Design of cold-formed stainless steel tubular T- and X-joints

This paper describes the numerical investigation of cold-formed stainless steel tubular T-joints, X-joints and X-joints with chord preload using finite element analysis. The stainless steel joints were fabricated from square hollow section (SHS) and rectangular hollow section (RHS) brace and chord members. The geometric and material nonlinearities of stainless steel tubular joints were carefully incorporated in the finite element models. The joint strengths, failure modes as well as load-deformation curves of stainless steel tubular joints were obtained from the numerical analysis. The nonlinear finite element models were calibrated against experimental results of cold-formed stainless steel SHS and RHS tubular T- and X-joints. Good agreement between the experimental and finite element analysis results was achieved. Therefore, an extensive parametric study of 172 T- and X-joints was then carried out using the verified finite element models to evaluate the effects of the strength and behaviour of cold-formed stainless steel tubular joints. The joint strengths obtained from the parametric study and tests were compared with the current design strengths calculated using the Australian/New Zealand Standard for stainless steel structures, CIDECT and Eurocode design rules for carbon steel tubular structures. Furthermore, design formulae of cold-formed stainless steel tubular T- and X-joints are proposed. A reliability analysis was performed to assess the reliability of the current and proposed design rules. It is shown that the design strengths calculated using the proposed equations are generally more accurate and reliable than those calculated using the current design rules.     

Experimental investigation of cold-formed stainless steel tubular T-joints

This paper describes a test program on a wide range of cold-formed stainless steel welded tubular T-joints fabricated from square and rectangular hollow section brace and chord members. A total of 22 tests was performed. High strength stainless steel (duplex and high strength austenitic) and normal strength stainless steel (AISI 304) specimens were tested. The tests were performed by supporting the chord member of the specimen along its entire length with the pure concentrated force applied to the chord face by the brace member. The ratio of brace width to chord width (β) of the specimens varied from 0.5 to 1.0 so that failure modes of chord face failure and chord side wall failure were observed. The test results were compared with the design procedures in the Australian/New Zealand Standard for stainless steel structures, CIDECT and Eurocode design rules for carbon steel structures. It is shown that the design strengths predicted by the current design specifications are conservative for the test specimens calculated using the 0.1%, 0.2%, 0.5% and 1.0% proof stresses as the yield stresses. The 0.2% proof stress is comparatively more reasonable to predict the design strengths of stainless steel welded tubular T-joints for both ultimate limit state and serviceability limit state. In this study, it is shown that the ultimate limit state controls rather than the serviceability limit state for most of the test specimens.     

Tests of concrete-filled stainless steel tubular T-joints

This paper describes a test program on a wide range of concrete-filled cold-formed stainless steel tubular T-joints fabricated from square hollow section (SHS) and rectangular hollow section (RHS) brace and chord members. A total of 27 tests was performed. The chord member of the test specimen was filled with concrete along its full length. Both high strength stainless steel (duplex and high strength austenitic) and normal strength stainless steel (AISI 304) specimens filled with nominal concrete cylinder strength of 30 MPa were tested. The axial compression force was applied to the top end of the brace member, which was welded to the center of the chord member. Local buckling failure of brace member was the main failure mode observed during the tests. Hence, the axial compression force was then applied by means of a steel bearing plate to avoid failure of brace member. The failure modes of chord face failure and chord side wall failure as well as crushing of the concrete infill were observed. All the tests were performed by supporting the chord member of the specimen along its entire length to apply the pure concentrated force without any bending moment. The test results were also compared with design rules for carbon steel tubular structures, which is the only existing design guideline for concrete-filled tubular joints. It is shown that the design strengths predicted by the current design rules are quite conservative for the test specimens. It is also recommended that the contribution of stainless steel tubes should be included in the design rules since it has significant effects on the ultimate bearing capacity of concrete-filled stainless steel tubular T-joints.     

