Progressive Collapse of 3D Composite Floor Systems with Rigid Connections under External Column Removal Scenarios

Progressive collapse of building structures has been a hot research topic since the September 11, 2001, terrorist attacks. These works provide valuable information focused more on beam–column connections and two-dimensional (2D) substructures, and relative studies on three-dimensional (3D) structures are limited, especially high-quality experimental tests on 3D composite floor systems. As is known, external columns are entirely exposed to the outside environment, which makes them more susceptible to be damaged by extreme events like vehicular impact or explosion, as observed in the collapse of the Alfred P. Murrah Federal Building in Oklahoma City in 1995. In this study, two $1/3$-scale 3D specimens with four types of connections, including web unreinforced flange bolted (WUFB) connection, fin plate (FP) connection, reduced beam section (RBS) connection, and double angle cleat (DAC) connection, were tested quasi-statically up to failure under external column removal scenarios. Based on the test results, the vertical load–displacement curves, failure modes, deflection profile, and strain development of the structural components were obtained and discussed in detail. The reaction force redistribution in the remaining columns and surrounding restraints was also investigated. Special attention was paid to the contributions of the main load-resisting mechanisms, namely, flexural action (FA), catenary action (CA), and tensile membrane action (TMA), on resisting progressive collapse. The analysis results suggest that, after the failure of an external column, FA would dominate in resisting progressive collapse. Although TMA does contribute to the mitigation of progressive collapse at the large-deformation stage, it plays a secondary role because the total load-carrying capacity has already been severely deteriorated. By contrast, CA might be negligible.