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BD 37/01

LOADS FOR HIGHWAY BRIDGES

Click on the Clause No. for the commentary.

CLAUSE No

SUBJECT

Loads on slabs spanning transversely.

Loads due to vehicle collision with parapets.

Problem:

Is there a requirement to apply HA wheel loading?

Solution:

Yes.
The HA wheel load still needs to be considered even though the HA UDL and KEL may be replaced by HB loading (see clause 6.4.1.3)
If 45 units of HB are used then the factored wheel load is of similar magnitude to the factored HA wheel load (150 kN). Any HB vehicle less than 45 units will underestimate wheel load effects consequently the HA wheel needs to be considered.
The assessment code BD 21 gives guidance on loads on transverse spanning slabs (see clause 5.2)
The maximum wheel load on all vehicles over 7.5 tonnes is 100 kN. The 11.5 tonne axle with impact gives :
Wheel load = (11.5 x 9.81 x 1.8)/2 = approx 100 kN

Problem:

What is the ultimate design moment and shear resistance of the parapet?

Solution:

Although it is not stated, the design capacity of the parapet should be calculated with material factors = 1.0. If the material factors stated in the design code are included in the capacity then there is a possibility that the parapet will be stronger than the designed capacity. Consequently the load factors applied to protect the supporting structure may be greatly reduced.
TD 19/06 Clause 4.58 amends the value of γm to be used for the design of reinforced concrete parapets to avoid under-designing the parapet support.

Last Updated : 05/05/2007

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