M27BE Notes 03 Traffic Load- Oct 2011

M27BE ‐ Bridge Engineering

Neil Tsang – Bridge Temperature 1 October 2011

Coventry University Department of Civil Engineering, Architecture and Building

MSc BRIDGE ENGINEERING MODULE

TRAFFIC LOADS ON BRIDGES In Eurocodes, loadings on bridges are governed by BS EN 1990 ‐ Basis of structural design and BS EN 1991 – Actions on structures. BS EN 1991‐1‐7 General actions – Accidental actions and BS EN 1991‐2 Traffic loads on bridges are particular relevant to the evaluation of traffic loads on bridges. In this notes, only the most basic traffic loads are covered, please refer to the original EN document for further detail of various types of traffic loads and combinations.

INTRODUCTION Bridges can be subjected to many different loadings due to traffic (rail, road, pedestrian, etc) and environmental (wind, temperature and earthquake) as well as a wide variety of accidental loads. BS EN 1991 Actions on structures covers all aspects of design loading for highway, railway and pedestrian bridges. BS EN 1990 Basis of structural design covers partial safety factors on loads and load combinations. Various combinations of loads together with the load factors to be used in the ultimate limit state as well as in the serviceability limit state are prescribed. For bridges in the UK, the Eurocode effectively supersedes BS5400. DoT Standards Design Manual for Roads and Bridges will be updated to be consistent with Eurocodes. The traffic loading in BS EN 1991‐2 is significantly different to BS5400:Part2 for certain loads. The following notes are based on the recommendations of the Eurocodes. Highway bridges are usually designed to carry a combination of uniformly distributed loadings (UDL system) and axle loads (Tandem system). These loads together with other variable actions are specified in BS EN 1991‐2 and 1991‐1‐3 to 6 and applied in prescribed combinations. The UDL and tandem systems are not direct representation of actual vehicles but idealisations to model the worst combinations of actual vehicles and include dynamic magnification, i.e. the effect of axles bouncing on rough roads.

1. PERMANENT ACTIONS Action that is likely to act throughout a given reference period and for which the variation in magnitude with time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value.

1.1 Self-weight of structure

This comprises the weight of structural elements of the bridge but excludes superimposed materials. The unit weight of various materials can be found in 1991‐1‐1.

1.2 Self-weight of non-structural elements

This includes the weight of road surfacing, parapets, waterproofing, kerbs, lighting columns, rail track ballast, services, etc.



Figure 1 – Typical details

1.3 Settlement

Settlements are mainly caused by permanent loads and backfill. Variable actions may have to be taken into account for some individual projects. Settlements vary monotonically (in the same direction) with time and need to be taken into account from the time they give rise to effects in the structure (i.e. after the structure, or a part of it, becomes statically indeterminate). In addition, in the case of a concrete structure or a structure with concrete elements, there may be an interaction between the development of settlements and creep of concrete members. Where differential settlement is likely to affect the structure in whole or in part, the effects of this shall be taken into account.

1.4 Prestressing

Prestressing should be classified as a permanent action caused by either controlled forces and/or controlled deformations imposed on a structure. These types of prestress should be distinguished from each other as relevant (e.g. prestress by tendons, prestress by imposed deformation at supports).

2. VARIABLE ACTIONS Action for which the variation in magnitude with time is neither negligible nor monotonic.

2.1 Traffic actions

EN 1991‐2 defines models of traffic loads for the design of road bridges, footbridges and railway bridges. For the design of new bridges, EN 1991‐2 is intended to be used, for direct application, together with Eurocodes EN 1990 to 1999. The basis for combinations of traffic loads with non‐traffic loads are given in EN 1990 Annex A2 Application for bridges.

Forces due to vehicle or pedestrian traffic can be sub‐divided into: Vertical forces ‐ due to vertical live loads as the weight of the traffic (i.e. LM1, LM2, LM3, LM4,

Pedestrian, LM71, SW/0, SW/2, unloaded train, HSLM). Horizontal forces ‐ essentially horizontal loads due to changes in direction or speed of the vehicle

traffic (e.g. braking and acceleration, centrifugal and transverse forces, nosing force, actions due to traction and braking, collision forces, etc) with their centrifugal loads.



