Structural behavior of reinforced concrete frames subjected to progressive collapse

Abstract

With the threat of terrorist attack looming large, the ability of a building to mitigate progressive collapse is of key interest to government agencies. Alternate load path (ALP) approach is one of the direct methods to assess progressive collapse resistance by introducing a column removal scenario. An experimental program, comprising eight reinforced concrete (RC) beam-column sub-assemblages and seven RC frames, was carried out to investigate structural behavior and progressive collapse resistance of RC frame members subjected to a middle column removal scenario (MCRS), in particular, large-deformation behavior. Each sub-assemblage specimen consisted of a two-bay beam, a middle beam-column joint (just above a removed column) and two end-column stubs. Each frame specimen comprised a two-bay, a middle joint and two side columns, and most of frame specimens also include beam extensions. With increasing two-bay beam deflection, three different structural mechanisms, i.e. flexural action, compressive arch action (CAA) and catenary action, can be sequentially mobilized in a load-deflection history. Flexural action capacity is determined with a conventional plastic hinge mechanism. CAA is developed accompanied by beam axial compression and catenary action is mobilized when beam axial force changes from compression to tension. On top of flexural action capacity, both CAA and catenary action can significantly increase structural resistance, suggesting large potential of structures. However, compared with CAA, catenary action involves much large deflections and requires large rotation capacities of RC beams. In the sub-assemblage tests, the effects of specimen detailing, top and bottom reinforcement ratios at the middle joint regions, and span-to-depth ratios of two-bay beams on structural behavior were investigated. In the frame tests, the effects of boundary conditions of the frame specimens and specimen detailing are studied. In particular, three special detailing techniques were introduced, targeting at increasing beam-end rotation capacities.