2-D Analysis of Composite Steel - Concrete Beams in Fire

Abstract

This report investigates the behaviour of composite steel - concrete beams at elevated temperatures using the finite element program SAFIR. The finite element analysis carried out in this report is two dimensional and investigates the effects of an envelope of support conditions under varying thermal exposure. Composite steel - concrete construction is a common and popular form of construction used around the world. It is well understood that this form of construction has good inherent fire resistance. At this stage, it is not well understood how the fire resistance mechanisms work and how changes in material properties influence the behaviour of the composite beam. It is the intention of this report to provide some detail on single span, two dimensional, beam behaviour in relation to material properties, support conditions and thermal exposure. The analysis of this report was conducted using SAFIR, a non-linear finite element program developed at the University of Liege, Belgium. A 610 UB 101 steel beam with a 120mm thick composite concrete floor slab is exposed to three sided heating to simulate the effects of a compartment fire. The composite beams with moment and axial restraint perform poorly in comparison to beams with only moment restraint, axial restraint or no restraint in linear heating rates. In the ISO 834 fire, the beams with axial restraint performed poorly in comparison to those without axial restraint due to the high axial forces experienced because of thermal elongation. The axially restrained - moment resisting case performed poorly in both scenarios due to high compression stresses in the steel section caused by thermal bowing and thermal elongation. It was also found that when the EC3 Proportional and EC3 Yield Limit stresses were reached in the steel section, that displacements, axial force and bending moments along the section were affected.