Section moment capacity tests of LiteSteel beams

The LiteSteel beam (LSB) is a new cold-formed hollow flange channel section developed by OneSteel Australian Tube Mills using their patented dual electric resistance welding and automated continuous roll-forming process. It has a unique geometry consisting of torsionally rigid rectangular hollow flanges and a relatively slender web. In addition to this unique geometry, the LSB sections also have unique characteristics relating to their stress–strain curves, residual stresses, initial geometric imperfections and hollow flanges that are not encountered in conventional hot-rolled and cold-formed steel channel sections. An experimental study including 20 section moment capacity tests was therefore conducted to investigate the behaviour and strength of LSB flexural members. The presence of inelastic reserve bending capacity in these beams was investigated in detail although the current design rules generally limit the section moment capacities of cold-formed steel members to their first yield moments. The ultimate moment capacities from the tests were compared with the section moment capacities predicted by the current cold-formed and hot-rolled steel design standards. It was found that compact and non-compact LSB sections have greater moment capacities than their first yield moments. The current cold-formed steel design standards were found to be conservative in predicting the section moment capacities of compact and non-compact LSB sections, while the hot-rolled steel design standards were able to better predict them. This paper has shown that suitable modifications are needed to the current design rules to allow the inclusion of available inelastic bending capacities of LSBs in design.     

冷成型双相不锈钢截面材料特性

给出了6个不同截面冷成型双相不锈钢的特性,其中2个为圆形中空截面,4个为矩形中空截面。试样为冷轧双相不锈钢带。确定方形和矩形中空截面高强度冷成型双相不锈钢的材料特性。对每种型材的薄弱和转角处进行拉伸试验,由此测量每种型材的弹性模量、0.2%弹性极限、1.0%弹性极限、抗张强度、断裂延伸率和Ramberg-Osgood参数(n)。通过短柱试验获得冷轧状态全截面的材料特性。测量6种型材的初始局部几何缺陷,绘制每种型材含初始几何缺陷的横截面图。采用断面法测量150×50×2.5截面的残余应力,测量并绘制截面上薄膜屈曲残余应力分布图。此外,给出适用于短柱的有限元模型,并与试验结果进行对比。将不锈钢短柱的试验强度与美国规范、澳大利亚/新西兰规范和欧洲规范的设计强度进行对比。总体看来,三种规范的计算结果都较为保守,其中欧洲规范的计算结果最为保守。     

Strength and stability criteria for thin-walled stainless steel circular hollow section members under bending

A series of tests consisting of various cross-section geometries were performed on structural stainless steel circular hollow sections (CHS) subjected to bending. The test program comprised a total of eight tests on CHS in two grades of stainless steel, namely 304 and Duplex 2205. For each grade four sections, each with a different slenderness, were tested, in order to cover a range of structural responses. Measurements of overall geometric imperfections and material properties were conducted. The test strengths are compared with the strengths predicted using the American, Australian and European specifications for cold-formed stainless steel structures. In the light of the test results and code recommendations, strength and cross-section classification criteria for stainless steel circular hollow section members in bending are examined.     

Tests of cold-formed high strength stainless steel compression members

The paper describes a test program on cold-formed high strength stainless steel compression members. The duplex stainless steel having the yield stress and tensile strength up to 750 and 850 MPa, respectively, was investigated. The material properties of the test specimens were obtained from tensile coupon and stub column tests. The test specimens were cold-rolled into square and rectangular hollow sections. The specimens were compressed between fixed ends at different column lengths. The initial overall geometric imperfections of the column specimens were measured. The strength and behaviour of cold-formed high strength stainless steel columns were investigated. The test strengths were compared with the design strengths predicted using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures. Generally, it is shown that the design strengths predicted by the three specifications are conservative for the cold-formed high strength stainless steel columns. In addition, reliability analysis was performed to evaluate the current design rules.     

Residual stresses in cold-rolled stainless steel hollow sections

Stainless steel exhibits a pronounced response to cold-work and heat input. As a result, the behaviour of structural stainless steel sections, as influenced by strength, ductility and residual stress presence, is sensitive to the precise means by which the sections are produced. This paper explores the presence and influence of residual stresses in cold-rolled stainless steel box sections using experimental and numerical techniques. In previous studies, residual stress magnitudes have been inferred from surface strain measurements and an assumed through-thickness stress distribution. In the present study, through-thickness residual stresses in cold-rolled stainless steel box sections have been measured directly by means of X-ray diffraction and their effect on structural behaviour has been carefully assessed through detailed non-linear numerical modelling. Geometric imperfections, flat and corner material properties and the average compressive response of stainless steel box sections were also examined experimentally and the results have been fully reported. From the X-ray diffraction measurements, it was concluded that the influence of through-thickness (bending) residual stresses in cold-rolled stainless steel box sections could be effectively represented by a rectangular stress block distribution. The developed ABAQUS numerical models included features such as non-linear material stress-strain characteristics, initial geometric imperfections, residual stresses (membrane and bending) and enhanced strength corner properties. The residual stresses, together with the corresponding plastic strains, were included in the FE models by means of the SIGINI and HARDINI Fortran subroutines. Of the two residual stress components, the bending residual stresses were found to be larger in magnitude and of greater (often positive) influence on the structural behaviour of thin-walled cold-formed stainless steel sections.     

