Cold-formed Stainless Steel Sections Research Articles

Web crippling resistances of cold-formed stainless steel sections: A proposal for EN 1993-1-4

The current EN 1993-1-4 does not have supplementary web crippling design provision for cold-formed stainless steel sections under local transverse forces. Therefore, designers are generally referring to the guidance in EN 1993-1-3 for cold-formed carbon steel sections. Over the past decades, quite a substantial number of tests have been carried out worldwide to study the behaviour of cold-formed stainless steel sections undergoing web crippling, among which the majority of the data were focused on C-sections as well as square and rectangular hollow sections (SHS and RHS). To this end, tailor-made design provisions for these stainless steel sections can be developed. In this study, up-to-date web crippling test data on cold-formed stainless steel C-sections and SHS/RHS were firstly collected. The applicability of the web crippling design provisions as per the new version of EN 1993-1-3 to stainless steel sections was assessed. New design provisions for cold-formed stainless steel unstiffened C-sections (unlipped channels), stiffened C-sections (lipped channels) as well as SHS and RHS under local transverse forces are proposed in this study. The newly developed web crippling design provisions for C-sections and SHS/RHS were derived based upon reliability requirements as regulated in EN 1990 using available experimental data reported on austenitic, ferritic and duplex stainless steel types of various grades. The proposed design provisions for stainless steel sections have been made in line with the development of the new version of EN 1993-1-3 provisions and therefore are recommended to EN 1993-1-4 for calculating web crippling resistances under local transverse forces.

Finite element modelling and design of welded stainless steel I-section columns

Stainless steel is widely used in construction due to its combination of excellent mechanical properties, durability and aesthetics. Towards more sustainable infrastructure, stainless steel is expected be more commonly specified and to feature in more substantial structural applications in the future; this will require larger and typically welded cross-sections. While the structural response of cold-formed stainless steel sections has been extensively studied in the literature, welded sections have received less attention to date. The stability and design of conventionally welded and laser-welded austenitic stainless steel compression members are therefore the focus of the present research. Finite element (FE) models were developed and validated against a total of 59 experiments, covering both conventionally welded and laser-welded columns, for which different residual stress patterns were applied. A subsequent parametric study was carried out, considering a range of cross-section and member geometries. The existing experimental results, together with the numerical data generated herein, were then used to assess the buckling curves given in European, North American and Chinese design standards. Following examination of the data and reliability analysis, new buckling curves were proposed, providing, for the first time, design guidance for laser-welded stainless steel members.

Strength and ductility of corner materials in cold-formed stainless steel sections

The cold work from the manufacturing process of cold-formed steel members can enhance the strength but reduce the ductility of materials. Due to a high cost of stainless steels, it is desirable to utilize this enhanced strength and avoid the early fracture in cold-formed stainless steel members. The paper is concerned with the prediction of the enhanced stress–strain behaviour and reduced ductility of corner materials in cold-formed stainless steel sections. The enhanced strength of corner materials has been traditionally determined using empirical models. However, most of these empirical models are only able to predict the enhanced 0.2% proof strength, but are neither capable of predicting the enhanced ultimate strength nor able to determine the reduced ductility. This paper first presents a modified weighted-average method for predicting the post-ultimate stress–strain behaviour and the fracture strain for stainless steels. An advanced numerical approach is next presented for predicting the full-range stress–strain behaviour of corner materials in cold-formed stainless steel sections, in which the modified weighted-average method is incorporated. The accuracy of this approach is demonstrated by comparing its predictions with test results. The proposed approach is generally applicable to cold-worked materials for predicting their enhanced strength, reduced ductility and full-range stress–strain behaviour. The proposed method and numerical results can explain why and how the ultimate strength of cold-formed steels can be increased and how the post-ultimate stress–strain behaviour can be utilized through cold working.

Combined Distortional and Overall Flexural-Torsional Buckling of Cold-Formed Stainless Steel Sections: Design

This paper provides a critical summary of the European standards and design methods available for calculating the strength of cold-formed stainless steel compression members. The standards considered (so far mainly applied to columns failing by flexural buckling) are applied to the case of lipped channel section columns failing by combined distortional and overall flexural-torsional buckling. As observed during the tests presented in a companion paper, distortional buckling is found to have a considerable effect on the cross section resistance and is prone to interact with the global flexural-torsional mode. It is investigated how the different formulations account for the two buckling modes and their interaction and how accurate strength predictions they provide. A new direct strength method taking into account these failure modes is then presented and compared to the test results.

Combined Distortional and Overall Flexural-Torsional Buckling of Cold-Formed Stainless Steel Sections: Experimental Investigations

This paper presents a series of 48 full-scale tests on press-braked stainless steel lipped channel section columns subjected to concentric compression. The tests were carried out between fixed ends in the Structures Laboratory of the University of Liège and the test specimens were designed such that distortional buckling developed in the section prior to overall flexural-torsional buckling. The stainless steel alloy was 1.4003 chromium weldable steel, popularly known as 3Cr12. Three different geometries were tested using the same experimental setup. A critical summary of the standards and methods for calculating the carrying capacity of cold-formed stainless steel compression members is presented in a companion paper by the same writers.

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.

