Research On Post-fire Mechanical Properties Of Structural Steel And Bearing Capacity Deterioration Of Steel Bridge

In recent years,many fire disasters occurred on bridges in China.Most of these disasters were either caused by vehicle and ship impacts or secondary disasters of earthquakes.Both the material and members in the steel structures may be damaged,but few of them collapse directly in the fire hazards.As the demolition or reconstruction result in great waste,safety evaluation of a fire-damaged structure is very neeessary.There are few guidelines about the post-fire mechanical properties of steels in the current specifications to be followed.Therefore,post-fire bearing capacity of steel bridge is still unclear.In this paper,based on the research about the residual mechanical properties of steel after extreme eonditions in the National Natural Science Foundation of China(No.5178485),the post-fire mechanical properties of steel are studied,and the elastic-plastic constitutive models of steel under monotonic and repeated loads are established.On this basis,the applicability of finite element model is analyzed,and the strength degradations of components and structures caused by material damages are discussed.The main research work and conclusions can be drawn as follows:The post-fire mechanical properties experiments of steel Q345,which are commonly used in the bridge constructions in China,were earied out.The degradation of mechanical properties such as the yield strength,ultimate strength,modulus of elasticity and elongation at breaking after exposure to fire and cooling down were studied.The effects of maximum temperature and cooling method on the degradation of properties were discussed.By analyzing the existing experimental data,the law of post-fire mechanical properties reduction was obtained.On this basis,the elastic-plastic constitutive models of steel under monotonic and repeated loads were established,which can be utilized in the evaluation of structural bearing capacity.The results show that,the mechanical properties of steel decline after being exposed to an elevated temperature over 500?.However,larger discreteness is observed in the test.The residual strength of steel cooled in air is lower than that cooled in water.Through the analysis of post-fire simple steel structures,the change of the residual mechanical properties were investigated and effect of cooling methods was considered.By comparing the calculation results,the applicability of fiber element model and shell element model in elastic-plastic analysis of thin-walled structures after fire were discussed.The results show that the load-carrying capacity and plastic energy dissipation capacity of post-fire steel piers decrease obviously and the maximum can reach more than 50%.The load-carrying capacity and ductility of portal frames decrease and the minimum can be reduced to 50%.The accuracy of fiber element model is lower than shell element model when applied in the elastic-plastic analysis of thin-walled structures after fire.The stress and deformation of a truss steel bridge under static dead and live loads in a fire accident caused by vehicle impact was studied.Through the temperature distribution analysis of the bridge under fire hazard,the highest temperature experienced by each component was determined.Then,the steel constitutive relationship model under fire is established.On that basis,the residual deformations of bridge under different fire conditions were analyzed,which provides theoretical basis for post-fire load-bearing capacity analysis of structures.The results show that the degradation of material properties under local fire is relatively large,while the deterioration of bearing capacity of the steel bridge is small.Finally,the residual bearing capacity of the bridge structure after fire is evaluated,considering the influence of fire temperature,cooling method and fire location on the bearing capacity of the structure.The results show that fire in different locations have different influence on the bearing capacity of the structure;when the fire occurs near the support,the residual bearing capacity of the structure is the smallest;when the fire occurs in somewhere else,the influence on the ultimate bearing capacity is small.