Cold-formed Steel Design Standards Research Articles

Design of sheathed built-up nested CFS channel studs in load-bearing LSF walls

Cold-formed steel built-up nested channel (NC) sections have been increasingly used to replace the commonly used lipped channel sections as the load-bearing studs of light gauge steel framed (LSF) walls with higher compression capacity. However, the related knowledge is limited and no research study has investigated such NC stud walls yet. In this study, an investigation was undertaken on the compression behaviour and capacity of LSF walls made of slender NC studs sheathed with gypsum plasterboards on both sides. Compression tests of sheathed NC studs were conducted first, which were then used in developing and validating suitable finite element models. The results obtained from tests and finite element models showed that plasterboards and stud-to-board screw connections had significant influence on the failure mode and compression capacity of sheathed NC studs, while the end fastener groups recommended in the current cold-formed steel design standards and the stud-to-stud screw connections had only marginal effects. Finally, a simple and conservative design method was proposed based on the Direct Strength Method equations in the current design standards. This paper presents the details of this research study, including the results, discussions and main findings.

Tests of cold-formed steel built-up open section members under eccentric compressive load

An experimental study was performed on cold-formed steel built-up open section members under eccentric compressive load. The specimens with member lengths varied from 300 to 1500 mm were manufactured by composing two identical channels using self-tapping screws. The channels with longitudinal stiffeners were fabricated from the steel grades G500 and G550 zinc-coated sheets with nominal 0.2% proof stresses of 500 and 550 MPa, respectively. A total of thirty-three combined minor axis bending and compression tests were carried out under pin-ended supports to explore the buckling behaviour of the newly designed built-up section members. Six test series with a relatively wide range of axial load-to-moment ratio were included in this study to examine the interaction relationship of the built-up section beam-columns. The test loading capacities, full-history responses and failure modes were obtained for all the test specimens. Since the novel built-up open sections are not covered in the current cold-formed steel design standards, the comparisons of experimental loading capacities with nominal strengths predicted by the North American Specification, Australian/New Zealand Standard, European Code and American Specification were conducted to assess the appropriateness of the existing design rules. The applicability of axial load-moment interactive formulae specified in the aforementioned design standards with the nominal pure compressive resistance and nominal pure bending resistance determined by direct strength method was evaluated for the built-up section beam-columns. It is found that the axial load-moment interactive formulae generally underestimate the strengths of the cold-formed steel built-up open section beam-columns.

Built-up nested cold-formed steel compression members subject to local or distortional buckling

The current cold-formed steel (CFS) design standards have not yet provided adequate design methods for built-up nested CFS channel compression members, which may fail by either local or distortional buckling, despite several research studies in recent times. These research studies have proposed improved DSM based design methods that led to capacity predictions with varying accuracy. Besides, there were some contradicting observations regarding the effects of screw connections on their compression behaviour and capacities. Hence, this research study investigated the behaviour and capacity of nested CFS channel compression members. It included experimental tests of short single channel and nested channel compression members failing by local-distortional interaction and local buckling, respectively, development and validation of suitable finite element models of nested channel compression members and finally a parametric study to (1) show that distortional buckling is not the critical failure mode of nested channel members, and (2) examine the effects of screw characteristics and spacing on their behaviour and capacities. Importantly, the accuracy of currently available design equations in the CFS design standards and recently published research papers was investigated and a simple DSM based design method is recommended. This paper presents the details of this research study, its results and findings.

Built-up back-to-back cold-formed steel compression members failing by local and distortional buckling

The current design methods for built-up back-to-back cold-formed steel (CFS) channel compression members are considered inadequate despite several research studies in recent times. These research studies have proposed improved DSM based design equations, however, they have in general led to predictions of varying accuracy. Further, there were some contradicting observations regarding the effects of screw connections on their compression behaviour and capacities. Therefore, this research focused on the compression behaviour and capacity of built-up back-to-back CFS channel sections subject to local and/or distortional buckling. It included compression tests of short single and back-to-back channel members failing by local-distortional interaction buckling, development and validation of finite element models and finally a parametric study to examine the effects of screw fastener characteristics and arrangements on the compression behaviour and capacities of back-to-back channel members failing in local and/or distortional buckling. Using the results, the accuracy of currently available design equations in the CFS design standards and recently published research papers was investigated and a simple DSM based design method is recommended. This paper presents the details of this research and the results including its findings.

