Numerical investigation of channel columns with complex stiffeners—part II: parametric study and design

A parametric study of cold-formed steel channel columns with complex stiffeners is performed using finite element analysis. An accurate and reliable finite element model is used for the parametric study in which different sizes of complex stiffeners are investigated. Column strengths predicted by the finite element analysis are compared with the unfactored design column strengths calculated using the American Specification and the Australian/New Zealand Standard for cold-formed steel structures. It is shown that the design strengths obtained from the specification and standard are generally conservative for fixed-ended cold-formed steel channel columns with complex stiffeners for the more slender sections having a plate thickness of 1 mm with the flat flange width to thickness ratio of 57, but unconservative for sections having a plate thickness of 2 mm with the flat flange width to thickness ratio of 27.     

直接强度法计算冷弯薄壁斜卷边槽钢轴压柱的承载力

通过计算得到了适合各种弯起角度的斜卷边槽形截面特性计算公式,方便了手算求解构件的屈曲应力。采用直接强度计算法对斜卷边槽钢轴压固支柱的稳定承载力及破坏模式进行了分析,并与已有的试验资料进行了对比。结果表明:直接强度法可以有效地预测斜卷边槽钢轴压固支构件的稳定承载力,从而说明了建立在简支试验结果基础上,经回归得到的直接强度法同样适用于固支柱稳定承载力的计算。     

腹板开孔复杂卷边槽钢双肢拼合构件轴压性能试验研究

通过12根腹板开孔复杂卷边槽钢和Σ形复杂卷边槽钢双肢拼合工字形简支轴压构件的轴压试验，研究了构件承载能力、失稳模式和拼合作用。采用有限元软件ANSYS对试验进行了模拟，验证了分析模型准确性。并通过有限元变参数分析研究了孔高、孔宽和孔间距对拼合构件承载力的影响。结果表明：复杂卷边槽钢截面双肢拼合开孔构件的腹板多波失稳现象明显，屈曲发生时两腹板间的相互支撑作用较强；Σ形复杂卷边槽钢截面双肢拼合开孔构件能有效地控制腹板局部屈曲的发生并显著提高短柱、中长柱的承载力；设置腹板加劲肋有助于提高孔洞周围板件变形的约束作用，同时也减弱了两单肢腹板间的相互支撑作用；相同条件下，Σ形复杂卷边槽钢截面双肢拼合开孔构件的轴压承载效率与复杂卷边槽钢截面双肢拼合开孔构件相比，短柱提高了32%，中长柱提高了10%，长柱提高了2%；非加劲截面构件在不同长度下，孔高为腹板高度1/2(69 mm)时构件的稳定承载力较孔高为25 mm时下降约7%；而孔宽、孔间距对上述两类截面构件稳定承载力影响不大；此外，采用直接强度法预测非加劲截面双肢拼合构件的承载力结果偏于保守，而对加劲截面双肢拼合构件则略显不安全。     

Research on cold-formed steel columns

The paper summarises research on cold-formed steel columns performed by the author. Cold-formed steel members are either cold-rolled or brake-pressed into structural shapes. As a result, cold-formed steel open sections are usually singly-point- or non-symmetric. The most common types of singly-symmetric sections are channel and angle. The research focused on cold-formed steel open sections, such as plain and lipped channels, channels with simple and complex edge stiffeners as well as plain and lipped angles, and unequal angles. In addition, cold-formed steel built-up closed sections with intermediate stiffeners were investigated. Both experimental and numerical investigations into the strength and behaviour of cold-formed steel columns were conducted. The column strengths obtained from these investigations were compared with the design strengths obtained using various international standards for cold-formed steel structures. Furthermore, the behaviour and design of cold-formed steel lipped channel columns at elevated temperatures were also investigated. The paper also summarises the design recommendations for cold-formed steel columns.     

Design of thin-walled plain channel section columns against flexural buckling

The paper presents a comparison of design strengths of thin-walled plain channel section columns, which have slender flanges and may undergo local buckling, with tests and analytical ultimate loads. The design strengths are obtained using the British, American and Australian specifications for cold-formed steel structure. The comparison is shown for fixed-ended and pin-ended columns.

A simple design procedure is presented for plain channel section columns. By introducing a ‘pin-ended stub column strength’, it is possible to calculate accurately the strength of concentrically loaded pin-ended columns without resorting to a beam-column design approach. It is also possible to account for the different effects of local buckling on the strengths of pin-ended and fixed-ended columns.     

