Behaviour of axially and rotationally restrained reinforced concrete beams in fire

Abstract

This paper presents a numerical study on the fire response of restrained reinforced concrete (RC) beams using the general finite element software ABAQUS. A comprehensive systematic study of the effects of different levels of axial and rotational restraints on the whole range behaviour of RC beams in fire was conducted, including combined bending and compression due to restrained thermal expansion, bending failure, transition from compression to tension when catenary action develops and complete fracture of reinforcement at ultimate failure. The numerical results show that different bending failure modes (middle span sagging failure, end hogging failure due to fracture of tensile reinforcement, end hogging failure due to concrete crushing) can occur under different levels of boundary restraints. Furthermore, release of a large amount of energy during the rapid transition phase from compression to tension in a beam prevents formation of a three hinge mechanism in the beam under bending. The numerical results have also revealed that reliable catenary action can develop in RC beams at large deflections following bending failure only if bending failure is governed by concrete crushing near the beam ends whereby a continuous tensile catenary action path can develop in the top reinforcement.