Numerical study of cold-formed steel channel sections under combined web crippling and bending action

Cold-formed steel sections used in floor systems are vulnerable to web crippling, and with increasing span lengths, they are likely to fail under combined web crippling and bending action. A numerical study was therefore undertaken to investigate the combined web crippling-bending interaction behaviour of unlipped channel sections used as bearers in floor systems with fastened supports. Web crippling finite element models developed and validated by the authors in a recent study were extended to investigate the behaviour of unlipped channels under combined action of web crippling and bending while new finite element models were developed for bending and validated using available experimental results. All three types of finite element models were used in a detailed parametric study to obtain the capacities of 12 unlipped channel sections made of G250 and G450 steels under pure and combined web crippling and bending actions. Comparison of the combined web crippling and bending capacities obtained from finite element analyses with the interaction equations in three cold-formed steel design standards, AISI S100, AS/NZS 4600 and Eurocode 3 Part 1.3, showed that the current design equations are accurate for 50 mm bearing length. However, they can also be used to predict the combined web crippling-bending capacities conservatively for bearing lengths of 100 and 150 mm. A new design equation with a suitable capacity reduction factor was then proposed to improve the accuracy of predicting the mid-span load capacity of channel sections subject to combined web crippling and bending actions. • Investigated the behaviour of unlipped channels with fastened supports under pure and combined web crippling and bending. • Developed and validated finite element models for web crippling and bending actions. • Recommended ABAQUS quasi-static analysis based on explicit conditions instead of general static method. • Current web crippling and bending interaction equations were shown to be conservative. • Proposed new equations to predict the mid-span load capacity under combined web crippling and bending action.

Numerical Modeling and Design of Lipped Channel Beams Subject to Web Crippling under One-Flange Load Cases

Web crippling failure governs the behavior of thin cold-formed steel lipped channel beams (LCBs) used in floor systems. This paper describes a numerical modeling–based research study undertaken to investigate the web crippling behavior of LCBs under one-flange load cases and to develop improved design equations for possible inclusion in the cold-formed steel design standards. Finite-element models were developed to simulate the web crippling behavior of LCBs and their accuracy was verified using 36 web crippling tests of LCBs conducted under one-flange load cases using the new standard test method. A detailed numerical parametric study was then undertaken to investigate the web crippling behavior of LCBs using the verified finite element models of LCBs. This numerical parametric study provided an extensive web crippling capacity database and improved the understanding of the effects of key web crippling parameters such as inside bent radius, bearing length, and yield stress on the web crippling capacity. Using these results, new and improved web crippling design equations were proposed in this paper for LCBs under one-flange load cases. They include both unified web crippling equations and the direct strength method–based equations. This paper demonstrated the improved accuracy of the proposed equations and their potential for inclusion in the cold-formed steel design standards.

Finite element analysis and design for section moment capacities of hollow flange steel plate girders

This paper describes a numerical study on the section moment capacity of a doubly symmetric hollow flange steel plate girder (HFSPG) for use in long span applications. The HFSPG is manufactured by welding industrially available cold-formed rectangular hollow sections to a steel web plate. Suitable finite element models were developed first to simulate the section moment capacity tests conducted on the proposed HFSPGs using a four-point bending test arrangement. The experimental conditions in terms of boundary conditions, lateral restraints, mechanical properties, initial geometric imperfections and residual stresses were modelled carefully. The comparison of ultimate moment capacities, moment versus deflection plots and failure modes proved the validity of the developed finite element models to investigate the section moment capacities of HFSPGs. However, the slenderness of the tested HFSPGs was limited to the inelastic region. Hence the validated finite element models were used in a detailed parametric study to extend the results to a wider slenderness range. The parametric study results of the section moment capacities were compared with the predictions of the relevant Australian, American and European hot-rolled and cold-formed steel design standards and suitable recommendations were made in relation to their accuracy in predicting the capacities of compact, non-compact and slender HFSPGs. Finally this paper proposes a new Direct Strength Method based design for HFSPGs.

Design of SupaCee Sections Subject to Web Crippling under One-Flange Load Cases

Cold-formed steel sections are generally used as flexural members in lightweight steel construction. The SupaCee section is an innovative section introduced recently to the Australian building sector. It provides superior flexural strength than the traditional channel sections due to unique ribbed web and curved lip elements. However, the web crippling behavior and strength of the high strength SupaCee sections has not been investigated yet. Current web crippling design methods given in cold-formed steel design standards do not include any provision for SupaCee sections. Hence, detailed experimental and finite element studies were conducted to investigate the web crippling behavior and strengths of SupaCee sections under one-flange load cases with their flanges unfastened from the supports. Experimental and finite element analysis results showed that the web crippling capacities of SupaCee sections are reduced relative to lipped channel sections. Therefore, the current web crippling design equations in the American and Australian/New Zealand cold-formed steel design standards were modified by including suitable web crippling coefficients for SupaCee sections. Finite element models of tested SupaCee sections were also developed and validated using the experimental results. This paper presents the details of experimental and numerical web crippling studies of SupaCee sections under one-flange load cases and the results. It also presents the details of direct strength method-based design equations developed in this research.

