Failure modes and robustness of beam-to-column joints for steel tubular frame resisting progressive collapse

To investigate the failure mode and assess the robustness of steel tubular column-H beam joints under the column removal scenario simulating the progressive collapse, steel beam-to-column assemblies, consisting of a beam-to-column connection with two half-span beams, were statically tested. Specimens were designed using the same H section beam, and the beams were connected to the circular hollow-section column via outer-diaphragms or to the square tubular column via inner-diaphragms. Various connection methods, web bolt layouts and span-to-height ratios of the beam were considered in the specimens. The test results indicate that, with the increase of vertical displacement of the middle column, the failures occur at the connection region and the failure processes vary distinctly due to different connection configurations. Three failure modes are observed in the test, namely the beam-end continuous failure, beam-end interrupted failure and the column-wall failure. When failure occurs at the beam-end, the welded web connection displays a continuous failure process while the bolted web connection performs as an interrupted failure process. The column-wall failure mode merely appears after the bottom inner-diaphragm separates from the adjacent square-column wall and the failure process is little affected by the web connection method. In both beam-end interrupted failure and column-wall failure mode, the joint still develops catenary mechanism after the initial fracture via the sufficient link between the beam and the column, thereby acquiring higher capacity in later stage and showing good robustness. In the beam-end continuous failure process, the catenary mechanism can't be effectively utilized due to the decreasing beam section area, which indicates unsatisfactory robustness of the joint