Current live load specifications Introduction

The basic philosophy of the normal and abnormal loading is common throughout the world, but there are, of course, variations to account for the range and weights of vehicles in use in any given country.

In this section normal and abnormal traffic loads specified in UK, USA and Eurocodes will be referred to.

British specification

The current UK Code is, by agreement with the British Standards Institution, Department of Transport Standard BD 37/01 (DoT, 2001) which is based on BS 5400: Part 2 (BSI, 1978).

Normal load application

The normal load consists of a lane UDL plus a lane KEL. The UDL (HAU) is based on the loaded length and is

Figure 8 British Standard normal loading curve

defined by a two-part curve as shown in Figure 8, each defined by a particular equation, one up to 50 m loaded length and the other for the remainder up to 1600 m. The KEL (HAK) has a value of 120 kN per lane.

The application and intensity of the traffic loads depends upon:

■ the carriageway width

■ the loaded length

■ the number of loaded lanes.

The carriageway width is essentially the distance between kerb lines and is described in Figure 1 of BD 37/01. It includes the hard strips, hard shoulders and the traffic lanes marked on the road surface.

The two most prominent load applications are defined as HA only, and HA + HB. HA is applied as described previously to every (notional) lane across the carriageway attenuated as defined in Table 14 of BD 37/01.

The attenuation of the curve in Figure 8 takes account of vehicle bunching along the length of a bridge. Lateral bunching is taken account of by applying lane factors fl to the load in each lane (both the UDL and the KEL). Generally this amounts to fl = 1.0 for the first two lanes and fl = 0.6 for the remainder. Thus nominal lane load = flHAU + flHAK.

The number of lanes (called notional lanes, and not necessarily the same as the actual traffic lanes defined by carriageway marking) is based on the total width (b) of the carriageway (the distance between kerbs in metres) and is given by Int[(b/3.65) + 1] where 3.65 is the standard lane width in metres. Notional lanes are numbered from a free edge.

Local effects

For parts of a bridge deck under the carriageway which are susceptible to the local effects of traffic loading, a wheel load is applied equivalent to either 45 units of HB or 30 units of HB as appropriate to the bridge being considered.


Alternatively an accidental wheel load of 100 kN is applied away from the carriageway on areas such as verges and footpaths. The wheel load is assumed to exert a pressure of 1.1 N/mm2 to the surfacing and is generally considered as a square of 320, 260 or 300 mm side for 45 units of HB, 30 units of HB or the accidental wheel load respectively.

Allowance can also be made for dispersal of the load through the surfacing and the structural concrete if desired.

Abnormal, HB loading

The loading for the abnormal vehicle is concentrated on 16 wheels arranged on four axles as shown in Figure 9. Its weight is measured in units per axle, where 1 unit = 10 kN. The maximum number of units applied to all motorways and trunk roads is 45 (equivalent to a total vehicle weight of 1800 kN), and the minimum number is 30 units applied to all other public roads. The inner axle spacing can vary to give the worst effect, but the most common value taken is 6 m. (It is worth noting that vehicles with this configuration are not considered in the Construction and Use Regulations because it is a hypothetical vehicle and used only as a device for rating a bridge in terms of the number of HB units it can support.) Each wheel area is based on a contact pressure of 1.1 N/mm2.

Load application

All bridges are designed for HA loading and checked for a combination of HA + HB loading. HA and HB are applied according to Figure 13 of BD 37/01 with the HB vehicle placed in one lane or straddled over two lanes (depending upon the width of the notional lane). Since such a load would normally be escorted by police, an unloaded length of 25 m in front and behind is specified, with HA loading occupying the remainder of the lane. The other lanes are loaded with an intensity of HA appropriate to the loaded length and the lane factor.

Exceptional loads

Road hauliers are often called upon to transport very heavy items of equipment such as transformers or parts for power stations which can weigh as much as 750 t (7500 kN) or

1.0 m 1.0 m 1.0 m

1.8 m 1 Varies from 6.0 m-26 m 1 1.8 m in increments of 5 m

1.8 m 1 Varies from 6.0 m-26 m 1 1.8 m in increments of 5 m

Figure 9 Abnormal HB vehicle more. Special flat-bed trailers are used with multiple axles and many wheels to spread the load so that the overall effect is generally no more than that of HA loading, and contact pressures are no more than 1.1 N/mm2, but where this is not possible, then any bridges crossed en route have to be strengthened. The loads on the axles can be relieved by the use of a central air cushion which raises the axles slightly and redistributes some of the load to the cushion. Heavy diesel traction engines placed in front and to the rear are used to pull and push the trailer. Some typical dimensions are shown in Figure 10.

Figure 11 shows a catalytic cracker installation unit 41m long and 15.3m in diameter weighing 8251 being transported from Ellesmere Port to Stanlow Oil Refinery via the M53 in 1984. The load was spread over 26 axles and 416 wheels.

US specification

The US highway loads are based on American Association of State Highway and Transportation Officials (AASHTO) Standard Specification for Highway Bridges (AASHTO, 1996) or more recently the AASHTO LRFD Bridge Design Specifications (1996, 3rd edition) which are similar. These specify standard lane and truck loads.

Lane loading

The commonly applied lane loading consists of a UDL plus a KEL on 'design lanes' typically 3.6 m wide placed centrally on the 'traffic lanes' marked on the road surface. The number of 'design lanes' is the integer component of the carriageway width/3.6. Traffic lanes less than 3.6m wide are considered as design lanes with the same width as the traffic lanes. Carriageways of between 6 m and 7.3 m are assumed to have two design lanes.

The lane load is constant regardless of the loaded length and is equal to 9.3 kN/m and occupies a region of 3 m transversely as indicated in Figure 12. Frequently the lane load is increased by a factor of between 1.3 and 2.0 to reflect the heavier loads than can occur in some regions.

Truck loading

Acting with the lane loading there are three different design truck loadings, namely:

1 tandem

2 truck

3 lane.

The new (AASHTO, 1994) tandem and the truck loadings are shown in Figure 13 compared with the old (AASHTO, 1977) standard H and HS trucks.

To account for the fact that trucks will be present in more than one lane, the loading is further modified by a multiple presence factor, m, according to the number of design lanes,

4.381 m

4.381 m

978 mm (3' 2/'') 1.184 m (3' 105-8'') 978 mm (3' 2/'')

40 t tractor

4.800 m , 23.927 m (78' 6'') bolster centres can be increased by , 4.800 m

914 mm (3' 0'') and/or 1.930 m (6' 4'') 14.326 m (47' 0'')

40 t tractor

978 mm (3' 2/'') 1.184 m (3' 105-8'') 978 mm (3' 2/'')

■ Direction of travel

9 t 15.5 t 15.5 t 9 t 15.5 t 15.5 t 40 t tractor 40 t tractor

Exceptional heavy vehicle

33.528 m (110' 0'') [35.458 m (116' 4'')] 7 axles on 1.600 m (5' 3'') crs 14.326 m (47' 0'') [16.256 m (53' 4'')]

4.267 m (14' 0'') [4.521 m (14' 10'')]

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  • feorie
    How to apply table 14 ha lane factor bs 5400?
    7 years ago

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