A structural fire strategy for an exposed weathering steel-framed building

Performance-based structural fire design can play an important part in delivering iconic buildings. Early design team engagement with critical issues, including fire safety considerations, allows a holistic solution to be developed, in contrast to post-applied fire engineering which could severely compromise the original design intent. Four Pancras Square (London) is a successful example of where early fire engineering engagement has helped deliver an iconic design. The building features an external, fully load-bearing, weathering steel frame, with a storey-deep Vierendeel transfer truss. To allow the weathering steel to develop its protective patina and achieve the desired aesthetic, it must be left untreated and exposed; this is in conflict with traditional fire resistance solutions. This paper documents the key steps undertaken in realising the building and describes how they were tackled at the interface of the fire and structural engineering disciplines. These include quantification of the design goals, selection of the fire constraints, understanding the behaviour of unconventional steel, quantifying thermal exposure for external elements and, finally, quantification of structural response in fire.