Imposed traffic loading

Bridge traffic can be vehicular, rail or pedestrian/cycle or indeed any combination of these. Vehicular and rail traffic are considered in subsections below. While pedestrian/cycle traffic loading on bridges is not difficult to calculate, its importance should not be underestimated. Bridge codes commonly specify a basic intensity for pedestrian loading (e.g. 5 kN/m2in the draft Eurocode and the British standard and 4 kN/m2in the American code). When a

structural element supports both pedestrian and traffic loading, a reduced intensity is allowed by some codes to reflect the reduced probability of both traffic and pedestrian loading

reaching extreme values simultaneously. Most codes allow a reduction for long footpaths. 2.3.1 Imposed loading due to road traffic

While some truck-weighing campaigns have been carried out in the past, there has been a scarcity of good unbiased data on road traffic loading until recent years. Bridge traffic loading is often governed by trucks whose weights are substantially in excess of the legal maximum. In the past, sampling was carried out by taking trucks from the traffic stream and weighing them statically on weighbridges. There are two problems with this as a means of collecting statistics on truck weights. In the first place, the quantity of data collected is relatively small but, more importantly, there tends to be a bias as drivers of illegally overloaded trucks quickly learn that weighing is taking place and take steps to avoid that point on the road.

In recent years the situation has improved considerably with the advent of weigh-in-motion (WIM) technology which allows all trucks passing a sensor to be weighed while they travel at full highway speed. WIM technology has resulted in a great increase in the availability of truck weight statistics and codes of practice are being revised to reflect the new data.

Bridge traffic loading is applied to notional lanes which are independent of the actual lanes delineated on the road. In the Eurocode, the road width is divided into a number of notional lanes, each 3 m wide. The outstanding road width between kerbs, after removing these lanes, is known as the ‘remaining area’. The AASHTO code also specifies notional lanes of fixed width. The British Standard on the other hand (for carriageway widths in excess of 5 m) allows the lane width to vary within bands in order to get an integer number of lanes without having any remaining area.

The AASHTO code specifies a traffic lane loading which consists of a knife-edge load plus a uniformly distributed lane loading. Alternatively, a truck of specified dimensions and axle weights must be considered. A dynamic factor is applied to the truck to allow for the

increased stresses which result from the sudden arrival of a speeding vehicle on a bridge. In general, the imposed traffic loading specified by AASHTO is considerably less onerous than that specified by both BD37/88 and the Eurocode.

BD37/88 and the draft Eurocode specify two types of traffic loading, ‘normal’ and ‘abnormal’. Normal traffic loading or Highway A (HA) represents an extreme

combination of overloaded trucks of normal dimensions. This could be a traffic jam involving a convoy of very heavy trucks as would tend to govern for a long bridge. On the other hand, it could be a chance occurrence of two overloaded moving trucks near the centre of a short bridge at the same time, Particularly on roads with rough surfaces, there can be a considerable dynamic component of truck loading which is deemed to be included in the specified normal load. Eurocode normal loading consists of uniform loading and a tandem of four wheels in each lane as illustrated in Fig. 2.1(a). In addition, there is uniform loading in the remaining area. While there are a number of factors which can vary between road classes and between countries, the standard combination is a load intensity of 9 kN/m2in Lane No. 1 and 2.5 kN/m2elsewhere. The four wheels of the tandems together weigh 600 kN, 400 kN and 200 kN for Lanes 1, 2 and 3, respectively. In the British standard, ‘full’ HA lane loading consists of a uniform loading whose intensity varies with the loaded length and a ‘knife edge’

concentrated loading of 120 kN. For bridges with many notional lanes, a number of

possibilities must be considered, a typical one being full HA in Lanes 1 and 2 combined with 60% of full HA in the other lanes as illustrated inFig. 2.1(b). The AASHTO code allows similar reductions in lane loading for multi-lane bridges to account for the reduced probability of extreme loading in many lanes simultaneously.

The possibility of abnormal or Highway B (HB) loading must also be considered in British and Eurocode designs. This consists of an exceptionally heavy vehicle of the type which is only allowed to travel under licence from the road/bridge authority. Different countries have different classes of abnormal vehicle for which bridges must be designed. A large number of alternative abnormal vehicle classifications are specified in the draft Eurocode from which individual countries can select combinations for which roads of specified classes are to be designed. In BD37/88, only one abnormal vehicle is specified but it may have a length of 9.6, 14.6, 19.6, 24.6, or 29.6 m. Illustrated in Fig. 2.2, the vehicle is known as the Highway B or HB vehicle. It is scaled in gross ‘units’ of 40 kN so that a minor road bridge can be designed, for example, to take 25 units (a 1000 kN vehicle) while a highway bridge can be designed for 45 units (a 1800 kN vehicle).

Combinations of normal traffic and an abnormal vehicle must be considered in bridge design. While there are exceptions, the abnormal load in BD37/88 is

Fig. 2.1 ‘Normal’ road traffic loading: (a) Eurocode normal loading; (b) British standard HA loading

Fig. 2.2 British standard abnormal (HB) vehicle consisting of 16 wheel loads of F=2.5 kN per unit

generally taken to replace the normal loading throughout the length of the vehicle and for a distance of 25 m before and after it. Normal load is placed throughout the remainder of the lane and in the other lanes.

2.3.2 Imposed loading due to rail traffic

The modelling of railway loading is considerably less onerous than that of road traffic loading as the transverse location of the load is specified. This follows from the fact that the train can generally be assumed to remain on the tracks. However, there are some aspects of traffic loading that are specific to railway bridges which must be considered.

The weights of railway carriages can be much better controlled than those of road vehicles with the result that different load models are possible depending on the railway line on which the bridge is located. However, bridges throughout a rail network are generally designed for the same normal load model. The standard Eurocode normal load model consists of four vertical point loads at 1.6 m intervals of magnitude 250 kN each and uniform loading of intensity 80 kN/m both before and after them. In addition, the Eurocode provides for an alternative abnormal load model. In BD37/88, the normal load model, known as Railway Upper (RU), is similar in format. On passenger transit ‘light rail’ systems, less onerous load models can be applied. A standard light rail load model, Railway Lower (RL), is specified in the British standard. However, less stringent models have been used for the design of bridges on some light rail networks.

The static loads specified for the design of railway bridges must be increased to take account of the dynamic effect of carriages arriving suddenly on the bridge. This factor is a function of the permissible train speed and of the natural frequency of the bridge. Railway tracks on grade are generally laid on ballast. On bridges, tracks can be laid on a concrete ‘track slab’ or the bridge can be designed to carry ballast and the track laid on this. There are two disadvantages to the use of track slabs. When used, an additional vertical dynamic load is induced by the change from the relatively ‘soft’ ballast support to the relatively hard track slab. This effect can be minimised by incorporating transition zones at the ends of the bridge with ballast of reducing depth. The other disadvantage to the use of track slabs depends on the method used to maintain and replace ballast. If this is done using automatic

equipment, a considerable delay can be caused by the need to remove the equipment at the start of the bridge and to reinstall it at the end.

Another aspect of loading specific to railway bridges is the rocking effect. It is assumed for design purposes that more than half of the load (about 55%) can be applied to one rail while the remainder (about 45%) is applied to the other. This can generate torsion in the bridge.

Horizontal loading due to braking and traction is more important in railway bridges than in road bridges as the complete train can brake or accelerate at once. While it is possible in road bridges for all vehicles to brake at once, it is statistically much less likely. Longitudinal horizontal loading in bridges can affect the design of bearings and can generate bending moment in substructures and throughout frame bridges.