Progressive Collapse Behavior of a Beam–Column Substructure of a Modular Steel Building under an Interior Cluster-Column Loss Scenario

A modular steel building (MSB) can maximize assembly levels and greatly improve construction efficiency, offering numerous benefits such as high industrialization, green energy-saving, and environmental friendliness. Unlike conventional steel frame structures, MSB structures feature a discontinuous structural frame and discrete floor diaphragms, which result in poor overall integrity and an increased susceptibility to collapse under extreme events. However, the structural responses to these events remain unclear. Accordingly, with an emphasis on the beam–column substructure, this research aims to conduct an experimental investigation on the progressive collapse behavior of MSBs. First, a semirigid intermodular connection—a typical bolted connection with cover plate as available in historical literature—was selected for the current specimen. An appropriate experimental method was developed to perform the vertical loading test under an interior cluster-column loss scenario. The related experimental methodologies were introduced, and test results were reported in detail. It was found that the specimen had failed as a result of fractures in the intramodular beam–column connections, with the floor beams (FBs) fracturing earlier and the ceiling beams (CBs) fracturing afterward. Further, the CBs experienced severe axial compression throughout the early loading stage. A thorough analysis of the double-beam system’s load transfer mechanism and resistance contributions was conducted throughout the entire loading process. A comprehensive finite element (FE) model was subsequently established and validated against the experimental results, enabling further discussion of the test results. Finally, several conclusions have been reached that may provide helpful references and guidance for enhancing the reliability and safety of MSB structures under extreme events.