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The high yielding strength of advanced high-strength steel (AHSS) provides great opportunities for cold-formed steel (CFS) members with much higher load-carrying capability. However, if manufactured into the traditional cross-section shapes, such as C and Z, the material advantage cannot be fully exploited due to the cross-section instabilities. The purpose of this study was to establish a shape optimization method for cold-formed sections with AHSS and explore the potentially material efficiency that AHSS could provide to these sections in terms of their axial strength. In this study, the insights provided from the elastic buckling analysis and nonlinear finite element (FE) simulations of a set of traditional CFS sections were employed to determine the appropriate section size and length for optimization. Then, the optimization method was established using the particle swarm optimization (PSO) algorithm with the integration of computational analysis through CUFSM and the design approach (i.e., the direct strength method, DSM). The objective function is the maximum axial strength of the CFS sections manufactured with AHSS using the same amount of material (i.e., the same cross-section area). Finally, the optimal sections were simulated and verified by FE analysis, and the characteristics of the optimal cross-sections were analyzed. Overall, the optimization method in this paper achieved good optimization results with greatly improved axial strength capacity from the optimal sections. [ABSTRACT FROM AUTHOR]

The parametric study of the ultimate strength of perforated cold-formed steel columns is presented in a companion paper. The parameters include five cross-section types (i.e., C, stiffened C, Z, rack, and hat sections), five hole widths, four hole lengths, four hole spacings, three material properties, two magnitudes and various directions of geometric imperfections, and three buckling modes (i.e., L, D, and G) and all the possible interactions between them (i.e., LD, LG, DG, and LDG). Based on the extensive range of strength data obtained, four design methods based on the direct strength method (DSM) are evaluated, including the existing AS/NZS 4600 DSM (Australia Standard 2005), Design Option 4 by Moen and Schafer (2011), and two sets of proposed methods. All these methods are complemented by a reliability analysis deriving appropriate resistance factors. The proposed methods are based on Design Option 4 and include DG interaction. They only require the inputs of elastic local and distortional buckling loads based on gross section properties, which is a significant simplification compared to the methods based on net section properties. Additionally, Proposed Method 1 uses a uniform penalty factor to ensure the satisfaction of the overall resistance factor of 0.85, while Proposed Method 2 adopts a factor based on a regression analysis involving parameters such as the dimensions of holes and cross-sections. Proposed Method 2 outperforms all the other methods in terms of a reduced overall scatter in prediction and an increased overall reliability. This method can also be used for nonperforated columns with a better performance than the current AS/NZS 4600 DSM (Australia Standard 2005). [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive parametric study on the ultimate strength of perforated cold-formed steel columns using the finite element (FE) package ABAQUS. FE models were developed for columns predisposed to local (L), distortional (D), and global (G) buckling failures. The parametric study involved five cross-section types (i.e., C, stiffened C, Z, rack, and hat sections), five widths of holes, four lengths of holes, four different sizes of hole spacings, three material properties, two magnitudes and various directions of geometric imperfections, and three buckling modes (i.e., L, D, and G) and all of the possible interactions between them (i.e., LD, LG, DG, and LDG). Ultimate strengths, as well as failure modes, were obtained from the analyses and are plotted against the current codified direct strength method (DSM) in AS/NZS 4600. Significant discrepancies between the numerical results and the DSM-predicted strengths were observed due to the influence of holes, the interaction of buckling modes, and occasionally the inherent limitations of the DSM itself. The effects of column length, material properties, and centroid shift due to perforation are also discussed. A data pool including 146,207 column-ultimate strengths were obtained, most of which were used to formulate new design equations for perforated thin-walled columns based on the direct strength method, as presented in a companion paper. [ABSTRACT FROM AUTHOR]

The direct strength method (DSM) is a newly developed design method for cold-formed steel members due to its reliability and consistency. It has been well developed and incorporated in the North American Specification and the Australian/New Zealand Standard for the design of cold-formed steel members under compression, bending, and recently, in shear. To date, there are no DSM rules for the design of cold-formed members under localized loading resulting in the web-crippling phenomenon. Recent literature has attempted to propose the DSM design equations for four localized loading cases including the interior one-flange (IOF), end one-flange (EOF), interior two-flange (ITF), and end two-flange (ETF) loading cases specified in the design specifications/standards. However, these proposed DSM equations were calibrated differently depending on the types of loading cases and geometric shapes of the cross-sections by varying coefficients and exponents in the DSM equations. In this paper, consistent and simplified DSM equations previously proposed by the authors are explained and calibrated for use in the design specifications/standards. They include new plastic mechanism models developed for determining the yield load and also cover the design of sections in the inelastic reserve range as observed. Detailed explanations of the yield load (Py) component and references to computing the buckling load (Pcr) component are given. A design example is also included. [ABSTRACT FROM AUTHOR]

This paper investigates the buckling behaviour and load-carrying capacity of G550 high-strength cold-formed steel multi-limb built-up section columns under axial compression. Each built-up section consists of multiple lipped and unlipped channel sections connected by self-tapping screws. The C3U1 section is formed by three C-sections (i.e. lipped channel section) and one U-section (i.e. unlipped channel section), while the C2U2 section is formed by two C-sections and two U-sections. The experimental programme involved tensile coupon tests, measurements of initial geometrical imperfection, and column tests on ten C3U1 and ten C2U2 multi-limb built-up section columns. The numerical modelling programme was then performed, with the nonlinear finite element models created and validated against the experimental results. The applicability of the codified Effective Width Method and Direct Strength Method for predicting the strengths of G550 high-strength cold-formed steel multi-limb built-up section columns, as specified in the American Specification and Australian Standard, was then evaluated. The evaluation results showed that the codified Effective Width Method led to rather conservative and less accurate failure load predictions for G550 high-strength cold-formed steel multi-limb built-up section columns, while the codified Direct Strength Method provided accurate and consistent failure load predictions and thus applicable to the design of G550 high-strength cold-formed steel multi-limb built-up section columns. Concerning the lack of design guidelines specifically for multi-limb built-up sections, a modified Direct Strength Method was thus proposed as an alternative design method and shown to provide accurate failure load predictions. • The structural performance of G550 high-strength CFS multi-limb built-up section columns was investigated. • 20 axial compression tests on G550 high-strength CFS multi-limb built-up section columns were conducted. • FE models were developed and validated against test results and then used for future analysis. • Codified design methods were evaluated for their applicability to G550 high strength CFS multi-limb built-up section columns. • A modified DSM was proposed as an alternative design method and shown to provide accurate failure load predictions. [ABSTRACT FROM AUTHOR]

This research paper presents a new flexural design approach for cold-formed steel (CFS) built-up sections failing in local buckling, developed following an extensive numerical investigation involving two novel closed built-up sections. At first, a finite element (FE) model using the widely adopted ABAQUS software was constructed and carefully validated against pertinent experimental data available in the literature. A comprehensive validation process which included comparing flexural strengths, deformed shapes, moment-curvature, and moment-displacement curves was performed. Afterwards, the validated FE model was extended to a numerical parametric investigation comprising of 108 simulations involving both three-point and four-point bending cases. Key parameters, including cross-sectional shapes, slenderness, and screw spacing, were varied primarily to facilitate the formulation of the novel design method. The outcomes of this investigation revealed that the current direct strength method (DSM) available in the North American Specifications (NAS) underestimated the bending strength, particularly for the ultra-thin sections. Considering this observation which is consistent with the findings of past research on similar cross-sections, a Generalised Direct Strength Method (DSM-G) was developed by introducing a new mathematical model, which differs from the traditional slenderness limit equations, instead relying on a conservativeness degree-based approach used for modifying the original DSM equations. The DSM-G method demonstrated a better accuracy in predicting the flexural strengths of different built-up sections (including the ones investigated by other researchers), all failing by local buckling. Moreover, the reliability of DSM-G equations was assessed, satisfying the prescribed threshold index limit suggested in NAS. A comprehensive set of guidelines, along with a design example for implementing the DSM-G method, has been presented to facilitate practical application. • FE investigation on novel CFS closed built-up sections is carried out. • Key parameters like cross-sectional profiles/sizes and screw spacing are varied. • Current DSM predictions of the novel sections are inconsistent. • A new mathematical model is proposed for a new generalized DSM (DSM-G). • DSM-G is reliable and is verified against results from current and previous studies. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on the performance of cold-formed stainless steel built-up closed section stub columns. The built-up sections were composed by assembling two channel components at the flanges, which were brake-pressed from high-strength austenitic and duplex stainless steel thin sheets. Three types of built-up sectional profiles were formed by connecting two unstiffened channels (i.e., unlipped channels), two edge-stiffened channels (i.e., lipped channels) as well as one unstiffened channel and one edge-stiffened channel, respectively, using self-plugging rivets. Four sectional series were devised to cover nominal overall web height-to-plate thickness ratio values of 50, 60, 67 and 80. In order to obtain the actual material properties considering the cold-work effect, tensile coupon tests were carried out on both flat and corner longitudinal coupon specimens. In addition, a total of 29 fixed-ended stub columns including 19 specimens without holes and 10 specimens with circular web holes were tested under axial compression. The experimental results involving failure modes, ultimate capacities and responses of load versus axial shortening were obtained and fully documented. The effects of sectional profile types, overall web height-to-plate thickness ratio and hole diameter-to-overall web height ratio on the performance of cold-formed stainless steel built-up closed section stub columns were examined. Moreover, the appropriateness of design rules as prescribed in the recently updated American Specification ASCE/SEI 8–22 was evaluated. It was revealed that the codified design provisions generally provided unconservative strength predictions for the high-strength austenitic and duplex stainless steel built-up closed section stub columns both without and with circular web holes. Reliability analyses were performed in this study, and the compressive strength predictions of the built-up closed sections according to the existing design rules were found to be unreliable for the high-strength austenitic stainless steel stub columns without holes as well as the duplex stainless steel stub columns without and with circular web holes. • Compression behaviour of cold-formed stainless steel built-up closed sections was experimentally investigated. • Twenty-nine stub column tests having three built-up sectional profiles were carried out. • Resistances of stainless steel built-up closed section stub columns without and with circular web holes were both examined. • Codified design provisions as per ASCE/SEI 8-22 were assessed. • Reliability analysis was conducted to evaluate the applicability of current design rules. [ABSTRACT FROM AUTHOR]