Behaviour of cold-formed singly symmetric columns

An experimental investigation into the behaviour of cold-formed plain and lipped channel columns compressed between fixed and pinned ends is presented in this paper. It is shown experimentally that local buckling does not induce overall bending of fixed-ended singly symmetric columns, as it does of pin-ended singly symmetric columns. Consequently, local buckling has a fundamentally different effect on the behaviour of pin-ended and fixed-ended singly symmetric columns. In order to show this fundamental different effect caused by local buckling, a series of tests was performed on plain and lipped channels brake-pressed from high strength structural steel sheets. Four different cross-section geometries were tested over a range of lengths which involved pure local buckling, distortional buckling as well as overall flexural buckling and flexural-torsional buckling. The different effects of local buckling on the behaviour of fixed-ended and pin-ended channels are investigated by comparing strengths, load–shortening and load–deflection curves, as well as longitudinal profiles of buckling deformations. The purpose of the paper is to demonstrate experimentally the different effects of local buckling on the behaviour and strengths of fixed-ended and pin-ended channels.     

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.     

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。     

用我国规范计算高强度冷弯薄壁型钢柱的适用性

将《冷弯薄壁型钢结构技术规范》(GB 50018-2002),用于计算比Q345钢强度更高的钢材制成的冷弯薄壁非卷边等边角钢和槽钢柱。对计算结果与试验、有限元分析结果及国外规范计算结果进行比较发现,对两端固支的槽钢和宽厚比大于25的等边角钢,计算偏于安全,对宽厚比小于25的等边角钢和两端铰支槽钢的安全性有待研究。提出采用多条柱子曲线以适应不同截面、不同壁厚、不同强度的高强度冷弯薄壁型钢柱。     

Experimental behaviour of stiffened concrete-filled thin-walled hollow steel structural (HSS) stub columns

Test results on concrete-filled steel tubular stub columns with inner or outer welded longitudinal stiffeners under axial compression are presented in this paper. The research was mainly focused on square hollow section (SHS) columns; two rectangular hollow section (RHS) columns were also tested. A longitudinal stiffener was provided on each side of the stiffened SHS column, while only two stiffeners were welded to the longer sides of the stiffened RHS column. The main experimental parameters considered were the height-to-thickness ratio and stiffener rigidity. In addition, empty tubes with or without stiffeners, as well as unstiffened concrete-filled steel tubes were also tested for comparison. Requirements for stiffener rigidity are developed by modifying a formula presented in the literature. Existing theoretical model and design codes were used to predict the load versus axial strain relationships and load-carrying capacities of the adequately stiffened composite sections respectively; reasonable results were achieved.     

Inelastic behaviour and design of cold-formed channel sections in bending

The aim of this paper is to investigate the inelastic bending capacity of cold-formed channel sections, and provide design rules to account for such behaviour. An extensive experimental and analytical analysis of 42 cold-formed channel sections in three different geometric categories is conducted, including simple channel sections, channel sections with simple edge stiffeners and channel sections with complex edge stiffeners. The sections were cold-formed from G450 steel with nominal thickness of 1.6 mm, and varying theoretical buckling stresses ranging between elastic to seven times the yield stress. The results of the pure bending experimental investigations show that current international cold-formed steel specifications are conservative, for channel sections with low slenderness values. The experimental results are used to propose revisions to current international cold-formed steel specifications.     

Experimental investigation on concrete-filled stainless steel stiffened tubular stub columns

This paper presents an experimental investigation on concrete-filled normal-strength stainless steel stiffened tubular stub columns using the austenitic stainless steel grade EN 1.4301 (304). The stiffened stainless steel tubes were fabricated by welding four lipped angles or two lipped channels at the lips. Therefore, the stiffeners were formed at the mid-depth of the sections. In total, five hollow columns and ten concrete-filled columns were tested. The longitudinal stiffener of the column plate was formed to avoid shrinkage of the concrete and to behave as a continuous connector between the concrete core and the stainless steel tube. The behavior of the columns was investigated using two different nominal concrete cubic strengths of 30 and 60 MPa. A series of tests was performed to investigate the effects of cross-section shape and concrete strength on the behavior and strength of concrete-filled stainless steel stiffened tubular stub columns. The measured average overall depth-to-width ratios (aspect ratio) varied from 1.0 to 1.8. The depth-to-plate thickness ratio of the tube sections varied from 60 to 90. Different lengths of columns were selected to fix the length-to-depth ratio to a constant value of 3. The concrete-filled stiffened stainless steel tubular columns were subjected to uniform axial compression over the concrete core and the stainless steel tube to force the entire section to undergo the same deformations by blocking action. The column strengths, load–axial strain relationships and failure modes of the columns are presented. Several comparisons were made to evaluate the test results. The results of the experimental study showed that the design rules, as specified in the European specifications and the ASCE, are highly conservative for square and rectangular cold-formed concrete-filled normal-strength stainless steel stiffened stub columns.     

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.     

