AD 557: Robustness and fire

SCI’s Advisory Desk has become aware of some requests to apply the “robustness” rules found in BS 5950 and BS EN 1991 in the fire limit state. This AD clarifies the requirements.

The requirement to verify a structure in the fire limit state is entirely separate to the avoidance of disproportionate collapse (the “robustness” rules).

Fire design (e.g. provision of fire protection) is intended to ensure that elements of structure do not lose their ability to support design loads for the duration of the specified fire period. The possibility of fire is a “known” accidental situation and must be designed for, as required under the “Basic requirements” of BS EN 1990 clause 2.1(3).

Robustness rules are there to ensure the structure does not suffer disproportionate collapse if an accidental action causes an element of structure to lose its ability to support design loads. BS EN 1990 clause 2.1(4) covers the necessity to avoid disproportionate collapse. In contrast to the specified actions in the fire limit state, the actions potentially leading to disproportionate collapse are unidentified.

To apply the robustness rules in the fire limit state is the equivalent of assuming that during one accidental situation (fire) there is a second, unidentified accidental event. Two accidental situations are not assumed to be concurrent.

However, while this approach is appropriate for typical buildings, there may be cases, possibly for higher-risk buildings, where collaboration between the structural engineer, fire engineer and other relevant parties is necessary to determine whether the risk of disproportionate collapse in fire warrants further assessment.

Contact: David Brown
Telephone: 01344 636555
Email: advisory@steel-sci.com