The direct strength method (DSM) design rules for cold-formed steel members in shear have been incorporated recently into the North American specification for the design of cold-formed steel structural members and are being implemented in the Australian cold-formed steel structures standard. The method, which was calibrated for unperforated members only, requires two inputs including the buckling load Vcr and the shear yielding load Vy. For members with square web cutouts, Vcr can be computed by either the spline finite strip method or the tabulated values based on the shear buckling coefficients kv as in previous studies or the finite element method. However, Vy has not been accurately formulated including holes. This paper represents a practical model to obtain Vy for members with central openings subjected predominantly to shear. The model ranges from very small holes where traditional shear yielding predominates to large holes where Vierendeel action dominates. The model is verified with the DSM design formulas using the predominantly shear tests recently conducted at the University of Sydney and Queensland University of Technology with both square and circular web openings and for shear spans with aspect ratio of 1.0. [ABSTRACT FROM AUTHOR]

The recently incorporated Direct Strength Method (DSM) rules for shear in the North American Specification for Cold-Formed Steel Structural Members (NAS S100:2012) consist of design equations for unperforated sections with and without Tension Field Action (TFA). The TFA is mobilised as a result of the development of diagonal tension due to the full depth bolt restraints. The design shear equations based on a reduction factor α to account for reduced TFA were proposed by Pham and Hancock in cases of partly bolted connections with lateral flange restraints. This paper presents an experimental program to further investigate the TFA of cold-formed C-sections in shear. A total of twelve tests were carried out at the University of Sydney. Three different bolted connection configurations were chosen with and without lateral restraints of the top flanges at the supports. Numerical simulations using the Finite Element Method (FEM) were also performed to validate test results and extend data range. For tests without lateral flange restraints, combined twisting and shear failure modes were observed. The lack of lateral flange restraint leads to a significant reduction of the ultimate load especially when more bolts were removed. Based on these experimental and FEM modelling data, a further reduction factor β dependent upon the slenderness of the sections is introduced in this paper to account for the lack of lateral flange restraint along with the previously proposed reduction factor α which accounts for bolt reduction. [ABSTRACT FROM AUTHOR]

This paper presents the details of an experimental investigation on the section moment capacities of rivet fastened Rectangular Hollow Flange Channel Beams (RHFCB). The rivet fastened RHFCB is a new type of cold-formed steel section, an extension to the widely researched hollow flange beams, shown to have capacities more typically associated with hot-rolled steel beams. It is characterized by a unique geometry, consisting of two rectangular hollow flanges and a web. Unlike other cold-formed sections, the RHFCB has improved capacity due to its mono-symmetric shape and the absence of free edges. The RHFCB manufacturing involves fastening two hollow flanges to a web, utilizing inexpensive intermittent self-pierced rivet fastening. This enables designers to develop optimum sections with varying web and flange thicknesses, therefore delaying the onset of other buckling failures due to slender web element. Various researches have been carried out to investigate the behavior of continuously welded hollow flange beams but little is known on the behavior of RHFCBs. Hence this paper investigates the section moment capacities of rivet fastened RHFCBs. Fifteen section moment capacity tests were conducted to investigate the behavior of RHFCB flexural members. The reduction in section moment capacities from having continuous weld to intermittent rivet connection at its web-flange junction was also investigated. The ultimate moment capacities from the tests were compared with current steel design standards: AS/NZS 4600, AS 4100 and AISI S100. The paper also explores the suitability of the Direct Strength Method, with modification being proposed, from the results obtained from this investigation. [ABSTRACT FROM AUTHOR]

This paper presents a study on the behavior of cold-formed steel built-up box-shape columns with longitudinal stiffeners (CFS-BCL) under axial compression. The specimens are fabricated using a pair of channel sections and lipped channel sections with longitudinal stiffeners, which are expected to enhance the local buckling resistance of the web. Sixteen specimens were prepared for compression tests, with varying lengths and screw arrangements. Prior to the tests, initial geometric imperfections of the specimens, material properties, and shearing-resistance behavior of the screw fasteners were measured. The results indicate that screw arrangements have a significant effect on simply supported specimens; reducing the screw spacing from 300 mm to 100 mm can increase load-bearing capacity by up to 11%. Finite element models were developed using ABAQUS and validated against test results. The parametric analysis revealed that screw arrangements had minimal impact on the local buckling capacity of specimens, with an increase in capacity of <8%. However, global buckling behavior was significantly affected by screw arrangements, resulting in an increase in capacity of up to 20% for slender columns. The test and analytical results were compared with the Direct Strength Method (DSM), and indicate that the DSM is conservative when the non-dimensional slenderness ratio λ g and λ l is higher than 1.2 and 1.4, respectively. The coefficients of the DSM were modified based on the test and simulation results, and its predictive accuracy was found to be highly increased. • Sixteen compression tests were conducted on cold-formed steel box-shaped built-up columns with longitudinal stiffeners. • Numerical models were developed and calibrated against the tests, and parametric analyses were conducted to investigate the influence of screw arrangements. • The Direct Strength Method was calibrated using both test and parametric analysis results. [ABSTRACT FROM AUTHOR]

A parametric investigation and design proposal for the back-to-back built-up cold-formed steel (CFS) channel sections with V-stiffeners and Σ-stiffeners under concentric and eccentric loading are presented in this paper. A nonlinear finite element model considering the initial geometric imperfection was developed and validated against experimental results. A total of 594 simulations were conducted involving a variety of slenderness ratios, eccentric axis directions, eccentricity, screw spacing and number of longitudinal holes. Simulation results including the load-carrying capacity, failure modes, ultimate bearing capacity versus eccentricity relationships, and load versus lateral displacement relationships were discussed. The influence of several member parameters on the load-carrying capacity and buckling modes was analyzed. Additionally, the applicability of the design strengths calculated from Chinese standard GB50018–2002 and North American specifications AISI S100–16 was evaluated based on the preliminary experimental results and simulation results. It was shown that both methods are conservative. Finally, a proposed DSM-based interaction equation considering the relationship between P/P nd and M/M nd was proposed and evaluated. • Nonlinear FE model of built-up I-sections under eccentric compression was developed and validated. • Concentric and eccentric compression of 594 simulates considering the screw spacings and holes effect were discussed. • Specifications applicability to V-stiffened & Σ-stiffened sections were evaluated by adopting the tests and simulates. • Modified DSM considering the relationship between P/P nd and M/M nd was proposed and evaluated. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the buckling behavior of cold-formed steel (CFS) lipped channel members under combined compression and minor axis bending (CaMAB). The numerical models are validated against existing tests. The influences of eccentricity on buckling behaviors are studied, and the results indicate that for the eccentricity toward the web side or lipped side, the failure modes of structural members are mainly controlled by local-global buckling interactions or distortional-global buckling interactions, respectively. The maximum ultimate load capacity may occur at the eccentricity toward the lipped side. Furthermore, a comparison of the load capacity obtained from the interaction formula of the standard EN 1993-1-3 and finite element analysis (FEA) is presented. In addition, the current direct strength method (DSM) curves are modified by considering the effect of the buckling interaction. • Buckling behavior of CFS lipped channel members under CaMAB is investigated. • For eccentricity toward the web or lipped side, failure modes of CFS members are mainly controlled by LB+GB or DB+GB, respectively. • The maximum ultimate load capacity may occur at the eccentricity toward the lipped side. • The modified DSM curves are proposed to accurately estimate the strengths. [ABSTRACT FROM AUTHOR]

