恒定压力作用下工字型和箱型截面压杆的局部屈曲效应、材料轴向刚度屈服和失效

局部屈曲显著影响薄壁受压构件的性能。局部屈曲将会引起薄壁构件失去轴向受压刚度,并导致受压承载力显著降低。用有限元模拟薄壁工字型和箱型截面压杆的屈曲后效应,并考虑了构件几何初始缺陷和材料非线性影响,给出了局部屈曲后应力发展和应力重分布、荷载作用下开始屈服和屈服发展的详细计算。详细介绍了构件截面平面内板件交点的平面内位移边界条件的影响和基于屈曲后刚度的屈服和破坏。结果显示,几何初始缺陷在屈曲和屈服几乎同时发生的杆件和设计最终破坏和卸载的杆件中的影响非常突出。一般地,几何初始缺陷影响与截面板件交点沿压杆长度的屈服发展和沿截面板件的中部发展有同等重要的地位。     

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.     

Interaction of global and local buckling in welded RHS compression members

This paper presents the results of a research project analysing some aspects of the stability failure of thin-walled compression members that are likely to exhibit combined global and local buckling. In particular, the influence of various imperfections, namely residual stresses due to welding and local as well as global geometrical defects, on the load-carrying capacity was subjected to detailed experimental and numerical investigations. The insights obtained into the structural behaviour led to a proposal for modification of the non-dimensional slenderness used in the global buckling verification, and to the recommendation of a shift from the currently applied European buckling curve to a more favourable one in the case of non-stiffened members with welded box sections subjected to pure compression.     

A finite strip method for the elastic—plastic large dlisplacement analysis of thin-walled and cold-formed steel sections

A large deflection elastic—plastic analysis has been developed using the finite strip method of structural analysis to determine the non-linear local buckling behaviour of thin-walled and cold-formed sections in compression. The analysis accounts for plate geometric imperfections, the variation of yield stress around a section, the stress—strain characteristics of the material forming the section and complex patterns of residual stress produced by the cold-forming process.

The analysis is verified against reliable solutions for the non-linear buckling behaviour of plates and plate assemblies in axial compression and the non-linear overall buckling behaviour of a strut. The analysis is further compared with the results of plates with a rounded stress—strain curve typical of cold-formed steel and aluminium.     

Der Einfluss von Imperfektionen in dünnwandigen geschweißten Rechteckquerschnitten unter Druckbeanspruchung

On imperfections in thin‐walled welded rectangular hollow section compressive members. This contribution presents the results of a research project analysing some aspects of stability failure of thin‐walled RHS compression members that are likely to exhibit combined global and local buckling. In particular the influence of various imperfections, namely residual stresses due to weld‐ing and local as well as global geometrical defects, on the load‐carrying capacity is subject to detailed experimental and numerical investigations. The insight into the structural behaviour leads to recommendations for standardized imperfections that should be used in the context of the numerical modelling of class‐4 square and rectangular hollow section members subjected to compression.     

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.     

Strength and ductility of high strength concrete-filled steel tubular beam-columns

The ultimate strength and ductility of high strength thin-walled concrete-filled steel tubular (CFST) beam-columns with local buckling effects, are investigated in this paper, using a performance-based analysis (PBA) technique. The PBA technique accounts for the effects of geometric imperfections, residual stresses, strain hardening, local buckling and concrete confinement on the behavior of high strength thin-walled CFST beam-columns. The accuracy of the PBA technique is further examined by comparisons with experimental results. The PBA program is employed to study the effects of depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility of high strength thin-walled CFST beam-columns under combined axial load and biaxial bending. The results obtained indicate that increasing the depth-to-thickness ratio and axial load levels significantly reduces the stiffness, strength and ductility of CFST beam-columns. Increasing concrete compressive strengths increases the stiffness and strength, but reduces the axial ductility and section performance of CFST beam-columns. Moreover, the steel yield strength has a significant effect on the section and strength performance of CFST beam-columns but does not have a significant effect on their axial and curvature ductility.     

The flexural behaviour of fixed-ended channel section columns

The paper presents a study of the behaviour of thin-walled channel sections compressed between fixed ends. The paper emphasises the differences between the behaviour of fixed-ended and pin-ended channel sections, arising mainly from the fact that the local buckling induces bending of pin-ended channel sections but not of fixed-ended channel sections.As a result of the different effects of local buckling, the strength of a fixed-ended channel section column exceeds that of a pin-ended column of the same effective length. The increase in strength is quantified for a particular channel section, and is studied through the transition from pinned to fixed-end support by analysing columns which are elastically restrained against rotations at the ends.The paper also investigates the imperfection sensitivity and post-ultimate behaviour of fixed-ended channel section columns. It is shown that fixed-ended columns are less sensitive to local imperfections and that they exhibit greater post-ultimate ductility than pin-ended columns.The study is confined to columns whose overall buckling mode is the flexural mode.     

