BS EN 1993-1-10 presents limiting thicknesses for steelwork, but notes that if the element is not subject to tension, welding or fatigue, the rules can be conservative. The conservatism arises because initial flaws are assumed to grow under fatigue loading, which will not be the case if fatigue is not a design consideration for the structure. The background document to the Eurocode notes that the use of the Eurocode rules would be extremely safe-sided in building where fatigue plays a minor role. In the UK, additional modifications are made in the National Annex and the rules are applied both to structures subject to fatigue, and to those that are not.
SCI have been commissioned by BCSA and Steel for Life to prepare a guide for so-called quasi-static structures, considering only nominal fatigue, to reduce the conservatism in the current rules. The result is that the limiting thicknesses presented in P419 are much less onerous, representing a real opportunity for designers to select an alternative sub-grade and relieve a frequent problem when specifying thick material. The guide follows the calculation process in the Eurocode precisely, excepting only that a smaller crack growth is calculated. The full provisions of the UK National Annex are also implemented, meaning the document is appropriate for structures built in the UK.
The effect of the reduced crack growth is very significant. A maximum thickness for S355 J0, used externally, with a stress ratio of 0.5 and a “moderate” detail type is 37.5 mm if the structure is subject to fatigue. In P419, for structures where fatigue is not a design consideration, the thickness limit rises to 188.8 mm. For structures in this category, P419 facilitates a significant easing of the limits in the Eurocode.