Improvement Of Progressive Collapse Resistance And Seismic Evaluation Of Composite Frame With Concrete Filled Steel Tubular Columns And Infill Walls

Huge and unpredictable accidental loads such as earthquakes,fires,explosions,and impacts may occur throughout the life cycle of the building structures.Some local structural components may be damaged and failure,which may cause the progressive collapse of the entire building structure.At present,scholars have studied the progressive collapse resistance of building structures through experiments,numerical simulations and theoretical analysis.Existing research mainly focuses on the resistance mechanism of upper structural components after the failure of vertical key componentss.Recently,there have been some studies on the progressive collapse resistance of RC frames and steel frames with infill walls,while there are few studies on the anti-collapse of the composite frame with concrete filled steel tubular(CFST)columns and masonry infill walls.Masonry infill walls are prone to break and even collapse during the collapse of the structure,which seriously affects the stiffness and bearing capacity of the structure.So far,the reinforcement of the collapse resistance of the composite frame with infill walls has not been reported.Therefore,the experimental study on the collapse resistance mechanism of the composite frame with CFST columns and infill walls is carried out in this paper.On this basis,ABAQUS is employed to carry out numerical simulation study,and the reinforcement mothods of the collapse resistance of the infilled composite frame is proposed.The collapse resistance and seismic comprehensive defense performance of the composite frame before and after reinforcement are evaluated.The main research contents of this paper are as follows:(1)The quasi-static progressive collapse test of the specimens of the composite frame with CFST columns and infill walls and the bare composite frame were carried out.The overall and local failure modes of the frame specimens were observed.The vertical resistance-displacement curves of the frame and the strain curves of the key sections of each component were obtained.The influence of the infill wall on the collapse resistance of the frame was analyzed.The test results show that the infill wall changes the failure modes of the CFST composite frame.The infill wall can be regarded as an equivalent strut,which significantly improves the resistance of the specimen in the small deformation stage,while the resistance contribution is small in the large deformation stage due to the serious damage of the wall.(2)Through the reasonable selection of material constitutive models,the finite element(FE)software ABAQUS/Explicit was used to establish the fine model of the composite frame with CFST columns and infill walls,and the anti-collapse mechanism was analyzed.The failure process and load redistribution mechanism of the infill wall during the collapse process were explored,and the resistance contribution of the infill wall was studied.The results show that the failure of the infill wall is brittle,and the diagonal cracks and horizontal cracks are easy to produce in the diagonal direction under vertical load.The mechanical mechanism of the infill wall in the collapse process can be explained by the effect of equivalent compressive struts.In the small deformation stage,the load of middle column is transferred to the adjacent span through the diagonal strut formed in the wall,while in the large deformation stage,the diagonal strut fails and the secondary strut plays a role.(3)Based on the failure modes of infill wall,the anti-collapse reinforcement motheds of the composite frame with CFST columns and masonry infill walls is proposed.The results show that increasing the diameter of the tie rebar in the range of 2mm-8mm can effectively improve the progressive collapse resistance of the frame.The the V-type oblique arrangement of tie rebars has better tie effect on the blocks,and the model has stronger collapse resistance.Pasting the CFRP sheet and spraying the ECC material to reinforce the infill wall can significantly improve the bearing capacity of the frame during the whole loading process.The peak bearing capacity of the frame can be increased by 25.1% and 22.9% respectively,and the ultimate bearing capacity can be increased by 96.2% and 74.5% respectively.In addition,a simplified calculation model of the composite frame with CFST columns and infill walls is established by equivalently treating the infill walls as equivalent compressive struts to predict the collapse resistance.The calculation results are compared with the experimental results and FE simulation results,and the relative errors are less than10%.(4)Based on the anti-collapse reinforcement methods of the frame,the seismic performance of frame models before and after reinforcement is evaluated.The anti-collapse bearing capacity,seismic ultimate bearing capacity,ultimate displacement,ductility coefficient,stiffness degradation coefficient,equivalent viscous damping coefficient and energy dissipation coefficient of each model are integrated to analyze the anti-collapse and seismic comprehensive defense performance of the models before and after reinforcement.The results show that the X-type pasting CFRP sheet has better anti-collapse and anti-seismic comprehensive defense performance for strengthening the infill wall.Therefore,this method can be given priority when strengthening the infilled composite frame in practical engineering.