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。     

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.     

剖分T型钢压杆的屈曲性能和应用

剖分T型钢由热轧H型钢一分为二得来,属单轴对称截面。在承受轴线压力时,它的屈曲性能有两个特点。其一是绕对称轴屈曲时必然伴随有扭转,从而降低其承载力。另一方面,当压力移至截面剪心时,屈曲不再出现扭转,承载力达到弯曲屈曲荷载。杆件承载力的这种提高,只有在绕另一主轴屈曲有多余承载力,足以承担压力移至剪心产生的偏心力矩时才能加以利用。本文分析表明,截面高宽比不小于1．25的剖分T型钢满足或部分满足这一条件。有时,荷载作用在形心与剪心距离的一半处较为有利。第二个特点表现在压杆腹板局部屈曲承载力和宽厚比限值。本文揭示出腹板的临界应力总是高于杆件弯扭屈曲的临界应力,因此,只要杆件通过弯扭屈曲验算,就不必顾虑腹板屈曲。然而,在工程实践中有些杆件扭转受到约束,从而不去验算其弯扭屈曲。此时,腹板有可能作为一纵边嵌同、另一纵边自由的板件屈曲。建议的腹板宽厚比限值就是为保证在这种情况下的稳定性而给出的。     

Some recent research on thin-walled members at the University of Strathclyde

Details of some recent research projects on thin-walled structural members carried out at the University of Strathclyde are given in this paper. The projects are, in the main, short duration programmes carried out by students to examine various aspects of the behaviour of thin-walled members under various loadings. The topics consist of the effects of end fixity on plain channel column behaviour, the effects of transverse impact on struts and the damaged strut capacity and the large deflection behaviour of slender rings under diametrically opposed point loads.     

Plastic mechanisms database for thin-walled cold-formed steel members in compression and bending

Short members of thin-walled cold-formed (TWCF) steel sections, in compression and bending, fail by forming local plastic mechanisms. Taking into account the localised buckling pattern, the collapse of slender members, due to the interaction between local and overall buckling modes, is always characterised by local plastic mechanism failure mode. Based on these two observations, the ultimate strength in interactive buckling of these members can be regarded as an interaction between localised plastic mode and overall elastic one.The yield line mechanism method has been widely used to predict the sectional strength (e.g. local) of thin-walled cold-formed steel members that involve failure mode characterized by local collapse mechanisms. This method can be also used to study post-collapse behaviour and to evaluate the load-carrying capacity, ductility and energy absorption.This paper is based on previous studies and some latest investigations of authors, as well as the literature collected data. It represents an attempt to make an inventory, classify and range geometrical and analytical models for the local-plastic mechanisms aiming to characterize the ultimate capacity of some of the most used cold-formed steel sections in structural applications.     

Cellular buckling in I-section struts

An analytical model that describes the interactive buckling of a thin-walled I-section strut under pure compression based on variational principles is presented. A formulation combining the Rayleigh–Ritz method and continuous displacement functions is used to derive a system of differential and integral equilibrium equations for the structural component. Numerical continuation reveals progressive cellular buckling (or snaking) arising from the nonlinear interaction between the weakly stable global buckling mode and the strongly stable local buckling mode. The resulting behaviour is highly unstable and when the model is extended to include geometric imperfections it compares excellently with some recently published experiments.     

椭圆环形截面钢柱的设计

对椭圆环形截面钢柱进行数值模拟和设计。建立准确的有限元模型,模拟两端固接的椭圆环形钢柱。对拉伸试验得出的材料非线性及初始局部(整体)几何缺陷都进行了考虑,通过收敛性研究,以获得最佳的单元网格尺寸。采用此数值模型,对100根柱试件进行参数化研究。对材料屈服、局部屈曲、弯曲屈曲及局部屈曲与弯曲屈曲同时发生的破坏模型进行了分析。将柱的承载力试验值和数值分析结果与基于北美规范、澳大利亚规范、新西兰规范和欧洲规范的计算值进行对比。另外,还采用了不适用于椭圆环形截面钢杆的直接强度法进行分析。对这些设计方法均进行了可靠度分析。     

