【作者】 王辉；

【导师】 聂诗东；

【作者基本信息】 重庆大学 ， 土木工程， 2022， 博士

【摘要】 21世纪后建筑火灾发生起数仍居高不下,而火灾不仅引起建筑材料性能的退化,而且直接影响钢结构建筑的安全性。从世贸中心的真实火灾事件和卡丁顿真实火灾试验中可以知晓钢结构中最薄弱的部分是节点域,其在保持结构的鲁棒性方面起着决定性作用。在现有标准火和自然火试验研究中,发现传统半刚性节点由于碳钢螺栓的高温敏感性而可能过早失效,这样其他连接构件的性能未能完全发挥。然而,奥氏体螺栓不仅在常温下表现出显著的延性,而且在高温下具备显著耐火性能。这些性能潜在性可以促使奥氏体螺栓用于钢结构连接,可以显著改善螺栓连接的延性和耐火性能。因此,从材料层面和节点层面亟待展开高温和过火条件下奥氏体螺栓及其配置的高强双角钢节点热力耦合响应的试验研究和理论分析,主要研究工作涵盖以下5个方面。(1)对10组A2-70、10组A4-70、10组A4-80奥氏体螺栓实施了常温和高温试验,获得了它们的常温和高温应力-应变曲线以及关键力学参数(杨氏模量、屈服强度和极限强度),讨论了奥氏体螺栓相对于碳钢螺栓的高温残余性能,并从微观结构角度解释了两类螺栓的高温性能的退化机理。其次,收集了这两类螺栓过火性能的现有研究,比较了这两类螺栓的过火折减因子,评估了这两类螺栓高温-冷却后性能的恢复程度。最后,利用统计学原理建立了两类螺栓的统一化高温或过火折减模型。总的来说,在高温或过火条件下,具有稳定微观结构的奥氏体螺栓材料刚度和强度的折减率在600°C之后小于碳钢螺栓的,这使得奥氏体螺栓可进一步应用于柔性节点或半刚性节点,进而改善节点的耐火性能。同时,新提出的螺栓高温或过火折减模型不仅保证螺栓耐火设计的安全性,而且不过于保守。(2)收集了34组热轧高强钢和16组冷轧高强钢的高温性能试验数据、34组热轧高强钢和21组冷轧高强钢的过火性能试验数据,比较了热轧和冷轧高强钢高温或过火性能的差异,分析了在某一温度下高强钢高温或过火折减因子表现出高度离散性的原因,并解释了不同热处理工艺下对高强钢微观结构的影响。基于收集的试验数据,利用K-S检验和小样本的t-分布提出了两类高强钢的标准化高温或过火折减模型。结果表明:由马氏体或贝氏体微观结构表征的热轧高强钢的杨氏模量、屈服强度和极限强度衰减率基本上慢于冷轧高强钢的,而提出的两类高强钢高温或过火折减公式的预测精度相对于文献中的经验折减公式得到了显著性的提高。(3)为了在螺栓连接节点中同时体现受拉螺栓和受剪螺栓,设计了8个奥氏体螺栓或碳钢螺栓装配高强双角钢节点,开展了4个常温节点和4个高温节点的试验研究,获得了节点的M-θ_R曲线,比较了不同螺栓配置节点在650°C时承载力、延性和失效模式。同时,探论了相同螺栓配置下高温节点的转动刚度和强度相对于常温节点的劣化程度。通过常温和高温M-θ_R曲线可知,在650°C时奥氏体螺栓配置节点相对于碳钢螺栓型节点表现出更高的弯矩和延展性;并且与相同螺栓配置的常温节点相比,前者的强度退化率明显小于后者;常温和高温条件下奥氏体螺栓节点的失效模式由角钢控制,而碳钢螺栓型节点的失效模式则从常温下的角钢失效过渡为高温下的螺栓失效。(4)在高温试验的基础之上,对4个奥氏体螺栓或碳钢螺栓装配双角钢节点的过火性能实施了试验研究,对比不同螺栓配置节点在高温-冷却后的力学性能(M-θ_R曲线)和失效模式,同时分析了相同螺栓配置下过火节点的转动刚度和强度相对于常温和高温节点的恢复能力。根据试验结果可知,从650°C冷却到常温时奥氏体螺栓连接节点的耐火性能也显著于碳钢螺栓型节点的。对于相同螺栓配置的节点而言,过火节点的转动刚度和强度相对于高温节点有所恢复,但相较于常温节点未能完全恢复。其中,奥氏体螺栓连接节点连接性能的恢复程度大于碳钢螺栓节点的。(5)在双角钢节点的现有组件模型基础之上,建立考虑了角钢与钢柱之间的撬力作用高强双角钢节点的改进组件模型,基于各组件间相互作用的受力平衡和变形协调条件推导了节点转动刚度的预测公式。结合为奥氏体螺栓和高强钢所建立的高温下或高温-冷却后的统一折减模型,利用常温、高温和过火节点的实测M-θ_R曲线验证所建立的改进组件模型的有效性。所建立的组件模型有能力预测常温、高温或过火节点实测M-θ_R响应,试验和预测所获得的M-θ_R曲线之间相关性较好,反映了实测曲线的发展趋势,但偏于保守。更多还原

【Abstract】 The number of building fires remains high after the 21^st 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-θ_Rcurves,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-θ_R 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-θ_Rcurves 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-θ_R 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-θ_R 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-θ_R curves obtained from the tests and predictions.更多还原