Axially restrained steel beams with web openings at elevated temperatures, part 1: Behaviour and numerical simulation results

Steel beams with web openings are frequently used as floor and roof beams in building construction to achieve attractive, flexible and optimised design solutions for passage of services. In practical construction, such beams may be axially restrained by the surrounding structure. The presence of axial restraint can drastically change the behaviour of such beams in fire, but the behaviour of axially restrained steel beams with web opening has received little attention. This paper investigates the effects of openings on axially restrained steel beams at elevated temperatures through extensive numerical simulations. The examined parameters include opening shape, opening size and opening position, load ratio, level of axial restraint and cross-section temperature distribution profile. The results of this numerical investigation identify the key stages and quantities that should be evaluated in fire resistance design of axially restrained perforated steel beams. The simulation results show that axially restrained steel beam with web opening may enter catenary action at a much lower temperature than the commonly accepted critical temperature of the beam calculated assuming no axial restraint. This happens when the opening is slender (long and deep) and when it is placed at high bending moment regions of the beam. The effect is more severe as the level of end axial restraints increases. An important implication of this finding is that additional tensile forces may exist in such beams at the critical temperatures calculated without considering axial restraint. The results of this paper also indicate that when the maximum axial tension in the beam is reached, the top tee section has almost zero axial force. At the same temperature, the bottom tee-section (in tension) has reached its maximum tensile capacity. Hence the maximum axial force in the beam is the same as the tensile capacity of the bottom tee-section. This finding can be used to determine the maximum tensile force against which the connections. The companion paper presents an analytical solution. (C) 2016 Elsevier Ltd. All rights reserved.