Behaviour of heated steel beams with lateral and axial restraint in steel-timber hybrid structures

Amidst the climate emergency and ambitions to decarbonise the built environment, hybrid systems consisting of a steel structural frame supporting cross-laminated timber floors have emerged as a new paradigm in sustainable construction. While this form of hybridisation is beneficial in many ways, it also presents certain challenges, particularly regarding its structural performance in fire. A critical concern in fire is the risk of lateral-torsional buckling of the steel beams connected to the timber slabs with self-tapping screws, which goes beyond the scope of current design guidance. Using a numerical approach, this study has developed finite element modelling capacity to investigate the thermo-mechanical response of a 9 m unprotected steel beam with screws at 250 mm spacing under various levels of lateral restraint from the screws and axial restraint from the end connections. The imposed heating regimes have also explored the influence of the temperature on the compression flanges of the steel beam. When axial restraint is present, indicating a realistic support condition in fire, the beam initially behaves similarly to a corresponding fully laterally restrained beam. This response continues until the first screw fails in the midspan region, which eventually triggers instability by lateral-torsional buckling. The results highlight the significance of lateral restraint in enhancing the fire performance of steel-timber hybrid structures. However, it is not realistic to assume a full lateral restraint condition in fire design.