Post-fire Residual Bearing Capacities Of Steel Members And Effects On Holistic Performances Of Grid Structures

The research on fire residence of large span steel structures is always one of the hottest topics in structural engineering.At present,there have been adequate researches on fire residence design and structural performance in fire of large span structures in order to ensure structural safety and avoid structural collapse in fire.Therefore,it is of great significance to explore the residual mechanical properties of large span spatial structures after fire for evaluation,identification and reinforcement.However,the research on the residual bearing capacity of pipes which are used in grid structures is limited as well as study on the whole structures.In this paper,taking orthogonal square pyramid space grid structure as an object of study,the variation law of residual deformation and residual bearing capacity about post-fire steel pipes was studied by a combinational method of experimental research and numerical simulation.And then,an accurate model of grid structure was built,the residual deformation was introduced to structural analysis,and a way of evaluating the safety of grid structure after fire based on direct analysis method was presented.The main research contents and achievements are as follows:(1)The material properties of Q235 and Q345 steel subjected to nonuniform temperature field treatment were tested and the effect of non-uniform temperature field in actual fire was studied.The value of highest temperature is the main factor influencing material properties,while nonuniform temperature distribution has little influence.(2)The axial compression tests of 32 different steel pipe members after exposure to different high temperature treatment which are used in grid structures commonly were completed.Test results show that high temperatures lead to vary deformations of steel pipe members,especially when the temperature is above 600?.And the higher fire temperature is and the greater the residual deformation is,the lower the residual stability bearing capacity of steel pipe is.(3)Through parametric analysis,the main factors influencing residual stability bearing capacity were found which were highest temperature during fire,slenderness ratio and residual deformation after fire.Based on the results of finite element analysis,a practical method for calculating the residual stability bearing capacity of steel pipes after exposure to high temperature was presented by fitting parameters.(4)A way of evaluating the safety of grid structure after fire based on direct analysis method was put forward.An accurate model of grid structure was built by introducing the residual deformations to structural analysis,and the overall structure was calculated by direct analysis method.Finally,the residual performance of whole structure was obtained.