Study On Performance Of RC Frame Substructures To Resist Progressive Collapse Considering The Influences Of Boundary Conditions

In the past decades,domestic and foreign researchers had conducted extensive studies on structures to resist progressive collapse and achieved a series of achievements.However,in their experimental studies,the boundary conditions were over-simplified and hardly to reflect the realistic behavior of the frames.Therefore,in this study,taking the interior and exterior substructures from a peripheral frame as the research object,two half-scaled double-span beamcolumn substructures,which simulated different column missing scenarios,were tested under pushdown loading method to quantify their progressive collapse behavior.Different with the specimens tested in previous studies,the investigated specimens were designed to consider the effects of surrounding joints and side columns.In consideration of the number of test specimens was relatively little,the numerical model was built based on finite element software ANSYS/LSDYNA.The numerical results were compared with the test one to validate the accuracy of the numerical models.After that,the validated numerical models were utilized to conduct parametric study to evaluate the effects of longitudinal reinforcement ratio,reinforcement strength,concrete strength,compressive axial force ratio at the side column,and the size of side column on the performance of RC sub-assemblages to resist progressive collapse.Based on experimental and numerical studies,the conclusions were drawn:1.The sub-assemblages under either the interior or exterior column missing scenarios performed three stages:yield stage,compressive arch action(CAA)stage,tensile catenary action(TCA)stage.It was found that failures were concentrated in regions near the joint interfaces,including bar fracture and concrete crush.2.As the interior sub-assemblage has overhanding beams beyond both side columns,which could provide additional horizontal constraints for the beams,the interior sub-assemblages achieved higher CAA strength and TCA strength.However,as the side column for tested specimens has enough horizontal constraints and it was inevitable that gap existed in the test rigs,the interior subassemblages could upgrade the CAA and TCA strength only by 5.3%and 6.5%.3.Based on the commercial software LS-DYNA,high fidelity finite element analysis results indicated that the numerical models could accurately simulate the load-displacement curves,failure modes of test results well,as long as the concrete and rebar constitutive model,bond-slip model between concrete and reinforcements,and boundary conditions were well simulated.4.Based on the validated numerical model,parametric studies were carried out.The parametric study indicated that the beam longitudinal reinforcement ratio,strength of reinforcements have significant effects on CAA and TCA strength.However,the concrete strength only has considerable effects on CAA strength.For the specimens with side column of 250mm×250mm,the compressive axial force ratio has little effects.However,for the specimens with 200mm×200mm,the effects of compressive axial force ratio could not be ignored.