• The web crippling behaviour of cold-formed steel channel section having elongated edge-stiffened web holes under interior-two-flange loading condition was studied numerically. • The results of a total of 1,227 new finite element analysis were reported. • Design formulae for the web crippling strength reduction factor were suggested. • A direct strength method (DSM) equation was also presented to evaluate web crippling strength. In the past decade, cold-formed steel (CFS) channel sections having circular edge-stiffened web holes have been developed in New Zealand. Such edge-stiffened holes increase the strength of the CFS channel sections, compared to an equivalent section having unstiffened holes, while still allowing full service integration. In the case of web crippling, previous research has found that use of edge-stiffened holes almost results in the same strength of an equivalent channel-section having a plain web. Such circular edge-stiffened web holes can now be extended to elongated edge-stiffened web holes. However, for such elongated holes, no experimental tests have been reported in the literature. In this paper, a numerical investigation was carried out, and non-linear finite element (FE) analyses were used to investigate the web crippling behaviour of CFS channel sections having edge-stiffened web holes under the interior-two-flange (ITF) loading condition. The cases of both flange fastened and flange unfastened were considered. The FE models were validated against test results of sections having circular edge-stiffened web holes; good agreement in terms of the load–displacement curves and deformed shapes was shown. Using the validated FE models, a parametric study was carried out on CFS channel sections having elongated un-stiffened and edge-stiffened web holes, comprising 1,227 finite element analyses (FEA) results. Compared to sections having a plain web, for the case of an elongated opening, without any edge-stiffener, having an aspect ratio of two and three, the average reduction in web crippling strength was 39% and 49%, respectively. However, for an edge-stiffened hole, the reduction in the web crippling strength was reduced to only 2% and 16%, respectively. Finally, the design equations in the form of the web crippling reduction factor (R p) and the equations based on the direct strength method (DSM) for CFS sections with elongated web holes were proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The behavior of cold-formed steel built-up closed cross-section columns is investigated. • 595 built-up column test results were used for assessment. • The current DSM design predictions are largely scattered. • The results indicated that the primary influential parameters were not incorporated in the DSM approach. • Future directions for the accurate design of built-up columns are presented. Structural behaviour of cold-formed steel (CFS) built-up closed cross-section columns subjected to local, global, and local–global interactive failure is investigated. The objective is to assess the appropriateness of six different Direct Strength Method (DSM) based designs proposed by various researchers including the current AISI codification. A total of 595 built-up column test results from the existing literature were used for assessment. The comparison of test results and design predictions indicated that the primary influential parameters that inhibit or induce the local–global interaction failure in closed cross-section built-up columns were not well incorporated into the designs. Owing to that, the DSM design predictions are largely scattered; conservative by a maximum of 258% and unconservative by 60%. The reliability analysis was carried out to statistically assess the design methods. It is found that five out of six designs are reliable but do not represent the actual structural behavioral trend. Following this, representative sample results and failure modes were analyzed for identifying the influential parameter that causes design methods to result in inaccurate failure load. The analysis shows that the strength of the column increases with a decrease in intermediate fastener spacing (less than the local buckling halfwave length) due to the flange overlapping effect while the column strength decreases due to local–global interactive buckling caused by a larger intermediate fastener spacing (higher than the local buckling halfwave length). Finally, the paper presents future directions on how the DSM should evolve for the accurate design of built-up closed cross–section columns. [ABSTRACT FROM AUTHOR]

The North American Standard (NAS) and the European Standard (EC3) for cold-formed steel design provide limited guidelines for the strength predictions of built-up columns. The existing design equations are limited to flexural buckling and local-flexural interactive buckling. The present study evaluates the local-flexural interactive design equations for cold-formed steel built-up back-to-back I-section columns against the results of experimental and numerical studies. The results show that the existing design guidelines for local-flexural interactive buckling are unconservative by about 17%, 5%, and 15% for DSM-NAS, EWM-NAS, and EWM-EC3, respectively. Furthermore, a maximum reduction in the built-up column strength by local-flexural interaction is observed when its nominal local and flexural buckling loads are close. Based on the results and observations, new design equations are proposed by including the product of nominal local and flexural buckling loads for the North American and European standards, which capture the local-flexural interactive buckling in built-up I-section columns. The effect of fastener spacing on the ultimate load for local-flexural interactive buckling is also evaluated, and its effect is found to be small, unlike for global buckling. The minimum fastener spacing criteria of NAS are assessed, and suitable recommendations are provided for fixed-end columns. Overall, the present study provides insights into the local-flexural interactive buckling behavior of built-up I-section columns and proposes reliable design equations to predict the ultimate load. [ABSTRACT FROM AUTHOR]

The direct strength method (DSM) of design of cold-formed sections has been recently extended in the North American Specification for Cold-Formed Steel Structural Members NAS S100:2012 to include shear. The two new features of the DSM rules for shear researched are the effect of full-section shear buckling as opposed to web-only shear buckling and tension field action (TFA). The prequalified sections in the rules include sections with flat webs and webs with small intermediate longitudinal stiffeners. In order to extend the range to larger intermediate stiffeners as occurs in practice, a series of fourteen shear tests have been performed at the University of Sydney for C-sections with rectangular stiffeners of varying sizes. Six different types of stiffeners were tested with an additional preferred plain section. Each type of section was tested twice to ensure accuracy. As the web stiffener sizes increase, the shear buckling and strength of the sections are expected to improve accordingly. However, the tests show that the shear ultimate strengths only increase slightly in association with the respective increases of stiffener sizes. The test results are compared with the DSM design rules for shear and found to be lower than those predicted by the DSM curve for shear with TFA. The test failures were observed mainly due to the combined bending and shear modes. The effect of the bending is therefore significant and starts to govern when the shear capacity is significantly strengthened by adding the large longitudinal web stiffener. The test results are subsequently plotted against the DSMinteraction curves between bending and shear, where the interaction is found to be significant. Recommendations for prequalified sections with longitudinally stiffened web channels in combined bending and shear are validated in the paper. [ABSTRACT FROM AUTHOR]

Shear behaviour of cold-formed steel beams with an aspect ratio (shear-span/web-depth) of 1.0 has been studied thoroughly, mainly using central point load tests. However, for beams with higher aspect ratios, the effect of bending causes a reduction of shear capacity and alters the failure modes. This paper introduces a new test setup, the dual actuator test rig, which is equipped with two actuators moving independently at customizable displacement rates to control and minimize applied bending moments in the shear spans of interest. Shear strength close to pure shear capacity can be therefore reached even at an aspect ratio of 2.0. Experimental series recently performed at the University of Sydney on channel section members using the new test configuration is presented. The experimental results together with other available experimental data are combined and used to validate and re-calibrate the Direct Strength Method design formulae incorporated in the AISI S100:2016 Specification and the AS/NZS 4600:2018 Standard. • An experimental program of cold-formed channel sections in shear with aspect ratio of 2.0 is performed • A new dual actuator test rig for testing channel sections in shear with minimum bending effect is developed and conducted • Test results affirmed the applicability of the DSM shear design provisions with shear span aspect ratios up to 2.0 • All test results on CFS channels with aspect ratios of 1.0 and 2.0 are used to recalibrate the DSM shear design rules • Resistance factors of 0.95 (AISI S100-16) and 0.9 (AS/NZS 4600:2018) for the DSM shear strength predictions are confirmed [ABSTRACT FROM AUTHOR]