Carriageway Width

The carriageway width, w, should be measured between kerbs of height more than 100mm (The UK NA 75mm) or between the inner limits of vehicle restraint system. It should not include the distance between fixed vehicle restraint systems or kerbs of a central reservation nor the widths of these vehicle restraint systems.

Figure 2 – Carriageway width

Notional and Traffic Lanes The lanes that are marked on the running surface of the bridge and are normally used by the traffic are termed as traffic lanes. Notional lanes, on the other hand, are defined as the notional part of a carriageway used solely for the purpose of applying specific live loads. These are measured in the direction at right angles to the line of raised curbs, lane markers or edge marking. The width w1 of notional lanes on a carriageway and the greatest possible whole integer number n1 of such lanes on this carriageway are defined in Table 4.1 of the code and shown on table 1 below. Table 1 – Number and width of notional lanes

For a carriageway width of 11 m, n1 = integer(w/3) = 3, and the width of remaining area is 11‐ 3x3 = 2 m.



The number of notional will be: 1 where w < 5.4 m 2 where 5.4 m ≤ w <9 m 3 where 9 m ≤ w < 12m The whole carriageway including central reservation should be divided into notional lanes if the carriageways are separated by temporary road restraint system.

Figure 3 ‐ Lane numbering

The lane giving the most unfavourable effect is numbered Lane Number 1, the lane giving the second most unfavourable effect is numbered Lane Number 2, etc. Where the carriageway consists of two separate parts on two independent decks, each part should be considered as a carriageway. Separate numbering should then be used for the design of each deck. If the two decks are supported by the same piers and/or abutments, there should be one consecutive numbering system for the 2 decks together for the design of the piers and/or the abutments.

2.1.1 Road bridges

Load models defined in this section of the code should be used for design of road bridges with loaded lengths less than 200 m. The UK National Annex allows for loaded lengths of up to 1500m for Load Model 1.

2.1.1.1 Load Models

For road bridges design, 4 load models are used: a) Load Model 1 (LM1): Concentrated and uniformly distributed loads, which cover most of the

effects of the traffic of lorries and cars. This model should be used for general and local verifications.

b) Load Model 2 (LM2): A single axle load applied on specific tyre contact areas which cover the

dynamic effects of the normal traffic on short structural members. c) Load Model 3 (LM3): A set of assemblies of axle loads representing special vehicles (e.g. for

industrial transport) which can travel on routes permitted for abnormal loads. It is intended for general and local verifications.

d) Load Model 4 (LM4): A crowd loading of 5kN/m2 intended only for general verification for

footways. It should be used only for some transient design situations.



a) Load Model LM1 (main model)

LM1 is intended to cover flowing, congested or traffic jam situations with a high percentage of heavy lorries. In general, when used with the basic values, it covers the effects of a special vehicle of 600 kN as defined in annex A. Load Model 1 should be applied on each notional lane and on the remaining areas. Load Model 1 consists of two partial systems: i) Double‐axle concentrated loads (tandem system: TS) Each axle having the following weight: αQQk, where αQ are adjustment factors No more than one tandem system (only complete tandem system) should be taken into account per notional lane. Each axle of the tandem system should be taken into account with two identical wheels, the load per wheel being therefore equal to 0,5αQQk. The adjustment factors are given in the National Annex. The contact surface of each wheel should be taken as 0,4 x 0,4 m2. ii) Uniformly distributed loads (UDL system) The following weight per square metre of notional lane to be used: αqqk, where αq are adjustment factors. The uniformly distributed loads should be applied only in the unfavourable parts of the influence surface ,longitudinally and transversely. The characteristic values of Qik and qik which includes dynamic amplification are given in Table 2 below, together with the UK adjustment factors. Table 2 – Load model LM1 : characteristic values



Figure 4 – Application of load Model LM 1

For local verifications, a tandem system should be applied at the most unfavourable location. Where two tandem systems on adjacent notional lanes are taken into account, they may be brought closer, with a distance between wheel axles not to be less than 0.5 m.

