Experimental Study And Analytical Analysis On Load Resisting Mechanisms Of Precast Concrete Frame Sub-assemblage To Resist Progressive Collapse

In the past several years,after the General Office of the State Council and Ministry of Housing and Urban-Rural Development Office issued the guidelines for rapid developing the prefabricated buildings,the "industrialized precast concrete structures housing" is vigorously promoted right now.The precast concrete(PC)structure with monolithic joint is an important part of the prefabricated structure.The cast-in-place monolithic joint is also called as "wet joint",in which the longitudinal reinforcements from the PC beams and columns were connected or anchored first.Then,high-grade concrete was poured in the cast-in-place connections.To date,domestic experimental and theoretical studies on PC structures with monolithic joints were mainly focused on their seismic performance based on low-cycle repeated tests.There are few studies involved on the progressive collapse performance of such PC structures under accident events.The compressive arch action(CAA)has been widely known as an important load resisting mechanism to resist collapse for a frame.However,existing studies on CAA normally still on the qualitative level by experimental tests or numerical analysis.More accurate and reliable analytical models are few.Therefore,it is necessary to quantitatively analyze the generation,development,and influencing factors of CAA to help a frame to resist progressive collapse.After that,the CAA could be applied to the practical design for concrete frames to mitigate progressive collapse conveniently.For aforementioned research problems,six PC frame sub-assemblages with cast-in-place monolithic joints were designed and tested under quasi-static pushdown loading regime.The test data were obtained:such as the load resisting functions of the PC sub-assemblages,the strain varying along the longitudinal reinforcement,and the varying of internal force at the critical sections near the joint region.In the theoretical part of this paper,a simplified two-dimensional(2D)analytical model,which relied on nonlinear iterative operation for the moment-curvature relationship of the beam-end critical section in the joint region,was proposed to determine CAA strength.The calculation of the analytical model was implemented relied on Matlab program.Based on the proposed analytical model,the CAA strength of the tested specimens and the specimens tested by other researchers were predicted and compared with test results to validate the accuracy and reliability of proposed analytical.The main conclusions from the experimental and analytical analysis were drawn as following:(1)Under the premise that the connection and anchorage of longitudinal reinforcement in the joint zone are fully guaranteed,the PC and RC frame substructures performed similar characteristics to resist progressive collapse when similar boundary constraints,identical span,similar reinforcement ratio in the joint zone,and similar concrete strength were applied.(2)The span-to-depth ratio has significant effects on the mobilization of load resisting mechanisms.The larger span-to-depth ratio leads to slower growth of the plastic rotation of the beam end and reduce the efficiency of CAA strength.However,the span-to-depth ratio has little effects on the development of TCA strength.(3)Under the experimental conditions,the boundary constraints of the quasi-static collapse process of the frame substructure can be characterized by the initial free lateral displacement t0,initial free rotation angle ω0,axially constrained stiffness Ka,and rotationally constrained stiffness Kr.The t0 andω0 caused by the gap existing in the experimental rigs,which will affect the development of CAA directly.Therefore,the gap in the experimental rig should be minimized to reduce unnecessary deviation.(4)The axially constrained stiffness Ka and the rotationally constrained stiffness Kr play a crucial role in the CAA strength of frame structures.The normalized factors γa and γr are used to represent the magnitude of the axially constrained stiffness and the rotationally constrained stiffness,respectively.When γa≥5.0 and γr≥3.0,it is called "strong constraints".If not,it is called "weak constraints".The current tests using rigid reaction frame to provide horizontal constraints prone to called "weak constraints".Therefore,it is recommended that the researchers engaged in progressive collapse tests need to measure the boundary constraints for objectively quantifying the influence of boundary constraints on behavior of the structures to resist progressive collapse.(5)Under the "weak constraints" condition provided by the test device,the improvement of concrete strength will not upgrade the CAA of the sub-assemblages significantly.Under the absolute "strong constraints" condition,which is provided by the interior bays of the frame in realistic frame,the improvement of concrete strength plays a significant role in increasing the CAA strength.(6)Amplification factor for CAA γ is related to the concrete strength fcu,the reinforcement ratios in the beam-end critical section in the joint region pT and ρB,and the span-to-depth ratio ln/h.Lower concrete strength(fcu≤C30),larger span-to-depth ratio(ln/h≥15.0),and the higher reinforcement ratios of the beam-end critical section in the joint region will detriment the amplification factor for CAA,the CAA strength is close to that of the yield strength of the frame.