| Slab-column structure is a kind of tradit ional architectural structure with many advantages,which is widely used in warehouses,garages,residential buildings and so on.However,the brittle punching shear failure of connection area between plate and column(named slab-column connection)is easy to occur,which may lead to the serious consequences such as catastrophic continuous collapse.Therefore,the punching shear failure mechanism and punching capacit y calculation method of slab-column connection have long been one of the most important issues concerned by researchers all over the world.In this paper,by means of experimental research,finite element simulation and theoretical analysis,a systematic study on punching shear mechanical performance and capacit y calculation method of slab-column connection is finished.The main research contents of this paper are as follows:(1)Punching shear failure tests of 7 concrete slab-column connections(The length,width and thickness were 2550/2650mm×2550mm×180mm)wit hout shear reinforcement were designed and completed.The test variables included longitudi nal reinforcement ratio(0.71%,0.93%,1.71%),reinforced method(orthogonal reinforcement,radial-circular reinforcement),column shape(square,circle)and eccentric load.In the experiment,the load,deflection,concrete and steel strain in the whole process of punching shear failure tests were measured,and the crack s morphology under different load levels was recorded in detail.At the same time,industrial endoscope and strain dynamic monitoring rods were used to observe the development and propagat ion of inclined cracks in the plate.(2)Based on the test results of the specimens,the mechanical characteristics of reinforced concrete slab-column connections in the whole process of punching shear failure were systematically analyzed.Based on the morphological characteristics of cracks at the bottom and saw-cut section of the slabs,load-deflection curves and deflection distribution curves,load-strain curves and strain distribution characteristics,and the variation trend of strain measuring rods representing the change of plate thickness at the moment of failure and inclined cracks propagation during loading,the mechanical characteristics and punching failure mechanism of slab-column connections were studied comprehensively.It was found that the formation and propagation of inclined cracks in the surrounding area of the slab-column connections was an important cause of punching shear failure.The ratio of reinforcement,the reinforced method,the shape of column section and the way of loading have different effects on punching shear failure.(3)Finite element software ABAQUS was used to simulate the mechanical properties of the slab-column connections,and the suggestions of parameter selection of the punching shear finite element simulation were proposed.The correctness of the simulation model was calibrated by the load-deflection curves,deflection distribution curves,strain of radial reinforcement and concrete,bending crack s at the bottom of the plate and inclined cracks in the plate.Besides,the applicabilit y of the simulation model was verified by more experimental result s.Than,the calibrated finite element simulation model was used for studying the influences of the concrete strength,longitudinal reinforcement ratio,reinforcement yield strength,column size and effective thickness of slab.The results showed that they have obvious influence on the mechanical property of the slab-column connections,and even change the failure mode of the connections.Therefore,reasonable selection of these parameters in the design is of great significance to balance the performance of slab-column connections to achieve the target performance.(4)The punching shear failure process,shear transfer and failure mechanism of slab-column connections are analyzed.The differences of concrete strength distribution and internal defects lead to crit ical inclined cracks in the slabs are not uniform around the column,and the concrete compression zone of the most developed area of inclined crack will take the lead in shear damage when the punching shear failure occur.Then the failure cracks propagate to the tensile area and the surrounding area rapidly,until the punching shear failure cone is completely formed.According to the experiment and finite element simulation results,the main mechanism s to resist punching shear force of slab-column connections are fromed the shear resistance of the concrete compression zone and the crit ical inclined cracks due to aggregate interlock action,while the dowel action of the longitudinal reinforcement can only be a negligible minor mechanism due to the unreliable force transferring path.At the same time,the failure modes and mechanisms of slab-column connection punching failure and beam shear failure were compared.It is found that the mechanism of punching shear is similar to the shear-compression failure of beam,but the failure mode does not change with the increase of span-height ratio.These differences are related to the spatial mechanical characteristics of the slab-column connections.(5)Based on the process and mechanism of punching shear failure summarized in this paper,and focusing on the main key shear transfer mechanism,a semi-theoretical and semi-empirical calculation model for the punching slab-column connections was established by reasonable assu mptions and simplified analysis.Based on the database of punching shear tests for slab-column connections without shear reinforcement which including 320 tests,the accuracy and dispersion of prediction results advised in this paper were evaluated and compared with those of four concrete structure design codes including GB50010-2010 code,as well as the empirical fitting formula or theoretical formula proposed by several other scholars.Meanwhile,the recommended calculation results were compared wit h the finite element simulation results.The results showed the same trend for the changes of different parameters,especially for the changes o f concrete strength and reinforcement ratio,which proves the rationalit y and accuracy of the recommended calculation model again. |
PDF Full Text Request