Structural behavior of centrally loaded steel columns at elevated temperature

A series of experimental studies is conducted to examine the behavior of SN490 steel column subjected to axial load in the fire condition. This experimental work is aimed at examining the effect of the width-to-thickness ratio of flanges, the slenderness ratio of steel columns and residual stresses on the ultimate strength of an SN490 steel column at a specified temperature. From these studies, it is found that the column strength decreases with the increase of width-to-thickness ratio and slenderness ratio. Column behavior is sensitive to width-to-thickness ratios and the slenderness ratio at temperatures below 550 ^°C. However, the influence of width-to-thickness ratios and the slenderness ratios is not severe when the temperature is higher than 550 ^°C. When the temperature is above 550 ^°C, the column strength deteriorates rapidly. It is also found that the failure mode of steel columns changes from inelastic global buckling at room temperature to inelastic local buckling at elevated temperature, due to the release of residual stress in a fire. Based on the results of this study, local buckling criteria and column strength at specified temperatures are suggested.     

Experimental study of fire-resistant steel H-columns at elevated temperature

This paper presents the experimental studies of axially loaded fire-resistant steel columns under elevated temperature. With the advancement of metal production, fire-resistant steel with enhanced mechanical properties at elevated temperatures has been developed recently. However, extensive research work is needed in order for the application of fire-resistant steel in building structures. In this study, a series of fire-resistant steel columns was loaded to their ultimate states at specified temperature. The effects of width-thickness ratios, slenderness ratios and residual stress on the performance of fire-resistant steel H-columns are examined. Based on this study, it is found that the section property of fire-resistant H-columns should be at least a non-compact section in order to prevent local buckling. Column strength is sensitive to slenderness ratio at elevated temperature. The strength of a slender column decreased sharply especially for temperatures above 600 ^°C. It is also found that the failure mode of steel columns changed from inelastic global buckling at room temperature to local buckling at elevated temperature, due to the release of residual stress in fire. An analytical model is proposed which is able to predict the behavior of fire-resistant steel H-columns under elevated temperature. Design guidelines are also proposed for the design of fire-resistant steel columns in fire conditions.     

Experimental study on local buckling of fire-resisting steel columns under fire load

The structural behavior of stub columns using fire-resistant steel has been investigated experimentally under uniform fire load. The newly-developed fire-resistant steel is proven to have higher strength at elevated temperature than that of conventional steel. Also, the requirement of fire-protection in the fire-resisting steel can be released or relaxed as compared with conventional steel structures. However, the design criteria for the application of the fire-resisting steel in steel columns are still limited. To examine the structural behavior of this type of steel columns under fire load, a total of 24 stub column specimens, including both box columns and H columns, reached their limit states due to axial load under fire condition. The main purpose of these studies is to evaluate the variations of the ultimate strength of steel columns due to different width-to-thickness ratios under specified elevated temperature; in addition, to investigate the reduction effects on column strength resulting in the increasing temperature; and finally, to establish the design guidelines of steel columns using fire-resisting steel. From the experimental results, it is found that the ultimate loads of the stub columns decrease while the width-to-thickness ratios or the temperature increases. However, with the increase of temperature, the effect of the width-to-thickness ratio on ultimate strength decreases. It is also found that the effect of the width-to-thickness ratio on the ultimate strength of box column at elevated temperature is more significant than that of H column. Based on these studies, design guidelines are proposed for the requirement of the width-to-thickness ratios of both box column and H column fabricated from fire-resisting steel.     

约束高强钢柱受火后轴压剩余承载力影响参数研究

利用经试验验证的ABAQUS有限元模型,对轴心压力作用下的轴向约束高强钢柱受火后的剩余承载性能进行了参数分析,考虑了过火温度、荷载比、轴向约束刚度比、长细比以及高强钢材强度等级等参数的影响。参数分析结果表明：当过火温度小于屈曲临界温度时,受火过程对轴向约束钢柱的轴压承载力没有影响;当过火温度大于屈曲临界温度时,钢柱的轴压剩余承载力会明显减小,且随着过火温度的升高,剩余承载力逐渐降低;轴向约束刚度比和长细比对约束钢柱的过火温度为屈曲临界温度时对应的轴压剩余承载力影响显著;荷载比对约束钢柱的过火温度为破坏临界温度时对应的轴压剩余承载力影响明显;高强钢材强度等级对约束钢柱受火后轴压承载力影响较小。根据参数分析所得数据及规律,提出了超500MPa高强钢轴向约束柱受火后轴压剩余承载力的简化计算方法,将计算结果与有限元分析结果对比,验证了该简化计算方法的可靠性。     