Column design of cold-formed stainless steel slender circular hollow sections

This paper describes the design and behaviour of cold-formed stainless steel slender circular hollow section columns. The columns were compressed between fixed ends at different column lengths. The investigation focused on large diameter-to-plate thickness (D/t) ratio ranged from 100 to 200. An accurate finite element model has been developed. The initial local and overall geometric imperfections have been included in the finite element model. The material nonlinearity of the cold-formed stainless steel sections was incorporated in the model. The column strengths, load-shortening curves as well as failure modes were predicted using the finite element model. The nonlinear finite element model was verified against test results. An extensive parametric study was carried out to study the effects of cross-section geometries on the strength and behaviour of stainless steel slender circular hollow section columns with large D/t ratio. 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 Australian/New Zealand and European specifications are generally unconservative for the cold-formed stainless steel slender circular hollow section columns, while the American Specification is generally quite conservative. Therefore, design equation was proposed in this study.

Distortional Buckling of Cold-Formed Stainless Steel Sections: Finite-Element Modeling and Design

A recent experimental and finite-element research program was conducted on the distortional buckling of axially compressed, cold-formed stainless steel simple lipped channels. Austenitic 304 (1.4301), ferritic 430 (1.4016), and ferritic-like 3Cr12 (1.4003) chromium weldable steel were considered. Finite-element analyses demonstrate that material anisotropy can be ignored. A total of 281 distortional buckling analyses of simple lipped channels with r∕t ratios of 1 and 2.5 show that enhanced corner properties may be important for sections which have a slenderness λd<1.0 and at least 10% corner area but have little effect on sections with λd>1.0. Evaluation of current stainless steel design codes show that the Australian/New Zealand Standard AS/NZS 4673 and the ASCE Specification effective width procedures are unconservative for austenitic and ferritic stainless steels. The Eurocode 3 prEN Part 1-4/preEN Part 1-3 effective width procedures are unconservative for austenitic stainless steel but provide reasonably conservative results for ferritic 430 and 3Cr12 alloys, provided enhanced corner properties are ignored. Direct strength design guidelines for the distortional buckling mode are proposed for austenitic and ferritic stainless steels.

Distortional Buckling of Cold-Formed Stainless Steel Sections: Experimental Investigation

This paper describes the experimental investigation of cold-formed, thin-walled stainless steel sections subject to distortional buckling under compression. Austenitic 304, ferritic 430 stainless steel, and ferritic-like 3Cr12 chromium weldable steel sheets were brake-pressed into simple-lipped channels and lipped channels with intermediate stiffeners. A full set of coupon tests reveal material nonlinearity with low proportionality stress and low n-parameter, anisotropy, and yield strength enhancements up to 2.33 times due to cold-forming. A total of 19 channel sections were tested and failed by distortional buckling at average ultimate stresses greater than the proportionality stress, and hence were influenced by material nonlinearity, with greater nonlinearity evident for sections with intermediate stiffeners. This paper presents comprehensive procedures to determine stainless steel mechanical properties and their influence on the distortional buckling mode of cold-formed sections. Experimental data required to calibrate finite element models and assess current design guidelines for distortional buckling of stainless steel compression members are provided herein.

Cold-Formed Stainless Steel Sections Subjected to Web Crippling

An experimental investigation of cold-formed stainless steel hollow sections subjected to web crippling is presented in this paper. Tests were conducted on square and rectangular hollow sections of austenitic stainless steel type 304. Tensile and compression coupon tests were performed to obtain the longitudinal tension and transverse compression material properties. The web crippling tests were conducted under two loading conditions for end-two-flange and interior-two-flange specified in the current American Specification and Australian/New Zealand Standard for cold-formed stainless steel structures. The test strengths obtained from this study and the test strengths of stainless steel lipped channel sections under end-one-flange and interior-one-flange loading conditions conducted by other researchers are compared with the design strengths obtained using the current American Specification, Australian/New Zealand Standard and European Code for cold-formed stainless steel structures. In addition, the test strengths are also compared with the design strengths obtained using the unified web crippling equation as specified in the North American Specification for cold-formed carbon steel structural members. A unified web crippling equation for cold-formed stainless steel sections with single web is proposed in this paper.

Design of Stiffened Elements in Cold-Formed Stainless Steel Sections

The paper presents strength equations for single stainless steel stiffened elements applicable to the range of alloys included in the American, Australian, and European standards for stainless steel structures. Cold-formed sections with stiffened elements are considered and the enhanced mechanical properties of the corners are taken into account to show that the Winter equation may or may not conservatively predict the strength, depending on the corner area relative to the area of the flat part. The paper shows that the Winter equation may be used safely for the design of stiffened elements provided half of the area of the adjacent corners exceeds 10.15% of the flat area, depending on the alloy and whether the section is cold-rolled or brake-pressed. An equation is derived for determining the reduction factor to be applied to the Winter strength when the minimum corner area requirement is not met. The reduction factor is less than 10% for practical sections.

Strength enhancement of the corner regions of stainless steel cross-sections

Stainless steel strain hardens to a considerably greater extent than carbon steel resulting in significant changes in material behaviour during a cold-forming process. This paper presents a method for predicting the enhanced strength of the corner regions of cold-formed stainless steel sections. All available test results have been collected and used to devise generalised models to predict strength enhancements for different cold-forming processes.