Figure 5 – Application of tandem systems for local verifications

b) Load Model LM2

Load Model 2 consists of a single axle load βQQak with Qak = 400 kN (including dynamic amplification), which should be applied at any location on the carriageway. However, when relevant, only one wheel of 200βQ (kN) may be taken into account. It is recommended that βQ = αQ1. The UK National Annex recommends βQ = αQ1 = 1.0 The contact area of each wheel should be taken as 0.35 m x 0.60 m. The UK National Annex recommends a square contact surface of 0.40 x 0.40 m.



Figure 6 – Load model LM2

d) Load Model LM4

Crowd loading, if relevant, should be represented by a Load Model consisting of a uniformly distributed load (which includes dynamic amplification) equal to 5 kN/m2. LM4 should be applied on the relevant part of the length and width of the road bridge deck, including central reservation where relevant.

2.1.1.2 Dispersal of concentrated loads

The dispersal of concentrated loads through the pavement and concrete slabs should be taken at a spread‐to‐depth ratio of 1 horizontally to 1 vertically down to the level of the centroid of the slab as shown on Figure 7 below. The dispersal through the pavement and orthotropic decks should be taken at a spread‐to‐depth ratio of 1 horizontally to 1 vertically down to the level of the middle plane of the structural top plate.

Figure 7 – dispersal of concentrated loads

2.1.1.3 Horizontal forces

a) Braking and acceleration forces A characteristic force, Qlk, is a longitudinal force acting at the surface level of the carriageway. Qlk is limited to 900 kN for the total width of the bridge. It should be calculated as a fraction of the total maximum vertical loads corresponding to Load Model 1, applied on Lane Number 1. Refer to the EN document for formula (not within the scope of this module) b) Centrifugal force



The centrifugal force Qfk should be taken as a transfer force acting at the finished carriageway level and radially to the axis of the carriageway. Table 4 below gives the the charateristic value (including dynamic effects) of Qfk, a point load at any deck cross‐section. Table 3 – Centrifugal forces

r = horizontal radius of curvature of the carriageway centreline (m) Qv = total maximum weight of vertical concentrated loads of the tandem systems of LM1 Σi αQi(2Qik). Where relevant, lateral forces, Qtrk from skew braking or skidding should be taken into account. From the UK National Annex Qtrk = 0.5 Qlk for loaded lengths up to 120m and Qtrk = 280 kN for loaded

lengths ≥ 120m.

2.1.1.4 Groups of traffic loads on road bridges

Each of the groups of loads in section 2.1.1.1 to 2.1.1.3 above, which are mutually exclusive, should be considered as defining a characteristic action for combination with non‐traffic loads. Table 4 (UK NA Table NA.3) – Assessment of groups of traffic loads



The frequent action should consist only of either the frequent value of, LM1 or LM2 or loads on footway or cycle tracks in group gr1a, gr1b and gr3 respectively. For verification in transient design situations, the characteristic values associated with the tandem system should be taken equal to 0,8αQiQik, and the other values and forces without any modification.

2.1.1.5 Fatigue load model

Out of the scope of the module

2.1.1.6 Accidental actions

Accidental action, usually of short duration but of significant magnitude, that is unlikely to occur on a given structure during the design working life. EN1991‐1‐7 section 4 gives principles and general rules for the accidental actions caused by road vehicles and trains. a) Collision forces from vehicle on piers and other supporting members Forces due to vehicular impact on bridge supports over or adjacent to roads are given in NA EN 1991‐1‐7 Table NA.1 Table 7 (Table NA.1 UK NA) – Vehicle impact forces on members supporting bridge over or adjacent to roads

For impact from cars and lorries the main component of the collision force F1 may be applied at any height h1 between 0,75 m to 1,5 m above the level of the carriageway; the residual component F2 may be applied at any height h2 between 1 m and 1.5 m above the carriageway level.



