Design of cold-formed steel unequal angle compression members

Cold-formed steel unequal angles are non-symmetric sections. The design procedure of non-symmetric sections subjected to axial compression load could be quite difficult. The unequal angle columns may fail by different buckling modes, such as local, flexural and flexural–torsional buckling as well as interaction of these buckling modes. The purpose of this study is to investigate the behaviour and design of cold-formed steel unequal angle columns. A nonlinear finite element analysis was conducted to investigate the strength and behaviour of unequal angle columns. The measured initial local and overall geometric imperfections as well as the material properties of the angle specimens were included in the finite element model. The finite element analysis was performed on fixed-ended columns for different lengths ranged from stub to long columns. It is demonstrated that the finite element model closely predicted the experimental ultimate loads and the behaviour of cold-formed steel unequal angle columns. Hence, the model was used for an extensive parametric study of cross-section geometries. The column strengths obtained from the parametric study were compared with the design strengths calculated using the North American Specification for cold-formed steel structural members. It is shown that the current design rules are generally unconservative for short and intermediate column lengths for the unequal angles. Therefore, design rules of cold-formed steel unequal angle columns are proposed.     

GBT-based buckling analysis of thin-walled members with non-standard support conditions

This paper reports on the use of a recently developed Generalised Beam Theory (GBT) formulation, and corresponding finite element implementation, to analyse the local and global buckling behaviour of thin-walled members with arbitrary loading and support conditions — this formulation takes into account longitudinal normal stress gradients and the ensuing pre-buckling shear stresses. After presenting an overview of the main concepts and procedures involved in the performance of a GBT-based (beam finite element) member buckling analysis, one addresses in detail the incorporation of non-standard support conditions, such as (i) full or partial localised displacement or rotation restraints, (ii) rigid or elastic intermediate supports or (iii) end supports corresponding to angle connections. In order to illustrate the application and capabilities of the proposed GBT-based approach, one presents and discusses numerical results concerning cold-formed steel (i) lipped channel beams and (ii) lipped I-section beams and columns with various “non-standard” support conditions — while the beams are acted by uniformly distributed or mid-span point loads, applied at the shear centre axis, the columns are subjected to uniform compression. In particular, it is possible to assess the influence of the different support conditions on the beam and column buckling behaviour (critical buckling loads and mode shapes). For validation purposes, most GBT-based results are compared with values yielded by shell finite element analyses carried out in the code Ansys.     

Study on the buckling behaviour of cold-formed angles in transmission towers

Considering the structural characteristics of transmission towers, four sections of cold-formed angles with different slenderness ratios and constrained types were selected for the experimental and numerical study. Experiments and finite element analysis for the ultimate loads of the compression cold-formed angles were carried out. The finite element model well predicts the buckling behaviour of the cold-formed members. For the axial compression members, the load-strain curves as well as the ultimate loads were analyzed, and the experimental ultimate loads were compared with those of calculated by the applicable standards. It shows that the applicable standards aren’t adaptive to the strength design of the compression cold-formed members in transmission towers. Through the analysis of the experimental and FEA results, the fitting curve of the stability coefficients for the cold-formed members in transmission towers was determined. The ultimate loads calculated by the fitting curve are well agreed with the experimental values, especially for the members with relatively low slenderness ratios. Some modification suggestions were proposed for the calculation of the ultimate load for the axial compression cold-formed angle. Experimental analysis on the cold-formed members of the other five constrained types was completed, and the modification formulas of the slenderness ratios for members of different constrained types were obtained.     

新型冷弯箱形组合构件力学性能研究

基于箱形截面具有双轴对称、抗弯扭刚度大的特点,提出将冷弯∑形截面两两翼缘相对,并将其卷边焊接形成一种新型冷弯箱形组合截面——DS。应用非线性有限元方法分析新型截面构件在轴压、弯矩及压弯荷载作用时的诸如屈曲模式、承载力、刚度、延性及相关曲线等方面力学性能。同时,还对相同参数的新型截面构件与C形截面构件的单位承载力耗钢量进行比较。分析表明:新型截面构件在承受轴压力时具有高承载力及较高的截面模量,截面加劲充分而使子板件局部屈曲不易出现等力学性能优势。特别适宜承受轴压荷载,同时也适宜承受弯矩和压弯荷载的作用。轴压构件的单位承载力耗钢量一般仅为相同参数C形构件的50%左右,经济效益明显。新型箱形组合构件适于深入试验研究并合理应用于实际工程。     

