The Investigation On The Properties And Stress-strain Model Of High-strength Bolts And Stainless Steel Bolts During And After Fire

Since the 9.11 attacks,the safety of steel structures in fires and after fires has received increasing attention from structural engineers and researchers.It is necessary to propose a new structural fire-resistant design method to replace the traditional passive(spray fire-retardant coating)and active(automatic spray system)fire-resistance methods by studying the performance of steel structures in fire and after fire.Even though,bolts connections are known to be a key component of steel structures,the study concerning the fire resistance of bolts is very limited.In this paper,the experimental research on high-strength bolts and stainless steel bolts under high temperature and over-fire as well as the fire resistance performance data of high-strength bolts is conducted and the lack of fire resistance performance data of stainless steel bolts is also further studied.Finally,the stress-strain curve models of two kinds of bolts under high temperature and over-fire were established.This paper lays the foundation for the subsequent research on the fire resistance performance of nodes and the establishment of new design methods of fire resistance of structural bolts.The main research contents are as follows:(1)High-temperature steady-state tests were carried out on high-strength bolts(8.8,10.9,12.9)and stainless steel bolts(A2-70,A4-70,A4-80).The key parameters such as elastic modulus,yield strength and ultimate strength were extracted from the stress-strain curves of the test,and compared with the performance of high-strength steel and stainless steel base materials at high temperature.The study found out that the performance of high-strength bolts produced by different materials is basically the same at high temperatures,while the performance of stainless steel bolts is affected by the material.The ultimate strength of high-strength bolts begins to decrease after 300oC,but the yield strength is significantly attenuated below 300oC due to nonlinear enhancement.Compared with high-strength bolts,stainless steel bolts have better resistance to high temperatures and similar with the base metals,the strength begins to decrease after over 500oC.(2)The post-fire performance of high-strength bolts(grade 8.8,grade 10.9,grade 12.9)and stainless steel bolts(A2-70,A4-70)was studied.The bolts were heated to the specified temperature and held for 25 minutes,then cooled to room temperature in two ways(cooling in water and cooling in air),and then processed into a standard sample and subjected to a tensile test at room temperature.The test results show that,the cooling method has a great influence on the performance of high-strength bolts after the post-fire temperature exceeds 700oC.The bolt becomes brittle under the effect of cooling in water and becomes soft when cooling in air.However,the performance of stainless steel bolts is not affected by the cooling method even to 900oC.In addition,after the overheating temperature exceeds 400oC,the strength of the high-strength bolt begins to decrease,while the ultimate strength of the stainless steel bolt can be maintained and the yield strength will increase significantly at 200-500 oC,which is caused by the weakening of the nonlinearity.(3)Based on the model proposed by Tao et al.[73],a three-stage stress-strain model for stainless steel bolts at high temperature and after exposure in fire is proposed,which includes a necked down section.A complete stress-strain curve can be obtained by calculating the predicted values of all the parameters through the known five parameters(Es、fy、fu、?u、n)of bolts at room temperature or without fire.(4)On the basis of the models proposed by Wang et al.[110],Tao et al.[73],Mander [143] and Tao et al.[109],the stress-strain models of high strength bolts at high temperature and after exposure to fire are proposed respectively.Similarly,only five normal parameter values(Es、fy、fu、?u、?f)can be used to calculate all the parameters of high strength bolts at high temperature or after fire,and then the predicted stress-strain model can be obtained.The results show that the prediction by the proposed model is in good agreement with the experimental results.The innovations of this research are as follows:(1)For the first time,the high temperature performance and post-fire performance of stainless steel bolts were studied,and the prediction formulas of each parameter were formulated.(2)The prediction formulas of key parameters of high strength bolts at high temperature and after overheating are proposed.(3)The full-stage stress-strain model of stainless steel bolts at high temperatures is also formulated as part of this study.(4)The stress-strain model of stainless steel bolts in all stages after overheating is first proposed and its reliability is also subsequently analyzed.(5)The full-stage stress-strain model of high strength bolts at high temperature is proposed.(6)The full-stage stress-strain model of high-strength bolts after overheating is proposed,and the stress-strain curves under two cooling modes are fitted by the same set of formulas.