Abstract Significant progress has been made on structural behavior of hot-finished (HF) steel members with elliptical hollow sections (EHS) in the past few years. However, there is very limited research on cold-formed (CF) steel members with EHS. In this study, both CF and HF steel elliptical tubular stub columns are investigated. The investigation includes laboratory testing, numerical analysis and design evaluations. The geometric imperfection measurements, full load-end shortening relationships and load-strain curves of the stub columns are detailed in this paper. A non-linear finite element model was developed and validated against test results obtained from the present study as well as from the literature. The validated numerical model was employed to perform an extensive parametric study, where the CF and HF stub columns with EHS were carefully designed to include a broad range of the cross-section slenderness and aspect ratio. The cold-forming effect on the CF sections was taken into account in a conservative manner. The ultimate strengths obtained from the tests and numerical analysis were compared with the predictions in the design strengths predicted by the design methods proposed in the existing literature and the Direct Strength Method (DSM). The comparisons showed that the strengths predicted by the equivalent diameter method, the DSM and the modified DSM are generally conservative and reliable. The modified DSM provides the most accurate and the least scattered predictions among these design methods. Therefore, the modified DSM is recommended for the design of CF and HF steel elliptical tubular stub columns. Highlights • Cold-formed and hot-finished steel elliptical tubular stub columns were tested. • Non-linear FEM was developed and validated against the test results. • Parametric study was performed on cold-formed and hot-finished steel elliptical tubular stub columns. • The equivalent diameter method, the Direct Strength Method (DSM) and the modified DSM were assessed. • The modified DSM is recommended for the design of cold-formed and hot-finished steel elliptical tubular stub columns. [ABSTRACT FROM AUTHOR]

Highlights • Flexural response of cold-formed steel floor trusses is studied experimentally. • CFS section thickness and diagonal-chord connection detail are considered as test variables. • Elastic analysis overpredicts truss service stiffness due to neglect of connection flexibility. • Experimentally determined strain profiles indicate nonuniform strains in truss chords. • Considering local buckling of CFS section with unfolded lips results in accurate strength prediction. Abstract This paper reports findings of an experimental study on floor trusses fabricated from cold-formed steel lipped channel sections. In design, the members of these trusses are generally considered as pin-ended and their capacities are assessed under pure compressive and tensile loads by making use of commonly accepted design specifications. Seventeen full-scale floor trusses were experimentally investigated through four point bending tests. Thickness of lipped channels, number of connection fasteners, and type of diagonal connection were considered as the prime variables. Test results indicate that there are marked differences between the design and measured values in terms of both the strength and stiffness. The service load stiffnesses of the trusses were found to be lower than the stiffnesses obtained using two dimensional truss models due to the neglect of connection flexibility in analysis models. Additional experiments on screw connections were conducted to find out their axial stiffness. The results from connection tests were used to develop analytical expressions which can be used to simulate connection flexibility. Similar to the stiffness, the strength of the trusses were found to be lower than the predicted strength using the pin-ended member under compression approach. In addition, the observed failure modes are markedly different than the code predictions. The sources of these differences were found to arise from the unfolding of lips at the diagonal to chord member connections and the global bending effects. The strength and failure mode of trusses can be more accurately predicted by considering local buckling of channel sections with unfolded lips. [ABSTRACT FROM AUTHOR]

Highlights • Cold-formed ferritic stainless steel hollow sections undergoing web crippling were investigated. • A total of 44 tests was carried out under EOF, IOF, ETF, and ITF loading conditions. • Finite element models were developed and validated against the test results. • A parametric study of 154 specimens for both slender and stocky sections was performed. • Existing web crippling design provisions are assessed and improved design rules are proposed. Abstract This paper presents experimental and numerical investigations on cold-formed ferritic stainless steel (CFFSS) square and rectangular hollow sections undergoing web crippling. A total of 44 web crippling tests was carried out under the four codified web crippling load cases as per the American cold-formed stainless steel specification. Numerical model of each load case was built and verified with the web crippling test results. After the verification, a parametric study comprised of 154 finite element analyses was undertaken thereafter to gain further insight into the behaviour of the CFFSS sections undergoing web crippling. The current design provisions in the American, Australian/New Zealand and European stainless steel codes of practice were assessed. Moreover, the North American Specification (NAS) provisions for carbon and low-alloy steel sections were evaluated. Furthermore, design recommendations in the literature for stainless steel sections were examined. Improved design rules are proposed in this study for CFFSS square and rectangular hollow sections by modifying NAS and Direct Strength Method. Reliability analysis was also undertaken to assess the reliability levels of the existing and modified provisions. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes a framework to predict the buckling limit states of C-section purlins under wind uplift load with top flange restrained by the sheeting. The prediction framework includes a nonlinear twisting model to account for the flexural-torsional behavior and the application on the Direct Strength Method. The torsional effect induced by load eccentricity and the diaphragm effects provided by sheeting are incorporated into the flexural-torsional differential equations of the nonlinear twisting model. The biaxial bending moments and bi-moment, the flexural-torsional buckling strength and the stress distribution of the purlin can be obtained from the proposed model. On the basis of the Finite Strip Method (FSM), the elastic buckling load can be calculated through reducing the applied bending moments by the load factor. Thereafter, the Direct Strength Method is modified to account for the buckling interaction behavior influenced by the warping-torsion and diaphragm effects. A database of 17 simple-span purlin tests is used to validate the proposed model. Graphical abstract Unlabelled Image Highlights • Proposed a nonlinear twisting model to calculate the biaxial bending moments and bi-moment. • Elastic buckling load can be obtained based on the biaxial bending moments and bi-moment combined with FSM. • Proposed a modified Direct Strength Method to account for the interaction behavior considering the diaphragm effects. [ABSTRACT FROM AUTHOR]

Shear behaviour of cold-formed steel beams with an aspect ratio (shear span/web depth) of 1.0 has been studied thoroughly, mainly using central point load tests. However, for beams with longer aspect ratios, the effect of bending causes reduction of shear capacity and alters the failure modes. This paper summarises experiments recently performed at the University of Sydney on channel section members using a new test configuration to minimize bending moments. Shear strength close to pure shear capacity can be therefore reached even at an aspect ratio of 2.0 . The test results were compared with the strength predictions using the current direct strength method (DSM) of design for shear specified in the North American specification for the design of cold-formed steel structural members, AISI S100:2016 and the Australian/New Zealand Standard for cold-formed steel structures, AS/NZS 4600:2018. A good agreement between the experimental results and the predicted values has confirmed the viability of the DSM design rules for structures with aspect ratios up to 2.0 . Numerical models were developed to foresee potential issues prior to the experimental work, and were subsequently calibrated against the tests to produce reliable results. [ABSTRACT FROM AUTHOR]

• Tests on the behaviors of slotted perforated cold-formed steel (CFS) channels subjected to eccentric compression. • A nonlinear elasto-plastic FE model was developed and validated against the experimental results. • The extensive FE analyses were carried out for predicting the eccentricity impact coefficient formulas. • The simplified DSM formulas of slotted perforated CFS channels subjected to eccentric compression were proposed, without the need to calculate the elastic buckling critical stress for slotted perforated CFS channels. This paper aims to investigate the buckling behaviors of cold-formed steel (CFS) channels with slotted web holes determined by the Steel Stud Manufacturers Association (SSMA) under eccentric compression, and propose the simplified direct strength method (DSM) formulas for engineering practicing. An experimental study on 20 specimens was conducted, including 18 slotted perforated specimens with nine eccentricities increasing from −20 mm to 20 mm and 2 unperforated specimens subjected to axial compression. Two kinds of nominal lip lengths for cross sections were selected as 15 mm and 20 mm. The experimental results indicated that the failure modes of slotted perforated specimens subjected to eccentric compression depended on the position of the eccentricity. Compared with the slotted perforated specimens subjected to pure axial compression, the ultimate loads of specimens subjected to positive eccentric compression decreased by 21.70 % on average, while they decreased by 16.38 % on average for those subjected to negative eccentric compression. Moreover, a nonlinear elasto-plastic finite element model (FEM) was developed and validated against the experimental results. Furthermore, combined with the experimental results, extensive FE parametric analyses were conducted to predict the eccentricity impact coefficient formulas. Finally, based on DSM of unperforated CFS members subjected to pure axial compression in AISI S100-16 and the eccentricity impact coefficient formulas, the suitable DSM design formulas for slotted perforated CFS channels determined by the SSMA under eccentric compression were proposed, without the need to calculate the elastic buckling critical stress for slotted perforated CFS channels. [ABSTRACT FROM AUTHOR]

