VINAY.M.M

INTRODUCTIONAny civil engineering structure designed is expected to have a long service life, which is synonymous with durability . In a developing country like India, we can hardly afford to build structures, which will not provide a long service life if we do so, this will only weaken the countrys economic structure. Bridge structures are no exception.Bridge construction is one of the most important activities of infrastructure development and is also a very costly projectsteel bridges have been a preferable option for the authorities for more than last 100 years as it has satisfactorily established that only steel bridge can offer more than 100 years of service life.Ex Fig.1 it is a bridge constructed over Hoogly river in Kolkatta which is a rail bridge which has served over 121 years.

Steel Steel is iron refined until it is almost free from impurities up to 2.5% of carbon is left and other materials can be added to create different strength and hardness of steel.

DURABILITYDurability refers to the extent to which a material is proficient of resisting deterioration caused by exposure to severe conditions. It is not an intrinsic property of a material but depends largely on how the material would behave under specific environment conditions such as temperature, moisture etc..The right kind of material should be used. It should posses properties like Absorption power, Compressive strength, Abrasion resistance, Moisture penetration.

FACTORS INFLUENCING DURABILITY Steel bridges are durable but durability is not ensured by making the bridges with steel only. Several other factors are also to be taken care of they areSelection of materialDesignFabrication and erectionSurface preparationInitial protective coatingInspection and maintenanceProtective coating during service lifeStrengthening Replacement of girdersMiscellaneous

1.Selection of materialsStrength , elongation , corrosion resistance, ductility , brittleness , fatigue resistance are some of the properties are structural steel needed for application in the bridge. Use of weather resistance grades of steel along with provision of sacrificial thickness (wherever required), is also being considered for construction of steel business in very aggressive environments.Wrought irons were used as structural ferrous material during 1870-1900. Presently Indian and some foreign codes are followed for the production of steel in India. Important of them are IS: 1977, IS:2062, IS:8500, IS:11587, IS:1786, BS:4360 etc.,

2.DesignDesign of Bridge is another important factor for bridge durability. There are several design methodologies (Working Stress method, Limit State method, Limit State analysis method etc.,) and types of bridges are plate girder, semi through and arch types etc. Cable Stayed bridges a relatively new development is trying to take major share in construction of long span bridges. Table 1 gives standard design of bridges adapted by Indian railway indicating its type, quantity of steel used in painting area

2.DesignDesign detail for uncoated steel in bridges require careful consideration of the following Elimination of bridge joint wherever possible.If expansion joint are used, these must be able to resist incoming of water.On the deck, a trough under the deck joint may serve to drive the water away from the vulnerable elementsPainting of all super structures steel within a distance of 1.5 times the depth of girder from bridge Joint.If box girders are used, these should be hermetically sealed wherever possible are provide with weep hole to allow proper drainage and circulation of air. All opening in boxes that are not sealed should be covered or screened.Protecting pier capes and abutments wall to minimize staining.Sealing overlapping surface exposed to water to prevent the capillary penetration of moisture

Table 1 Standard Design of Indian Railway Bridges

Sl. No.Span (m)Girder typeWeight /Span(MT)Painting area (Sq mtr)176.2Open Web291.8834000261.0Open Web195.8172900345.7Open Web114.5351625430.5Open Web62.6241257530.5Open Web43.163760630.5Riveted Plate58.879605724.4Riveted Plate33.383390818.3Riveted Plate20.950290912.2Riveted Plate10.069135109.2Riveted Plate6.436100116.1Riveted Plate3.999501218.3Riveted Plate21.1292701318.3Riveted Plate17.1152221412.2Riveted Plate9.8521361512.2Riveted Plate8.107138

3.Fabrication and Erection After designing quality of fabrication and erection is also important in contributing durability following are some of situations, which happen during construction having its adverse impact during service life Lying of uncoated/unprotected steel structure in open atmosphere Structural Sections are joined generally by riveting , bolting or by welding. Lack of quality control at site of fabrication will result in major maintenance problem within very short service life , use of substandard consumables would easily significantly shorten the service life steel bridges Alignment of sections during fabrication is also important Lack of quality control during application of protective coating during fabrication is a common phenomenon

4.Surface PreparationA technician uses high-pressure water to clean a section of steel.Before applying any protective coating on steel, surface preparation is a must for removal of mill scale rust and dirt. Blast cleaning could be a preferable choice for components of bridge structure

If the steel surface of a bridge is not cleaned adequately before painting, the protective coating can fail prematurely. The steel then will develop corrosion, such as the rust shown on the underside of this bridge. If the bridge is not rehabilitated, the corrosion eventually will compromise its structural integrity.4.Surface Preparation

5.Initial Protective Coating Selection and application of initial protective coating is very important for bridge durability. Maintenance free service life of about 25 years is obtained, but due to negligence and improper work 6-8 years are normally achieved. The reasons are:Structural sections are not normally galvanized for bridgesOnly metal spraying is adopted (though recently) for the bridges in coastal areasSurface preparation before painting is not of expected quality.Very few types of painting systems with expected service life of maximum 10 years are commonly used for most of the steel bridgesPainting work is carried out even when the surrounding environment is not technically favorable.