冷弯厚壁型钢轴压构件设计可靠度分析

在材性试验的基础上对冷弯厚壁型钢进行了轴压承载力试验及有限元分析。试验中发现：对于长细比较大、板件宽厚比较小的试件,破坏模式为整体失稳;对于长细比较小、板件宽厚比较大的方管柱,其破坏过程首先是柱中部产生很小的弯曲变形,随着荷载增大,整体弯曲变形也逐渐增大,并引起方管板件的局部屈曲。采用ANSYS软件进行有限元分析,其结果与试验结果接近,表明将残余应力引入有限元模型对冷弯厚壁型钢构件进行分析具有较高的精度。在此基础上,利用收集到的26组不同牌号、不同截面冷弯厚壁型钢的试验数据建立有限元模型,并引入已有的残余应力模型进行了大量的有限元分析,得到了多组冷弯厚壁型钢轴心受力构件的承载力。对冷弯厚壁型钢构件进行了设计可靠度分析,提出了Q235、Q345冷弯厚壁型钢的抗力分项系数和强度设计值的建议值,可为相关技术标准的修编提供依据。     

Numerical modelling of light gauge cold-formed steel compression members subjected to distortional buckling at elevated temperatures

Fire safety design of building structures has received greater attention in recent times due to continuing loss of properties and lives during fires. However, fire performance of light gauge cold-formed steel structures is not well understood despite its increased usage in buildings. Cold-formed steel compression members are susceptible to various buckling modes such as local and distortional buckling and their ultimate strength behaviour is governed by these buckling modes. Therefore a research project based on experimental and numerical studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. Lipped channel sections with and without additional lips were selected with three thicknesses of 0.6, 0.8, and 0.95 mm and both low and high strength steels (G250 and G550 steels). More than 150 compression tests were undertaken first at ambient and elevated temperatures. Finite element models of the tested compression members were then developed by including the degradation of mechanical properties with increasing temperatures. Comparison of finite element analysis and experimental results showed that the developed finite element models were capable of simulating the distortional buckling and strength behaviour at ambient and elevated temperatures up to 800 °C. The validated model was used to determine the effects of mechanical properties, geometric imperfections and residual stresses on the distortional buckling behaviour and strength of cold-formed steel columns. This paper presents the details of the numerical study and the results. It demonstrated the importance of using accurate mechanical properties at elevated temperatures in order to obtain reliable strength characteristics of cold-formed steel columns under fire conditions.     

Design of cold-formed steel channels with stiffened webs subjected to bending

The objectives of this study are to investigate the structural behaviour and evaluate the appropriateness of the current direct strength method on the design of cold-formed steel stiffened cross-sections subjected to bending. The stiffeners were employed to the web of plain channel and lipped channel sections to improve the flexural strength of cold-formed steel sections that are prone to local buckling and distortional buckling. An experimental investigation of simply supported beams with different stiffened channel sections has been conducted. The moment capacities and observed failure modes at ultimate loads were reported. A nonlinear finite element model was developed and verified against the test results in terms of strengths, failure modes and moment–curvature curves. The calibrated model was then adopted for an extensive parametric study to investigate the moment capacities and buckling modes of cold-formed steel beams with various geometries of stiffened sections. The strengths and failure modes of specimens obtained from experimental and numerical results were compared with design strengths predicted using the direct strength method specified in the North American Specification for cold-formed steel structures. The comparison shows that the design strengths predicted by the current direct strength method (DSM) are conservative for both local buckling and distortional buckling in this study. Hence, the DSM is modified to cover the new stiffened channel sections investigated in this study. A reliability analysis was also performed to assess the current and modified DSM.     

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.     

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.     

Buckling analysis of thin-walled cold-formed steel structural members using complex finite strip method

In this paper, a generalised complex finite strip method is proposed for buckling analysis of thin-walled cold-formed steel structures. The main advantage of this method over the ordinary finite strip method is that it can handle the shear effects due to the use of complex functions. In addition, distortional buckling as well as all other buckling modes of cold-formed steel sections like local and global modes can be investigated by the suggested complex finite strip method. A combination of general loading including bending, compression, shear and transverse compression forces is considered in the analytical model. For validation purposes, the results are compared with those obtained by the Generalized Beam Theory analysis. In order to illustrate the capabilities of complex finite strip method in modelling the buckling behavior of cold-formed steel structures, a number of case studies with different applications are presented. The studies are on both stiffened and unstiffened cold-formed steel members.     

