Behaviour of composite frames with castellated steel beams at elevated temperatures

Abstract

The behaviour of composite frames with castellated steel beams at elevated temperatures is investigated, and the results obtained were compared with the companion results of simply supported composite castellated steel beams. Overall, it is aimed to investigate the effect of axial and rotational restraining of the steel beam via rigid connections with protected steel columns at elevated temperatures. A previously reported finite element model for the analysis of composite beams in fire was extended to perform the nonlinear analyses of composite frames at elevated temperatures. The finite element model has accounted for the frame geometries and boundary conditions, nonlinear material properties of steel, concrete, profiled steel sheeting, longitudinal and lateral reinforcement bars as well as shear connection behaviour at ambient and elevated temperatures. The thermal properties at the steel beam top flange/profiled steel sheeting and profiled steel sheeting/concrete elements interfaces were considered in the thermal heat transfer analysis that allowed the temperatures to be accurately predicted in the composite slab during fire exposure. The comparison of composite frames and composite beams behaviour has shown that if the columns were sufficiently protected and the connection maintained its rigidity during fire exposure, such that it can restrain the composite beam thermal expansions, this would result in considerable reduction in fire resistances of the composite frames compared with that of simply supported composite beams. This is attributed to the premature failure at elevated temperatures due to local buckling at the bottom flange of the steel beams in the frame connections. Furthermore, the variables that influence the fire resistance and behaviour of the composite frames comprising different load ratios during fire, different fire curves, presence of web openings and different steel grades were investigated by parametric studies. It is shown that the strength of steel and the fire scenarios have a considerable effect on the time–displacement behaviour and fire resistances of the composite frames.