| As a traditional type of building structure,reinforced concrete slab-column structures have been widely used in office buildings,shopping malls and underground garages because of its simple formwork,convenient reinforcement arrangement and high space utilization,which reduces the construction period and project cost.However,punching shear failure is prone to occur at the slab-column joints,and the resulting load redistribution may lead to a catastrophic continuous collapse of the building.Currently,for slab-column connections subjected to combined vertical and lateral loads,the problems,including the mechanical performance,failure mechanism,evaluation method of unbalanced moment and deformation capacity,have become the focus by scholars all over the world.Therefore,the above-mentioned problems of slab-column connections are discussed in this study using the methods of tests,numerical analysis and theoretical research,and the finished research work is as follows:(1)The seismic experiments of eight slab-column connections were designed and conducted under combined vertical and lateral loads.The dimension of length ×width × height is 2900 mm × 2900 mm × 150 mm.The main test variables include gravity shear ratios(0.35,0.50,0.65),reinforcement ratios(0.61%,0.81%),types of shear reinforcement and arrangement of hidden beams.During the test,the load,displacement,strain of longitudinal steel bars and shear reinforcements were measured,and the crack distributions under all levels of horizontal displacement were recorded in detail.(2)According to the test results of each specimen,the seismic performance of slab-column connections was studied in depth.The main contents included the characteristics of slab top crack distribution and punching failure surface,hysteretic curve and skeleton curve of unbalanced moment-drift,law of stiffness degradation,deformation capacity and strain distribution characteristics of longitudinal reinforcements and shear reinforcements.The test results show that the gravity shear ratio,longitudinal reinforcement ratio and shear reinforcement had obvious effect on the seismic performance of slab-column connections.When the gravity shear ratio is relatively high,the seismic performance of slab-column connections can be significantly improved using the shear reinforcement.The effect of different types of shear reinforcement was compared and analyzed by using the evaluation criteria that are the amount of shear reinforcement and the improvement of performance indexes.It is found that the stud is the most economical and effective measure to resist punching.When the hidden beams are designed,it is an economical anti-punching measure to adopt the four-legged stirrups with a width of the column size plus 0.8times slab thickness centered on the column and the connection with four-legged stirrups has good lateral deformation capacity before the failure.(3)Based on the concrete damage plasticity model in ABAQUS software,a finite element model was established to simulate the mechanical behavior of slab-column connections subjected to the combined vertical and horizontal loads.Firstly,the principles and suggestions were given to define the model parameters,which were calibrated according to the test results.And it is verified that the established model and methods to determine parameters are reasonable.Secondly,the calibrated finite element model was used to further simulate the test results obtained in this study and by other scholars,and the applicability of the calibrated finite element model was verified.Thirdly,a parametric study was performed,and the effects of vertical loads,longitudinal reinforcement ratios,concrete strengths and different layouts of shear studs on the unbalanced moment capacity,lateral stiffness and deformation capacity of slab-column connections were analyzed.Finally,by comparing the results of both different codes and the parametric study,it is further verified that the results of the parametric study obtained by the calibrated finite element model are reasonable and reliable.(4)Based on the test data of reinforced concrete slab-column connections,a simple and accurate moment-shear formula was proposed being suitable for all kinds of connections(interior connections,edge connections,corner connections),considering whether the connections were arranged with shear reinforcement.Furthermore,the simple calculation formula of ultimate shear stress on critical section was deduced for slab-column connections subjected to unbalanced moment.By comparing the calculation results with proposed formula and Chinese code formula,it was verified that the simplified proposed formula had better predictive capability.(5)The lateral deformation capacity of slab-column connections without shear reinforcement was studied under different failure modes,which are punching shear failure,flexure-punching failure and flexural failure.And a two-stage(curvilinear-linear)punching shear failure model was proposed.The rationality of the proposed model was verified by comparing with the existing calculation models.Based on the proposed model,a simple and effective method was proposed to determine whether the reinforced concrete slab-column connections need to be equipped with shear reinforcement.Moreover,the lateral deformation capacity of slab-column connections with different types of shear reinforcement was discussed,and the formulas for calculating the ultimate drift ratio of slab-column connections with studs and stirrups were proposed.The difference of lateral deformation capacity between the slab-column connections with and without shear reinforcement was analyzed by comparing the drift limit curves.(6)Based on the beam analogy concept and nonlinear model,a simplified model of relationship between unbalanced moment and drift ratio was proposed for slab-column connections with and without shear reinforcement,and the characteristic parameters of the model were calibrated by the test results.Combined with the experimental and finite element simulation results of slab-column connections,it was found that the relationships between unbalanced moment and drift ratio obtained by the proposed model were in good agreement with those of experiment and finite element analysis,which verified the accuracy and rationality of the proposed model. |
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