Study On Progressive Collapse Resistance Of Precast Reinforced Concrete Frame Structures Under Local Explosion Actions

In order to investigate the progressive collapse resistance of precast reinforced concrete(RC)substructures with different connection methods in beam-column joints,a series of static loading tests for substructure models were carried out.The test specimens included assembled monolithic RC two-span beam-column subassemblies,fully assembled RC two-span beam-column subassemblies,assembled monolithic RC Tbeam-column subassemblies(considering flange effects),a assembled monolithic RC cross-beam subassembly without slab(considering transverse beam effect)and a assembled monolithic RC cross-beam subassembly with slab(considering transverse beam and slab effects).The static loading tests of assembled monolithic RC substructures indicated that the compressive arch action(CAA)and tensile catenary action(TCA)were activated at the small deformation stage and large deformation stage,respectively.The initiation mechanism for the CAA was the movement of the neutral axises of the beam end sections.There were two initiation mechanisms for the TCA,one was the fracture of the bottom bars at the beam end sections near the middle joint,and the other one was concrete crushing.However,when considering the effect of a flange in a T-beam-column specimen,the fracture of the bottom steel bars in the middle joint was the only mechanism found to activate TCA.The mechanical performance of the assembled monolithic RC substructure with 90° bend of bottom bars in the middle joint was similar to that of the cast-in-situ substructure.The peak bearing capacity at the CAA stage of the assembled monolithic RC substructure with lap-spliced of bottom bars in the middle joint was30% smaller than that of the assembled monolithic RC substructure with 90° bend of bottom bars in the middle joint.However,the peak bearing capacity at the TCA stage of the assembled monolithic RC substructure with lap-spliced of bottom bars in the middle joint was 72% larger than that of the assembled monolithic RC substructure with 90° bend of bottom bars in the middle joint.When considering the transverse beam effect,the load-carrying capacity of a cross-beam system at the CAA stage was almost equal to the sum of the load-carrying capacities of these two crossed beams,whereas the transverse beam effects including RC slab could increase the beam action by as much as 369%.The static loading tests of fully assembled RC substructures indicated that the connection gaps,the anchorage forms of longitudinal reinforcement at the ends of precast beams and the on-site welding quality between steel plate and rebars reduced the CAA and TCA of fully assembled substructures,which led to the mechanical performance of a fully assembled RC substructure was worse than that of an assembled monolithic RC substructure.The initiation mechanism for the CAA of a fully assembled RC substructure with corbel-bolt connections was that the both ends of the precast beam were in compression state after contacting with the side column and the middle column,and the initiation mechanism for the TCA was concrete crushing.The initiation mechanism for the CAA of a fully assembled RC substructure with hidden corbel-flange connections was the movement of the neutral axises of the connected surface between steel plate and concrete,and the initiation mechanism for the TCA was the fracture of the bottom bars at the welding points between steel plate and rebars.Based on the failure mechanisms of the substructures observed in the tests and the comparison with those of the cast-in-situ substructures,simplified models of the static load-displacement relationships of axially constrained precast substructures with different connection forms were proposed.The calculation results were in good agreement with the test results in this thesis and the literatures,which indicated that the proposed simplified models can be used in the structural progressive collapse assessment based on the energy conservation method.Considering the performance of steel angle cleat,the anchorage length of longitudinal rebars at the beam ends,the performance of welded rebars,the bound strength of steel-concrete interface and the connection forms between precast slabs and beams,the collapse simulation software for precast frame structures was developed in this thesis.Based on the test results of substructures and the characteristics of precast RC frame structures,it was concluded that the assembled monolithic RC frame structures could adopt the same “beam-slab-column-node” discrete type as cast-in-situ frame structures,and the fully assembled frame structures could adopt the “beam-slab-column-node-special element” discrete type,in which the special elements included corbel element,steel plate unit,and so on.The comparison between the test and the simulation results indicated that the simplified spring models for the connections were reasonable and could be used to predict the static load-displacement curves of fully assembled RC subassemblies.By reducing the element damping linearly,the problem of failure for weak connections between precast slabs and beams at the static equilibrium stage was solved.The numerical simulation results based on the collapse simulation software for precast frame structures were in good agreement with the progressive collapse assessments based on the energy conservation method.The progressive collapse-resisting performance of an assembled monolithic RC frame structure was better than that of a fully assembled frame structure,and the laminated slab could improve the progressive collapse-resisting performance of an assembled monolithic RC frame structure.Compared with the middle column failure scenario,precast frame structures are more sensitive to the failure of corner column and edge column scenarios.Under the middle column failure situation,a structure is less likely to lead to progressive collapse.However,once the middle column failure leads to progressive collapse,the collapse duration is shorter,and the consequences are often more serious than the other two cases.For high-rise structures,when the asymmetric bottom bearing members such as corner columns or edge columns fail,the second-order effect is more obvious,which is easy to lead to the collapse of the whole structure.The use of laminated slabs in fully assembled frame structures can significantly improve the collapse resistance.However,the influence of precast slabs on the progressive collapse-resisting performance of a fully assembled frame structures is limited.Considering the effect of precast slabs will not affect whether the initial failure will lead to progressive collapse,but it can appropriately delay the occurrence time of large deformation of a structure,and more truly reflect the collision between components in the collapse process and the final accumulation state of components.