Built-up sections are increasingly used as structural elements in the cold-formed steel (CFS) industry. They are composed of two or more component sections connected by discrete fasteners, typically spaced evenly along the length and potentially with fastener groups at the ends. Conventionally, two singly symmetric C-sections are connected to form a doubly symmetric cross-section, and current design guidelines are limited to this particular application. As a means towards broadening the application of built-up CFS sections, the cross-sections in this study were composed of three or four lipped channel sections. The paper presents an experimental investigation of the strength and behaviour of built-up section columns of various lengths and cross-sectional shapes. As observed in the tests, singly-symmetric columns composed of three channel sections (3C) experienced either local, distortional and/or flexural-torsional buckling failure modes, while doubly-symmetric columns formed by four sections (4C) failed in local, distortional and/or flexural buckling modes. The current AISI Specification specifies the use of a modified slenderness ratio only for built-up sections composed of two sections connected back-to-back, whereas the design proposal in this paper recommends the use of an effective rigidity approach coupled with the Direct Strength Method for predicting the strengths of the test 3C and 4C built-up sections. This proposal also suggests a design procedure for the built-up sections experiencing flexural-torsional buckling, which is neither explicitly stipulated in current design standards nor in previous studies. [Display omitted] • First reported tests of built-up sections columns failing in global modes with 3 or more component sections. • 20 full-scale tests of built-up section columns are comprehensively reported. • First paper to present a design methodology for built-up section columns failing by flexuraltorsional buckling. • Proposed design procedure uses effective rigidities rather than modified slenderness. [ABSTRACT FROM AUTHOR]

Cold-formed stainless steel circular hollow section (CHS) columns have been increasingly used in construction, due to its aesthetic appearance, long life-span and good ductility. It is shown that direct strength method (DSM) is capable of predicting cold-formed steel column strengths accurately. However, the DSM is developed for cold-formed steel sections with plate rather than curved elements, and thus its applicability for cold-formed stainless steel CHS is worth investigating. This paper presents a numerical investigation of cold-formed stainless steel CHS columns. A non-linear finite element model was developed and verified against column tests. Extensive parametric study of cold-formed duplex, lean duplex and ferritic stainless steel CHS columns has been performed to obtain column strengths. A total of 273 experimental and numerical cold-formed stainless steel CHS column strengths, which are obtained from previous researches and parametric study obtained from this study, are compared with the design strengths predicted by the current DSM. Reliability analysis was performed to evaluate the reliability of the design rules. It is shown that the current DSM provides unconservative and not reliable prediction for cold-formed stainless steel CHS columns. Therefore, modified DSM is proposed for cold-formed stainless steel CHS columns. It is shown that the modified design rule is more accurate than the current DSM, and the modified design rule is considered to be reliable. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents an experimental program investigating the local and/or distortional (sectional) buckling of built-up cold-formed steel sections subjected to pure compression. The experiments were designed to study the influence of key parameters, including buckling mode, cross-section geometry and fastener spacing, on the ultimate sectional capacity of built-up columns. Three singly- or doubly-symmetric built-up sections were considered with two sets of component lipped channel sections, one buckling predominantly in the distortional mode and one buckling in the local mode. The component sections were connected using self-drilling screws at three different spacings. Material properties, geometric imperfections and residual stresses of the specimens were measured prior to testing. A total of 36 specimens with a length of 1 m were tested under concentric compression between fixed ends, which was achieved through a special end plate connection. The specimens failed in either distortional or the interaction of local and distortional buckling modes. Compared to the sum of the capacity of component sections, the ultimate capacity in built-up columns with distortional buckling mode increased significantly with the reduction of screw spacing, whereas the increase was minor for specimens that failed through local-distortional interaction. The ultimate strength of the specimens was predicted using the current Direct Strength Method, where either conservative or overly unconservative predictions were obtained depending on the considered composite action level in the elastic buckling analysis. • Experimental study of built-up sections with more than two component sections. • Fastener-constrained distortional and local buckling failure modes. • Influence on strength of cross-section geometry and fasteners spacing. • Comparison of experimental strengths to predictions by the Direct Strength Method. [ABSTRACT FROM AUTHOR]

Thin-walled cold-formed hollow flange channel (HFC) sections are increasingly becoming popular due to their potential benefits such as increase in buckling capacities provided by the presence of two torsionally rigid hollow flanges and the elimination of free edges. Past research studies of HFC sections were limited to their shear and bending capacities. This paper investigates their section compression capacities through a series of stub column tests, followed by finite element modelling of welded HFC columns. The developed finite element models were validated using experimental results, and then used to investigate the section compression capacity of HFCs made by welding rectangular sections to a steel plate or cold-forming and rivet/screw fastening to hollow flanges, where steel plates with different strengths and thicknesses were used as web and flange elements. Extensive structural performance data of HFC stub columns subject to local buckling was thus obtained covering the effects of varying slenderness of plate elements and the use of different strength steels for web and flange elements. Furthermore, the applicability of the available design rules such as effective width and direct strength methods to predict the compression capacity of such HFC sections was also evaluated. Suitable recommendations are then proposed to improve their accuracy. This study facilitates and advances the use of HFC sections as compression members. [ABSTRACT FROM AUTHOR]

The use of cold-formed steel (CFS) channel sections is becoming popular as the load-carrying members in building structures, and such channel sections are often perforated for the ease of installation of services. However, limited experimental investigation has been reported in the literature for such channel sections under pinned-pinned boundary conditions. In this paper, a total of 16 CFS channel sections with and without slotted web holes were tested under pinned-pinned boundary conditions, and it was shown that all specimens with short lips failed by distortional-local buckling interaction controlled by distortional buckling and all specimens with long lips failed by local buckling. Moreover, the slotted web holes caused the axial capacities to decrease slightly by 2.4% on average for specimens failed mainly by distortional buckling and 6.4% on average for specimens failed by local buckling. A nonlinear elasto-plastic finite element (FE) model was also validated against the test results, a parametric study was conducted using the validated FE model. In addition, the results obtained from the tests and parametric study were compared against the results predicted by the current direct strength method (DSM) in AISI S100–16 for CFS channel sections with web holes. It indicates that the results predicted by the DSM are un-conservative for most CFS channel sections with web holes under pinned-pinned boundary conditions, especially un-conservative by 27.0% on average for CFS channel sections failed by local buckling. Therefore, new design formulas were developed based on the current DSM formulas for CFS channel sections with web holes. • Compression tests on cold-formed steel (CFS) with slotted web holes under pinned-pinned boundary conditions. • A non-linear finite element (FE) model was developed and validated against tests. • The extensive FE analyses with different parameters were carried out for study on direct strength method. • The DSM formulas were proposed for CFS with web holes under pinned-pinned boundary conditions in compression. [ABSTRACT FROM AUTHOR]

This research highlights the possibility of LD interaction in the built-up columns and provides a detailed design procedure to account for the strength erosion due to LD interaction. For this, a comprehensive numerical study using finite element (FE) method is performed where the FE models are validated using the test results in the literature. In this study, three types of built-up sections, i) back-to-back I section made of two lipped channels, ii) nested section made of two lipped channels, and iii) nested section made of one lipped and one unlipped channel, are selected. The effects of LD interaction, fastener spacing, and end fastener group (EFG) on the ultimate strength of built-up columns are investigated. The study reveals that the LD interaction behaviour of the built-up back-to-back I section in terms of failure mode and normalized ultimate strength result (P u / P y) is similar to that of a lipped channel. For the built-up nested section, improved strength is observed when the lipped channel fails in distortional buckling. No improvement is observed in the LD strength of built-up sections with reduced fastener spacing or by using EFG. The study shows that the traditional direct strength method (DSM) based design procedure for LD interaction is conservative for built-up sections. Hence, a reliable design procedure is proposed based on the modified DSM equations obtained from the literature. The strength and failure mode predictions of the proposed method match well with those of experimental studies in the literature and the numerical study of this paper. • Evidence of local-distortional (LD) interaction in built-up columns is presented based on a detailed numerical study • Investigated the behaviour of both back-to-back and nested sections made of lipped channels and unlipped channels. • Explored the effects of built-up formation type, fastener spacing and end fastener group on the LD interaction strength. • LD interaction is significant in local buckling dominant sections than distortional buckling dominant sections. • New direct strength method based design procedure is proposed which is more accurate than the traditional design approach. [ABSTRACT FROM AUTHOR]

