The Influence of Tensile Membrane Action on Fire-exposed Composite Concrete Floor-steel Beams with Web-openings

Abstract

On the basis of a series of the full-scale fire tests carried out on the composite frame at Cardington, a design method is now well established to calculate the performance of composite flooring systems subject to fire. The method models in a simplified fashion the influence of tensile membrane action in composite floor slabs which are formed as an array of composite beams using solid-web steel downstand beams which are largely unprotected. The development of membrane action depends on the conditions of vertical support maintained around the boundaries of the fire-affected slab panels by protected beams. Cellular beams can achieve the same strength as unperforated I-beams of the same depth, with significantly reduced steel mass, and the ability to accommodate service ducts within the beam depth, and therefore the use of composite cellular beams as floor members is becoming increasingly popular in construction. Due to the general lack of research on perforated sections, the guidance about their design for fire conditions remains rather primitive. Moreover, tensile membrane action in composite floor slabs with cellular steel downstand beams could exhibit very different behaviour in fire from that when solid-web sections are used. A parametric study has been carried out as an initial investigation into the fire performance of cellular beams within composite slab systems, including the effects of tensile membrane action in enhancing the load-carrying capacity. The effects of changes to the edge support conditions are also investigated. The results show the protected perimeter beams maintained their load-carrying capacity and were subject only to small vertical displacements, although web-post buckling was observed to occur on the protected secondary beam near to a support. This suggests that maintenance of vertical support is not sufficient for a slab panel with cellular beams. Web-post buckling or the Vierendeel mechanism may govern the mode of structural failure, indicating that the sizes of openings and their positioning necessitate careful design.