箱形钢柱受力性能分析及抗震设计建议

基于箱形钢柱拟静力试验,通过有限元分析,研究腹板宽厚比、构件轴压比及平面外长细比、柱顶弯矩等因素对偏压箱形钢柱抗震性能的影响。研究结果表明：腹板宽厚比和平面外长细比对箱形钢柱抗震性能影响较大,随着腹板宽厚比、平面外长细比的增大,箱形钢柱延性和承载力均有所减小,抗震性能变差;柱顶弯矩使箱形钢柱的正向延性增大,而降低其反向延性,柱顶弯矩对箱形钢柱抗震性能的影响相对较小。在此基础上,对箱形钢构件提出与受力状态相关联的腹板宽厚比、平面外长细比限值建议。引入箱形钢柱位移角概念,进而提出大跨度空间钢结构的构件抗震等级划分方法。并给出箱形钢构件腹板宽厚比、构件平面外长细比的抗震验算流程。     

轻钢桁架柱轴压性能试验研究

通过10根轻钢桁架柱的轴压试验,研究了不同长细比和扣板间距等参数对轻钢桁架柱轴压承载受力性能的影响。结果表明:在轴向压力作用下,轻钢桁架柱主要发生弯曲屈曲破坏,且在没有任何侧向约束条件下双管柱不适于承受竖向荷载;轻钢桁架柱的承载力随着长细比的增加而显著下降;扣板间距对轻钢桁架柱的承载力影响不大,但要符合分肢长细比的要求;轻钢桁架柱可视为缀板式格构柱,双管柱按照实腹式轴心压杆的方法计算承载力,四管柱采用换算长细比的方法计算其绕虚轴弯曲屈曲破坏的极限承载力。     

高强度钢材箱形柱滞回性能试验研究

为研究Q460高强度钢材箱形柱的抗震性能,对5个足尺试件进行了水平往复加载试验研究,分析了板件宽厚比、轴压比等因素对试件的承载力、破坏模式、耗能能力、变形能力和延性的影响。试验结果表明,Q460高强度钢材箱形柱具有很好的耗能能力和抗震性能,适用于抗震钢框架;除试件HB-1外其他试件本身及其柱脚节点均未发生焊缝开裂,证明设计合理、质量合格的Q460高强度钢材焊缝连接具有足够的承载力和良好的抗震性能;板件宽厚比越大,试件局部屈曲出现得越早,最大荷载对应的位移级越小,达到破坏时的位移级也越小;试件发生局部屈曲的范围及屈曲中心位置相对于试件截面高度的比值依次减小,所有试件最大屈曲位置距固定端0.25B~0.50B（B为等边箱型截面外边长）,塑性区范围距离固定端0.72B~1.06B。根据试验结果,建议在轴压比不大于0.2时,Q460钢材箱形截面压弯构件板件宽厚比限值不应大于30;同时,钢框架柱在进行抗震设计时,其板件宽厚比限值应与轴压比相联系,轴压比越大,板件宽厚比限值应越小。     

轻钢轻混凝土柱轴心受压承载力试验研究

通过6根轻钢轻混凝土柱的轴压试验,研究了不同长细比、含钢率和缀板间距等参数对其轴压承载受力性能的影响.试验结果表明：含钢率一定时,长细比为57.2的轻钢轻混凝土柱发生整体失稳破坏,长细比为40.1和46.7的轻钢轻混凝土柱发生强度破坏,破坏时钢管均发生了局部屈曲;轻钢轻混凝土柱的承载力随含钢率增大而增大,随长细比增大而减小,而缀板间距对轻钢轻混凝土柱的承载力影响不大;轻钢轻混凝土柱与普通混凝土柱的抗压破坏机理和形态基本相似,其轴压承载力可按普通混凝土柱轴压承载力公式计算.     

