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Eurocodes and steel design

The introduction of Eurocodes should not result in radically different results compared to previous designs. The Eurocodes may pose challenges in the number of documents to be used, each with its own National Annex, with unfamiliar symbols and language, and in some cases different approaches compared to existing design standards, but there are few technical challenges in steel design. 

The reduction in ultimate loads is common for all materials, but this saving is one advantage of Eurocode design. For beams carrying gravity loads, the reduction in ultimate loads is around 8 per cent compared to BS 5950.  In steel design, BS 5950 is a mature standard, and designers would not expect significant changes.

Nevertheless, there are significant increases in resistance, particularly for unrestrained beams. For typical spans of unrestrained beams, the Eurocode resistance can be around 25-30 per cent higher than that calculated according to BS 5950. This is a quantum leap in resistance, and will be hugely beneficial when designing unrestrained beams. Of course, choosing a smaller beam may mean that deflection or floor dynamics become more significant.

Design to the Eurocodes can appear complicated, principally because of the unfamiliar symbols and subscripts, and secondly because the Eurocode generally presents formulae, not look-up tables as in BS 5950. Any designer who has looked at the appendices of BS 5950 would recognise that the formulae in both standards are essentially the same.

In practice, many designers hardly design orthodox members using the codified approach in the standard – they use books of member resistances such as the Blue Book. Designing steel members using the Blue Book is simple – and the obvious route for most designers. The Eurocode Blue Book is available in paper form, but also freely available in electronic form from the Tata website.

Crucial connections

Connections are a key part of steel structures, usually the responsibility of the steelwork contractor. As expected, connection components have a very similar resistance, whatever design standard is used, which means that connection resistances have not changed significantly.

The familiar standard details will remain. One potentially significant change is that the Eurocode for joints (BS EN 1993-1-8) requires connections to be classified, to demonstrate that the assumption of pinned or rigid behaviour is justified. Fortunately an alternative to calculation is allowed. Connections can be classified on the basis of experience of previous satisfactory performance. The UK National Annex is even clearer, describing connections designed to the familiar Green Book on simple connections as ‘pinned’ and those on moment connections as rigid. This assertion neatly avoids the complicated calculations.

A final word of warning, which applies to all materials including steel. The influence of the National Annex is critical, sometimes making very significant changes to the core Eurocode part – and should never be overlooked.

David Brown is the associate director of the Steel Construction Institute

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