# 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.

This means that both steel and concrete bridges are designed to the same number of HB units.

• Thanks @Hazzey. I'd completely forgotten that there might be something in the main text of BD37/01 - Appendix A is what is mostly used for design, so I just went straight there. As a note: concrete bridges aren't designed to BD37/01 Clause 4.1 - the requirement for HB Units is overridden by BS5400-4:1990 Clause 4.2.2. I suspect that there isn't an equivalent overriding clause for steel bridges though. – AndyT Feb 16 '15 at 20:25
• AndyT, the BDs over-ride the BS, not the other way round. Actually, Highways Agency concrete bridges are designed to BD24, which explicitly states that - "This Standard covers the use of BS 5400: Part 4: 1990, for the design of structural concrete in bridges and other highway structures. It sets out the Overseeing Department's particular requirements where these differ from, or are more comprehensive than, those given in the British Standard." – achrn Mar 14 '15 at 9:21