高强度等边单角钢轴压构件稳定设计研究回顾与问题探讨

高强度单角钢轴压构件的稳定性能因其材料的屈服强度等力学性能改变而发生变化,因而在稳定设计及其相关研究中会遇到许多新的问题。针对这一现状,结合我国GB 50017-2003《钢结构设计规范》、美国钢结构设计规范ANSI/AISC 360-05和ASCE 10-97等三部规范,对设计规范中宽厚比限值规定的适用性、高强度角钢宽厚比限值制定的必要性、角钢构件长细比的计算,以及现有角钢轴压构件稳定计算的折减方法等问题进行了探讨和算例分析,基于稳定的基本理论和数据结果的对比分析,解释并给出了上述几个问题的确定性结论和需要注意的方面,为高强度单角钢轴压构件稳定设计方法的研究提供了参考。     

高强度Q690钢柱耐火性能试验研究

为了获得高强度Q690钢柱的耐火性能，使用电炉对无防护足尺焊接H形Q690钢柱进行模拟ISO 834升温条件下耐火试验。测量得到不同荷载比下Q690钢柱温度、轴向位移、侧向位移与受火时间的关系，基于试验数据得到钢柱的临界温度和耐火极限。采用ABAQUS有限元软件建立钢柱耐火性能分析模型，考虑钢材高温蠕变和焊接残余应力的影响，模拟得到了钢柱的受火响应，其与试验结果吻合良好。利用验证的有限元模型分析了荷载比、长细比和升温速率对钢柱受力性能的影响。研究表明，无防护的Q690钢柱在受火20min左右发生破坏，破坏模式为整体失稳破坏；荷载比对临界温度影响较大，长细比和升温速率影响较小；Q690钢柱的临界温度比GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EN 1993-1-2的计算结果低60℃左右。最后提出了高强Q690钢柱抗火设计的简化方法。     

高强度Q690钢柱受火后受力性能试验研究

为了获得高强度Q690钢柱受火后的受力性能,采用电炉将2根高强度Q690钢焊接H形截面柱升温至800℃后自然冷却至常温,对受火后钢柱进行受压试验,得到钢柱的承载力和破坏模式。为考察受火对钢柱承载力的影响,对2根同尺寸不受火钢柱也进行受压试验。使用ABAQUS软件建立了试验钢柱有限元模型,考虑钢材受火后力学性能的退化和截面残余应力的影响,模拟试验构件的结构响应,并与试验结果进行对比。研究表明:钢柱均发生了绕弱轴的整体失稳破坏;经800℃受火作用后的钢柱承载力与不受火相比降低30%左右;有限元分析结果与试验结果吻合良好。进而采用有限元模型分析受火温度、冷却方式、长细比等参数对受火后Q690钢柱整体稳定系数的影响,有限元分析发现:受火后Q690钢柱稳定系数随受火温度升高呈先增大后减小的趋势,转折点为800℃左右;受火温度较低时冷却方式对Q690钢柱稳定系数影响较小,受火温度高于800℃后影响显著;受火后Q690钢柱稳定系数随长细比增大而降低,长细比小于80时,降低幅度较大,长细比超过80时,降低幅度减小。     

强约束轴心受压钢管柱耐火性能试验研究

为正确评估强约束钢管柱的耐火性能,利用自行研制的杆系结构构件温度轴力测量装置,采用恒载升温试验方法,设5级初应力水平,6种长细比,对30根Q345钢管柱进行试验研究,揭示轴心受压钢管柱在强约束下的耐火性能。试验结果表明:强约束钢管柱在温升作用下的温度应力相当大,对钢管柱破坏有决定性作用,在耐火设计与评估中必须考虑。钢管柱在高温作用下,破坏前其弹塑性性质较为明显。相同长细比的构件,在较高的初始应力水平作用下,极限承载力大、温度应力小、临界温度低;反之相反。在相同初应力水平下,长细比对强约束钢管柱的极限承载力和温度应力的影响并不敏感,但长细比大的试件为失稳破坏,破坏后变形很大;长细比较小的试件为强度破坏,破坏后变形较小。以试验数据为基础给出两端固定Q345钢管柱的下限临界温度回归计算公式,可用于火灾中约束刚度不变的钢结构耐火设计与评估。     

