Experimental Study On Progressive Collapse Behaviour Of RC Beam-column Sub-assemblages With Weak Restraints

In recent years,with the increasing popularity of high-rise and super-high-rise buildings,the damage of various accidental loads to building safety cannot be ignored,and a series of building collapse events caused by explosion,vehicle collision,fire and other reasons have caused wide attention in the academic and engineering circles around the world.It is a key problem for structural engineers to improve the progressive collapse resistance and ductility of building structures.In this paper,three 1:2-scale specimens of double-span beam-column sub-assemblage are designed and fabricated according to alternate path method.By conducting the quasi-static test,the progressive collapse resistance and force transfer mechanism of 2D-plane RC beam-column sub-assemblage with weak restraints are studied.On this basis,a theoretical model is used to make parameter analysis.The main research contents are as follows:(1)The collapse resistance curve is obtained through quasi-static loading test.During the collapse process,the flexural action,compressive arch action and catenary action are successively formed.Among them,the flexural action mainly bears the load by the pure bending action,the compressive arch action increases the flexural bearing capacity by generating the axial force in the beam,and the catenary action mainly bears the load by the vertical component of the axial tension of the beam.(2)The failure modes of sub-assemblage at different stages are recorded,and the reinforcement strain development of key sections are analyzed.Section tensile cracking,tensile yield of steel reinforcement at the bottom of beam at middle joint,concrete crushing at the top of beam at middle joint,tensile yield of steel reinforcement at the top of beam at side joint,shear cracks at side column joint,concrete crushing at side column joint,tensile failure of beam reinforcement,etc are occured in sequence.(3)The changing regular of internal force at key sections of beam-column sub-assemblage is determined,and the resistance mechanism and deformation capacity of 2D-plane sub-assemblage with weak restraints are discussed.(4)The vertical bearing capacity of beam-column sub-assemblage is decomposed,the contribution of bending moment at beam end and axial force to the bearing capacity of sub-assemblage against progressive collapse is studied respectively,and the influence of side column size and ratio of longitudinal bars on top of beam is analyzed.The results shows that the bending moment at beam end has a great effect on the resistance bearing capacity in the flexural action and compressive arch action stage,and the axial force plays a dominant role in the catenary stage.(5)Based on the Matlab theoretical analysis model considering the compressive arch action,the influence of ratio of longitudinal bars at beam bottom and the span-depth ratio on progressive collapse resistance are studied.The results shows that increasing the ratio of longitudinal bars at beam bottom could increase the bearing capacity of compressive arch action,but the impact on axial force is limited,and reducing the span-depth ratio could visibly improve the bearing capacity of compressive arch action and increase the axial force of the beam,but it will correspondingly reduce the ductility of the structure.