Study On Punching Shear Behavior And Capacity Of Reinforced Concrete Slab-column Connections

As a widely adopted structural system,reinforced concrete(RC)flat-slab are used in the building,hospital and underground garage structures.Due to the significant moment-shear interaction,the slab-column connections are prone to punching shear failure in a brittle manner.However,the mechanism of punching shear failure is still not clear,which brings in uncertainty into the calculation of punching shear capacity.To address these issues,the research carried out hereby incorporated the experiment study,finite element analysis and theoretical analysis.The work in the study including four sections:(1)Nine slab-column connections specimens were designed without shear reinforcement.The dimensions are 2,550mm×2,550mm×180mm.The design variables include concrete strength(C30,C50,C70)and longitudinal reinforcement ratio(0.71%,0.93%,1.71%).These nine specimens were conducted the punching shear test.The load,deflection,concrete strain and steel rebar strain throughout the loading process were recorded.The crack pattern for each loading stage was captured.The thickness variation,that is the development of internal micro cracks,was indirectly measured as well,by monitoring the dynamic strain of embedded rod.(2)Based on the test results of the specimens,the behavior of the punching shear failure has been systematically analyzed through four aspects: the crack pattern of section plane and bottom plane,the load-deflection curve and the deflection distribution along the span,the load-strain curve and strain distribution,and the slab thickness variation in the loading process and the failure moment.It is indicated that punching shear failure of RC slab-column connections without shear reinforcement may occur with or without the yielding of longitudinal reinforcement.At the failure moment,the diggonal were initialed in the concrete compression zone nearby the column.And the failure surface was formed with crack developing from the top face to the bottom.The failure process was similar to domino collapse with a “progressive” effects.The shear crack were impossible formed simultaneously along the column and the punching shear failure was trriggered by the crack in local areas.On the other hand,the phenomenon of the concrete strain decompression is a typical features for punching shear.(3)The parameters for finite element analysis of punching shear were proposed.Punching shear behavior of the slab-column connections was simulated in ATENA.The input parameters in ATENA were firstly calibrated based on the load-deflection curves for specimens with different failure modes.Then,14 specimens,with the variables of concrete strength,longitudinal reinforcement ratio and shear span to depth ratio,were numerically simulated by applying the calibrated parameters.Simulation results including load-deflection curves,load-strain curves and crack patterns demonstrated the finite element modeling is satisfactory.(4)The subtle difference between the shear failure of beams without shear reinforcement and one-way slab as well as the punching shear failure of the slab-column connections without shear reinforcement were discussed.Considering the inclined crack development and numerical stress field,the discussions are made based on the relevant provisions of several design codes,the key points of existed theoretical model,numerical stress field(from nonlinear finite element analysis),and representative experimental results.The difference in crack development,failure mechanism and shear resistance mechanism of the mentioned failure modes were defined.(5)The failure mechanism,failure process and shear resistance mechanism of the punching shear of slab-column connections was investigated in depth.A semi-empirical punching shear capacity formula was formed based on the proposed model.The proposed model considers the contribution of both the concrete compression zone and the aggregate interlock action,to the shear resistance.It is assumed that the parabolic Mohr-Coulomb criterion failure was used for concrete in compression.The aggregate interlock action is based on the method for the shear strength of cracked concrete proposed by Vecchio and Collins.The results of punching shear capacity obtained from different codes such as GB50010-2010,ACI 318-14,EC 2-04 and fib MC2010 are compared with the predictions from the proposed formula.The accuracy and the dispersion of these predications are evaluated based on 363 set of data from the new punching shear test database.