设分配梁与内环板传力构造的巨型钢管混凝土柱抗震性能试验研究

针对7个按1∶5缩尺的设置分配梁加内环板传力构造的巨型钢管混凝土柱试件进行拟静力试验研究,考察了轴压比、长细比、管壁宽厚比及纵向T形加劲肋等因素对此类构件抗震新能的影响。研究结果表明,管壁宽厚比大于60的试件,在达到极限荷载之前（约为极限荷载的63%）发生管壁局部屈曲,延性较差;设置T形加劲肋可有效减小管壁宽厚比,提高管壁局部屈曲强度,改善试件的延性性能及耗能能力;试件位移延性系数随轴压比和试件长细比的增大而降低,刚度退化越明显;在低周反复荷载作用下,同时设置分配梁与内环板传力构造的钢管混凝土柱试件破坏时的位移角超过了规范规定的弹塑性层间位移角限值,满足抗震设计要求,且钢管与核心混凝土变形协调,相应的截面属性和压弯承载力可按平截面假定计算。     

火灾下带肋薄壁方钢管混凝土柱承载力分析

对带肋薄壁方钢管混凝土柱在火灾下的承载力进行了分析,考察了各重要参数对构件承载力影响系数的影响。结果表明:在火灾作用下,带肋薄壁方钢管混凝土柱的承载力损失较为严重,在一定参数范围内,截面尺寸、构件长细比和防火保护层厚度对火灾下构件承载力影响系数的影响较大,荷载偏心率、截面含钢率的影响次之,钢管屈服强度、加劲肋屈服强度、混凝土抗压强度的影响较小,加劲肋宽厚比在荷载比≤0.3且宽厚比≤25的情况下有一定影响。在构件外表面采用防火保护能有效提高耐火极限,而且厚涂型钢结构防火涂料的效果明显优于水泥砂浆保护层。     

Distortional buckling tests of cold-formed steel compression members at elevated temperatures

In recent times, light gauge cold-formed steel sections have been used extensively since they have a very high strength to weight ratio compared with thicker hot-rolled steel sections. However, they are susceptible to various buckling modes including a distortional mode and hence show complex behaviour under fire conditions. Therefore, a research project based on detailed experimental studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. More than 150 axial compression tests were undertaken at uniform ambient and elevated temperatures. Two types of cross sections were selected with nominal thicknesses of 0.60, 0.80, and 0.95 mm. Both low (G250) and high (G550) strength steels were used. Distortional buckling tests were conducted at six different temperatures in the range of 20-800 ^°C. The ultimate loads of compression members subject to distortional buckling were then used to review the adequacy of the current design rules at ambient and elevated temperatures. This paper presents the details of this experimental study and the results.     

A numerical imperfection sensitivity study of cold-formed thin-walled tubular steel columns at uniform elevated temperatures

A numerical study is carried out on cold-formed rectangular hollow section columns to evaluate the sensitivity of column failure strength to initial imperfections, stress–strain relationships and to assess the existing design methods. It is shown that the magnitude of initial local buckling imperfection has a significant effect on the ultimate strength of short columns where failure is predominantly local buckling. Its effect on long columns is relatively small. Similarly the magnitude of initial global imperfection has more influence on the ultimate strength of a long column, whose failure is governed by global buckling, than on short columns, where local buckling controls. The shape of the stress–strain relationship of cold-formed steel will have noticeable effect on the column failure load. Current design methods, for high temperatures in ENV1993-1-2 and for ambient temperature in ENV1993-1-3, can provide a valid basis of calculation but modification will be necessary, depending on the exact model of stress–strain relationship of cold-formed steel at elevated temperatures.     

