AD 336: Execution class for bridge steelwork
This Advisory Desk Note provides background to the choice of ‘default’ execution class in the Model Project Specification (MPS) for the execution of steelwork in bridges (SCI publication P382).
The execution standard for steel structures, EN 1090-2 (Execution of steel structures and aluminium structures – Technical requirements for steel structures), provides for four execution classes for steel structures, EXC1 being the lowest class and EXC4 the highest class. The requirements for quality, testing and acceptance criteria are greater the higher the class. It is left to the project specification to choose the appropriate class for the structure, either by the choice of a single class or by specifying different classes for different elements, depending on the reliability required for the individual element. Clause 4.1.2 states that if no class is specified, EXC2 shall apply.
For bridge steelwork, the quality that has traditionally been specified in the UK (using BS 5400-6 and the Model Appendix 18/1, P170) corresponds generally to class EXC3. For elements that are particularly susceptible to fatigue loading, more stringent requirements have been applied and for lightly loaded bridges requirements have sometimes been relaxed. In drawing up the MPS, the Steel Bridge Group took the view that EXC3 should be specified as a default and that comment would be made about increasing or, occasionally, decreasing the execution class. It was felt that it was preferable to specify EXC3 as a default, with few exceptions, rather than advise the designer to consider the most appropriate class for each part of the bridge and then to expect the steelwork contractor to work to different quality levels for different details. It was also recognised that in some cases the requirements corresponding to EXC4 are impractical in an ordinary workshop and that, where requirements better than EXC3 are needed, they should be addressed in detail rather than simply classing the element as EXC4.
The Steel Bridge Group has also reviewed a new draft Annex for EN 1993-1-1 (Design of steel structures – General rules and rules for buildings) that would give the (normative) provisions for selection of the execution class, taking account of the requirements in EN 1990 (Eurocode – Basis of structural design) for achieving the required reliability. In that draft Annex, which is derived from informative Annex B of EN 1090-2, two new classifications are defined – service category (SC) and production category (PC); these are in addition to the definition of consequence class (CC) already given in EN 1990. Two service categories are defined, SC1 and SC2, the latter being the category for structures subject to fatigue. Two production categories are defined, PC1 and PC2, the latter being the category most likely to be appropriate to road and rail bridges (footbridges might in some cases be PC1).
Bridge structures are generally in consequence class CC2 or CC3 according to EN 1990 and thus the draft Annex recommends the use of EXC3 or possibly EXC4 (particularly for CC3+SC2+PC2). The choice of EXC3 in the MPS is thus consistent with the recommendations in the draft Annex.
The new draft Annex to EN 1993-1-1 is to be formally considered by TC250/SC3 in October 2009 and publication (and thus implementation) is therefore not likely until late 2010.
Contact: David Iles
Tel: 01344 636525