Execution classes explained: what EXC2 asks of your steelwork

Every structural steel package is built to an execution class, whether or not anyone wrote it on the drawings. The class, defined in EN 1090-2, sets how carefully the steel has to be made and checked — and it decides which fabricators can legally make it.

Four classes, one idea

EN 1090-2 defines four execution classes, EXC1 to EXC4, in order of increasing rigour. The higher the class, the more the standard asks for: tighter control of welding and welders, more inspection, more documentation and better traceability of materials. EXC1 covers simple, low-consequence structures; EXC4 is reserved for structures where failure would be severe, such as major bridges.

The class isn't picked from a list by feel. The designer derives it from the consequences of failure, whether the structure sees fatigue or seismic loading, and how it's made. For the great majority of buildings in the UK and Ireland that lands on EXC2 — and the National Structural Steelwork Specification (NSSS) makes EXC2 the default where nothing else is specified.

What actually changes between classes

• Welding coordination — the technical knowledge required of the person responsible for welding rises with the class.
• Inspection — the extent of supplementary non-destructive testing of welds increases.
• Traceability — higher classes require materials to be traced further through fabrication.
• Process limits — some shortcuts acceptable at lower classes, such as certain punched holes, are restricted at higher ones.

The fabricator's factory production control certificate under EN 1090-1 states the highest class they're certified to make. If a job is EXC3 and the fabricator is certified to EXC2, that's a conversation to have before the order, not after.

Why it matters to the detailing

The execution class belongs on the general arrangement drawings and in the specification, so the workshop, the inspector and the erector are all working to the same rules. It also shapes detail decisions the detailer makes every day: weld types and sizes, hole types and the information each part drawing has to carry.

When we detail a package, the class is one of the first things we confirm. If the drawings are silent, we ask — because "assumed EXC2" is a decision someone should make on purpose.

Three questions before steel is ordered

• What execution class has the designer specified, and where is it written down?
• Is the fabricator certified to that class under EN 1090-1?
• Does anything on the job — fatigue, a crane girder, an unusual consequence of failure — push part of it above EXC2?