| The number of building fires remains high after the 21st century,and fire disasters not only cause degradation of building materials,but also have a direct impact on the safety of constructional steel buildings.The weakest part of steel structures during a fire event is the joint domain from the explosion-induced fire event at the World Trade Centre and the simulated fire tests at Cardington,according to which beam-to-column connections play a decisive role in maintaining the robustness of steel structures.In the existing studies of the standard and/or natural fire tests,it was found that traditional semi-rigid beam-to-column connections may fail prematurely due to the sensitivity of carbon steel bolts at elevated temperatures,such that the potential performance of other connecting members is not fully exploited.However,austenitic bolts exhibit significant ductility at ambient temperature,possessing significant fire resistance at elevated temperatures.These performance potentials can lead to the application of austenitic bolts to bolted connections,which can significantly extend their ductility and improve their fire resistance.Therefore,the experimental investigations on how austenitic bolts and their configured double web angle cleat joints（WACJ）made of high-strength steels during and after furnace fire are urgently conducted from the material and joint level,based on which the main research covers the following five aspects.（1）Ambient and elevated temperature tests were implemented against austenitic bolts,including ten groups of A2-70,ten groups of A4-70,and ten groups of A4-80,based on which,furthermore,their stress-strain curves and key mechanical parameters（Young’s modulus,yield strength,and ultimate strength）were obtained at ambient and elevated temperatures.Besides,the residual properties of austenitic bolts were discussed relative to those of carbon steel bolts at elevated temperatures,and the degradation mechanism on behaviour of two types of bolts at elevated temperatures was also explained from the microstructural viewpoints.Also,the available studies on the post-fire properties regarding two types of bolts were collected to compare the post-fire reduction factors of two types of bolts,and the degree of recovery on the properties was evaluated for two types of bolts during heating-cooling.Using the available test data,a series of unified reduction model for two sets of bolts were developed using statistical principles.In general,the material stiffness and strength of austenitic bolts with stable microstructure after 600°C are less than those of carbon steel bolts during or after exposure to elevated temperatures,which enables austenitic bolts to be further applied to flexible or semi-rigid nodes and thus improve the fire resistance of beam-to-column connections.Meanwhile,the newly proposed reduction models of bolts ensure the safety on fire resistance design of bolts without highly over-conservative prediction.（2）The test data of 34 groups of hot-rolled high-strength steels（HSS）and 16 groups of cold-rolled HSS during fire,as well as 34 groups of hot-rolled HSS and 21 groups of cold-rolled HSS after fire were collected to compare the behavioural differences in the of hot-and cold-rolled HSS under fire or post-fire conditions,to analyze the reasons why reduction factors of HSS exhibit a high degree of dispersion at a given temperature,and to explain the effects on the microstructure of HSS under different heat treatment processes.Based on the collected experimental data,a series of standardized reduction models for both types of HSS during or after fire were developed using the K-S test and t-distribution of small samples.The results show that the deterioration rates of Young’s modulus,yield strength and ultimate strength of hot-rolled HSS characterized by martensitic or bainitic microstructures are essentially slower than those of cold-rolled HSS,while the prediction accuracy of the proposed reduction equations for both types of HSS is more significantly improved compared to empirical reduction formulas in the literature.（3）In order to reflect both tensile and shear bolts in the bolted joints,a total of eight hot-rolled HSS-made WACJ configured with austenitic or carbon steel bolts were designed,and experimental studies of WACJ,including four at ambient temperature and four at elevated temperatures,were carried out to acquire M-θRcurves,according to which the load capacity,ductility and failure modes are compared for WACJ with different bolt configurations at 650°C.Meanwhile,the degree of deterioration concerning the rotational stiffness and strength of WACJ with the same bolt configuration at 650°C were also discussed relative to those of WACJ at ambient temperature.Based on the obtained M-θR curves at ambient and elevated temperatures,WACJ configurated by austenitic bolts exhibited higher bending moments and ductility at 650°C relative to those done by carbon steel bolts,and the strength degradation rate of the former is significantly less than that of the latter compared to WACJ with the same bolt configuration at ambient temperature.Failure modes of the former were dominated by the angles at 20°C and 650°C,while the those of the latter transitioned from angle failure at ambient temperature to bolt failure at650°C.（4）Based on the elevated temperature tests of WACJ at 650°C,the post-fire performance of four WACJ assembled with austenitic bolts or carbon steel bolts was investigated to compare the mechanical properties characterized by M-θRcurves and failure modes of WACJ with different bolt configurations,and to analyze the recovery of the rotational stiffness and strength of WACJ with the same bolt configurations with respect to those at ambient and elevated temperatures.According to the test results,the fire resistance of the austenitic bolted WACJ was also significantly higher than that of carbon steel bolted ones when they were cooled from 650°C to ambient temperature.For the WACJ with the same bolt configurations,the rotational stiffness and strength of the post-fire WACJ were partially recovered for comparison with WACJ at 650°C,but failed to be recovered completely compared to WACJ at 20°C.Among them,the recovery of the connection performance of the austenitic bolted WACJ was greater than that of the carbon steel bolted ones.（5）Based on the existing component model of WACJ,an improved component-based model of WACJ was developed following that the prediction formulas for the rotational stiffness of WACJ were derived based on force equilibrium and deformation coordination conditions of the interaction between the components,and the modified component-based model considers the prying force effect between the angle and the steel column.Combined with the unified reduction models developed for austenitic bolts and hot-rolled HSS during or after fire,the measured M-θR curves of WACJ assembled by austenitic bolts are used to verify the validity of the improved component-based model.The formulated component models have the ability to predict the experimentally measured M-θR response of WACJ under normal,in-fire,and post-fire conditions,and to reflect the development trend of the measured curve,with excellent but conservative correlation between the M-θR curves obtained from the tests and predictions. |
