Disproportionate collapse of 3D steel-framed structures exposed to various compartment fires

This paper numerically investigates disproportionate collapse resistance of three-dimensional steel-framed structures exposed to compartment fires. The effect of fire protections (low, medium, high) as well as fire locations (corner, edge and interior) on collapse modes and load redistribution schemes is studied. The results show that the frames do not collapse immediately after this local failure but experience a relatively long withstanding period of at least 60 min. This is attributed to the increasing deflection of heated slabs, resulting in increased lateral displacements of adjacent cool columns which governs their buckling. This indicates that the “fire rating” of a structure against global collapse is somewhat 1-hour longer than that of individual members. It is found that the fire protection of steel members has significant effect on the resistance of structures against fire-induced disproportionate collapse. The frames with a medium level of fire protection (2-hour fire rating for columns) withstand the fire. A comparison between 2D and 3D models shows that the 2D model produces conservative results by underestimating the collapse resistance of structures. It cannot capture the load redistribution in a 3D model where more loads are distributed along the short span than those along the long span. The presence of slabs for delaying the global collapse cannot also be simulated by a 2D model. It is recommended that the fire protection of perimeter columns should be enhanced to 2-hour fire rating and slabs should be protected to delay and prevent the collapse of structures.