| The initial local failure of the structure caused by accidental loads may lead to failure of adjacent elements or progressive collapse of the entire structure.In recent years,progressive collapse accidents have occurred frequently,causing huge casualties and property losses,which has caused academics to attach great importance to progressive collapse.After local vertical load-bearing elements failure,improving the bearing capacity of the remaining structure is considered to be one of the most effective methods to resist progressive collapse of the structure.Therefore,studying the force transmission mechanism of the remaining structural elements and enhancing the capacity of the remaining structure to resist progressive collapse,which is of great significance to improve the safety of the building structure,prevent the occurrence of major engineering disasters,and reduce the corresponding economic losses and casualties.Existing experimental studies mostly consider beam-column sub-assemblages tests with strong boundary constraints after the failure of the middle column.In real engineering,the surrounding elements can only provide limited boundary constraints,and the compressive arch effect and catenary effect of progressive collapse of the structure may also affect the surrounding elements and cause them to break.In this paper,three RC frame sub-assemblages with a removed middle column are designed.Various performance indexes of the frame sub-assemblages under quasi-static loading are systematically studied using a new test method,and the resistance mechanism of the RC frame sub-assemblages with weak boundary constraints is revealed.In addition,the interaction of beams,columns and joints is analyzed,the failure mode and bearing capacity of progressive collapse were determined,and the influence of different engineering parameters on progressive collapse are analyzed.The main research contents and conclusions of this article are as follows:(1)The collapse process of the frame sub-assemblages can be divided into flexural action stage,compressive arch action stage and catenary action stage.Flexural action stage and compressive arch action stage are mainly subjected to bending failure,the axial pressure is mainly transmitted by the bottom support,and catenary action stage is mainly subjected to tensile failure,the axial tension is mainly transmitted by the top support.The final failure mode of the frame sub-assemblages is bottom steel bar fracture of the middle joints with the tensile failure of the side column joints.(2)The vertical resistance of the frame sub-assemblages can be decomposed into the contribution of the bending moment resistance and the axial force resistance.Before the catenary action stage,the vertical resistance was mainly provided by the bending moment,and the axial pressure contributed negatively to the vertical resistance;The vertical resistance in the catenary action stage is mainly based on the resistance provided by the catenary tension.(3)Using the M-N curve analysis of the eccentric compression member,it was found that the tensile longitudinal steel bars reached yield at the end of flexural action stage,and the concrete in the compression zone was crushed when the axial pressure reached its peak.The failure of the beam end section is shown by the large eccentric compression failure.The existence of axial pressure increases the ultimate bearing capacity of the beam end section.(4)The seismic design of the joints has little effect on the bearing capacity.Increasing the reinforcement ratio of the longitudinal reinforcement at the top of the beam can significantly improve the collapse resistance of the structure,but the development of the collapse resistance of the sub-assemblages in the catenary action stage is restricted by the damage of the side columns and joints.(5)Based on the existing compression arch model to predict the bearing capacity of the test piece,it is found that the model has a higher accuracy in the prediction of bearing capacity of compressive arch effect.Taking the test piece in this article as an example,the influence of boundary constraint stiffness and concrete strength on bearing capacity of compressive arch effect is analyzed based on the compression arch model,and it is found that increasing the axial constraint stiffness can enhance bearing capacity of compressive arch effect;Increasing the rotation constraint stiffness has little effect on bearing capacity of compressive arch effect;Increasing the strength of concrete can enhance bearing capacity of compressive arch effect,but the degree of enhancement is not significant. |
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