Compression tests of high strength cold-formed steel channels with buckling interaction

This paper describes a series of compression tests conducted on cold-formed simple lipped channels and lipped channels with intermediate stiffeners in the flanges and web fabricated from high strength steel plate of thickness 0.6 and 0.8 mm with the nominal yield stress 560 MPa. A range of lengths of lipped channel sections were tested to failure with both ends of the column fixed with a special capping to prevent local failure of column ends and influence from the shift of centroid during testing. The high strength cold-formed steel channel sections of intermediate lengths generally displayed a significant interaction between local and distortional buckling. A noticeable interaction between local and overall buckling was also observed for the long columns. A significant post-buckling strength reserve was shown for those sections that showed interaction between local and distortional or overall buckling. Simple design strength formulas in the Direct Strength Method for the thin-walled cold-formed steel sections failing in the mixed mode of local and distortional buckling have been studied. The strengths predicted by the strength formulas proposed are compared with the test results for verification.     

Compression tests of welded section columns undergoing buckling interaction

This paper describes a series of compression tests performed on welded H-section and channel section columns fabricated from a mild steel plate of thickness 6.0 mm with nominal yield stress of 240 MPa. The ultimate strength and performance of the compression members undergoing nonlinear interaction between local and overall buckling were investigated experimentally and theoretically. The compression tests indicated that the interaction between local and overall buckling had a significant negative effect on the ultimate strength of the thin-walled welded steel section columns. The Direct Strength Method (DSM), which was newly developed and adopted as an alternative to the effective width method for the design of cold-formed steel sections recently by NAS (AISI, 2004), was calibrated by using the test results for application to welded steel sections. This paper confirms that the Direct Strength Method can properly predict the ultimate strength of welded section columns when local buckling and flexural buckling occur simultaneously or nearly simultaneously.     

Numerical study of built-up double-Z members in bending and compression

The use of the finite element method (FEM) for the design of composed, thin-walled, structural steel members is considered. The bolted double-Z frame member is an interesting and economical engineering solution, already used in practice [1]. However, the European recommendations for the design of steel structures do not consider built-up members from cold-formed steel profiles. Finite element analysis is used to capture the various buckling effects that shape the response of slender thin-walled members. From the finite element model, the importance of initial imperfections and stiffness of connections is identified. The experimentally validated model predictions show that a non-linear finite element analysis can predict the member behaviour, in terms of failure mode and ultimate load, yield line pattern, overall stiffness and local strain in the cold-formed profiles. To obtain a good prediction, overall and localised initial imperfections should be considered and included in the analysis.     

Recent research advances on ECBL approach.: Part I: Plastic–elastic interactive buckling of cold-formed steel sections

The objective of this two parts paper is to present some recent developments and applications of erosion of critical bifurcation load (ECBL) approach for the interactive buckling. Two different types of problems are analysed: (1) plastic–elastic interactive buckling which implements into the Ayrton–Perry interaction formula the plastic strength of the stub columns evaluated by means of local plastic mechanism analysis, and (2) elastic–elastic interactive buckling for members with perforations.The first part of the paper analyses the occurrence of local plastic mechanisms in cold-formed steel sections in compression, and how they can be implemented in the ultimate limit state analysis of the members. Actually, the failure of thin-walled cold-formed members in compression always occurs with a local plastic mechanism. Starting from this observation, the authors suggest to use in the interactive local-overall buckling analysis the sectional plastic mechanism strength instead of traditional ‘effective section’. The ECBL approach is used to implement the proposed interactive buckling model. Results are compared with those of other two recent methods, namely the direct strength method and plastic effective width approach. Relevant tests are used to evaluate the three methods. Comparisons with European and American design codes are also presented in the paper.     

