Progressive collapse analysis of a steel frame subjected to confined explosion and post-explosion fire

Abstract

Fire is a common secondary disaster following blast events. Because the strength of steel is sensitive to temperature, the steel structure that survives an explosion threat may collapse in the post-explosion fire. Thus, it is significant to investigate the progressive collapse mechanism of steel structures subjected to explosion and post-explosion fire. This article presents an accurate analytical model for evaluating the progressive collapse potential of a steel frame subjected to a confined explosion and post-explosion fire. The macro-model of the shear tab connection is established, and a material model that can reproduce the nonlinear dynamic behaviour of connections under combined hazard is compiled as subroutines. The finite element package AUTODYN is adopted to calculate the internal blast load to accurately analyse the effect of blast-induced damage on the fire resistance of the steel structure. The analytical results indicate that the gravity columns and columns in the interior moment resisting frames are key components for structural stability under combined hazards. The blast not only results in geometrical damage to the structural members, improving the load–displacement effect, but also leads to failure of the shear tab connections which increases the effective length of the column. Therefore, the failure time of the column under the combined hazard of blast and fire is earlier than the fire only case. Blast damage to the connection may also result in connection failure before column buckling in a fire, leading to local collapse prior to progressive collapse. Moreover, the collapse time–charge weight interaction diagram is generated to determine the weakest area of the steel frame. The membrane action of the floor is the only mode to redistribute load after the failure of the damaged column in a fire. Thus, increasing the reinforcement ratio in the floor may help mitigate the progressive collapse potential of the steel frame.