Experimental Study on Progressive Collapse Resistance of Glulam Post-and-Beam Substructures under a Middle Column Removal Scenario

The increasing growth of high-rise mass timber buildings and several notable collapses have highlighted the need to understand their progressive collapse resistance, but the underlying mechanisms and reliable reinforcement methods remain underdeveloped. This study investigates progressive collapse resistance mechanisms and enhancement strategies for bolted steel-plate beam-to-column connections in glued laminated timber (glulam) post-and-beam frames. Four groups of seven 1/2-scale 2-bay substructures were tested under a middle column removal scenario. The typical failure modes and flexural strain distribution across beam sections were analyzed to elucidate progressive collapse resistance mechanisms. The contributions of internal forces to structural resistance were quantified, and the mechanical performance of substructures with conventional and enhanced connections was systematically compared. The results indicated that glulam post-and-beam substructures assembled with bolts and steel plates resisted vertical loads primarily by compressive arch action (CAA) and catenary action (CA), with their contributions governed by beam-to-column rotational capacity and horizontal constraints. Three connection enhancement strategies, including self-tapping screws, supplemental postactivation bolts, and hold-downs, differentially enhanced the development of CAA and CA. Enhanced specimens exhibited maximum average displacements during CAA and CA, at least exceeding 1.19 and 1.03 times those of the conventional ones, respectively. Their ultimate, CAA, and CA load-bearing capacities increased by 32%–159%, 32%–45%, and 18%–197%, respectively.