冷弯薄壁卷边槽钢轴压构件畸变与局部相关屈曲试验研究

对卷边尺寸不同的两类腹板中间设置加劲卷边槽形截面,共18个冷弯薄壁型钢固支轴压试件进行畸变屈曲与局部屈曲相关作用的静力试验研究。得到试件的屈曲模式、相关屈曲行为、破坏模式以及极限荷载。试验结果表明：畸变屈曲与局部屈曲的耦合相关对试件的变形和极限荷载有不利作用;畸变屈曲与局部屈曲的耦合相关作用存有较大的屈曲后承载力;畸变屈曲与局部屈曲的耦合相关顺序,即畸变屈曲 局部屈曲耦合相关、局部屈曲 畸变屈曲耦合相关,对试件的变形、非线性平衡路径、破坏模式以及极限荷载的影响有所不同。采用ABAQUS有限元软件对试件进行模拟分析,计算结果与试验结果吻合良好。     

Hysteretic behaviors of cold-formed steel beam-columns with hollow rectangular section: Experimental and numerical simulations

This paper presents a comprehensive experimental and numerical investigation on the cyclic response of cold-formed steel columns with hollow rectangular sections. The present study examined the columns׳ post-buckling strength and rigidity degradation, deformation and failure modes, ductility, and energy dissipation capacity. The cold-formed steel members exhibited stable hysteretic performance up to the point of local buckling with considerable degradation in strength and ductility. The energy dissipation mechanisms from the in-plane plastic behavior and out-of-plane elastic buckling deformation were identified. The influence of the height-to-width ratio and axial-compression ratio on energy-dissipation and failure mode was also investigated.     

高强冷弯薄壁型钢柱畸变屈曲研究

随着冷弯薄壁型钢构件朝着高强、超薄和复杂截面形式的趋势发展,畸变屈曲成了近年来国内外冷弯薄壁型钢结构研究领域的重要课题。在查阅大量文献和已有试验结果的基础上,利用有限元着重研究了不同腹板高度、翼缘宽度、卷边高度、厚度、长度、初始缺陷值大小对高强冷弯薄壁型钢槽形截面轴压构件的屈曲形式和极限承载力的影响。     

G550高强冷弯薄壁Z形钢受弯构件承载力试验与直接强度法研究

为了研究高强冷弯薄壁型钢构件的受弯性能和完善直接强度法,对卷边形式为斜卷边、直卷边和复杂卷边的12个G550高强冷弯薄壁Z形钢受弯试件进行了静力试验。试验结果表明,卷边形式对试件屈曲模式和受弯承载力有显著影响,试件发生了局部与畸变相关屈曲或畸变屈曲,直卷边试件的承载力高于斜卷边试件的承载力,复杂Ⅱ型卷边试件的承载力高于复杂Ⅰ型卷边试件的承载力。根据试验结果并结合现有国外直接强度法公式,回归出G550高强冷弯薄壁Z形钢受弯构件发生局部与畸变相关屈曲变形时的承载力直接强度法修正公式,当此类构件发生畸变屈曲时,无需再修正现有直接强度法公式。建立有限元模型对试验进行了分析,分析结果与试验结果吻合良好。在此基础上,利用大量有限元分析结果验证了直接强度法修正公式比现有直接强度法公式对此类构件的承载力的预测更为安全可靠。     

Post-buckling behaviour and DSM design of web-stiffened lipped channel columns with distortional and local mode interaction

This paper examines the design and load-carrying capacity of fixed-ended web-stiffened lipped channel columns eroded by mode interaction behaviour combined with distortional and local deformations. Initially, the paper presents the results of an experimental investigation of compressive tests on web-stiffened lipped channel columns fabricated from cold-formed mild steel with a thickness of 1.50 mm, which is aimed at determining their failure load-carrying capacity; the experimental investigation also aims to provide experimental evidence of the occurrence of such coupling phenomena concerning distortional and local modes, namely, local-distortional interaction and distortional-local interactive failures. Then, the paper examines the ultimate strength data of experimental columns, both reported in this paper and collected from the literature, and concludes that the current direct strength method (DSM) provides very unsafe predictions concerning such a detrimental interaction nature. Next, two DSM-based design approaches, namely, the nominal strength against local-distortional (NLD) and distortional-local (NDL) procedures, are presented and evaluated on the basis of all available experimental ultimate strength data. Finally, proposals and design considerations based on the DSM-shape for the thin-walled cold-formed steel sections, which fail in mixed modes of distortional and local buckling, are presented.     