厚壁冷弯方矩管和热轧H型钢轴压极限承载力的对比研究

运用ANSYS有限元软件,在综合考虑初始几何缺陷、残余应力、冷弯效应等因素的基础上,分析了厚壁冷弯方、矩钢管和热轧H型钢轴心压杆的极限承载力,并将它们的极限承载力计算结果进行对比,得出相关结论,为轻钢结构框架柱构件截面选型提供了参考.     

Structural performance of cold-formed high strength stainless steel columns

This paper describes an accurate finite element model for the structural performance of cold-formed high strength stainless steel columns. The finite element analysis was conducted on duplex stainless steel columns having square and rectangular hollow sections. The columns were compressed between fixed ends at different column lengths. The effects of initial local and overall geometric imperfections have been taken into consideration in the finite element model. The material nonlinearity of the flat and corner portions of the high strength stainless steel sections were carefully incorporated in the model. The column strengths and failure modes as well as the load-shortening curves of the columns were obtained using the finite element model. Furthermore, the effect of residual stresses in the columns was studied. The nonlinear finite element model was verified against experimental results. An extensive parametric study was carried out using the verified finite element model to study the effects of cross-section geometries on the strength and behaviour of cold-formed high strength stainless steel columns. The column strengths predicted from the parametric study were compared with the design strengths calculated using the American Specification, Australian/New Zealand Standard and European Code for cold-formed stainless steel structures. The results of the parametric study showed that the design rules specified in the American, Australian/New Zealand and European specifications are generally conservative for cold-formed high strength stainless steel square and rectangular hollow section columns, but unconservative for some of the short columns.     

Experimental and numerical investigations of cold-formed stainless steel tubular sections subjected to concentrated bearing load

Experimental and numerical investigations of cold-formed stainless steel square and rectangular hollow sections subjected to concentrated bearing load are presented in this paper. A total of 124 data are presented that include 64 test results and 60 numerical results. The tests were performed on austenitic stainless steel type 304, high strength austenitic and duplex material. The measured web slenderness value of the tubular sections ranged from comparatively stocky webs of 6.2 to relatively more slender webs of 61.4. The tests were carried out under end and interior loading conditions. A non-linear finite element model is developed and verified against experimental results. Geometric and material non-linearities were included in the finite element model. The material nonlinearity of the flat and corner portions of the specimen sections were carefully incorporated in the model. It was shown that the finite element model closely predicted the web crippling strengths and failure modes of the tested specimens. Hence, the model was used for an extensive parametric study of cross-section geometries, and the web slenderness value ranged from 52.0 to 206.7. The test results and the web crippling strengths predicted from the finite element analysis were compared with the design strengths obtained using the American, Australian/New Zealand and European specifications for stainless steel structures. A unified web crippling equation with new coefficients for cold-formed stainless steel square and rectangular hollow sections subjected to concentrated bearing load is proposed. It is demonstrated that the proposed web crippling equation is safe and reliable using reliability analysis.     

带加劲肋的复杂截面薄壁柱的试验研究

进行了带加劲肋的复杂截面管柱的一系列试验研究。测量了试件的初始几何缺陷和材料特性。加劲肋能有效增强薄壁构件的局部稳定性。将试验强度与采用北美、澳大利亚、新西兰规范中针对冷弯型钢结构的直接强度法计算的设计强度进行对比,结果表明,采用有线条法确定屈曲荷载的直接强度法非常保守。因此,采用有限元法确定弹性屈曲荷载,结果表明,采用此直接强度法计算的设计强度与试验结果吻合较好。     

Stub column tests of thin-walled complex section with intermediate stiffeners

A series of stub column tests on complex sections with intermediate stiffeners is presented in this paper. Initial geometric imperfections and material properties of the test specimens were measured. It is shown that the intermediate stiffeners could effectively enhance the local buckling stress of thin-walled sections. The test strengths are compared with the design strengths calculated using the direct strength method in the North American Specification and Australian/New Zealand Standard for cold-formed steel structures. It is shown that the direct strength method using finite strip method to obtain the buckling stresses is very conservative. Therefore, finite element method was used to predict the elastic buckling stresses. It is shown that the design strengths calculated using direct strength method based on the buckling stresses obtained from finite element analysis results generally agree with the test results well.     