The main and residual load components should be applied simultaneously. Fdx does not act simultaneously with Fdy. The design forces given in Table NA.1 should be multiplied by an adjustment factor Fa (greater than 1) in accordance with NA.2.1.2.4, which is based on the risk assessment procedure given in NA.2.11.2.3.

Figure 11 – Collision forces on supporting structures of over bridges and other structures

b) Collision forces from vehicle on decks (superstructures)

The equivalent static design forces for road bridges due to vehicular impact on the superstructure are given in Tables NA.9 and NA.10 of the UK National Annex. Table 11 below is the combined table. Table 8 – Equivalent Static forces due to impact on superstructure

Fdx does not act simultaneously with Fdy. The value of the clearance between the road surface and the underside of the bridge deck, h1 should be taken as 5.7 m and the impact force above this height need not be considered.

2.1.2 Footways, cycle tracks and footbridges

Pedestrian and cycle traffic vertical actions are defined by means of load models. Three models of footbridge loading are given. They are mutually exclusive. The characteristic values of these load models should be used for both persistent and transient design situations. a) Uniformly distributed load, qfk The recommended value for qfk is 5kN/m2 where there is a risk of continuous dense crowd. In other cases the UK National Annex recommends

qfk = 2.0 + 120/(L + 10) kN/m2 where 2.5 kN/m2 ≤ qfk ≤ 5.0 kN/m2



L = loaded length in m b) Concentrated load Q fwk Qfwk should be taken as 10 kN acting on a surface area of 0.1 x 0.1 m

2. c) Service vehicle Qserv When service vehicles are to be carried on a footbridge, one service vehicle Qserv shall be taken into account. Recommended loads given in the UK National Annex are those for the accidental loading shown below.

Horizontal forces

For footbridges only, a horizontal force Q flk should be taken into account, acting along the bridge deck axis at the pavement level. The horizontal force is considered as acting simultaneously with the corresponding vertical load, and in no case with the concentrated load Qfwk. The characteristic value of the horizontal force should be taken equal to the greater of the following two values: i) 10% of the total load corresponding to the uniformly distributed load ii) 60% of the total weight of the service vehicle {i.e. 0.6(115+65) = 108 kN}

Group of traffic loads on footbridges

When relevant, the vertical loads and horizontal forces due to traffic should be taken into account by considering groups of loads shown below. Each of these groups of loads, which are mutually exclusive, should be considered as defining a characteristic action for combination with non‐traffic loads.

i) Group of loads gr1



Accidental presence of vehicle on footbridge

If no permanent obstacle prevents a vehicle from being driven onto the bridge deck, the accidental presence of a vehicle on the bridge deck shall be taken into account. The following load model provided by the UK National Annex, consisting of two axle load group, should be used. The braking force associated with the load model should be 60% of the vertical load.

figure 13 – Accidental loading

APPLICATION OF HIGHWAY LOADS

i) Single Span Bridges

A small number of effects need to be considered with single spans. These include:

Maximum moment in mid‐span

Maximum shear at supports

Maximum and minimum reactions at bearings

Transverse moments in the span

Combination of shear and bending for prestressed beams, e.g. 1/4 span.

Group gr1 – Maximum longitudinal bending moment



Group gr1 – Maximum shear at support

ii) Continuous Bridges

The method of Influence line has been used for many years at the analysis stage to enable the determination of the distribution of the primary traffic loads on the decks of continuous structures to provide the worst possible effect. The influence lines are generally used in a qualitative manner to determine the critical regions, although they can be used to calculate actual values of stress resultants. However, in recent years, due to the availability of high power and fast computer, the so‐called “Moving Load Analysis” facility in most computer software has replaced the manual Influence Line method. It is outside the scope of this module to discuss the Influence Line method.

M27BE Notes 03 Traffic Load- Oct 2011

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