Structural behaviour of isolated composite beam-slab system subject to elevated temperature

Abstract

Composite beam-slab systems are frequently used in buildings because they can reduce construction cost and time effectively, but the supporting steel beams are vulnerable under fire condition, weakening the structural performance. Therefore, fire safety should be carefully considered in the design of composite slab-beam system. In this study, the structural behaviour of isolated composite beam-slab system under fire condition has been experimentally and numerically investigated. Three specimens, viz. an isolated composite square slab without interior beam, and two isolated composite slab panels with an unprotected interior beam with aspect ratios of 1 and 1.5, respectively, were heated during which uniformly distributed load was applied. Temperature, centre deflection, crack patterns, failure time, and lateral movements and strains of supporting columns were recorded by conventional techniques. A novel non-contact approach based on photogrammetry was adopted to measure three-dimensional displacements of the specimens in fire testing for the first time, which could avoid direct contact between instrumentation and the heated specimen. However, some points may be obscured in the image view by either rising water vapour or load cell. Each specimen kept its integrity due to tensile membrane action (TMA) although large deformations occurred. Finite element (FE) models using concrete damage plasticity (CDP) model were also built up and verified by the experiments. The results of this study showed that the slab with larger aspect ratio had TMA in earlier stage, greater the cross-sectional tensile forces and lower fire resistance because the failure mechanism changed from two-directional TMA to one-way catenary action.