A theoretical investigation of the column behaviour of cold-formed square hollow sections

An experimental programme investigating the column behaviour of four sizes of square hollow section was undertaken at the University of Sydney using Australian produced cold-formed square hollow sections. Stub and pinended column tests were performed and detailed measurements of the yield stress and residual stress taken around the sections.

A large deflection elastic—plastic finite strip analysis including the measured distributions of yield stress and residual stress is used to investigate the behaviour of the stub and pin-ended columns. In particular, the influence of the measured through thickness residual stress components on the ultimate load and behaviour of the square hollow section columns is demonstrated. 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 the highly complex patterns of residual stress produced by the cold-forming process. Comparison of the analytical results with the test results is provided.     

考虑初始缺陷的冷弯矩型管轴压极限承载力研究

运用ANSYS通用有限元软件包,综合考虑初始几何缺陷、残余应力、冷弯效应等因素,分析了壁厚大于6mm的冷弯矩型管轴心压杆的极限承载力,得出其稳定系数,并与《钢结构设计规范》、《冷弯薄壁型钢结构技术规程》的计算结果进行对比,提出了冷弯中厚壁矩型管轴心压杆稳定系数的初步确定方法。     

Cold-formed high strength stainless steel cross-sections in compression considering interaction effects of constituent plate elements

This paper presents a numerical investigation of cold-formed high strength stainless steel square and rectangular hollow sections in compression. A non-linear finite element model which includes geometric and material non-linearities was developed and verified against experimental results. The model was then used for an extensive parametric study to investigate the interaction effects of constituent plate elements on Class 3 slenderness limit and section capacities of cold-formed high strength stainless steel square and rectangular hollow sections in compression.The numerical investigation shows that the interaction effects of constituent plate elements on cross-section response are quite obvious particularly for slender sections. The design provisions on Class 3 slenderness limit and effective width equations specified in the EC3 Code and proposed by Gardner and Theofanous are suitable for square hollow sections, but not for rectangular hollow sections since they do not take into consideration of interaction effects of constituent plate element. Hence, the new Class 3 slenderness limit and the section capacity design equations based on the whole cross-section response are proposed in this study, which carefully consider the interaction effects of constituent plate elements.     

Tests on concrete filled double-skin (CHS outer and SHS inner) composite short columns under axial compression

This paper presents stub columns tests on concrete-filled double skin sandwich tubes (CFDT) constructed using cold-formed steel tubes. The annulus is filled with micro high-strength concrete having compressive cylinder strength of 64 MPa. The outer skin is made of circular hollow sections (CHS), while the inner skin is made of square hollow sections (SHS). Eight different section sizes were used for the outer skin with diameter-to-thickness ratio ranging from 19 to 55. Three section sizes were chosen for the inner skin with width-to-thickness ratio in the range of 20 to 26. The CFDT construction was found to have significant increase in strength, ductility and energy absorption over the outer jacket. A simplified formula is derived to determine the compressive capacity of CFDT and compared against the current design rules. The proposed formula was found in good agreement with experimental results. This paper also verifies the yield slenderness limit (λ_ey) of 82 specified in AS 4100 for cold-formed CHS stub columns.     

Column curves for elliptical hollow section members

A thermo-mechanical finite element analysis model is developed to predict residual stress patterns in hot-rolled sections. The model is first verified against experimental measurements for residual stresses reported for I-sections reported in the literature. The method is then used to predict residual stresses in elliptical hollow sections. A sensitivity analysis is then conducted to assess the influence of various input parameters of the model on the predicted residual stress patterns. The effects of cross-section geometric parameters on the residual stress distribution are then investigated.A series of column curves is generated providing the compressive capacity of a column as a function of slenderness. The column curves are generated based on a) elasto-plastic geometrically nonlinear analyses, b) including the effect of residual stresses as predicted from thermo-mechanical analyses and c) incorporating initial geometric out-of straightness according to the fundamental buckling mode as predicted from an elastic buckling eigen value analysis. Generated column curves are then compared to those in current design codes. A best fit for the numeric results obtained is conducted to cast them in a format similar to that in the current codes.     

Experimental study of web crippling behaviour of hollow flange channel beams under two flange load cases

This paper presents the details of an experimental study of a cold-formed steel hollow flange channel beam known as LiteSteel beam (LSB) subject to web crippling under End Two Flange (ETF) and Interior Two Flange (ITF) load cases. The LSB sections with two rectangular hollow flanges are made using a simultaneous cold-forming and electric resistance welding process. Due to the geometry of the LSB, and its unique residual stress characteristics and initial geometric imperfections, much of the existing research for common cold-formed steel sections is not directly applicable to LSB. Experimental and numerical studies have been carried out to evaluate the behaviour and design of LSBs subject to pure bending, predominant shear and combined actions. To date, however, no investigation has been conducted on the web crippling behaviour and strength of LSB sections. Hence an experimental study was conducted to investigate the web crippling behaviour and capacities of LSBs. Twenty-eight web crippling tests were conducted under ETF and ITF load cases, and the ultimate web crippling capacities were compared with the predictions from the design equations in AS/NZS 4600 and AISI S100. This comparison showed that AS/NZS 4600 and AISI S100 web crippling design equations are unconservative for LSB sections under ETF and ITF load cases. Hence new equations were proposed to determine the web crippling capacities of LSBs based on experimental results. Suitable design rules were also developed under the direct strength method (DSM) format.     

