Execution class
EXC1 to EXC4
The classification in BS EN 1090-2 that sets how rigorously a steel structure must be fabricated and inspected, from EXC1 (low consequence) to EXC4 (extreme), specified by the designer.
The execution class is the designer's statement of how much can go wrong if the steelwork fails, translated into fabrication and inspection requirements. It is derived from the consequence class of the structure (CC1 to CC3, roughly agricultural buildings to stadia), the service category (static or fatigue loading) and the production category (welded or bolted, steel grade). EXC1 covers low-consequence, statically loaded structures with limited welding requirements; EXC2 is the default for most building steelwork; EXC3 applies to higher-consequence structures, bridges and fatigue-loaded members; EXC4 to extreme-consequence structures.
The class drives the quality requirements for welding (BS EN ISO 3834 Part 4, 3 or 2), the level of non-destructive testing of welds (from visual only at EXC1 to significant supplementary NDT at EXC3 and above), the qualification requirements for the welding coordinator, the traceability of materials (full traceability at EXC3 and above), the tolerances, and the documentation. A fabricator certified for EXC2 cannot make EXC3 work; a designer who specifies EXC3 for a portal frame shed has added cost for no safety benefit. If the drawings do not state an execution class, BS EN 1090-2 defaults to EXC2, and the fabricator should ask rather than assume.
Where it comes from
- BS EN 1090-2 Annex A and Annex B (formerly), now the guidance in BS EN 1993-1-1 Annex C and the UK National Annex on determining execution class.
- BS EN ISO 3834 Parts 2 to 4 (welding quality requirements by class).
Questions people ask
Who decides the execution class?
The designer, on the drawings or specification, from the consequence class, service category and production category. Not the fabricator. If it is missing, ask; do not default silently.
What changes between EXC2 and EXC3?
Full traceability of materials to the piece, a welding coordinator with comprehensive technical knowledge, more supplementary NDT of welds, tighter documentation and, usually, a formal welding plan. Enough that the price and the shop set-up are different.
Execution class, handled on a phone
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