Cold-formed Steel Structural Members Research Articles

Localized loading behavior of cold-formed stainless steel RHS under interior-one-flange load case at elevated temperatures

The localized loading behavior of cold-formed stainless steel (CFSS) rectangular hollow section (RHS) members at elevated temperatures is investigated in this paper. Fifteen localized loading tests conducted at various temperatures up to 800 °C were reported. The tests were carried out under the Interior-One-Flange (IOF) load case as specified in American Specification for the design of cold-formed stainless steel structural members. Tensile coupon tests were performed to obtain the material properties of the CFSS RHS at various temperatures. Detailed information on dimensions, temperature histories, failure modes, load-deformation curves, localized loading capacities of specimens at various temperatures were presented. In addition, a numerical investigation was supplemented, where a total of 192 finite element (FE) analyses were undertaken after validation of developed FE model against the elevated-temperature test results. The influence of various parameters on the localized loading capacities of CFSS RHS at different temperatures under IOF load case was revealed. The obtained results were utilized to assess the applicability of the localized loading design provision for CFSS RHS under IOF load case codified in the American Specification to elevated temperature conditions. Moreover, the results were also compared with predictions based on existing design rules in literatures for cold-formed RHS at elevated temperatures. Furthermore, reliability analyses were carried out to assess the reliability levels of the assessed design methods. It is shown that the existing design rules can generally provide conservative and reliable predictions of the localized loading capacities for the CFSS RHS under IOF load case at elevated temperatures.

Derivation of stainless steel material factors for European and U.S. design standards

Modern structural design standards are based on the Limit State Design (LSD) philosophy, also known as Load and Resistance Factor Design (LRFD), which involves the use of resistance functions with accompanying safety factors that are based on a statistical evaluation of relevant experimental data, carried out within the framework of a probabilistic reliability theory. This paper presents the derivation of material factors for the yield strength and ultimate tensile strength of austenitic, duplex and ferritic stainless steels, and their associated coefficients of variation (COV) for use in evaluating the safety factors for the European design code EN 1993-1-4 and the American design codes Specification for Structural Stainless Steel Buildings (ANSI/AISC 370–21) and Specification for the Design of Cold-Formed Stainless Steel Structural Members (ASCE/SEI 8–22). Separate material factors and associated COV values are required for the U.S. and European design codes due to the different minimum specified strength values that are given in the respective product standards. The material factors were derived by analysing a large amount of material data provided by different stainless steel producers, which encompassed a wide range of stainless steels and material thicknesses.

Numerical modelling of cold-formed steel built-up columns

Built-up cold-formed steel structural members can provide technically and economically advantageous solutions in cold-formed steel construction. However, there is a need for a deeper understanding of their stability behaviour, and this paper demonstrates that detailed, accurate and reliable finite element models provide an excellent means to achieve this. Apart from the typical issues to be considered in the high-fidelity modelling of thin-walled members, which include geometric and material nonlinearity, element type, mesh density, and the modelling of imperfections and boundary conditions, the paper focuses on two additional complications which arise in built-up members: the modelling of connectors, and contact between constituent parts. A number of alternative techniques to model connector behaviour are investigated and their effect on the predictions of the ultimate capacity quantified by benchmarking against experimental data. Various successful strategies to mitigate stubborn convergence problems, which almost inevitable originate when modelling complex contact problems, are also presented. It is demonstrated, however, that the inclusion of contact as a model feature does not have a significant effect on the predicted capacity in the majority of the considered cases, with its influence typically remaining limited below 10%. The validated models were further used in parametric studies which showed that once the connector spacing is short enough to prevent global instabilities of the component sections between connector points, its effect on the ultimate capacity of the built-up column is very small within the practical range of connector spacings. Modelling connector lines as continuous longitudinal constraints tying surfaces together, on the other hand, is bound to significantly overestimate the capacity.

Revision of ASCE 8 - Design of cold-formed stainless steel structural members

The American Society of Civil Engineers (ASCE) Specification for the Design of Cold-Formed Stainless Steel Structural Members was last updated in 2002 and significant advancements in research knowledge and companion design standards have occurred in the intervening 20 years. A dedicated group of volunteers re-formed the ASCE 8 specification committee and over the course of four years completely revamped the standard. The new version, ASCE 8–22, still parallels the cold-formed carbon steel standard AISI S100, and has improved and simplified design expressions for members and connections. Both the Continuous Strength Method and the Direct Strength Method are provided in ASCE 8–22, along with traditional effective width design approaches. The standard provides a robust set of provisions for slender austenitic, ferritic, and duplex cold-formed stainless steel structural members and their connections. This paper overviews major changes in the ASCE 8–22 standard highlighting areas of divergence from carbon cold-formed steel (AISI S100) and from stainless structural steel (AISC 370–21).