Design of cold-formed steel oval hollow section columns

This paper presents the numerical simulation and design of cold-formed steel oval hollow section columns. An accurate finite element model was developed to simulate the fixed-ended column tests of oval hollow sections. The material non-linearities obtained from tensile coupon tests as well as the initial local and overall geometric imperfections were incorporated in the finite element model. Convergence study was performed to obtain the optimized mesh size. A parametric study consisted of 100 columns was conducted using the verified numerical model. The failure modes of material yielding, local buckling and flexural buckling as well as interaction of local and flexural buckling were found in this study. The experimental column strengths and numerical results predicted by the parametric study were compared with the design strengths calculated using the current North American, Australian/New Zealand and European specifications for cold-formed steel structures. In addition, the direct strength method, which was developed for cold-formed steel members for certain cross-sections but not cover oval hollow sections, was used in this study. The reliability of these design rules was evaluated using reliability analysis.     

Nonlinear analysis of concrete-filled thin-walled steel box columns with local buckling effects

Local buckling of steel plates reduces the ultimate loads of concrete-filled thin-walled steel box columns under axial compression. The effects of local buckling have not been considered in advanced analysis methods that lead to the overestimates of the ultimate loads of composite columns and frames. This paper presents a nonlinear fiber element analysis method for predicting the ultimate strengths and behavior of short concrete-filled thin-walled steel box columns with local buckling effects. The fiber element method considers nonlinear constitutive models for confined concrete and structural steel. Effective width formulas for steel plates with geometric imperfections and residual stresses are incorporated in the fiber element analysis program to account for local buckling effects. The progressive local and post-local buckling is simulated by gradually redistributing the normal stresses within the steel plates. Two performance indices are proposed for evaluating the section and ductility performance of concrete-filled steel box columns. The computational technique developed is used to investigate the effects of the width-to-thickness ratios and concrete compressive strengths on the ultimate strength and ductility of concrete-filled steel box columns. It is demonstrated that the nonlinear fiber element method developed predicts well the ultimate loads and behavior of concrete-filled thin-walled steel box columns and can be implemented in advanced analysis programs for the nonlinear analysis of composite frames.     

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。     

拉链柱支撑钢框架结构静力推覆试验及数值模拟

为研究拉链柱支撑钢框架结构在水平荷载作用下的承载能力、抗侧刚度、破坏模式以及构件的传力模式等,对一个3层单跨1:2.6缩尺比例的拉链柱支撑钢框架进行了静力推覆试验及数值模拟。结果表明：在对应8度多遇及设防烈度地震作用工况的水平侧移下,结构基本处于弹性工作状态,构件未出现明显失稳现象。在对应8度罕遇地震作用工况的水平侧移下,由于支撑失稳导致结构抗侧刚度明显降低,但结构的承载力并无明显下降,随着加载的进一步进行,因支撑失稳产生的竖向不平衡力主要由拉链柱承担,且该不平衡力由此逐层向上传递,导致顶层支撑的轴压力逐渐增加;当反向卸载时,拉链柱构件承担部分压力,但并未失稳。总体上,结构各构件的受力状态及传力过程与设计目标基本一致,结构在静力推覆试验中表现出良好的受力性能。     

Numerical modelling and design of LiteSteel Beams subject to lateral buckling

The LiteSteel Beam (LSB) is a new hollow flange channel section developed using a patented dual electric resistance welding and cold-forming process. It has a unique geometry consisting of torsionally rigid rectangular hollow flanges and a slender web, and is commonly used as flexural members. However, the LSB flexural members are subjected to a relatively new lateral distortional buckling mode, which reduces their moment capacities. Unlike lateral torsional buckling, the lateral distortional buckling of LSBs is characterised by simultaneous lateral deflection, twist and cross sectional change due to web distortion. Therefore a detailed investigation into the lateral buckling behaviour of LSB flexural members was undertaken using experiments and finite element analyses. This paper presents the details of suitable finite element models developed to simulate the behaviour and capacity of LSB flexural members subject to lateral buckling. The models included all significant effects that influence the ultimate moment capacities of such members, including material inelasticity, lateral distortional buckling deformations, web distortion, residual stresses, and geometric imperfections. Comparison of elastic buckling and ultimate moment capacity results with predictions from other numerical analyses and available buckling moment equations, and experimental results showed that the developed finite element models accurately predict the behaviour and moment capacities of LSBs. The validated model was then used in a detailed parametric study that produced accurate moment capacity data for all the LSB sections and improved design rules for LSB flexural members subject to lateral distortional buckling.