冷弯不锈钢管T形节点的试验研究

对冷弯不锈钢管T形节点进行试验研究,这些节点由方矩形中空截面的支撑和弦杆构成。对高强不锈钢（双相和高强度奥氏体）和普通强度不锈钢（AISI304）构件进行了测试,共进行了22个试验。试验方法：将支撑对弦杆正面所导致的全部集中力,沿试件长度施加到弦杆上。支撑和弦杆的宽度比值（β）范围为0.5～1.0,这样可以观察到弦杆正面和侧面的失效模式。将试验结果与采用澳大利亚/新西兰不锈钢结构设计标准、CIDECT和欧洲碳素钢结构设计规范的设计方法相对比。对比结果显示：采用0.1%,0.2%,0.5%和1.0%的弹性极限应力作为屈服应力,按照这些规范计算出来的设计强度略微保守。相对而言,0.2%的弹性极限应力比较适用于预测不锈钢管T形节点在使用和极限状态下的设计强度。     

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.     

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.     

冷弯不锈钢管T型及X型节点设计

采用有限元方法,对冷弯不锈钢方管、矩形管支撑和弦杆中的T型、X型及预应力X型节点进行数值分析。考虑几何非线性和材料非线性,获得节点承载力、破坏模式及荷载-位移曲线。利用试验结果,对T型、X型矩形管、方管节点的非线性有限元模型进行修正,直到有限元结果和试验结果足够吻合。采用修正后的有限元模型对172个T型、X型节点进行参数分析,研究冷弯不锈钢管节点强度和性能的影响。将数值分析和试验中获得的节点承载力与按规范计算的设计承载力进行对比。对不锈钢管结构,采用澳大利亚规范、新西兰规范计算;对碳素钢管结构,采用国际管结构发展与研究委员会设计规范和欧洲设计规范计算。通过可靠性分析,分别评价本文提出的设计方法和现有规范的可靠度。结果表明:采用本文方法计算的设计承载力更准确、更可靠。     

Experimental investigation of concrete-filled cold-formed high strength stainless steel tube columns

This paper presents an experimental investigation of concrete-filled cold-formed high strength stainless steel tube columns. The high strength stainless steel tubes had a yield stress and tensile strength up to 536 and 961 MPa, respectively. The behaviour of the columns was investigated using different concrete cylinder strengths varied from 40 to 80 MPa. A series of tests was performed to investigate the effects of the shape of the stainless steel tube, plate thickness and concrete strength on the behaviour and strength of concrete-filled high strength stainless steel tube columns. The high strength stainless steel tubes were cold-rolled into square and rectangular hollow sections. The depth-to-plate thickness ratio of the tube sections varied from 25.7 for compact sections to 55.8 for relatively slender sections. The columns had different lengths so the length-to-depth ratio generally remained at a constant value of 3. The concrete-filled high strength stainless steel tube specimens were subjected to uniform axial compression. The column strengths, load-axial strain relationships and failure modes of the columns were presented. The test strengths were compared with the design strengths calculated using the American specifications and Australian/New Zealand standards that consider the effect of local buckling using an effective width concept in the calculation of the stainless steel tube column strengths. Based on the test results, design recommendations were proposed for concrete-filled high strength stainless steel tube columns.     

Buckling analysis of high strength stainless steel stiffened and unstiffened slender hollow section columns

This paper investigates the buckling behaviour of cold-formed high strength stainless steel stiffened and unstiffened slender square and rectangular hollow section columns. The high strength duplex material is austenitic-ferritic stainless steel approximately equivalent to EN 1.4462 and UNS S31803. The columns were compressed between fixed ends at different column lengths. A nonlinear finite element model has been developed to investigate the behaviour of stiffened slender square and rectangular hollow section columns. The column strengths, load-shortening curves as well as failure modes were predicted for the stiffened and unstiffened slender hollow section columns. An extensive parametric study was conducted to study the effects of cross-section geometries on the strength and behaviour of the stiffened and unstiffened columns. The investigation has shown that the high strength stainless steel stiffened slender hollow section columns offer a considerable increase in the column strength over that of the unstiffened slender hollow section 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. It is shown that the design strengths obtained using the three specifications are generally conservative for the cold-formed stainless steel unstiffened slender square and rectangular hollow section columns, but slightly unconservative for the stiffened slender square and rectangular hollow section columns.     

