Possible ‘panel instability’ in composite deck floor systems under fire

Abstract

This paper describes a possible ‘panel instability’ in composite deck floor slabs under fire conditions. The phenomenon is characterised by a rapid increase in deflection of the whole composite floor system (steel beams and concrete deck) forming part of a continuous floor in a compartment fire. This is accompanied by a sudden loss in the thermally induced axial compression in a primary steel beam. The instability occurs when the moment capacity at the end of the composite section (comprising the profiled deck slab and the primary beam) is achieved, and the connection at the column changes from moment resisting (because of the composite action) to pinned. The end moment is the sum of, moment due to imposed load (constant); moment due to increasing gradient over the composite (increasing with temperature); and moment from the increased loading caused by expanding and bowing secondary beams pushing down on the primary beam (increasing with temperature). The moment resisting connections at the ends of the primary beam prevent secondary beam(s) from reaching their natural thermally displaced shape (determined by the temperature distribution in the composite secondary beam–slab section). When the moment capacity at the composite primary beam–column connection is reached, the secondary beams are instantly free to deflect. To the authors’ knowledge, this phenomenon has not been reported in other previous studies. Also the authors’ discovery of this phenomenon is entirely based on computational modelling without any experimental evidence, and much further work is required to ascertain that it would indeed occur under normal fires, loading and restraint conditions.