Research On Fire Resistance Performance Of Stainless Steel Square Hollow Section Tubular X-Joints

With the rapid development of the national economy,people are increasingly demand for building appearance.Stainless steel materials are increasingly favored as a high-performance green building material due to their perfect appearance and excellent mechanical properties.Because of the beautiful shape of the hollow tube structure,the simple joints and the good performance in engineer,it is now widely used in the building structure.However,as one of the natural disasters of human beings,building fires seriously hinder social development and threaten public safety.The safety of building structures under fire is facing unprecedented challenges.At present,domestic and foreign research on the fire resistance performance of stainless steel structures is mainly concentrated at components,and related research is still lacking.Especially for the research on the fire resistance performance of stainless steel joints,there have been fewer reports,which is undoubtedly a huge technical loss for the promotion and application of stainless steel structures.Based on the above background,experimental research and finite element numerical simulation of stainless steel square hollow section?SHS?tubular X-joints under fire conditions were carried out and their behavioral response and failure mechanism under fire conditions were revealed.Finally,corresponding formulas for fire resistance performance of stainless steel SHS tubular X-joints is proposed.The mechanical properties of materials at room temperature and elevated temperature are the cornerstones of the research on fire resistance performance of structures or components.Based on the domestic cold-formed S30408 austenitic stainless steel material,the mechanical properties tests were firstly conducted,including the static standard tensile test at room temperature and the steady-state tensile test at elevated temperature.The test results show that,at room temperature,the two-stage material stress-strain curve proposed by Rasmussesn is in good agreement with the overall test data;at elevated temperature,the material stress-strain curve proposed by Chen is in good agreement with the overall test data.In addition,the cold forming process has an effect on the mechanical properties of stainless steel materials at room temperature and elevated temperature.According to the mechanical properties of the test materials,the experiment investigations of seven stainless steel SHS tubular X-joints under fire conditions were conducted.The fire test mainly investigated the influence that brace load ratio n,brace to chord width ratio?,and inclined angle between brace and chord member?may have on the fire resistance performance of stainless steel SHS tubular X-joints and revealed their mechanical performance and failure modes under fire conditions.Through fire test,the furnace temperature-time curve,the surface temperature-time curve,the vertical displacement-time curve,the chord web lateral displacement-time curve,the critical temperature,the failure temperature and the failure modes of test specimens were obtained.Fire test results show that:?1?Similar to failure modes of stainless steel SHS tubular X-joints at room temperature,failure modes of stainless steel SHS tubular X-joints can also be divided to three categories:chord face failure,chord face-side wall failure and chord side wall failure;?2?The failure process of stainless steel SHS tubular X-joints under fire conditions can be roughly divided into three stages:in the first stage,the specimen is heated to produce a expansion displacement;in the second stage,the vertical displacement of the specimen increases gradually due to the degradation of mechanical properties of stainless steel at elevated temperature;in the third stage,the specimen collapse rapidly because of the mechanical properties of stainless steel deteriorate to a critical state.?3?Combined with the test data,the failure criterion of stainless steel SHS tubular X-joints under fire conditions is proposed by referring to the existing failure criterion of stainless steel SHS tubular X-joints at room temperature.This criterion discriminates that when the chord flange indentation?u?reaches 0.03b₀?the chord width?under fire conditions,the specimen fails,and the corresponding surface temperature of chord at this moment is defined as"failure temperature".?4?Barce load ratio n and brace to chord width ratio?are important factors affecting the failure temperature of specimens;The increasing of the brace load ratio n will significantly reduce the failure temperature of specimens;The increasing of the brace to chord width ratio?will also significantly reduce the failure temperature of specimens.Based on finite element software ABAQUS,accurate finite element models of specimens SP-2^SP-7 in the fire test were established.Then the behavioral response of the test specimens under fire conditions was numerically simulated.The whole process of the fire test was reproduced.The mechanical performance and the failure process were investigated.Finally,the finite element analysis results were compared with the test results,and the results show that:the results of finite element analysis agree well with the test results,and the finite element analysis model established in this paper can simulate the mechanical behavior of stainless steel SHS tubular X-joints under fire conditions well.The parametric analysis was carried out by the verified finite element models to analyze the influence that seven factors?amplitude of out-of-plate initial imperfection e₂,brace to chord width ratio?,inclined angle between brace and chord member?,chord width to thickness ratio 2?,brace to chord thickness ratio?,brace load ratio n and compressive chord preload ratio n'?have on the fire resistance performance of stainless steel SHS tubular X-joints.The analysis results show that:?1?Brace load ratio n and compressive chord preload ratio n'are key external factors affecting the fire resistance of stainless steel SHS tubular X-joints;Brace to chord width ratio?and chord width to thickness ratio 2?are key internal factors affecting the fire resistance of stainless steel SHS tubular X-joints.?2?The increasing of chord width to thickness ratio 2?,the decreasing of brace load ratio n and compressive chord preload ratio n'will all rise the critical temperature of stainless steel SHS tubular X-joints under fire conditions.?3?The increasing of chord width to thickness ratio2?,the decreasing of brace to chord width ratio?,brace load ratio n and compressive chord preload ratio n'will all rise the failure temperature of stainless steel SHS tubular X-joints under fire conditions.Based on parametric analysis,coupling analysis among three vital factors?brace to chord width ratio?,chord width to thickness ratio 2?and brace load ratio n?was conducted.The analysis results indicate that the above three parameters have no significant coupling effect on the failure temperature of stainless steel SHS tubular X-joints.Finally,based on yield line mechanism model considering chord web deflection,the perdition formula for the failure temperature of stainless steel SHS tubular X-joints was deduced.The formula was also modified in combination with the finite element analysis results.At the same time,the adverse effect of the compressive chord preload on the failure temperature of stainless steel SHS tubular X-joints was investigated by numerical simulation,and the calculation formula for the failure temperature of stainless steel SHS tubular X-joints considering the adverse effect was given.