Robustness Of A Steel Frame-composite Floor System With Reverse Channel Connections Under An Exterior Column Removal Scenario

The building structures suffer accidental loads,resulting in initial failure,and then the loads carried by failure elements transfer to surrounding elements,which cause more elements failure.Finaly,the building structures collapsed or the structure failure is disproportionate to the initial local failure,which is so called progressive collapse.The911 event pushed the research on progressive collapse to a climax,including the research on beam-to-column joints,2-dimentional(2D)composite frames and3-dimentional(3D)integral structures.The beam-to-column joints are the key point connecting horizontal load-bearing elements with vertical load-bearing elements,whose rotation capacity and axial tensile capacity significantly influence the effectiveness of catenary action in the beams.At present,the robustness research on traditional weld joints and bolted joints are matured,while there are no related studies on reverse channel connections.3D integral structures could reflect the 3D effect of real structures,as well as flexural action,catenary action,compressive arch action and tensile membrane action.Howere,the studies on 3D integral structures are not matured enough and they are mainly on internal column removal scenarios.There are few studies on exterior column removal scenario,leading to knowledge lack on mechanical mechanism of a structure subjected to exterior column loss.In order to solve the aforementioned problem,this paper carried on qusi-static experiment to study the robustness of a steel frame-composite floor system with reverse channel connections under exterior column removal scenario.Also,component-based method was used to experimentally study the property of reverse channel connections.The main research processes and main conclusions are listed blow.(1)In the experiment,a 6-point loading system was used to simulate the uniformly distributed loads and a set of restraint system was applied to simulate the restraints of the residual structures.After the specimen destroyed,the tension cracks mainly distributed in a semicircular ring around the inside edge beams.Meanwhile,the compression cracks mainly distributed in a trilateral zone around the failure column.The failure of longitudinal reinforcing bars and profiled decking in the center part of composite slab led to structure failure.(2)The tensile membrane action of composite slab was difficult to quantify,because it partially transferred to steel beams by composite effect of shear stud,contributing to the flectural action and catenary action of composite beams.In the small deformation stage,the vertical loads were mainly carried by the flectural action of composite beams.Catenary action in the double-span primary beam was mobilized at about one depth of effective composite primary beam and developed rapidly,while there was no catenary action in the composite secondary beam.At large deformation stage,with the fracture of primary beam-to-column connection and composite slab,the flectural action of composite beams reduced rapidly.At the ultimate stage,flectural action of composite beams,catenary antion of composite beams and the action of residual composite slab contributed 62 percent,13 percent and 25 percent to the total vertical loads,respectively.(3)After the exterior column failure,the vertical loads were mostly transferred to the adjacent columns through composite primary beams and composite secondary beams.With the increase of vertical displacement,the reforcing bars under hogging moment fractured as well as the reverse channel connection under sagging moment,leading to the original load path was not so effective that the vertical loads started transfering to distant corner columns through the tensile membrane action of composite slab.(4)The effective width of middle row component of reverse channel connection was equal to the pich distance of bolts and the rest two parts of connection represented the residual two components,which was proved to be reliable.The good deformation capacity of reverse channel connection could delay the failure of beam-to-column connections,making it possible that each resistance mechanism worked simultaneously.(5)The force-based dynamic increased factor(DIF_p)in the paper decreased with the increasement of vertical displacement.After the vertical displacement reached about150mm,the DIF_p stayed around 1.15,meeting the requirements of DoD 2013.The displacement-based dynamic increased factor(DIF_u)increased with the increasement of equivalent uniform loads.When the equivalent uniform loads were below 25kN/m~2,DIF_u maintained in around 2.5.