Experimental dynamic progressive collapse assessment of post-and-beam mass timber substructures with two-bay cross-laminated timber floors under interior column removal

Mass timber buildings are emerging construction systems that present distinct design challenges, one of which is progressive collapse. Current design guidelines are largely based on studies conducted on steel and concrete structures, whereas experimental research on the progressive collapse behaviour of mass timber buildings has been predominantly limited to quasi-static testing. However, progressive collapse is a dynamic and nonlinear phenomenon that can lead to severe social and economic repercussions. This paper provides new insights into the progressive collapse behaviour of three-dimensional post-and-beam mass timber buildings with cross-laminated timber (CLT) panels spanning two-bays. Two scaled substructures were subjected to both quasi-static and dynamic testing under an interior column removal scenario. The load was incrementally increased until collapse. The structural responses, load redistribution mechanisms, dynamic increase factors (DIFs), failure modes, and associated collapse mechanisms are analysed and discussed. The findings highlighted that the substructures exhibited fewer alternative load paths and a less uniform load‑redistribution pattern compared with conventional concrete systems. The CLT panels spanning two-bays provided the primary alternate load path, only compressive arch action was observed, not catenary action. It is critical that the elements along the primary alternate load path are designed to sustain most of the redistributed load. The observed average DIF value was 1.69 and was 15.5% lower than the theoretical DIF value of 2.0 proposed in the DoD and Eurocode. The observed collapse mechanisms were characterised by the successive shear failure of the beam-to-column connectors, with those along the primary alternate load path failing first.