5.Initial Protective Coating(Above) A painter applies a protective coating to a bridge to protect it.

6.Inspection and Maintenance Maintenance in bridges is required for following reasons:To implement procedures to detect and minimize corrosion.To control and divert drainage system away from the bridge structures, to clean or reseal deck joints, to maintain deck drainage systemsTo remove dirt, debris and other deposits, which trap moistureTo remove all vegetation and other matter that can prevent the natural drying of wet steel surface.To maintain covers and screens over access holes. Inspectibility of all bridge members and connections is an essentially design-stage consideration especially when the structure includes enclosed sections, such as box girders.

7.Protective Coating During Service LifeProtective coating during service life of bridges is another most important factor for durability. It is not only painting but also includes other coating like thermal spraying. For other type of bridges, painting may be for aesthetic reason but for steel bridges, it gives protection from deterioration due to several causes. In India, painting systems adopted for the bridges are as per IS: 8629-1977. The code recommends three grades of painting systems suitable for different environments

8.Strengthening / Replacement of GirdersAs durability of steel bridges is about 100-125 years, it cannot be expected that loading will remain same throughout the service life. Even today, while constructing a bridge, it is impossible to predict what could be the loading on the brides after 100 years. So it has been experienced that technical inspection is carries out in a steel bridge after about 50 years of service life to assess the condition of the bridge to compare with required condition of the bridge. In most of the cases, authorities suggest for re-strengthening (by adding new structural members) without discarding the existing

9.MiscellaneousThere are several miscellaneous factors, which are generally not common but happen occasionally. Some of such factors are flood, earthquake, derailment (in case of Rail Bridge), unexpected short life of painting, localized corrosion, structural members failure etc

FACTORS AFFECTING DURABILITYMain factors affecting the durability of steel bridges are indicated below:Constructional NegligenceFatigueStrengthChemical ReactionsTemperature VariationMoisture

Constructional NegligenceFlowchart1: Constructional factors

FatigueIt is a phenomenon, which results in the sudden fracture of a component after a period of cyclic loading. Failure is the result of a process involving the initiation and growth of a crack. Fatigue strength is determined by the application of different levels of cyclic stress and measuring the number of cycles to failure

StrengthThe ability to withstand either gravity or cyclical structural loads whether it is vertical or lateral relies heavily on the material strength. Structural analysis may require higher strength in order to resist the applied loads. If a structure member or connection is subjected to cyclically varying load, it may fail after a certain number of load applications even if the maximum nominal stress in a single cycle is much less than the yield stress of the material. This initiates the designer to take such aspects into consideration if durability has to be taken care of.

StrengthFlowchart2: Factors affecting strength

DAMAGE GROWTHPast experience have indicated that the time for initiation of cracks is relatively short and the majority of life is spent on the growth of resultant crack. Thus to ensure structural safety, the consideration of initial damage in the form of crack is absolutely obligatory. The inter-relationship between the crack length, loading and strength results in structural failure. The service life of the structure is the number of cycles required to force the crack from the initial length to the final length. The residual strength (i.e. the load carrying capacity of the crack structure) is inversely proportional to the crack length when this residual strength decays to the level of the maximum Stress fracture occurs.Experiment have shown the service life of the structure depends on several factors some of them important amongst them are Preliminary cracks sizeProperties of the materialLoading conditionCritical Crack Size

DAMAGE GROWTH Some effective methods of improving fatigue performance are eliminating or reducing stress raisers by streamlining the part, avoiding the sharp surface tear, preventing the development of the surface discontinuities during construction, reduction in tensile residual stresses caused by manufacturing, improving the detail of fabrication and fastening procedures. Two major vulnerable factors of steel are Corrosion andWeld cracks

SCOPE OF IMPROVING DURABILITY For Existing BridgesFollowing some of the steps if taken durability of steel bridges will be improvedImproving quality control in painting systems, so that gap between estimated and achieved service life of paints is minimized Remedial/corrective steps may be taken immediately after noticing the problemsMaintenance the system is to be improved by computerizing the system and inspecting the members of bridges by modern requirement. Improved coating system may be introduced to the bridges, where corrosion problem is severe Steps may be taken to strengthen the old bridges wherever necessary For new BridgesFollowing steps if taken will improve the durability the new steel bridgesGrade of steel selected for bridges must have corrosion resistance and high strength Initial protective coating system for bridges is required to adapted in such a way in 25 yrs of maintenance free life can be achieved

Use of galvanized steel for bridge construction Sharp fabrication instead of site fabrication is preferable as it will ensure improved quality controlMinimum thickness of steel section should always be governed in bridges by slenderness ratios or maximum width- thickness and depth-thickness ratios

CONCLUSIONService life of steel bridges for more than 100 yrs can be ensured with some exceptions such durability cannot be achieved for bridges with other type of material. The durability will be much improved if suitable step are taken for re-strengthening and implementing improved maintenance system so factors influencing the durability for each bridge may be identified and actions should be taken accordingly

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