Experimental and numerical investigation of high strength stainless steel structures

The paper summarises research on high strength stainless steel tubular structures conducted at the University of Hong Kong, and the Hong Kong University of Science and Technology. Square and rectangular hollow sections were investigated. The test specimens were cold-rolled from high strength austenitic and duplex stainless steel sheets. The material properties of the test specimens were determined by tensile coupon tests at normal room and elevated temperatures. The initial geometric imperfection and residual stress of the specimens were measured. The experimental and numerical investigation focused on the design and behaviour of cold-formed high strength stainless steel structural members. The results were compared with design strengths calculated using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures.     

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.     

Stress concentration factors of cold-formed stainless steel tubular X-joints

This paper describes experimental and numerical investigations on stress concentration factors (SCFs) of cold-formed stainless steel square and rectangular hollow section (SHS and RHS) tubular X-joints. Both high strength stainless steel (duplex and high strength austenitic) and normal strength stainless steel (AISI 304) specimens were investigated. The SCFs were experimentally determined under static loading by measuring the strains at typical hot spot locations using strip strain gauges. The corresponding finite element analysis was performed to simulate the non-uniform stress distribution along the brace and chord intersection region. Good agreement between the experimental and finite element analysis results was achieved. Therefore, an extensive parametric study was then carried out by using the verified finite element model to evaluate the effects of the SCFs of cold-formed stainless steel tubular X-joints. The SCFs at the hot spot locations obtained from the experimental investigation and parametric study were compared with those calculated using the design formulae given in the CIDECT for carbon steel tubular X-joints. It is shown from the comparison that the design rules for the SCFs specified in the CIDECT are generally quite unconservative for cold-formed stainless steel tubular X-joints. In this study, a unified design equation for the SCFs of cold-formed stainless steel tubular X-joints is proposed. The proposed design equation was based on the CIDECT design equation for carbon steel tubular X-joints. It is shown that the SCFs calculated from the proposed unified design equation are generally in agreement with the values predicted from finite element analysis.     

Flexural behaviour and strength of cold-formed steel L-headers

Cold-formed steel headers are structural components used over wall openings in cold-formed steel residential and light commercial construction. Recently, there has been an increased interest in cold-formed L-headers among homebuilders primarily due to their ease of installation and low material cost. The findings from an extensive laboratory testing program, of full-scale single and double cold-formed steel L-headers are presented in this paper. The objective of the research was to investigate the flexural behaviour and strength of L-headers under both gravity and uplift loads. Based on the results improved ultimate strength design expressions and new deflection expressions for a wide range of L-header assemblies have been proposed.     

Strength of arc-welded T-joints between equal width cold-formed RHS

The paper describes laboratory tests of arc-welded T-joints between equal width rectangular hollow sections (called “matched box connections” in ANSI/AWS D1.1-2000 Structural Welding Code). The brace and chord members were cold-formed with a nominal yield stress of 350 MPa. The welds were laid using MMAW and GMAW processes without profiling the brace ends. The brace of each specimen was loaded in tension to failure with the chord supported continuously so as not to induce significant bending effects. The test results showed that the joint strength can be improved by using backing strips for the butt (or groove) welds, while backing rods (or filler rods) should not be used as they led to larger variation in joint strengths, and often, inferior strengths. The test strengths are compared with the design strengths obtained using the IIW Recommendations and Eurocode 3, Part 1.8. It is shown that for cold-formed tubes with a nominal yield stress of 350 MPa (or above), a design check on the strength of the butt (or groove) weld is required in addition to the checks on the strengths of the chord and brace members specified in the current design guidelines. An equation is proposed for calculating the strength of the weld.     

Residual stress influence on material properties and column behaviour of stainless steel SHS

The paper describes the influence of forming-induced residual stresses in stainless steel SHS (Square Hollow Sections) on both the material itself and the behaviour of compressed members. The residual stress contribution to the stress–strain diagram concerning the initial modulus of elasticity and non-linearity is shown by the comparison of as-delivered and stress-relieved material. An analytical model covering the influence of residual stresses on material behaviour was developed and verified numerically. The FE study using the Abaqus software determines the influence of residual stresses in compressed members both on local and global buckling. Finally, the study on behaviour of member includes the influence of a varying degree of material non-linearity as an independent parameter representing the behaviour of various steels in long and stub columns with residual stresses.     