Built-up back-to-back cold-formed steel channel (BC) members are increasingly used as studs for increasing the compression capacity of light gauge steel framed (LSF) walls, even though the knowledge of their behaviour is limited. In this study, the behaviour and capacities of LSF walls made of slender BC members sheathed with gypsum plasterboards on both sides were investigated using a series of compression tests and detailed finite element studies based on validated models. The numerical and experimental results showed that (1) screw connections of BC members had marginal effects on the compression capacities of the investigated sheathed BC members with non-staggered stud-to-board screw arrangement, and the capacities were mostly equal to twice the capacities of their individual members, (2) the restraints provided by the plasterboards and stud-to-board screw connections effectively prevented sheathed BC members from in-plane flexural buckling and considerably improved their compression capacities, and (3) the Direct Strength Method in the current design standards in combination with the guidelines and recommendations proposed in this study conservatively predicted the compression capacities of these members. Details of this research study, its results and findings are presented in this paper. [Display omitted] • Investigated the behaviour of built-up back-to-back channel (BC) studs sheathed with gypsum plasterboards. • Plasterboards and stud-to-board screws restrained sheathed studs from in-plane flexural buckling and improved the capacity. • The capacity of sheathed BC studs with non-staggered stud-to-board screws was about twice the individual member capacity. • The effect of stud-to-stud screw arrangement was marginal. • Conservative DSM-based design guidelines and recommendations are proposed. [ABSTRACT FROM AUTHOR]

This paper proposed and investigated the performance of cold-formed thin-walled steel open-section lipped channel columns reinforced with plain channel sleeve(s). The novel reinforcement strategy was intended to improve the post-distortional ultimate bearing capacity of the reinforced lipped channel columns. A total of 24 reinforced columns at four different lengths (400 mm, 1000 mm, 1600 mm, and 2200 mm) were tested under axial and eccentric loads. Initial geometric imperfections, load-displacement curves, load-strain curves, ultimate bearing capacity, and collapse modes from the experimental study were summarized and discussed. A finite element model was developed and validated against the experimental results. Then a total of 300 finite element simulations considering a variety of parameters, such as sleeve spacing, axial and eccentric compression about minor (y-y) and major (x-x) axis, and lip length, were performed and investigated. The results demonstrated the effectiveness of channel sleeves in restraining or inhibiting the occurrence and development of distortional buckling. The results were also evaluated with the current design provisions, such as the Effective Width Method in AISI S100–16, AS/NZS 4600, and EN1993-1-3, and the Direct Strength Method (DSM) in AISI S100–16. Finally, due to conservativeness in these methods, a modified design method based on DSM in AISI S100–16 was proposed. • Installing channel sleeve at the open side of C-sections to constrain distortional behavior was proposed. • Concentric and eccentric compression of 24 tests and 300 simulates of sleeve reinforced C-sections were investigated. • Performance enhanced C-sections under axially, x-x positive and negative eccentric, and y-y eccentric compression was examined. • Influence on distortional half-wavelength and component characteristics was discussed. [ABSTRACT FROM AUTHOR]

The cold-formed steel (CFS) double-lipped equal-leg angle is widely used in modular container houses and cold-formed steel buildings. To study the buckling behavior and bearing capacity design method of the cold-formed steel (CFS) double-lipped equal-leg angle under axial compression, 24 CFS double-lipped equal-leg angles with different sections and slenderness ratios the axial compression were conducted. The test results showed that the distortional buckling occurs for specimens with a small width-to-thickness ratio and small slenderness ratio. The buckling interactive with distortional and global flexural buckling was observed for the specimens with small width-to-thickness ratios and large slenderness ratios. The specimens with large width-to-thickness ratios and small slenderness ratios showed interactive buckling with local and distortion buckling. The specimens with large width-to-thickness ratios and large slenderness ratio developed interactive buckling with local, distortional, and global flexural buckling. The finite element model established by ABAQUS software was used to simulate and analyze the test. The buckling modes and the load-carrying capacities analyzed by the finite element model agreed with the test results, which showed that the developed finite element model was feasible to analyze the buckling and bearing capacity of the CFS double-lipped equal-leg angles. The experimental results were compared with those calculated by the direct strength method in the North American standard and the effective width method in the Chinese standard. The comparisons indicated that the calculated results are very conservative with maximum value 36% and 51% for direct strength method and effective width method, respectively. The coefficient of variation was 0.276 and 0.397, respectively. Finally, the modified direct strength method and the modified effective width method were proposed based on the experimental results. The comparison on the ultimate strength between test results and calculated results by using the modified method showed a good agreement. The modified method can be as a proposed desigh method for the ultimate strength of the CFS double-lipped equal-leg angles under axial compression. [ABSTRACT FROM AUTHOR]

Cold-formed steel channels with web openings in shear have been thoroughly investigated mainly with circular and square holes. However, web cut-outs are likely to be enlarged along the span length of members due to a limited web depth to provide more access for the building services in practice. The design of cold-formed sections with elongated web holes in shear was investigated and retained in the AISI S100–16 and the AS/NZS 4600:2018 using the empirical approach. This paper presents an experimental investigation of perforated channel sections with rectangular and slotted web openings in shear using a dual actuator test rig to minimise the effects of bending moments and obtain the predominantly shear capacity with an aspect ratio up to 2.0. The commercially available plain C-lipped channel sections with a depth of 200 mm and a thickness of 1.5 mm were chosen with varying sizes of central cut-outs. A new strategy for determination of the shear yield loads (V y) together with the Vierendeel mechanism approach is presented in this study to propose a modified design for perforated channels with web openings having aspect ratios of holes up to 3.0 for the Direct Strength Method (DSM) of design of channel members in shear with elongated web openings. In addition, as required in the DSM method, the shear elastic buckling loads (V cr) are extracted from the linear elastic buckling analyses using finite element method in this paper. • An experimental investigation of channel sections with rectangular and slotted web openings in shear is performed. • A dual actuator test rig to minimise the effects of bending moments and obtain the predominantly shear capacity is used. • Shear elastic buckling loads are extracted from the linear elastic buckling analyses using finite element method. • New strategy for determination of the shear yield loads together with the Vierendeel mechanism approach is presented. • Direct Strength Method modified design rules for perforated channels with elongated web openings in shear are proposed. [ABSTRACT FROM AUTHOR]

The intermittently rivet fastened Rectangular Hollow Flange Channel Beam (RHFCB) is a new cold-formed hollow section proposed as an alternative to welded hollow flange channel beams. It is a monosymmetric channel section made by intermittently rivet fastening two torsionally rigid rectangular hollow flanges to a web plate. This process enables the end users to choose an effective combination of different web and flange plate sizes to achieve optimum design capacities. Recent research studies focused mainly on the shear behaviour of the most commonly used lipped channel beam and welded hollow flange beam sections. However, the shear behaviour of rivet fastened RHFCB has not been investigated. Therefore a detailed experimental study involving 24 shear tests was undertaken to investigate the shear behaviour and capacities of rivet fastened RHFCBs. Simply supported test specimens of RHFCB with aspect ratios of 1.0 and 1.5 were loaded at mid-span until failure. Comparison of experimental shear capacities with corresponding predictions from the current Australian cold-formed steel design rules showed that the current design rules are very conservative for the shear design of rivet fastened RHFCBs. Significant improvements to web shear buckling occurred due to the presence of rectangular hollow flanges while considerable post-buckling strength was also observed. Such enhancements to the shear behaviour and capacity were achieved with a rivet spacing of 100 mm. Improved design rules were proposed for rivet fastened RHFCBs based on the current shear design equations in AISI S100 and the direct strength method. This paper presents the details of this experimental investigation and the results. [ABSTRACT FROM AUTHOR]

This paper presents an investigation on interaction equation between the required flexural strength, M, and the required shear strength, V, of cold-formed C-channels with web holes and bearing stiffeners. The primarily shear condition test was employed to study total 8 back to back lipped C channel sections of 95 and 100 mm depth when bearing stiffeners and circular holes were placed at center and both ends of specimens. The interaction equation were evaluated via Direct Strength Method, DSM, in accordance with the American Iron and Steel Institute for the design of cold-formed steel structural members, AISI 2007. A nonlinear finite element model was developed and verified against the test results in terms of failure buckling modes. It was concluded that the M-V interaction equation for specimens with web stiffeners was conservative where these specimens experienced plastic failure mode rather than local (Msl) or distortional (Msd) buckling mode. Moreover, the results indicated that proposed M-V interaction equation calculated by local buckling strength (Msl) adequately predicted the behavior of specimens with circular web holes. [ABSTRACT FROM AUTHOR]