Geometric imperfections in CFS structural members: Part I: A review of the basics and some modeling strategies

Unavoidable geometric imperfections complicate the load–displacement response of thin-walled cold-formed steel (CFS) structural members. While the inclusion of geometric imperfections in shell finite element (FE) collapse simulations has long been the subject of research, no consensus exits on modeling their full three-dimensional spatial pattern. This paper is the first part in a two-part series and provides a review of the basics and a summary of some efforts geared towards modeling geometric imperfections. We discuss the necessary ingredients for systematic characterization of elastic buckling mode shapes, due to their frequent use in approximating geometric imperfections, and make a conscious effort to express some of the existing geometric imperfection models through three components: imperfection shape, magnitude, and combination coefficient. Different choices and selections of these components, pertaining to either the inherent buckling behavior of the members and/or physical measurements are critically examined. The details required to develop high fidelity nonlinear FE models of CFS members seeded with geometric imperfections are presented. We then perform a comparative study of collapse analysis results obtained from adopting different imperfection modeling strategies. The noticeable variability in the load carrying capacity and the stability behavior highlights an urgent need for reliable probabilistic frameworks for this class of problems. A stochastic framework to generate samples of imperfect shell FE models from limited available data and validate the prescribed or practice-oriented geometric imperfection models is presented in a companion paper (see Part II Farzanian et al., (2023)).

Geometric imperfections in CFS structural members, Part II: Data-driven modeling and probabilistic validation

This paper is the second part in a two-part series that examines the role of geometric imperfections in load–displacement response and collapse behavior of cold-formed steel (CFS) structural members. In part I (Farzanian et al., 2023) a review of the basics and a summary of efforts geared towards modeling geometric imperfections was provided. This included the ingredients needed for systematic identification of elastic buckling mode shapes, due to their frequent use in approximating geometric imperfections, and the development of high fidelity shell finite element (FE) nonlinear collapse analysis of CFS members seeded with different geometric imperfection models. Part II is the companion to the analysis in part I that revealed a noticeable variability in the collapse behavior and load carrying capacity of CFS members as a result of adopting different imperfection modeling strategies. In this part we provide the necessary steps to build a probabilistic framework that rests on machine learning, polynomial approximation, and parametric and non-parametric statistical inference and uses imperfection data to build a consistent stochastic field model of geometric imperfections. This data-driven model is then used to generate the much needed realizations of geometric imperfections that are faithful to the underlying statistics of measured data and therefore can be seeded, directly, in nonlinear collapse analysis of CFS members. Nonlinear shell FE analysis, within a stochastic simulation framework, is finally used to (statistically) quantify the impact of geometric imperfections and to enable probabilistic validation of geometric imperfection models.

Compression test results of CFS structural members

The subject of the study is the stress-strain state of a cold-formed steel structural members under axial compression. The aim is to experimentally study the behavior of a sample from a cold-formed thin-walled profile in axial compression with stress control. A mechanical method was used to test compression samples on a universal testing machine, a strain gauge method for measuring relative deformations by strain gauges with data collection in the registration unit. The processing of the test results was carried out using the methods of the theory of elasticity. As a result of the tests carried out, a qualitative picture of the deformation of the sample was obtained, the values of relative deformations at specific points of the cross sections of the sample; as a result of processing the data obtained, the main normal stresses at the points of the cross sections of the element were determined. As conclusions, an assessment of the reliability of the results obtained based on the nonlinear theory of plates is given; a conclusion is made about the inapplicability of the strain gauge method to the analysis of the behavior of thin-walled steel profiles. A promising study is the study of fields of relative deformations by non-contact optical methods.

Searching for a compromise solution in cross-section size optimization problems of cold-formed steel structural members

A parametric optimization problem of cross-sectional sizes for cold-formed steel lipped channel structural members subjected to axial compression has been considered by the paper. An optimization problem is formulated as to define optimum cross-sectional sizes of cold-formed structural member taking into account post-buckling behavior (web and flange local and distortional buckling) of the member as well as structural requirements when the profile perimeter (strip width), profile thickness, design lengths of the structural member as well as material properties are constant and specified in advance. Maximization of the load-carrying capacity of the cold-formed structural member has been assumed as purpose function. The formulated parametric optimization problem has been solved by exhaustive search method using the software written in Python. As optimization results the cold-formed steel lipped channels with optimum cross-sectional dimensions have been obtained depending on the profile thickness and design lengths of the structural member. In order to obtain optimum solutions for cross-sectional dimensions of the CFS lipped channel structural members which are independent from the design flexural lengths and profile thickness, searching for a compromise solution has been performed by exhaustive search method. The obtained cold-formed steel lipped channel structural members with optimum cross-sectional sizes have higher design buckling resistance under the axial compression at the same material consumption (stripe width) comparing with the cold-formed steel lipped channels proposed by the manufacturer. Web local buckling phenomenon has been occurred in all obtained CFS lipped channel cross-sections with optimum sizes.