Section moment capacity design rules for rivet fastened rectangular hollow flange channel beams

The rivet fastened Rectangular Hollow Flange Channel Beam (RHFCB) is a new type of cold-formed steel section, made of two torsionally rigid rectangular hollow flanges, connected to a web via intermittent rivet fastening. The hollow flanges and the absence of free edges in the RHFCB contribute to improved structural performance. The structural behaviour of the RHFCB is unique compared to other conventional cold-formed steel sections and its moment capacity reduces with increasing rivet spacing. The current cold-formed steel design standards do not provide a calculation method to include the effects of intermittent fastening. In this research an extensive parametric study was conducted using validated finite element models to investigate the section moment capacity of RHFCBs. This paper presents the findings from the parametric study and proposes new design equations for the section moment capacity of RHFCBs in the Direct Strength Method format. The parametric study considers various slenderness regions, section dimensions and rivet spacing. In the new design equations, a reduction factor parameter is included to calculate the section moment capacity of RHFCBs at any rivet spacing up to 200 mm. Optimum rivet spacing was also recommended for RHFCBs.

Pull-out capacity of multiple screw fastener connections in cold-formed steel roof battens

Light gauge steel roofing systems of low rise buildings are made of high strength steel roof cladding, battens and purlins/rafters/trusses and fastened with self-drilling screws. Localised failures occurring at these screw connections under wind uplift loading often lead to catastrophic losses of steel roofing systems during severe storm events. Pull-out failures occurring at the two bottom flanges of roof batten to purlin/rafter screw connections is one of them. Multiple screw fasteners connecting the bottom flanges of roof battens to purlins/rafters pull out prematurely from the light gauge steel purlins/rafters. Since the structural behaviour of multiple screw connections differs significantly from the behaviour of single screw connections, the pull-out capacity of multiple screw connections cannot be calculated directly based on the number of screw fasteners. Hence a series of 80 small scale pull-out tests was conducted to investigate the pull-out capacities of two and four-screw fastener connections in roof battens. The pull-out failure loads from the tests were first compared with the pull-out capacities predicted using the design equations in the current cold-formed steel design standards. Suitable design modifications were then proposed to accurately calculate the pull-out capacities of roof battens with two and four-screw connections. This paper describes the details and the results of this study.

Web crippling studies of SupaCee sections under two flange load cases

Cold-formed steel channel sections are generally used as flexural members in light weight steel construction. Improved channel section profiles such as SupaCee sections with longitudinal web stiffeners and curved lips are also used instead of the conventional lipped channel sections. Web crippling capacities of these innovative sections can be different from those of conventional lipped channel sections. However, the web crippling behaviour and strength of these high strength SupaCee sections has not been investigated yet. Current web crippling design methods given in cold-formed steel design standards do not include any design procedures for SupaCee sections. Hence an experimental study involving 36 web crippling tests was first undertaken to investigate the web crippling behaviour and strengths of SupaCee sections under two flange load cases with their flanges unfastened to the supports. Comparison of experimental results showed that the web crippling capacities of SupaCee sections are reduced in comparison to lipped channel sections. Therefore the current web crippling design equations in the American and Australian/New Zealand cold-formed steel design standards were modified by including suitable web crippling coefficients for SupeCee sections. Finite element models of tested SupaCee sections were also developed and validated using the experimental results. This paper presents the details of the experimental and numerical web crippling studies of SupaCee sections under two flange load cases and the results. It also presents the details of direct strength method based design equations developed in this research.

