Study On Progressive Collapse Behavior And Resistance Of Precast Concrete Frame Structures

Progressive collapse of building structures refers to the phenomenon that the initial failure caused by the accidental action spreads from component to component and finally leads to the disproportionate collapse or overall collapse of the structure.All types of public and private buildings would be affected by accidental extreme events.In recent years,researchers at home and abroad have conducted a lot of experiments and numerical studies on the progressive collapse behavior of building structures.Most of the existing researches focused on reinforced concrete structures,but the progressive collapse performance of precast concrete structures has been rarely studied.The progressive collapse danger of precast concrete structures is higher than that of cast-in-place concrete structures owing to the discontinuity of reinforcement at beam-column joints.For this reason,the progressive collapse resistance mechanism of precast concrete structures needs to be studied,particularly considering the background that building industrialization is vigorously promoted in China.Therefore,in this study,the progressive collapse resistance of precast concrete frame structures is investigated through static and dynamic loading tests,simplified analysis model,and finite element simulation.The main contents of this study are summarized as follows:(1)Static loading tests on the progressive collapse resistance of precast concrete moment sub-frames are performed.The load transfer mechanism of the sub-frame and the effects of various connection detail,rebar development length,rebar ratio,and beam section size on the progressive collapse resistance of precast concrete moment sub-frames are evaluated.The test results show that,similar to the cast-in-place reinforced concrete frame,the precast concrete frame structure with wet connection can also generate two lines of defense against progressive collapse.When the rough surface is set on the joint surface between the precast beam and the laminated layer according to the code and the keyway is set on the end face of the precast beam,the new and old concrete bond well and show good integrity.(2)Dynamic loading tests on the progressive collapse performance of the precast concrete moment sub-frames are performed.In dynamic tests,the specimens corresponding to the static loading tests are adopted for dir ect comparative study on the progressive collapse resistance of the sub-frames.According to the static loading test results,a total of five load conditions are designed for the dynamic loading tests,so as to study the progressive collapse resistance of the sub-frames under different load levels.The test results show that al though the specimen DPCF-1 has reached the ultimate strength of compressive arch action(CAA)under the fifth load condition,the successful mobilization of catenary action(CTA)prevents the progressive collapse of the sub-frame.However,the specimen DPCF-3 collapsed under the fourth load condition,and the sub-frame did not form an effective CTA.By comparing the dynamic load test results with the pseudo-static resistance curves obtained based on the energy balance method,it can be seen that the energy balance method is a reliable method to evaluate the dynamic capacity of structures.Besides,the dynamic increase factor(DIF)of the two specimens ranges from 1.03 to 1.55,both of which are lower than the recommended value 2.0 in the current design guide lines,indicating that the current design guidelines are relatively conservative.(3)Based on the study of the progressive collapse mechanism of the moment sub-frames,a simplified analysis model is established addressing the force equilibrium condition,compatibility condition,and material constitutive relationship.The model can consider the effect of actual boundary conditions and bond strength of reinforcement,which can directly evaluate the load-displacement relationships of the moment sub-frames with pin and fixed boundary conditions,using four points to simply simulate the strength and corresponding deformation of CAA and CTA without iterative calculation.The validity of the proposed method is verified by comparing with the existing test results.Based on the verified model,a parametric study is conducted to study the effects of concrete strength,reinforcement ratio,beam section size,and beam span on the progressive collapse resistance of the moment sub-frames.In general,increasing the beam height is the most effective way to increase the strength of CAA,and increasing the beam top rebar ratio is the most effective way to increase the strength of CTA.Increasing the reinforcement ratio of beam top rebar ratio can also improve the strength of CAA to some extent.Besides,the different design strategies of beams will also have a significant impact on the progressive collapse resistance of the structure.(4)The nonlinear finite element(FE)software LS-DYNA is adopted to study the progressive collapse performance of precast concrete moment sub-frames.The load transfer mechanism and failure mode of the specimens are studied,and the simulation results are compared with the experimental results.Based on the verified FE model,the parameter extension analysis is performed.Firstly,the influence of side span of the plane frame on the progressive collapse resistance is studied,and the boundary conditions are simplified to study the ration ality of different simplified constraint conditions.Then,on the premise of reasonably considering the effect of side span,the effect of three-dimensional space effects(including transverse beam and floors)on the progressive collapse resistance is furt her studied.Eventually,under various column removal scenarios,the progressive collapse resistance of space frame structures is studied,and the contribution of the floor to the progressive collapse resistance is quantified.The results show that the existence of side span will improve the CAA strength of the substructure,and the horizontal rigid constraint on the end of the extension section of the beam can reflect the influence of edge spans on the progressive collapse resistance of the substructure to the greatest extent.The existence of transverse beam and slab will significantly improve the progressive collapse resistance of the substructure.The different column removal scenarios will significantly affect the boundary constraint conditions of the s ubstructure,thus further affecting the progressive collapse resistance.Compared with the external column removal scenario,the progressive collapse resistance of the substructure will be significantly improved when the internal column is removed.