BS 5400 Part 2 : 2006

Clause 6.2 Type HA Loading

Problem: How do you work out the HA loading and bending moment for a bridge

deck ?

Example: Carriageway = 6m wide

Deck span = 34m (centre to centre of bearings for a simply supported

single span)

Design for a metre width of deck :

Cl 3.2.9.3.1. Number of notional lanes = 2

Notional lane width = 6.0/2 = 3.0m

Cl 6.2.1. Loaded length = 34m

W = 336(1/L)0.67 kN/m (per notional lane)

W = 31.6 kN/m (per notional lane)

Cl 6.2.2. Knife Edge Load = 120 kN (per notional lane)

Cl 6.4.1.1. α2 = 0.0137[bL(40-L)+3.65(L-20)]

α2 = 0.0137[3.0(40-34.0)+3.65(34.0-20)] = 0.947

Note: For loaded lengths less than 20m the load is proportioned to a

standard lane width of 3.65m, i.e. 0.274bL = bL/3.65.

For a metre width of deck :

W = (31.6 x 0.947)/3.0 = 10.0 kN/m

KEL = (120 x 0.947)/3.0 = 37.88 kN

Cl 6.2.7. γfL = 1.50 (Ultimate limit state - combination 1)

Design HA loading for a metre width of deck :

W = 1.5 x 10.0 = 15.0 kN/m

KEL = 1.5 x 37.88 = 56.82 kN

Maximum mid span Bending Moment with KEL at mid span = Mult

Mult = (15.0 x 342)/8 + (56.82 x 34)/4

Mult = 2170 + 480 = 2650 kNm

Note: Use of γf3

BS 5400 Pt.3 & Pt.5 - γf3 is used with the design strength so Mult = 2650

kNm.

BS 5400 Pt.4 - γf3 is used with the load effect so Mult = 1.1 x 2650 = 2915

kNm.

Clause 6.3 Type HB Loading

Assume the road over the bridge is not a Principal Road then we need to

check for 30 units type HB loading (see BD 37 Chapter 4).

Cl 6.3.1 Nominal load per axle = 30units x 10kN = 300kN

The maximum bending moment will be achieved by using the shortest HB

vehicle i.e. with 6m spacing (see BD 37 Fig 12).

The maximum moment for a simply supported span occurs under the inner

axle when the vehicle is positioned such that the mid span bisects the

distance between the centroid of the load and the nearest axle.

With a 34m span and the 6m HB vehicle with equal axle loads, the inner

axle is placed at 1.5m from the mid span.

HA and HB Bridge Loading Example http://www.childs-ceng.demon.co.uk/tutorial/exha.html

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γfL = 1.30 (Ultimate limit state - combination 1)

Design HB moment for a metre width of deck :

Mult = 1.3 x 2175 = 2828 kN/m (compared to 2650 for HA load)

Hence in this case HB load effects would govern although a grillage or

finite element type distribution would reduce the HB moment

considerably.

HA and HB Bridge Loading Example http://www.childs-ceng.demon.co.uk/tutorial/exha.html

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