Web crippling experiments of high strength lipped channel beams under one-flange loading

Web crippling is often a critical design problem in cold-formed steel flexural members. Lipped channel beams (LCBs) are commonly used as floor joists and bearers in the construction industry and are often subjected to concentrated loads. Design capacity predictions from most of the cold-formed steel design standards such as AISI S100 [1], AS/NZS 4600 [2] and Eurocode 3 Part 1-3 [3] are empirical, developed based on past experimental studies. They were found to be either unconservative or conservative in most cases. Inconsistencies in design capacity predictions are considered to be due to the fact that the specimen length and support conditions pertaining to the test set-up varied among past experimental studies. In 2008, American Iron and Steel Institute introduced a standard test method for conducting web crippling studies [4]. However, limited web crippling studies have been conducted to date for LCB sections under EOF and IOF load cases. Therefore a detailed experimental study consisting of 36 tests was conducted to investigate the web crippling behaviour of high strength cold-formed steel LCB sections under EOF and IOF load cases based on the AISI web crippling standard test method. This paper presents the details of this experimental study of LCBs unfastened to supports, using which it proposes suitable modifications to the current unified web crippling design equation. It also presents suitable direct strength method based design equations and associated predictive equations for elastic bucking and yield loads of LCBs under EOF and IOF load cases.

New design rules for lipped channel beams subject to web crippling under two-flange load cases

Lipped channel beams (LCBs) are commonly used as floor joists and bearers in the construction industry. These thin-walled LCBs are subjected to specific local and global failures, one of them being web crippling. Several experimental and numerical studies have been conducted in the past to study the web crippling behaviour and capacities of different cold-formed steel sections under different concentrated load cases. However, due to the nature of the web crippling phenomenon and many factors influencing the web crippling capacities, capacity predictions given by most of the cold-formed steel design standards are either unconservative or conservative. Therefore a detailed experimental study was conducted to study the web crippling under End Two-Flange (ETF) and Interior Two-Flange (ITF) load cases based on the new AISI S909 standard web crippling test method. Finite element models were developed and validated using the test results. These models were then used in a detailed parametric study to investigate the web crippling capacities of a wide range of LCB sections including different sectional geometric parameters such as section depth, inside bent radius, thickness and bearing length. This paper presents the details of the numerical study of LCBs subject to web crippling under ETF and ITF load cases. Using the extensive web crippling capacity data obtained from both numerical and experimental studies, improved unified web crippling design equations were developed. Suitable web crippling design rules were also developed under the direct strength method format.

Shear behaviour and design of Lipped Channel Beams with non-circular web openings

Cold-formed steel Lipped Channel Beams (LCB) with web openings are commonly used as floor joists and bearers in building structures. Their shear behaviour is influenced by the presence of web openings and the shear capacities are considerably reduced. However, the shear behaviour and capacity of LCBs with non-circular web openings (square, rectangular and elliptical web openings) has not been investigated adequately. Hence a detailed numerical study was conducted to investigate the shear behaviour and capacity of LCBs with non-circular web openings. Finite element models of simply supported LCBs under a mid-span load with aspect ratios of 1.0 and 1.5 were developed and validated by using the available shear test results. They were then used in a detailed parametric study to investigate the effects of various influential parameters. Numerical results showed that the current shear design equations in cold-formed steel structures design standards are conservative or unsafe. New shear design equations were therefore proposed for the accurate prediction of the shear capacity of LCBs with non-circular web openings. This paper presents the details of finite element modelling of LCBs with unreinforced non-circular web openings and the development of new shear design rules. The proposed shear design equations in this paper can be considered for inclusion in the future versions of cold-formed steel design standards. Suitable design equations were also developed under the direct strength method.

Local buckling studies of cold-formed steel compression members at elevated temperatures

Cold-formed steel members have been widely used in residential and commercial buildings as primary load bearing structural elements. They are often made of thin steel sheets and hence they are more susceptible to local buckling. The buckling behaviour of cold-formed steel compression members under fire conditions is not fully investigated yet and hence there is a lack of knowledge on the fire performance of cold-formed steel compression members. Current cold-formed steel design standards do not provide adequate design guidelines for the fire design of cold-formed steel compression members. Therefore a research project based on extensive experimental and numerical studies was undertaken to investigate the local buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. First a series of 91 local buckling tests was conducted at ambient and uniform elevated temperatures up to 700°C on cold-formed lipped and unlipped channels. Suitable finite element models were then developed to simulate the behaviour of tested columns and were validated using test results. All the ultimate load capacity results for local buckling were compared with the predictions from the available design rules based on AS/NZS 4600, BS 5950 Part 5, Eurocode 3 Parts 1.2 and 1.3 and the direct strength method (DSM), based on which suitable recommendations have been made for the fire design of cold-formed steel compression members subject to local buckling at uniform elevated temperatures.