Seismic performance of floor-by-floor assembled steel braced structures with stiffened connections

This paper presents an overview of a full-scale testing on a tension-only concentrically braced beam-through frame (TCBBF). The implementation of TCBBF facilitates the construction of low-rise industrialised residential steel houses by means of floor-by-floor assembling. This type of TCBBF system features cold-formed hollow structural section columns connected to H-section through beams by end plate with bearing-type high-strength bolts. A two-storey, four-span by one-span TCBBF subjected to vertical loads was cyclically loaded horizontally to examine the seismic behaviour. Stable behaviour was observed up to a storey drift angle of 1/10. The cyclic behaviour was characterised by a linear response, a slip range and a significant hardening response. Deteriorating pinched hysteretic behaviour was notable for cyclic loading primarily because of cyclic brace compression buckling and tension yielding. TCBBF incorporates very slender bracing members that are unable to bear much axial load when subjected to compression. Alternating brace compression buckling and tension yielding induce unrecoverable plastic deformation, which results in a sharp decrease in the lateral system stiffness of TCBBF when lateral displacement becomes zero or around zero. Additionally, bracing members and frame members share different proportions of horizontal force although the dual systems bear the lateral forces collaboratively. The variation philosophy of distribution proportion of bracing and frame members is evaluated. Pushover analysis is undertaken to duplicate the test results and develop an analytical model, which is able to predict the elastic stiffness and the strength reasonably.     

高强冷弯薄壁型钢轴压柱畸变屈曲研究

550MPa高强冷弯薄壁型钢卷边槽形截面轴压柱发生畸变屈曲时会出现三种模式（O—O,O-I,I—I）,考虑材料和几何双重非线性的有限元法对三种畸变屈曲模式的构件承载力、变形模式、畸变屈曲应力、横截面应力的分布等相关性能进行比较分析,可看出三种畸变屈曲模式诸性能存在较大不同,同时畸变屈曲与局部屈曲、整体屈曲在相关性能方面也存在不同。采用直接强度法对畸变屈曲轴压柱极限承载力进行计算,计算结果与试验结果较吻合,故在计算畸变屈曲轴压柱极限承载力时建议采用直接强度法．     

高强冷弯型钢骨架墙体立柱轴压性能试验研究及有限元分析

对12根550 MPa高强冷弯型钢骨架墙体立柱进行了足尺轴压试验研究,其中包括6根单面带肋钢板墙体立柱和6根双面板(带肋钢板+石膏板)墙体立柱,在试验研究的基础上建立了墙体立柱有限元分析模型。在有限元分析模型充分得到试验验证的情况下,对影响墙体立柱轴压承载力的钢墙面板、墙体柔性拉条、连接螺钉间距等因素进行参数分析,分析结果可供设计参考。分析结果表明:相同用钢量的平钢板代替带肋钢板对单面钢板墙体立柱轴压承载力有较大的降低作用,但对双面板墙体立柱轴压承载力影响甚微;设置柔性拉条可以提高单面带肋钢板墙体立柱轴压承载力,但对双面板墙体立柱轴压承载力没有影响;连接立柱与墙板的螺钉间距不大于300 mm时,减小螺钉间距并不能提高墙体立柱轴压承载力;墙体周边螺钉间距小于150 mm时,减小墙体周边螺钉间距对墙体立柱轴压承载力提高幅度不大。     

Ultimate strength of bolted moment-connections between cold-formed steel members

The behaviour and design of bolted moment-connections between cold-formed steel members, formed by using brackets bolted to the webs of the section, is considered. The particular problem of the moment-capacity of such joints being lower than that of the cold-formed steel sections being connected because of web buckling, caused by the concentration of load transfer from the bolts, is addressed. In this paper, a combination of laboratory tests and finite element analyses is used to investigate this mode of failure. It is demonstrated that there is good agreement between the measured ultimate moment-capacity and that predicted by using the finite element method. A parametric study conducted using the finite element model shows that the moment-capacity of a practical size joint can be up to 20% lower than that of the cold-formed steel sections being connected. Web buckling so-caused must therefore be considered in the design of such connections.     