Aluminum alloy tubular columns—Part I: Finite element modeling and test verification

A numerical investigation on fixed-ended aluminum alloy tubular columns of square and rectangular hollow sections is described in this paper. The fixed-ended column tests were conducted that included columns with both ends transversely welded to aluminum end plates using the tungsten inert gas welding method, and columns without welding of end plates. The specimens were extruded from aluminum alloy of 6061-T6 and 6063-T5. The failure modes included local buckling, flexural buckling, interaction of local and flexural buckling, as well as failure in the heat-affected zone (HAZ). An accurate finite element model (FEM) was developed. The initial local and overall geometric imperfections were incorporated in the model. The non-welded and welded material nonlinearities were considered in the analysis. The welded columns were modeled having different HAZ extension at the ends of the column of 25 and 30 mm. The nonlinear FEM was verified against experimental results. It is shown that the calibrated model provides accurate predictions of the experimental loads and failure modes of the tested columns. The load-shortening curves predicted by the finite element analysis are also compared with the test results.     

槽钢加劲钢板剪力墙受力性能研究

竖向槽钢加劲钢板剪力墙由内嵌钢板、边缘构件和墙板两侧对称布置的竖向槽钢加劲肋组成。槽钢加劲肋具有较高的抗弯、抗扭刚度,可为墙板提供有效的面外约束和轴向支撑作用。利用有限元分析软件ABAQUS对槽钢加劲钢板墙进行屈曲分析和静力弹塑性分析,研究槽钢距边缘构件距离、钢板宽厚比、高厚比、肋板刚度比和柱刚度对钢板墙及边缘构件力学性能的影响。结果表明,槽钢加劲肋至边缘构件的距离主要影响钢板墙的平面外变形,对钢板墙屈曲应力和承载力的影响较小。槽钢加劲肋可以有效提高墙板的屈曲应力。随着宽厚比和高宽比的减小,结构的承载力和刚度均显著提高。增大肋板刚度比可提高钢板墙的初始刚度和屈曲应力,肋板刚度比应大于20。为避免边框柱变形过大或过早破坏,边框柱应具有足够的刚度,增大柱刚度可以提高钢板墙的初始刚度,减缓墙板刚度的退化。     

加劲薄壁钢管混凝土柱的试验性能

对加劲薄壁钢管混凝土柱的结构性能进行试验研究。通过在钢管内表面焊接纵向加劲肋实现加强的作用。同时对12个不带加劲肋的钢管混凝土柱进行试验,部分柱的填充混凝土中加有钢纤维。试验结果表明,加劲肋可以有效延迟钢管局部屈曲现象的出现。当加劲构件达到最大承载量时钢板才出现屈曲,所以相比普通钢管混凝土,加劲钢管混凝土具有更高的适用性。     

Experimental behaviour of concrete filled thin walled steel tubes with tab stiffeners

This paper presents an experimental investigation into the structural behaviour of concrete filled, thin walled, steel tubular stub column with tab stiffeners. The stiffening was attained by welding together four pieces of lipped angle, whereupon two parts of the lips were notched and folded vertically in order to form the tab stiffeners. The effects of the tab stiffeners on the bond and compressive strengths were investigated experimentally using 18 and 10 specimens respectively. It was observed that the tab stiffener does enhance both the bond strength and the axial load capacity of the concrete filled thin walled steel tubular stub column tested.     

Bifurcation of singly symmetric columns

The paper derives the governing equations for the fundamental and bifurcated states of members with singly symmetric cross-sections that locally buckle in the fundamental state. The members are subject to pure compression and assumed to be geometrically perfect in the overall sense. It is shown using the fundamental state equations that fixed-ended columns exhibit overall bifurcation behaviour while pin-ended columns do not. The bifurcation equation is applied to plain channel sections and the results are compared with tests of fixed-ended columns. The variation of the bifurcation loads with the length is shown to be in good agreement with the tests. The results are shown to be sensitive to the magnitudes of local and overall geometric imperfections.     

带肋方钢管混凝土轴压短柱试验研究及有限元分析

以方钢管宽厚比和加劲肋高厚比为主要变化参数,进行了14个带肋方钢管混凝土轴压短柱试验研究;同时采用有限元软件ABAQUS对带肋方钢管混凝土轴压短柱的荷载-变形关系进行了计算,计算结果与试验结果吻合良好。同时从应力-应变关系、核心混凝土和钢管的纵向应力分布及其相互作用等方面对比分析了无肋、单肋和双肋方钢管混凝土轴压短柱的受力性能。分析结果表明：设置加劲肋不仅提高了核心混凝土的纵向应力,而且明显减小了钢管管壁的拉应力区范围,改善了管壁的稳定性;带肋试件的约束作用主要集中在钢管角部和加劲肋处,随着每边加劲肋数量的增加,角部约束力明显增大。图13表1参11     

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.