Post-Fire Performance Of Structural Steels

Steel structure occupies an increasingly important position in the field of structural engineering because of its light weight,good plasticity and toughness,convenience for standardized production,convenient construction process,and short construction time.While,except the above advantages,steel structure also has some weeknesses.The fire hazard of steel structure is one of them.Although steel is non-combustion material,its fire resistance is unsatisfactory.With the widespread use and promotion of steel structure,more and more cases of steel structure fires have emerged.The post-fire performance evaluation of steel structure gives out higher requirements for existing researches and standards.While the researches and standards on the post-fire performance of steel structure are far from enough in China and abroad,the post-fire performance of steel structure need more research.The post-fire mechanical properties of Chinese structural steel Q235 and Q345 steel materials and welds under different cooling conditions were investigated by experimental research,literature investigation,and fitting analysis in this thesis to meet the need of standards compilation.And the possibility of using Leeb hardness as an index to evaluate the post-fire mechanical properties of steel structure without distrcution.In the mean time,formulas for reduction factors of post-fire mechanical properties of Chinese structural steels are given out based on the data from experiments and literatures.The research works in this thesis are as follows:(1)Mechanical properties and its reduction factors of Q235 and Q345 materials including the stress-strain curve,elastic modulus,yield strength,ultimate strength,elongation after fracture,section reduction,appearance characteristics,Leeb hardness are studied based on experimental research and fitting analysis.The effects of the elevated temperature and cooling methods on the post-fire mechanical properties of materials were also investigated.The fitting formulas are verified with the data collected from literatures.(2)Mechanical properties and its reduction factors of Q235 and Q345 wields including the stress-strain curve,elastic modulus,ultimate strength,section reduction are studied based on experimental research and fitting analysis.The effects of the highest heating temperature and cooling methods on the post-fire mechanical properties of wields were also investigated.The fitting formulas are verified with the data collected from literatures.(3)The relationships between Leeb hardness and yield strength,Leeb hardness and ultimate strength,Leeb hardness and elongation after fracture were studied based on the experiments,existing researches and fitting analysis.New fitting formulas are given out.(4)Based on the fitting analysis on data from experiments and literatures,fitting formulas of the reduction factors of post-fire mechanical properties in polynomial form were given out considering the effect of the highest heating temperature and the standard value of the yield strength of the steel.