Behaviour of a small composite steel frame structure in a “long-cool” and a “short-hot” fire

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This paper describes the results of finite element analyses on a small generic composite steel and lightweight concrete frame. It compares the structural behaviour of the frame during two different single floor compartment fires. The fire scenarios are modelled using Pettersson's (Fire Engineering design of Steel Structures, Swedish Institute of Steel Construction, Publication 50, Stockholm, 1976) post-flashover temperature-time curves assuming a fire load density typical of offices and opening factors of 0.02 and 0.08 m 1/2 . With an opening factor of 0.08 m 1/2 the model fire is characterised by high temperatures but a relatively short post-flashover duration (“short-hot fire”). In contrast an opening factor of 0.02 m 1/2 provides less ventilation leading to a post-flashover fire with lower maximum atmosphere temperatures but a longer post-flashover duration (“long-cool” fire). The two fire scenarios create contrasting structural behaviour because the duration of the fire dictates the gradient through the depth of the composite slab. In the “short-hot” fire the steel beams achieve high temperatures but only the exposed face of the concrete begins to respond to heating. The rest of the slab depth stays cool. In the “long-cool” fire an extended post-flashover duration allows heat to penetrate further through the thickness of the slab. The slab has a higher mean temperature, thus the gradient through the composite is lower. During the analyses the columns and edge beams were protected to provide 60 min fire resistance, primary and secondary beams were unprotected. The structural response to each fire is compared and explained.