Experimental tests have previously shown that the strength of bolted moment-connections between cold-formed steel channel members, where the connections are formed through an array of bolts in the web, is dependent on the length of the bolt-group. This reduced strength has been observed in tests on portal frame joints as well as overlapped purlin joints. For a long bolt-group length, an upper bound would be the full in-plane major axis moment-capacity of the section, which can be predicted using the conventional Direct Strength Method (DSM). For a short bolt-group length, in the order of the depth of the section, a reasonable lower bound to the reduced strength can be predicted by using the DSM, modified to include the effect of the bimoment at the connection. In this paper, a parametric study is performed covering sections of different values of slenderness. [ABSTRACT FROM AUTHOR]

Light gauge cold-formed steel sections have been developed as more economical building solutions to the alternative heavier hot-rolled sections in the commercial and residential markets. Cold-formed lipped channel beams (LCB), LiteSteel Beams (LSB) and Triangular Hollow Flange Beams (THFB) are commonly used as flexural members such as floor joists and bearers while Rectangular Hollow Flange Beams (RHFB) are used in small scale housing developments through to large building structures. However, their shear capacities are determined based on conservative design rules. For the shear design of cold-formed steel beams, their elastic shear buckling strength and the potential post-buckling strength must be determined accurately. Hence experimental and numerical studies were conducted to investigate the shear behaviour and strength of LCBs, LSBs, THFBs and RHFBs. Improved shear design rules including the direct strength method (DSM) based design equations were developed to determine the ultimate shear capacities of these open and hollow flange steel beams. An improved equation for the higher elastic shear buckling coefficient of cold-formed steel beams was proposed based on finite element analysis results and included in the design equations. A new post-buckling coefficient was also introduced in the design equations to include the available post-buckling strength of cold-formed steel beams. This paper presents the details of this study on cold-formed steel beams subject to shear, and the results. It proposes generalised and improved shear design rules that can be used for any type of cold-formed steel beam. [ABSTRACT FROM AUTHOR]

• Buckling behaviour of cold-formed steel perforated members under combined compression and minor axis bending is studied. • The beam-column tests and numerical modelling are performed for the study. • The specimens generally failed in the mode of distortional-global buckling interaction. • The applicability of existing design approaches is assessed and discussed. • A modified DSM distortional strength curve is proposed to accurately estimate the structural strengths. In this paper the buckling behaviour of cold-formed steel (CFS) members with web perforations under combined compression and minor axis bending is experimentally and numerically studied. A total of sixteen specimens with web openings were examined, and extra three specimens without perforation were also tested for comparisons. The load–displacement performances of all specimens are discussed. The specimens generally failed in the mode of distortional-global buckling interaction. Moreover, the effect of perforations on buckling behaviour of CFS members is studied. Comparisons of buckling behaviour between members with different perforation shapes, different perforation sizes, different perforation areas and various lengths are provided. The numerical models are developed and verified by the experimental data, and the comprehensive numerical analyses are conducted to further investigate the buckling behaviour of perforated members. The strengths obtained from the tests and the numerical analyses reveal that the current Direct Strength Method (DSM) curve overestimates the load-carrying capacity of perforated beam-columns experiencing distortional buckling and a modified DSM is then proposed. [ABSTRACT FROM AUTHOR]

• A total of 41 cold-formed steel built-up I-sections were tested under ETF, ITF, EOF and IOF loading conditions. • Finite element models were developed to replicate the experiments. • Parametric study of 258 specimens was performed. • The existing codified web crippling design provisions were assessed. • Design rules were proposed based on the unified equation and direct strength method. This paper reports the experiments, numerical modelling and design of cold-formed steel built-up I-sections subjected to localised loadings. The built-up I-sections were made up of two equal-sized plain channels fastened back-to-back in the web. The web crippling tests were undertaken on 41 built-up I-section members under Interior-Two-Flange (ITF), End-Two-Flange (ETF), Interior-One-Flange (IOF) and End-One-Flange (EOF) loading conditions. The effects of the arrangement of screws along the web height were investigated. After the experiments, finite element (FE) analyses considering the initial local imperfections of the web of the specimens were performed and the validity of the FE models was assessed using the test results. A parametric investigation including 258 numerical simulations was conducted to expand the database and evaluate the influences of the screw arrangements in the web height. The test and numerical results indicated that the failure modes of the specimens with different screw arrangements were generally similar in each loading condition, and the screws located close to the flange-web junctions of the sections could generally provide maximum web crippling resistances of the built-up I-sections. The design recommendations on the arrangements of screws along the web height were proposed. The experimentally and numerically generated web crippling resistances were utilized to assess the current North American specification (AISI S100), Australian and New Zealand standard (AS/NZS 4600) and European code (EN1993-1-3). The results show that the existing codified design formulas lead to either conservative or highly unsafe results. Therefore, two sets of design formulas were derived on the basis of unified equation and direct strength method. [ABSTRACT FROM AUTHOR]

The experimental investigation and numerical analyses are presented in this paper on cold-formed steel built-up open and closed section beams under either uniform or non-uniform minor axis bending. To the best knowledge of the authors, this is the first study devoted to exploring the effect of non-uniform bending on the behaviour of cold-formed steel built-up section flexural members. In the experimental investigation, the built-up section specimens were assembled by fastening two individual channels as press-braked from G500 and G550 steel sheets. Four-point bending tests of eighteen specimens with varying moment distributions were conducted to examine the flexural performance of the built-up section members experiencing uniform bending and moment gradients. In the numerical simulation, the finite element models were established for the built-up section flexural members and calibrated against the experimental results in terms of moment-deflection response, failure mode and moment capacity. Based on the validated finite element models, additional 300 numerical data of the built-up section beams under uniform and non-uniform minor axis bending were generated through a parametric study. Subsequently, the comparison of moment capacities obtained from the 18 tests and 300 numerical predictions with nominal strengths was carried out to evaluate the appropriateness of the current direct strength method as prescribed in the North American Specification for the built-up section flexural members. Aimed to achieve more suitable strength predictions, the modified design rules were recommended in this study and proven to be generally reliable as well as accurate for the design of cold-formed steel built-up section beams. [Display omitted] • Four-point bending tests were conducted on cold-formed steel built-up sections under uniform and non-uniform bending • Finite element models were established and calibrated against experimental results • Numerical investigation was performed on built-up open and closed section flexural members • Current direct strength method was evaluated based on experimental and numerical data • Design rules were proposed for cold-formed steel built-up section beams experiencing uniform bending and moment gradients [ABSTRACT FROM AUTHOR]

Built-up sections are commonly used in cold-formed steel (CFS) structures. The built-up sections are either welded or mechanically fastened based on the need and necessity. However, the current design standards lack specific design methods for CFS built-up members as the development is in progress. The American Iron and Steel Institute (AISI) specifies that only the beams fabricated from back-to-back connected C-channel built-up section as a prequalified geometry for design. This paper presents experimental and numerical investigation on the CFS built-up beams fabricated from back-to-back sigma sections. A total of 15 tests were carried out, containing three different lengths and five different intermediate connection spacing. The test specimens were laterally braced, webs and flanges were stiffened to avoid lateral torsional and local buckling so as to exhibit failure due to distortional buckling. Further, parametric studies were carried out using numerically validated model with five different lengths, two different sheet thicknesses and varying distortional slendernesses totaling to 160 specimens. The numerical results were summarized and compared with the design strength predictions by AISI's direct strength method (DSM). The comparison indicates that the design predictions in general are conservative, however, over conservative for low slenderness range (λ d ≤ 0.673). Therefore, a new limit is suggested for increasing the inelastic reserve buckling strength based on the experimental and numerical results. [Display omitted] • Structural behaviour of Cold-formed Steel back-to-back connected built-up beams is investigated. • A total of 15 tests and 160 parametric studies were carried out. • The influence of intermediate longitudinal connection spacing was reported. • The test results are compared with the current direct strength method design predictions. • New limitation is suggested for increasing the inelastic reserve distortional buckling strength [ABSTRACT FROM AUTHOR]

• The research on cold-formed steel columns with built-up box sections was conducted. • Tests on the built-up box section members were carried out. • The parametric analyses were conducted by the numerical simulation. • A simplified design method was proposed for the built-up box section columns. The built-up box section members in this paper consist of single cold-formed steel (CFS) C-section and U-section basic components, which are connected by self-drilling screws at the flanges. This study incorporates experimental and theoretical research on the buckling behavior and ultimate strength of axially loaded pin-ended CFS built-up box section columns as affected by local and local-flexural interactive buckling modes. In total, 28 specimens were prepared and tested under axial compression. The test results revealed visible local buckling or local-flexural interactive buckling for specimens. Thereafter, finite element models were established by the ABAQUS software and validated with the structural behavior of the test specimens. The effects of the slenderness ratio, web depth-to-thickness ratio and screw spacing on the ultimate strength of the built-up columns were investigated based on the numerical analysis. Finally, based on the test results of this research as well as previous works in the literature, a simplified design method was proposed to predict the ultimate strength of built-up box section columns. By comparing the predicted results against the test results, the proposed method was subsequently validated. [ABSTRACT FROM AUTHOR]