冷成型不锈钢管轴心受压柱试验研究

为研究冷成型不锈钢管轴心受压柱的稳定性能,进行了国产304牌号冷成型不锈钢方管、矩形管和圆管截面,共43根轴心受压柱试验。通过对不锈钢材料、轴压短柱试件和轴压长柱试件的试验研究,得到了试件的材料力学性能和极限荷载。分析了试件的长细比、宽厚比（径厚比）对其破坏模式及变形性能的影响。结果表明：试件的宽厚比（径厚比）对其破坏模式及变形性能有较大的影响。采用GB 50018-2002《冷弯薄壁型钢结构技术规范》、欧洲不锈钢结构设计规范、美国冷成型不锈钢结构设计规范中的计算方法以及Rasmussen提出的设计方法对试验试件进行了计算,并与试验数据对比结果表明,对于圆管试件,采用三本规范计算得到的承载力均高于试验值,偏于不安全,采用Rasmussen 提出方法的计算结果与试验值较为接近;对于方矩管试件,各种方法计算结果相近,除短柱试件试验结果高于计算结果外,其余试件试验值均与计算结果吻合较好;GB 50018-2002《冷弯薄壁型钢结构技术规范》中的计算方法不能直接用于计算不锈钢管轴心受压柱承载力。     

Cross-sectional flexural capacity of cold-formed laterally-restrained steel rectangular hollow flange beams

This paper presents the results of a comprehensive experimental-numerical study aimed at determining the flexural performance of cold-formed laterally-restrained steel rectangular hollow flange beams (RHFBs). Two RHFBs of different dimensions were considered as representative of typical secondary beams in small steel-framed houses. Results of the experimental study that consisted of (i) material characterisation and (ii) tests on full-scale specimens are thoroughly presented. Moreover, a numerical work was performed in order to develop a model able to reproduce the experimental outcomes and used to expand the available findings over a wider slenderness range through parametric studies.     

Strength and behaviour of cold-formed steel offset trusses

Thirteen full-scale truss specimens fabricated with cold-formed steel C-sections were tested to study various practical strengthening techniques in order to achieve a desired behaviour and an increased capacity. Specimens were subjected to concentrated panel point loading simulating the realistic loading condition of a roof truss. Results revealed that local buckling of the top chord (LBTC) adjacent to the heel plate was the predominant failure mechanism. In instances where the heel plate was not adequately stiffened, the distortion or crippling of the plate occurred at failure. For the investigated pitches, an increase in specimen pitch resulted in an increase in the capacity. Strengthening both the top chord adjacent to the heel plate and the heel connection with both a shallow member and a heel plate stiffener resulted in the most significant increase in the capacity compared with the original truss configuration. Beam-column analysis of the top chords in the failure region was performed using both the conventional interaction design equations and the direct strength method as suggested in CSA S136-07.     

Experimental behaviour of cold-formed steel welded tube filled with concrete made of crushed crystallized slag subjected to eccentric load

This paper presents results of tests conducted on thin welded rectangular steel stubs filled with concrete that gravel was substituted by 10 mm crushed crystallized slag stone. The studied section was made of two cold steel plates with U shape and welded with electric arc to form a steel box section. The cross-section dimensions were: 100×70×2 mm³. the main studied parameters were the stub height (200, 300, 400, 500 mm), the effect of the in filled concrete, the continued weld and the eccentric force. The tests were carried out 28 days after the date of casting. A total of 20 stubs were tested in a 50 tf machine up to failure, 4 stubs subjected to axial load compression and 16 stubs subjected to eccentric load compression along the minor and major rigidity axis. The aim of the study is to provide some evidences that the use of crushed slag could be integrated in the manufacturing of non-conventional concrete. All failure loads were predicted by using the Euro code 4 and the design method proposed by Z. Vrcelj and B. Uy. From test results, it was confirmed that the length of stubs and the eccentric load had a drastic effect on the load carrying capacity. The failure mode of composite stubs was a local buckling mode with all steel sides deformed outwards. The Euro code 4 loads predictions were generally in good agreement compared with experimental loads and on safe side. The loads results of design method proposed by Vrcelj and B. Uy were generally on safe side compared with experimental load except the columns subject to eccentric load with 400 mm and 500 mm height.