Behaviour and design of restrained steel column in fire: Part 2. Parameter study

This paper investigates behaviours of the restrained steel column in fire. For the restrained steel column under axial load only, investigated parameters include the axial load, the axial restraint stiffness, and the column slenderness; for the restrained steel column under combined axial load and bending moment, studied parameters included the axial load, the bending moment load, the axial restraint stiffness, the column slenderness and the end moment ratio.The results of parametric studies show that (1) the axial restraint causes a reduction in the failure temperature of the restrained column. The reduction increases with the increase in the axial restraint stiffness. However, when the axial restraint stiffness ratio is greater than a certain value, no further reduction occurs; (2) the difference between failure and buckling temperatures of a restrained column is great for columns with great axial restraint stiffness or great slenderness or small load ratio. This means that in this situation, the fire resistance of the restrained column can be increased from the column buckling temperature by considering the post-buckling behaviour; (3) an increase in the column axial load ratio or bending moment ratio causes both the column buckling and failure temperatures to decrease; (4) with an increase in the column end moment ratio, the failure temperature of restrained column decreases. The results of parametric studies will form the basis of a simplified calculation method to be presented in the companion paper.     

火灾下轴心受压H型截面约束钢柱整体稳定设计

利用验证的有限元模型分析了轴向约束刚度比、轴力荷载比和钢柱长细比对火灾下轴心受压H形截面约束钢柱屈曲温度和破坏温度的影响,给出了其屈曲温度和破坏温度的计算方法。轴向约束刚度比对约束钢柱屈曲温度和破坏温度的影响可用指数函数表示,轴力荷载比和钢柱长细比对约束钢柱屈曲温度和破坏温度的影响可用多项式表示。采用有限元方法对计算公式进行了验证,设计公式计算结果与有限元分析结果吻合较好,且设计公式给出偏于安全的结果。     

Residual strength of concrete-filled RHS columns after exposure to the ISO-834 standard fire

The residual strength of a composite column may be used to assess the potential damage caused by fire and help to establish an approach to calculate the structural fire protection for minimum post-fire repair. The behavior of six rectangular hollow structural steel (RHS) columns filled with concrete, with or without fire protection, after exposure to the ISO-834 standard fire (ISO 834, 1975), subjected to axial or eccentric loads have been experimentally investigated and the results presented in this paper. Comparisons are made with predicted column strengths using the existing codes such as LRFD-AISC-1994, AIJ-1997, EC4-1996 and GJB4142-2000. It was found, in general, that the loss of the strength of the specimens without protections was significantly greater than that of columns with fire protection. A mechanics model is developed in this paper for concrete-filled RHS columns after exposure to the ISO-834 Standard Fire (ISO 834, 1975), and is a development of the analysis used for ambient condition (Han et al., 2001). The predicted load versus mid-span deflection relationship for the composite columns is in good agreement with test results. Based on the theoretical model, influence of the changing strength of the materials, fire duration time, sectional dimensions, steel ratio, load eccentricity ratio, depth-to-width ratio and slenderness ratio on the residual strength index (RSI) is discussed. It was found that, in general, the slenderness ratio, sectional dimensions and the fire duration time have a significant influence on the residual strength index (RSI). However, the steel ratio, the depth-to-width ratio, the load eccentricity ratio and the strength of the materials have a moderate influence on RSI. Finally, formulas suitable for incorporation into a building code, for the calculation of the residual strength of the concrete-filled RHS columns after exposure to ISO-834 standard fire is developed based on the parametric analysis results.     

冷弯薄壁复杂卷边槽钢轴压固支构件稳定性能分析

为研究复杂卷边槽钢轴压固支构件的稳定性能,选取不同的翼缘宽厚比、截面形式、构件长度、卷边长度等几何参数,对共计292个构件进行非线性有限元分析,研究上述参数对复杂卷边槽钢固支构件稳定性能的影响。结果表明:固支构件较易发生畸变屈曲,翼缘宽厚比较大时,构件的整体失稳模式多为弯扭屈曲。构件的极限承载力随着翼缘宽厚比、构件长细比的增大而减小,卷边对构件承载效率的影响随翼缘宽厚比的不同而呈现两种不同的规律。