Experimental investigation of asymmetrical reinforced concrete spatial frame substructures against progressive collapse under different column removal scenarios

The progressive collapse resistance design approach is generally applied in the context of a "column-removal" scenario to assess the structural vulnerability to progressive collapse. To obtain a better understanding of the complex progressive collapse resistance of 3D asymmetrical column-beam-slab systems, five one-third scale 2 x 2 bay asymmetrical reinforced concrete (RC) spatial frame substructure specimens were tested to analyze their collapse mechanisms under five different column removal scenarios, namely, an interior column removal scenario (INT), an exterior column removal scenario in the asymmetrical direction (EXT-X), an exterior column removal scenario in the symmetrical direction (EXT-Y), a corner column removal scenario at the long bay (COR-L), and a corner column removal scenario at the short bay (COR-S), which are among the most critical scenarios for analyzing structural resistance against progressive collapse. The test results showed that INT had the highest progressive collapse resistance capacity among the scenario substructures and exhibited two progressive collapse-resisting mechanism stages: a primary mechanism stage (beam and compressive membrane mechanism) under small deformations and a secondary mechanism stage (catenary and tensile membrane mechanism) under large deformations in both the X-direction and the Y-direction. In EXT-X and EXT-Y, the collapse resistance was mainly provided by the double-span beam at both the primary mechanism stage and the secondary mechanism stage. In COR-L and COR-S, the tensile membrane mechanism could not be mobilized, as the single-span beams in both the X-direction and the Y-direction behaved like cantilevers. Additionally, the asymmetrical span design reduced the resistance of the structure against progressive collapse.