Stainless steel offers a range of benefits over conventional carbon steel in structural applications. This paper presents the detailed numerical modelling of shear response of cold-formed stainless steel hollow flange sections using finite element software package, Abaqus. The effect of geometric parameters such as section height and section thickness, and the influence of different steel grades were investigated following the validation of finite element models. From numerical results, the formation of diagonal tension fields can be clearly observed in the webs of rectangular hollow flange sections while more even distribution of the stresses in the webs is seen in triangular hollow flange sections. Further, a plastic hinge type mechanism is formed in triangular flanges at the post-failure region. The evaluation of Eurocode 3 and the direct strength method shear design provisions for stainless steel hollow flange beams is found to be significantly conservative. Therefore, modified provisions were proposed and the comparison of those with finite element results confirmed the accurate and consistent shear resistance predictions over the codified provisions. Unlabelled Image • Numerical investigations were undertaken to study the shear response of cold-formed stainless steel hollow flange sections. • Developed finite element models simulate the shear behaviour of hollow flange sections with reasonably good accuracy. • Numerical parametric studies were conducted for rectangular and triangular hollow flange sections using validated models. • The current shear design provisions are significantly conservative for stainless steel hollow flange sections. • Proposed shear design rules provide accurate and consistent predictions over the existing design provisions. [ABSTRACT FROM AUTHOR]

Holes in steel roof decks are often provided to allow for the passage of services. Published information on structural properties and design methods of steel deck with holes is limited. To shed light on this topic, a numerical parametric study of the flexural strength of steel decks with single square and rectangular holes was performed. This paper presents results of the study. Nonlinear finite element deck models were developed in ANSYS and validated using published test results. Flexural strengths and load-deflection curves predicted by the developed models showed good agreement with the experimental data. The following parameters were varied in the study: deck type and thickness, hole width and length-to-width ratio, hole location along and across the deck span, and load type. Ultimate moment reductions due to the holes have been presented and discussed. The ultimate moments obtained numerically were compared with those predicted by the effective width and direct strength methods. Modifications of the direct strength method were proposed to allow for better predictions of the flexural strength of steel deck with holes. A method to account for the effects of hole location along and across deck span on the ultimate moment was also presented. The design methods predicted the simulation results well when the proposed design procedures were considered. It was shown that the direct strength method predictions for distortional buckling of solid decks depend on the compression-to-tension flange width ratio. The conservatism of the DSM predictions increases when the compression-to-tension flange width ratio increases. Unlabelled Image • FE models of steel deck with single holes were developed and validated. • A numerical parametric study of deck with holes was conducted. • Effects of several parameters on flexural strength of decks with holes were studied. • DSM and EWM procedures for predicting strength of deck with holes were proposed. • Proposed design procedures showed good agreements with the FE simulation results. [ABSTRACT FROM AUTHOR]

Cold-formed steel members are commonly perforated with elongated web openings in practice to allow more access for the building service systems. The presence of such extended web holes in the members, when subjected to high shear forces, influences the shear buckling characteristics, the shear failure modes and generally leads to a reduction of shear capacity as well. In order to provide a better insight into the shear behaviour of cold-formed members with elongated web cut-outs, this paper presents a numerical study using finite element method (FEM) models employing ABAQUS/Standard to investigate the elastic shear buckling and post-buckling behaviour of cold-formed members in the form of thin channel sections. A full non-linear FEM model is developed and calibrated against previous shear tests with elongated web holes carried out using a dual actuator test rig. Based on the successful calibration against the tests, a parametric investigation is conducted to extend the resulting database. Further, a new proposal for the Direct Strength Method (DSM) of design of perforated channels in shear with both non-elongated and elongated web holes is further validated by using the results from the parametric study. A comparison between the empirical approach (" q s " method) and the new DSM of design derived from the Vierendeel mechanism approach is also provided to enhance the applicability of the new proposal to different cases of perforated channel members in shear with a wider range of sectional dimension and thicknesses. Unlabelled Image [ABSTRACT FROM AUTHOR]

The direct strength method (DSM) simplified the design of cold-formed steel (CFS) beams and columns compared with the traditional effective width method (EWM). In this paper, DSM equations for the distortional buckling (DB) strength are evaluated using the results of the experimental study on channel compression members with edge and intermediate stiffeners as well as nonlinear finite-element analysis (FEA) of the stiffened lipped channel compression members with various configurations, which fail after only DB. A total of 14 fixed-ended stiffened lipped channel compression members with intermediate stiffeners in both the web and flanges were tested, which failed after only DB. Further, additional data on the DB behavior is generated based on a parametric study using the nonlinear FEA, which was initially calibrated with the test data. A study on effect of parameters, such as the type of cross section, the dimensions of the cross section, the lip depth to flange width ratio (), the web height to the flange width ratio (), the yield stress (), end boundary condition (EBC), and the failure modes, shows that the nondimensional ultimate strength, expressed as the ratio of the ultimate strength to yield strength under compression, , is adequately addressed through the nondimensional DB slenderness, , alone as assumed in the DSM. The test and the analysis results show that the DSM equations generally evaluate the strength of stiffened lipped channel members under DB conservatively. Modifications to the DSM equations are suggested to evaluate the DB strength of stiffened lipped channel members more accurately. [ABSTRACT FROM AUTHOR]

This paper presents the details of an experimental and numerical study on the effect of warping on the flexural-torsional buckling (FTB) behaviour of axially loaded cold-formed steel lipped channel members. Eleven controlled experiments on two different lipped channel sections are conducted with simply-supported flexural end conditions. A unique scheme of compression loading is evolved, which allows the ends of the member to warp freely. Experimental investigations reveal that warping end conditions play a significant role in the compressive strength and failure modes. The results from the experiments are used to calibrate the numerical model developed using ABAQUS. Parametric studies are conducted on different cross-section sizes of the members to quantify the effect of warping restraint on the FTB strength under compression. Results are compared with the current AISI S100–16 specification using both the effective width method (EWM) and the direct strength method (DSM). The comparison of results reveals that for warping restrained condition, the present DSM framework based on the finite strip method (FSM) predicts FTB strength conservatively. A possible correction factors are identified for members with warping restrained ends, which can be applied over the traditional strength curve in DSM. A reliability study is also carried out on the modified design procedure for the FTB strength of CFS members. • New experimental setup for applying axial compression on lipped channel section under warping free condition. • Experimental data pool on the axial compressive behaviour of members under warping free and warping restraint condition. • Concepts for numerical models to create warping free and warping restrained conditions under axial compression. • Influence of warping on the Flexural torsional buckling (FTB) strength of the cold-formed steel members. • Proposed design procedure to incorporate the effect of warping in the DSM framework. [ABSTRACT FROM AUTHOR]

Cold-Formed Steel (CFS) members with slotted perforations in webs are used in civil construction to amplify the thermal and energy performance of structures. However, the slotted webs reduce the structural performance of the element, prominently their shear, bending and combined bending and shear strengths. Many research studies have been undertaken to examine the behaviour of CFS channel sections subject to bending. Yet, no research has been performed to investigate the distortional buckling behaviour of slotted perforated CFS flexural members. Finite Element (FE) models of CFS channels with staggered slotted perforations were developed herein to investigate their distortional buckling under bending stress. A parametric study was conducted in detail by developing 432 slotted perforated CFS FE models based on the validation process with available experimental results. In particular, this paper presents the FE analysis details of CFS flexural members with slotted perforations subject to distortional buckling and results. The reliability of the current Direct Strength Method (DSM) for CFS flexural members with web holes subject to distortional buckling in accordance with the North American Specification (AISI S100) (2016) and the Australian/New Zealand Standards (AS/NZ 4600) (2018) was investigated. Modified DSM formulae for slotted perforated CFS flexural members subject to distortional buckling were also proposed. • Distortional buckling behavior of CFS beams with staggered slotted perforations have been investigated. • Finite element models showed good agreement with available test data. • All influencing parameters were considered in the parametric studies. • Staggered slotted perforations reduced the distortional buckling flexural capacity up to 23%. • New direct strength method based design equations were proposed for distortional buckling. [ABSTRACT FROM AUTHOR]