# How many units of HB are used for steel bridges?

Bridges in the UK can be assessed (to standard BA34/90) using an HB vehicle (to standard BD37/01). The HB vehicle is 16 wheel loads, each of 2.5kN times "units".

For concrete bridges the number of units is normally defined as 45 units at Ultimate Limit State (ULS) and 0 units at Serviceability Limit State (SLS); although the number of units for SLS is 30 if the bridge spans transversely. Reference: BS5400-4:1990 Clause 4.2.2 as modified by BD24/92.

For steel bridges there doesn't seem to be an equivalent clause. So how many units should be used at ULS, and how many at SLS?

• Forgive my ignorance but I don't know what HB stands for? It's something that seems to come in units and vehicles... – Trilarion Feb 4 '15 at 15:23
• I know of "HB" as meaning "hardness on the Brinell scale", but I don't think that's what's being referred to here. – Mark Feb 5 '15 at 0:12
• "HB" isn't this, is it? – HDE 226868 Feb 5 '15 at 1:10
• For those of you who aren't British Engineers: BD37/01 can be found [here] (standardsforhighways.co.uk/dmrb/vol1/section3/bd3701.pdf) and BA34/90 is replaced with BD101/11 here – thomasmichaelwallace Feb 5 '15 at 13:36
• n.b. I've always understood that HA/B stand for "Highways" A and B; with A being the effect of normal vehicles, and B being the effect of exception industrial loads. – thomasmichaelwallace Feb 5 '15 at 13:38

Section 4.1 gives the HB loading required for various bridges uses. It doesn't segregate based on superstructure type; it only segregates based on roadway class: Motorway and Trunk Road, Principal roads, and Other public roads.