Experimental study on behavior of top-and-seat-angle with double web-angle (TSDW) connections under fire conditions

Steel beam-to-column connections are more prone to fracture due to tension in the heating and cooling phase of a fire, imposing an important issue on progressive collapse of steel structures in fire. Experiments are conducted in this study on four top-and-seat-angle with double web-angle (TSDW) connections under room-temperature, heating, heating-cooling and after-fire conditions. The effect of temperature histories and loading states on the failure modes and displacement curves of connections is investigated. A numerical model of full-scale TSDW connections is established and validated, and parametric studies are carried out to study the effect of scale ratios, material properties, angle thickness and stiffeners. The experimental results show that the connection may fail due to tearing of bolt holes or shear failure of bolts in the seat angle. The critical temperature of connections is 710 degrees C for a load ratio of 0.5, and its residual capacity reduces by 23 % after experiencing a maximum temperature of 980 degrees C. Loading during fire has a significant effect (60 %) on the residual capacity of connections after fire. It is found from numerical simulation that the scale ratio affects failure modes, critical temperatures, residual capacities and failure displacements of connections, and further work is still needed to quantify structural fire similarity. The yield strength and fracture strain are important factors affecting the behavior of steel connections at both ambient and elevated temperatures. Increasing the angle thickness and applying stiffeners can effectively improve the load-bearing capacity of TSDW connections at room temperature. There is an optimal angle thickness (e.g., 8 mm) for improving residual capacity of connections, and the selection of stiffeners should consider its matching with angles. It is recommended to exclude stiffeners for thick angles (e.g., 12 mm) to avoid adverse effect on the fire resistance and post-fire residual capacity of TSDW connections.