Experimental investigation and design of lipped channel beams in shear

Cold-formed high strength steel members are increasingly used as primary load bearing components in low rise buildings. Lipped channel beam (LCB) is one of the most commonly used flexural members in these applications. In this research an experimental study was undertaken to investigate the shear behaviour and strengths of LCB sections. Simply supported test specimens of back to back LCBs with aspect ratios of 1.0 and 1.5 were loaded at mid-span until failure. Test specimens were chosen such that all three types of shear failure (shear yielding, inelastic and elastic shear buckling) occurred in the tests. The ultimate shear capacity results obtained from the tests were compared with the predictions from the current design rules in Australian/NewZealand and American cold-formed steel design standards. This comparison showed that these shear design rules are very conservative as they did not include the post-buckling strength observed in the shear tests and the higher shear buckling coefficient due to the additional fixity along the web-flange juncture. Improved shear design equations are proposed in this paper by including the above beneficial effects. Suitable lower bound design rules were also developed under the direct strength method format. This paper presents the details of this experimental study and the results including the improved design rules for the shear capacity of LCBs. It also includes the details of tests of LCBs subject to combined shear and flange distortion, and combined bending and shear actions, and proposes suitable design rules to predict the capacities in these cases.

Pullout Strength of Steel Pins in Cold-Formed Steel Framing

Steel pins are an attractive and efficient alternative technology to screws when connecting cold-formed steel or other materials to cold-formed steel members. However, unlike screws, pins remain an essentially proprietary connection technology based on manufacturer claims of special pin shank design. As such, current cold-formed steel design standards and specifications do not include provisions for their direct use and designers have no simple, rational means for making informed design decisions beyond information provided by manufacturers. This paper reviews the current design model for screws and examines the basic tension pullout/withdrawal limit state of helical knurled steel pin connections for cold-formed steel thicknesses common in residential construction. Included in this paper is a determination of material properties using a nondestructive test method as an alternative to traditional methods for estimation of steel base metal thickness. A simple lower bound friction model is proposed for helical steel pins and the model is calibrated using data from 32 physical pin tests. The calibrated model is shown to provide an acceptable degree of reliability and safety consistent with that of screws in current cold-formed steel design standards and specifications.

Flexural-torsional buckling tests of cold-formed steel compression members at elevated temperatures

Current design standards do not provide adequate guidelines for the fire design of cold-formed steel compression members subject to flexural-torsional buckling. Eurocode 3 Part 1.2 (2005) recommends the same fire design guidelines for both hot-rolled and cold-formed steel compression members subject to flexural-torsional buckling although considerable behavioural differences exist between cold-formed and hot-rolled steel members. Past research has recommended the use of ambient temperature cold-formed steel design rules for the fire design of cold-formed steel compression members provided appropriately reduced mechanical properties are used at elevated temperatures. To assess the accuracy of flexural-torsional buckling design rules in both ambient temperature cold-formed steel design and fire design standards, an experimental study of slender cold-formed steel compression members was undertaken at both ambient and elevated temperatures. This paper presents the details of this experimental study, its results, and their comparison with the predictions from the current design rules. It was found that the current ambient temperature design rules are conservative while the fire design rules are overly conservative. Suitable recommendations have been made in relation to the currently available design rules for flexural-torsional buckling including methods of improvement. Most importantly, this paper has addressed the lack of experimental results for slender cold-formed steel columns at elevated temperatures.

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.

Bolted Connection Tests of Thin G550 and G300 Sheet Steels

Cold-formed structural members are fabricated from sheet steels that must meet the various material requirements prescribed in applicable national design standards. These requirements ensure that (1) stress concentrations can be redistributed; and (2) members and connections can undergo a minimum amount of displacement without a loss in structural performance. The 1996 Standards Australian/Standards New Zealand (\IAS/NZS) 4600\N, and North American Canadian Standards Association \I(CSA)-S136\N and American Iron and Steel Institute (AISI) cold-formed steel design standards allow for the use of thin (\it<0.9 mm in \IAS/NZS 4600\N), high-strength (\if\i\dy = 550 MPa) sheet steels if the yield stress and ultimate strength are reduced to 75% of their minimum specified values. This paper provides a summary of results detailing the behavior of thin G550 and G300 sheet steel, single overlap, screwed connections. The accuracy of the \IAS/NZS 4600, CSA-S136\N, and AISI design standards diminishes when they are used to estimate the bearing resistance of screwed connections that are loaded in shear. Hence, it is necessary to reduce the bearing coefficient that is used for screwed connections to limit the unconservative nature of these design standards. A proposed design formulation that can be used to more accurately predict the resistance of screwed connections that fail in the combined bearing/tilting mode is presented.