Inelastic buckling of channel columns in the distortional mode

Thin-walled channel sections may buckle in a distortional mode when subjected to compression. A variety of cold-formed channel columns were tested recently to study distortional buckling in the inelastic range. This paper describes a comparison of a theoretical method for predicting the inelastic distortional buckling strengths of thin-walled columns with the column tests. The method is based on a spline finite strip buckling analysis which takes into account the non-linear stress-strain behaviour of the steel making up the test sections, and the fix-ended test conditions. The inelastic buckling analysis is found to produce accurate estimates of the test failure loads and failure modes     

Local and distortional buckling of cold-formed steel beams with both edge and intermediate stiffeners in their compression flanges

The true buckling behaviour of cold-formed steel beams with both edge and intermediate stiffeners in their compression flanges has been predicted with the aid of advanced numerical modelling. A series of nonlinear finite element analyses has been carried out to investigate the flexural behaviour of cold-formed Z sections with both edge and intermediate stiffeners in their flanges, when the failure is controlled by local and/or distortional buckling. The effect of the size and position of intermediate stiffeners as well as the effect of the edge stiffener/intermediate stiffener interaction on the buckling behaviour and ultimate strength of these sections has been studied. The knowledge gained from FE analyses was used to check the accuracy of the Eurocode design rules in predicting the ultimate strengths for these sections.     

受压翼缘外带卷边和中间加劲肋的冷弯型钢梁的局部和扭转屈曲特性

采用数值模型分析受压翼缘含卷边和中间加劲肋的冷弯型钢梁的屈曲特性。通过一系列非线性有限元分析,研究了由局部或扭转屈曲导致失稳的两侧翼缘处均含卷边和中间加劲肋的冷弯Z型钢的弯曲性能。研究了中间加劲肋的尺寸和位置以及卷边加劲肋和中间加劲肋的相互作用对型材的屈曲特性和极限强度的影响。有限元分析结果常可用来检验用欧洲设计规范在预测这类断面极限强度时的精确度。     

内配型钢方钢管混凝土构件压弯剪性能研究

为研究内配型钢方钢管混凝土构件压弯剪复合受力工作机理,完成10个内配型钢方钢管混凝土试件的压弯剪试验研究,考虑剪跨比、轴压比和型钢截面形式对构件承载力和破坏模式的影响.运用有限元对试验结果进行模拟,针对典型算例进行受力机理分析,并对影响水平承载力的不同因素进行参数分析和评估.结果表明:内配型钢方钢管混凝土压弯剪试件表现...     

冷弯薄壁不锈钢受压构件变形屈曲的分析

介绍了目前国外有关冷弯薄壁不锈钢设计中受压构件强度的计算，重点阐述了变形屈曲的计算方法。对冷弯薄壁不锈钢槽钢和工字钢受压构件的变形屈曲进行分析，并做了部分试验，用于理论分析方法、有限元法和实际值之间的比较。     

Analysis and design of cold-formed steel channels subjected to combined bending and web crippling

The channel failures due to combined bending and web crippling may occur at the highly concentrated interior loading when there is no load stiffener in cold-formed thin-walled steel beams. This paper presents accurate finite element models to predict the behavior and ultimate strengths of cold-formed steel channels subjected to pure bending as well as combined bending and web crippling. Both geometric and material nonlinearities are considered in the finite element analysis. The nonlinear finite element models are verified against experimental results of cold-formed steel channels subjected to pure bending as well as combined bending and web crippling. The finite element analytical results show a good agreement with the experimental results in terms of the ultimate loads and moments, failure modes and web load-deformation curves thus validating the accuracy of the finite element models. The verified finite element models are then used for an extensive parametric study of different channel dimensions. The channel strengths predicted from the parametric study are compared with the design strengths calculated from the North American Specification for cold-formed steel structures. It is shown that the design rules in the North American Specification are generally conservative for channel sections with unstiffened flanges having the web slenderness ranged from 7.8 to 108.5 subjected to combined bending and web crippling. It is demonstrated that the nonlinear finite element analysis by using the verified finite element models against test results is an effective way to predict the ultimate strengths of cold-formed thin-walled steel members.