Study On Mechanical Properties Of Q235 Steel And Square Steel Tubes After Fire

Steel structures are widely used in building structures by virtue of their good strength,toughness and weldability.As a support member of steel structure,square steel tube has good mechanical properties,good weldability,corrosion resistance and plasticity,and has long been widely used in steel structures.However,the fire resistance of steel is poor,and fire accidents are very destructive to buildings dominated by steel structure systems.After a fire occurs,the commonly used fire-fighting means in my country is to extinguish the fire by spraying water or foam fire extinguishing agent.After the fire is extinguished,some steel structure buildings can continue to be used after inspection,evaluation,repair and reinforcement,reducing unnecessary economic property losses and waste of resources.Therefore,in this paper,relevant tests and numerical simulations were carried out to analyse the changes in mechanical properties of Q235 steel and square steel tubes under different cooling methods after fire,and the main work is as follows:(1)Material properties test of Q235 steel after fire: Place the Q235 steel material test pieces in the environment of 20-900°C,set different temperature gradients to simulate different temperatures in a fire,and then use water immersion cooling and foam fire extinguishing agent cooling to simulate fire-fighting methods.Finally,the residual mechanical properties of Q235 steel after fire are obtained through static tensile test,and the change rules of Q235 steel after various temperatures and cooling modes are analyzed and discussed.Using Origin software,the simplified calculation formulas for fitting various residual mechanical properties with temperature and cooling methods were sorted out,and compared with the mechanical properties of Q345 and Q355 steels with similar labels after high temperature cooling.(2)Axial compression test of Q235 hollow square steel tubes after fire: Put the specimens of Q235 hollow square steel tube under the same temperature and cooling mode conditions as the material test,and then carry out the axial compression test on it,observe the damage form of the specimens,and obtain its residual value under axial compression.Bearing capacity,load-axial displacement curve,load-strain curve,the residual bearing capacity and deformation after the fire were studied,and the simplified calculation formulas of the residual bearing capacity of the Q235 hollow square steel tubes after the fire were proposed.(3)Finite element analysis of Q235 hollow square steel tubes after fire: According to the various mechanical properties obtained from the material tests of Q235 steel and the axial compression test data of the hollow square steel tube,the finite element model of the Q235 hollow square steel tubes after fire is established.The axial compression test of of Q235 hollow square steel tubes were simulated,and the numerical simulation results were compared with the test results to verify the reliability of the finite element model.The influence of different temperature,cooling method,slenderness ratio and width-thickness ratio on the residual bearing capacity of Q235 hollow square steel tubes after fire is studied by the verified finite element models.