Flexural behaviour of stainless steel oval hollow sections

Structural hollow sections are predominantly square, rectangular or circular in profile. While square and circular hollow sections are often the most effective in resisting axial loads, rectangular hollow sections, with greater stiffness about one principal axis than the other, are generally more suitable in bending. Oval or elliptical hollow sections (EHS) combine the aesthetic external profile of circular hollow sections with the suitability for resisting flexure of rectangular sections, whilst also retaining the inherent torsional stiffness offered by all tubular sections. This paper examines the structural response of recently introduced stainless steel oval hollow sections (OHS) in bending and presents design recommendations. In-plane bending tests in the three-point configuration about both the major and minor axes were conducted. All tested specimens were cold-formed from Grade 1.4401 stainless steel and had an aspect ratio of approximately 1.5. The full moment-rotation responses of the specimens were recorded and have been presented herein. The tests were replicated numerically by means of non-linear finite element (FE) analysis and parametric studies were performed to investigate the influence of key parameters, such as the aspect ratio and the cross-section slenderness, on the flexural response. Based on both the experimental and numerical results, structural design recommendations for stainless steel OHS in bending in accordance with Eurocode 3: Part 1.4 have been made.     

Lateral buckling strengths of cold-formed rectangular hollow sections

Code rules for designing steel beams against lateral buckling which are based on data for hot-rolled I-sections are unnecessarily conservative when used for cold formed rectangular hollow section beams.Cold-formed rectangular hollow section beams have different stress-strain curves, residual stresses, and crookedness and twist. The effects of residual stress on the inelastic buckling of I-section beams are not nearly as pronounced for hollow sections with two webs, while the strengthening effects of pre-buckling deflections are greater for hollow sections. Simplistic code rules for top flange loading are very conservative when applied to hollow sections.This paper reviews elastic lateral buckling behaviour and the strength rules used to design steel beams. It develops realistic models for cold-formed rectangular hollow beams which are analysed to predict the effects of moment distribution, load height and yield stress on their strengths. The results of the analyses are used to develop improved design rules which remove much of the conservatism of present design rules.     

Bending strength of hot-rolled elliptical hollow sections

The recent emergence of hot-rolled elliptical hollow sections (EHS) within the construction industry has attracted considerable interest from structural engineers and architects. Comprehensive structural design rules are now required to facilitate their wider application. This paper focuses on the bending strength of hot-rolled elliptical hollow sections; the results of detailed experimental and numerical studies are presented and structural design rules for EHS in bending about the major and minor axes are proposed. A total of 18 in-plane bending tests in three-point and four-point configurations have been performed. All tested specimens had an aspect ratio of two. Full moment-rotation and moment-curvature histories were derived, including into the post-ultimate range. The experimental results were replicated by means of non-linear numerical modelling. Following careful validation of the models, parametric studies were performed to assess the structural response of EHS over a wider range of aspect ratios (between one (CHS) and three) and cross-section slendernesses. For design, cross-section slenderness parameters have been proposed and a set of classification limits in harmony with those given in Eurocode 3 for circular hollow sections (CHS) has been derived. A new Class 3 limit has also been proposed for both EHS and CHS. An interim effective section modulus formula for Class 4 (slender) elliptical hollow sections based on BS 5950-1 has also been developed. Further investigation into effective section modulus formulations is currently underway.     

Experimental investigation for the behaviour of battened beam-columns composed of four equal slender angles

This study presents series of compression tests on battened columns that are composed of four equal slender angles. The angles are formed by bending thin steel sheets, such that the legs outstand width–thickness ratio is slender. Twenty specimens varied in their plate element width–thickness ratio as well as covered short and medium member slenderness were tested. The angles were assembled by batten plates by means of bolts. Measurements of residual stresses and geometrical imperfections were carried out. Moreover the specimens were simulated by a finite element model using shell element that accounts for both geometric and material non-linearities. The measured geometric imperfections and residual stresses were included in the numerical model. Finally, the test results have been compared with those of non-linear finite element model, and also with the predicted ultimate strengths determined by the American and European specifications. Results show that the interaction between slender outstanding width–thickness ratios, overall angle slenderness and overall column slenderness decrease the strength of battened columns. Also, the results of bolted finite element model were in reasonably good agreement with test results that neglect the effect of bolt holes.