高强冷弯薄壁型钢卷边槽形截面轴压构件畸变屈曲控制试验研究

针对屈服强度550MPa高强冷弯薄壁型钢轴压构件提出了一种控制畸变屈曲的构造措施--在卷边间设缀板,并通过试验对其有效性进行了验证。试验结果表明：屈服强度550MPa高强冷弯薄壁型钢卷边槽形截面轴压构件易发生畸变屈曲,通过构造措施能有效地阻止翼缘的转动,试件的畸变屈曲荷载和极限承载力都有很大的提高;随着缀板布置间距的不同,构件承载力的提高幅度也不同;缀板间距越小,试件承载力提高越多,但当缀板间距大于畸变屈曲半波长时,缀板不能起到提高承载力的作用。最后,在此基础上提出了构件不考虑畸变屈曲影响的若干条件,供实际设计参考。     

高强冷弯薄壁型钢卷边槽形截面偏压构件试验研究及承载力分析

进行了48根屈服强度550MPa高强冷弯薄壁型钢卷边槽形截面偏心受压构件试验,考虑了不同截面形式、厚度、长细比和荷载偏心方式的影响,研究了这类偏心受压构件的破坏模式、承载力影响因素以及构件承载力计算方法。结果表明：高强冷弯薄壁型钢偏压构件由于材料强度高,截面宽厚比较大,局部屈曲和畸变屈曲的影响较大,我国规范仅考虑了局部屈曲的影响而没有全面考虑畸变屈曲的影响,这使得部分发生畸变屈曲的试件计算结果偏于不安全,但又对不发生畸变屈曲的长细比较大的构件偏于保守。最后,在试验和现有规范方法比较分析的基础上,提出了一种适用于高强冷弯薄壁型钢偏压构件极限承载力的建议计算方法。该建议方法计算所得结果与试验结果吻合较好,且安全可靠,可供设计参考。     

Recent developments in the buckling and nonlinear analysis of thin-walled structural members

This paper describes recent work performed at the University of Sydney to develop buckling and nonlinear analyses of thin-walled structural members undergoing local, distortional and overall buckling. The analyses are based on the finite strip method of structural analysis and include elastic and inelastic buckling and the full nonlinear response with both post-local buckling and plasticity. Two variations of the finite strip method have been used, these being the semi-analytical and spline finite strip methods. A nonlinear beam-column analysis based on the influence coefficient method for including the local buckling behaviour in the overall member response is also described.

The analytical methods are compared with tests performed at the University of Sydney on cold-formed rectangular hollow sections, welded I-sections, welded channel sections and cold-formed channel sections. Spatial plastic collapse mechanismsdeveloped for the welded sections described above are also compared with the post-ultimate response of the test sections.     

Thin-walled cold-formed sections subjected to compressive loading

The weakening effects of local buckling on the compressive load carrying capability of thin-walled cold-formed sections is examined and discussed. Theoretical results are presented for the pin-ended support condition and a study is made of the local buckling and overall bending interaction behaviour of compression members with different cross-sectional shapes. It is shown that singly symmetric plain channel sections exhibit considerably different interactive equilibrium characteristics to those of I-sections for which the shear centre and section centroid are coincident.

The theoretical results given have been obtained through the solution of a differential equation which governs, approximately, the overall bending behaviour of a locally buckled compression member. Both local and overall imperfections are considered in the theoretical approach and it is shown that the effect of these is to reduce the ultimate compressive carrying capacity of cold-formed sections.

