Experimental Study On Progressive Collapse Of A Prestressed Fabricated PC Frame Subjected To A Middle-Column-Removal Scenario

The development and application of precast structure have been vigorously promoted in China for the past few years,whereas precast concrete structure is more prone to progressive collapse subjected to accidental load comparing with cast-in-place reinforced concrete structure.Most of the existing studies on progressive collapse resisting of structures focused on cast-in-place structure,and the studies of the progressive collapse resisting of precast structure are relatively scarce.A series of related previous studies conducted on the prestressed fabricated PC frame by the research group showed that it has good seismic behavior and reliable joint connection performance.Therefore,based on the support of the National Natural Science Foundation of China,this paper made a further study on the progressive collapse resisting of the prestressed fabricated PC frame.In this paper,the experimental,theoretical and numerical simulation studies were carried out on a two-story two-span prestressed fabricated PC planar frame.The crack development,deformation,progressive collapse resisting mechanisms,alternative load path and failure mode of the frame subjected to the removal of the middle column were studied in the experimental study.The theoretical analysis was carried out from the aspects of deformation and stress,and the nonlinear dynamic response was used to evaluate the progressive collapse resistance of the prestressed fabricated PC frame.A finite element model with the quasi-static process was established by ABAQUS/Explicit to obtain the stress state of the frame in the whole process,and the progressive collapse resisting performance and collapse mechanism were studied.The main research contents and results are as follows:(1)The static load test was carried out on the progressive collapse of the prestressed fabricated PC planar frame subjected to a middle-column-removal scenario.Based on the data analysis of load-displacement curve,inter-layer displacements,reinforcement strains,rotations and steel strand stress,the test results showed that the prestressed fabricated PC frame has a good capacity to resist progressive collapse.The frame experienced the beam mechanism at small deformation,including the elastic stage,the elastic-plastic stage and the plastic hinge development stage,and the effect of the bending moments at the beam ends and the arch action provided the main resistance.The frame experienced the catenary mechanism at large deformation,and the tension of prestressed tendons provided an alternative load path and the main resistance.(2)The smart steel strand was used for the first time in the progressive collapse test to real-time monitor the prestressing force,and the influence of the prestressing force on the progressive collapse behavior of the prestressed fabricated PC frame was studied.The precast frame formed integrated stress joint by prestressing force,which provided the bending moments at beam ends in the beam mechanism and tie force in the catenary mechanism.Bonded prestressed tendons ensured the integrity and safety of the structure in the final destruction,and improved the resistance and deformation at each stage of progressive collapse.(3)The theoretical analysis was carried on the important indicators to evaluate the progressive collapse of the prestressed fabricated PC frame.The beam mechanism-catenary mechanism transform displacement,critical collapse displacement,and the resistance of beam mechanism and catenary mechanism were analyzed and calculated.A simplified dynamic analysis method which based on the energy balance principle was used to analyze the progressive collapse resistance of the test frame under the dynamic load,and the dynamic response curve,the maximum dynamic load and dynamic amplification factor were obtained.It is different from the static test that the increase of the resistance under dynamic load provided by prestressing force was not obvious.The calculated dynamic amplification factor was low at the catenary mechanism stage,and the recommended value of 2.0 should be safe for analyzing the progressive collapse of the prestressed fabricated PC frame.(4)Considering the complex contact and strong nonlinearity in the progressive collapse of structures,the progressive collapse of the prestressed fabricated PC frame as a quasi-static process simulated by ABAQUS / Explicit to obtain the stress,deformation and failure mode of concrete,non-prestressed reinforcement and prestressed tendons from assembly to final destruction.The simulation results made up for the deficiency of the test,and the accuracy and reliability of the finite element model were verified by the experimental results and theoretical analysis.The deformation process and stress mechanism of progressive collapse resisting were further discussed.