Fire performance of connections between high-strength steel tubular members

An investigation is conducted into the behaviour of bolted endplate connections between high-strength steel tubular members at elevated temperatures. Experiments are described where assemblies comprising Grade S460 steel square hollow sections connected via bolted endplates are heated to a target temperature between 150°C and 750°C using ceramic heating pads. The assembly, which is inclined at 45° to the horizontal, is then loaded vertically, thus generating combined bending and axial thrust in the members. The in-plane vertical deflection, the applied load and temperatures at various points were measured. Results for the ultimate loads, equilibrium paths and the observed failure modes are discussed along with the influence of temperature on these features. A complementary nonlinear finite element analysis of the experimental assemblies is also described. Material models incorporating temperature-dependent strengths, stiffnesses and strain hardening relationships available from the literature are employed, with temperature fields calibrated using steel temperatures recorded during the experiments applied to the models. It is shown that good agreement exists between the ultimate loads, equilibrium paths and failure modes predicted by the numerical models and those observed in the experiments, thus validating the modelling approach. It is found that, at lower temperatures, buckling of the endplate was the governing mode of failure. With increasing temperature, the mode of failure transitions to flexure in the beam and column stubs, thus suggesting that a relative degree of fixity is offered by the connections at the higher temperatures.