An experimental investigation of the behaviour of concentrically loaded pin-ended I-section struts and columns is reported and the findings from this are compared with theoretical predictions. The theoretical predictions of load-deflection equilibrium behaviour and stress variations with load are shown to be in good agreement with those obtained from test.

The provisions made in the UK Code of Practice and the American Specification pertaining to the design of cold-formed compression members are briefly outlined and compared. It is shown that with regard to I-section struts and columns the American design procedure gives, in general, more conservative estimates of collapse.     

新型冷弯薄壁型钢墙体立柱轴压性能试验研究

提出一种在冷弯薄壁型钢骨架中放置聚苯乙烯泡沫板,并在骨架外侧喷涂轻质保温物料的新型轻质组合墙体。通过对1根无墙板的冷弯薄壁型钢立柱和2根新型冷弯薄壁型钢墙体立柱的轴压性能进行足尺试验,研究其受力过程、承载能力和破坏模式,并将试验结果与按规范公式计算的极限承载力进行对比分析。研究结果表明:无墙板立柱的破坏模式为构件的整体屈曲;新型冷弯薄壁型钢墙体立柱的破坏现象为柱顶截面的局部受压屈服,保温物料对立柱的支承作用使新型墙体立柱的承载能力显著提高。按照规范中关于轴心受压杆件的强度和稳定计算公式得出的极限承载力与试验的破坏荷载较为吻合。     

焊接H型钢局部纵弯失稳下抗弯强度计算

对由厚度为6.0mm、名义屈服强度为315.0MPa的钢板制成的H型钢进行了一系列弯曲试验,研究焊接H型钢的抗弯强度。截面几何形状和侧向边界条件决定了薄壁受弯构件的屈曲形式(局部屈曲、侧向扭转屈曲或交互屈曲)。翼缘或腹板宽厚比较大的受弯构件最先出现局部屈曲,继而发生侧向扭转屈曲,在交互屈曲作用下材料最终破坏。侧向扭转屈曲下局部屈曲对抗弯强度有负面影响。计算薄壁抗弯构件名义屈服应力时应将该现象考虑在内。对翼缘和腹板宽厚比不同的焊接H型钢梁进行了试验。进行有限元分析时将局部和侧向扭转屈曲模态的初始缺陷及残余应力考虑在内。基于考虑焊接型材局部和侧向扭转屈曲相互作用的试验和有限元分析结果,给出直接强度法(DSM)计算抗弯强度的简化公式。计算强度曲线与AISC规范(2005),EC3(2003)及试验结果进行比较,验证了DSM方法所计算的强度曲线的准确性。通过试验得出薄壁焊接H型钢的抗弯强度和结构性能的有关结论。     

冷弯薄壁型钢屋架受力性能及杆件计算长度研究

为研究低层冷弯薄壁型钢结构屋架的受力性能,对两个不同构造形式的屋架足尺试件进行了受弯性能试验研究,考察了冷弯薄壁型钢结构屋架的工作原理和破坏模式。试验结果表明：钢桁架的破坏主要是支座处节点连接的破坏,而杆件没有发生屈曲,不属于强度破坏。采用非线性有限元分析方法对钢结构屋架进行了变参数分析,结果表明：钢材强度、截面形式以及钢材厚度对桁架承载力都有较大的影响。采用柱挠度曲线(CDC)法分析了节点连接半刚性对冷弯薄壁型钢桁架压杆计算长度的影响,研究表明,冷弯薄壁型钢桁架压杆半刚性连接的计算长度与节点转动刚度和构件自身刚度有关,建议受压弦杆、端斜腹杆以及端竖腹杆平面内计算长度系数取为1.0,受压腹杆平面内计算长度系数取为0.9。     

