Performance in fire of long-span composite truss systems

A numerical investigation of the structural behaviour of long-span composite truss systems, typically used in multi-storey floor construction, under fire conditions is presented. The non-linear finite element program, Vulcan, which has been specifically developed at the University of Sheffield for the analysis of structures at elevated temperatures, has been used extensively throughout. The in-fire performance of both restrained and unrestrained composite trusses is determined. The contribution of each group of members (i.e. top chord and slab, web members, bottom chord and supporting column) is investigated whilst considering various parameters such as the level of fire protection, structural modifications and deflection patterns. It is shown that, for a truss which is restrained against horizontal movement at its ends, the load-carrying mechanism undergoes a transition from bending to catenary action. Furthermore, at elevated temperatures there is a possibility of progressive buckling in the compressive web elements, which may not have been the identified failure mechanism in design. This is seen to be initiated by a significant rise of the thermal stress in the web members located within the high-shear zones towards the ends of the truss. The knowledge obtained is suitable for use when implementing performance-based design.