Failure mechanism of steel frames with angle steel-bolted connections exposed to fire under progressive collapse condition

Abstract

Partially restrained connections comprise intricate systems of multiple components, each exhibiting varying degrees of performance degradation under fire conditions. Consequently, the damage patterns within these connections are reconfigured. Moreover, owing to the insulating effect of floor slabs, the thermal and mechanical states of remaining structures differ in hogging and sagging bending moment regions after removing critical members, thereby influencing the potential damage modes and collapse-resistant mechanisms. To elucidate this impact mechanism, a comprehensive analysis of the influence parameters on the failure mechanism and collapse-resistant action of a typical partially restrained connection (angle steel-bolted connection) steel frames exposed to fire was conducted based on multi-scale models. The results revealed that steel frame structures with different connection configurations all enter the compressive arching action (CAA) stage after the heating phase due to the thermal expansion behavior of beams. After the completion of load application, the deformations of all three connection models remain below 0.2 rad. The variation of beam load magnification factor α does not affect the structural failure mode, while a higher column load ratio causes the damage location to shift from the connection to the column under fire conditions and triggers a chain failure reaction. Compared to axial constraint kas, the rotational constraint krs is a critical factor influencing the damage mode. The negative axial force generated by the thermal expansion behavior of the fire-affected region can enhance the flexural action resistance in the unheated region by employing the CAA. Finally, a theoretical method is proposed to predict the failure mechanism of steel frames with angle steel-bolted connections exposed to fire under progressive collapse condition, providing a novel approach for identifying weak components of similar structures and improving anti-collapse capabilities.