550MPa高强冷弯薄壁卷边槽钢受弯构件畸变屈曲试验研究

为了研究屈服强度550MPa高强冷弯薄壁型钢受弯构件的畸变屈曲性能,分别对直卷边、斜卷边和复杂卷边3种卷边形式的12组高强冷弯薄壁槽钢受弯试件进行了静力试验研究,其中纯弯试验6组,非纯弯试验6组。试验结果表明,卷边形式是影响试件发生畸变屈曲或局部和畸变相关屈曲的重要因素。相同卷边形式下,非纯弯试件的承载力均高于纯弯试件的承载力,且提高幅度与试件屈曲破坏模式有关,只发生畸变屈曲的试件承载力提高幅度最大,而在发生局部和畸变相关屈曲的试件中,由畸变屈曲引起破坏的试件承载力提高幅度次之,由局部屈曲引起破坏的试件承载力提高幅度最小。     

Prediction of the flexural strengths of welded H-sections with local buckling

This paper describes the flexural strength of welded sections based on a series of flexural tests performed on H-sections fabricated from steel plates of thickness 6.0 mm with nominal yield stress of 315.0 MPa. Thin-walled flexural members undergo local, lateral-torsional or their interactive buckling according to the section geometries and lateral boundary conditions. Flexural members with the flanges or the web of large width-to-thickness ratios may undergo local buckling before lateral-torsional buckling and their interaction before the final collapse of the section. The local buckling has a negative effect on the flexural strength based on the lateral-torsional buckling. This phenomenon should be considered in the estimation of the nominal flexural strength of thin-walled flexural members. Welded H-section beams composed of the flanges and the web with various width-to-thickness ratios were tested to failure. The initial imperfections in local and lateral buckling mode, and residual stresses were included in the FE analyses. Simple design flexural strength formulas for the direct strength method (DSM) were proposed based on the test and FE results of welded sections to account for interaction between local and lateral-torsional buckling. The design strength curves were compared with the AISC specifications (2005), Eurocode3 (2003) and test results. The adequacy of the strength curve for the DSM was confirmed. A set of conclusions on the flexural strength and structural behavior of thin-walled welded H-sections was drawn from the experimental studies.     

冷弯薄壁型钢卷边槽形截面构件畸变屈曲控制研究

基于部分加劲板件的畸变屈曲和局部屈曲的稳定系数比较,提出了冷弯薄壁型钢卷边槽形截面构件畸变屈曲发生于局部屈曲之后或畸变屈曲不发生的临界控制条件;给出了通过构件畸变屈曲计算长度控制畸变屈曲的临界条件;提出一种控制畸变屈曲的构造措施,即在卷边间加设缀板,并通过已有试验对其有效性进行验证,同时推导了卷边间缀板的刚度需求。结果表明：通过构件截面尺寸控制畸变屈曲不发生或发生在局部屈曲之后,可以不考虑构件畸变屈曲的影响,简化冷弯薄壁型钢卷边槽形截面构件承载力的计算;计算长度小于畸变屈曲半波长一半的构件不发生畸变屈曲;通过在卷边间加设缀板的构造措施能有效阻止部分加劲板件的转动,构件的畸变屈曲荷载和承载力都有很大的提高,缀板布置间距不同,构件承载力的提高幅度也不同,缀板间距越小,构件承载力提高幅度越大。算例分析表明,满足一定间距和刚度需求的缀板能够提高构件的畸变屈曲承载力或避免畸变屈曲的发生。     

局部封闭和开口薄壁压弯构件的弯扭屈曲

单轴对称开口薄壁压弯构件在荷载作用于对称平面内时有可能发生弯扭屈曲。在这种情况下,其临界荷载总是低于平面内弯曲失稳破坏荷载,如果在构件的开口边加上缀板,使之形成若干断续的封闭截面,则弯扭屈曲临界荷载将显著提高,并有可能使破坏模式由弯扭屈曲转化为平面内弯曲失稳。本文提出了一种计算薄壁压弯构件弯扭屈曲荷载的方法,这种方法对局部封闭和开口截面都能适用。曾经做了213根具有不同长细比、偏心距、缀板间距(或无缀板)的冷弯薄壁型钢压杆试验,其结果与理论符合较好。