Research On Punching Shear Performance Of Slab-column Connections Under Fire Conditions

Slab-column structures are composed of floor and column which are serving as load-bearing system,and its materials are generally reinforced concrete.Reinforced concrete slab-column structure has the advantages of low interlayer height,fast construction speed,easy to separate the room and other advantages.It has been continuously developed with many advantages and widely used in underground garages,multi-storey buildings and other projects.In slab-column structure,slab-column connections with the huge internal forces,complex stress state,the combination of large bending moments and shear forces and concentrated stress,are easy to occur punching shear failure in this case which are the weak parts in slab-column structure.In addition,slab-column connections are the key transfer force parts of the whole structure,when a part of the nodes are destroyed,and it will lead to the redistribution of the vertical load,cause the sudden increase of the load on the rest of the nodes,set off a chain reaction easily.The strength of reinforced concrete materials decreases with the increase of temperature under fire.Slab-column connections are more likely to occur punching shear failure which lead to the continuous collapse of the structure frequently.Therefore,we need to further research on punching shear performance of reinforced concrete slab-column structures under fire.It is of great significance for the design and strengthening of slab-column structures.This thesis takes reinforced concrete slab-column connections as the object of the research,and carry out studying on punching shear of slab-column connections under fire.The main research work can be summarized as follows:(1)Firstly,this thesis carry out studying on punching shear of slab-column connections at room temperature.Based on previous test of Hunan University,numerical simulation of punching shear performance of slab-column connections was carried out by using ABAQUS finite element software.The accuracy of the model is proved by comparing the simulation results with the experimental phenomena and the experimental data.Secondly,we can set oblique steel bars with different relative reinforcement rates in the middle of the slab.Research on the improvement of the punching shear performance and penetration performance for oblique steel bars which are with the same diameter and the same type of bottom steel bars.It can be found from the research that oblique steel bars can increase the punching shear performance and penetration performance of the slab.And the higher the relative reinforcement rates is,the greater relative improvement ratio of punching shear performance and penetration performance is.(2)On the basis of detennining the thermal performance of steel bar and concrete,the finite element analysis model of temperature field of reinforced concrete slab-column structures are established by using finite element software ABAQUS to analyze the temperature field.According to the ISO-834 international heating curve along the top surface of slab heating for two hours,the top surface concrete of slab temperature can reach more than 1000 degrees,the slab bottom concrete is 200 degrees.It shows that the transfer of heat takes a certain amount of time.The temperature curves from finite element analysis correspond with ISO-834 international heating curve,which lay the foundation of thermal-mechanical coupling analysis.(3)Based on the reinforcements and concrete thermo-mechanical constitutive relation and temperature field analysis,this paper carry out research on punching shear of slab-column connections at high temperature by using thermal-mechanical coupling model of the finite element software ABAQUS.At the same time,oblique steel bars are placed in the middle of the slab at room temperature,and the punching shear performance of oblique steel bars at high temperature slab-column structure is analyzed.The results show that:the oblique reinforcements in the middle of the slab will also improve the punching shear performance and penetration performance of the slab under high temperature.When the relative of oblique steel bars at middle of slab is 100%,the most increasing rate of time to occur the penetration is.This thesis implies that oblique steel bars at middle of slab can increase the punching shear performance and penetration performance of the slab.We can set oblique steel bars with relative reinforcement rates of 100%in the middle of the slab under fire,the great number of oblique steel bars are equal to integrity bars of column strip,which can increase the time of penetration by 9.417%.It can delay the time of the slab to occur collapse,which can be provided some references for improving fire resistance of slab-column.