Direct Strength Method for Design of Cold-Formed Steel Structural Members

Direct Strength Method for Design of Cold-Formed Steel Structural Members

Design approach against column local–distortional interactive failures based on the Direct Strength Method

This paper deals with the development and merit assessment of an efficient and rational design approach intended to handle local–distortional (L–D) interactive failures in cold-formed steel columns (CFS) and based on the Direct Strength Method (DSM). Initially, a brief overview of the mechanical aspects underlying local–distortional interaction is provided, for the benefit of readers not familiar with this coupling phenomenon, and its relevance in CFS column design is addressed. Then, the available experimental failure load data, obtained for columns exhibiting various cross-section shapes (plain, web-stiffened and web-flange-stiffened lipped channels, hats and racks) are collected and used as the basis for the development, calibration and validation of this DSM-based design approach, which relies also on the current DSM local and distortional strength curves and existing numerical failure loads. It covers local–distortional​ interactive failures occurring in columns with several ratios between the distortional and local buckling loads, which are grouped in three families, denoted as “true L–D interaction”, “secondary-distortional bifurcation L–D interaction” and “secondary-local bifurcation L–D interaction”. The failure load prediction quality (accuracy and safety) and reliability of the proposed DSM-based design approach are assessed and compared with those exhibited by the existing methodologies to design CFS columns against L–D interactive failures. The reliability assessments follow the procedure prescribed by the current North American Specification for the Design of Cold-Formed Steel Structural Members and indicate that the design approach proposed in this work should be ready for codification in the near future.

Flexural behavior of delta and bi-delta cold-formed steel beams: experimental investigation and numerical analysis

Cold-formed steel (CFS) structural members retain their positions in the lightweight construction industry. This is due to the significant advantages of CFS. The optimization of these CFS elements will allow the construction of economical buildings with increased load capacities and solutions for stable and economical construction will be obtained. The main aim of this research was to evaluate the effectiveness of these new CFS sections with the estimation of remarkable instabilities and failure modes. This article deals with an experimental study on the behavior of CFS beams of open delta and bi-delta form stressed by four-point bending loads. These cross section shapes are often used in floors as main and secondary beams. The section properties are based on the effective width method designated by the Eurocode 3 standard. A nonlinear finite element (FE) analysis using the ABAQUS program is performed and the comparison between the experimental, numerical and theoretical results is done. Finally, the results showed above all that the breaking loads of the delta and bi-delta beams corresponded to the modes of local buckling and crushing of the web.

Numerical investigation of the strength and design of cold-formed steel built-up columns

The paper concerns built-up cold-formed steel (CFS) structural members composed of three or four component lipped channel sections. Finite element (FE) models are calibrated against recently conducted experimental results and used to perform a parametric study of the strength and behaviour of CFS built-up section columns, considering column the length, section thickness and screw spacing as independent variables. The strengths thus obtained combined with experimental ultimate strengths are compared with strength predictions obtained using the North American Specification for the Design of Cold-formed Steel Structural Members, which specifies the use of a modified slenderness ratio for built-up members composed of two sections connected back-to-back. The same data is used to assess a recently proposed design method for predicting the strengths of the studied 3C and 4C built-up sections, and potentially multi-cell built-up sections more generally, based on effective rigidities coupled with the Direct Strength Method. The proposal includes a design procedure for built-up sections experiencing flexural-torsional buckling, which is neither explicitly stipulated in current design standards nor discussed in-depth in previous studies, and generally provides accurate strength predictions. A reliability analysis is included in the paper to calculate resistance factors to accompany the proposed design provisions.

Cold-formed steel battened built-up columns: Experimental behaviour and verification of different design rules developed

Some of the past studies on cold-formed steel (CFS) battened built-up columns have resulted in the development of new design rules for predicting their axial strengths. However, the main drawbacks of such studies are that they are purely numerical and the numerical models developed for such parametric studies were validated using the test results on similar built-up column configurations, but not the exact ones. Therefore, experimental studies on CFS battened columns comprising of lipped channels are needed for verifying the accuracy of the proposed design rules for CFS battened columns. This paper reports an experimental study performed on CFS built-up battened columns under axial compression. Adequately spaced identical lipped channels in the back-to-back arrangement were used as chords and were connected by batten plates laterally with self-driving screws to form the built-up members. The dimensions of chords were fixed as per the geometric limits given out in the North American Specifications (NAS) for the design of CFS structural members. The sectional compactness of the chords and the overall slenderness of the built-up columns were varied by altering the thickness of the channels and height of the built-up columns, respectively. A total of 20 built-up sections were tested under uniform compression to investigate the behavioural changes in the built-up columns due to these variations. The behaviour assessment was made in terms of peak strengths, load–displacement response and failure modes of the test specimens. The current design standards on CFS structures were used to determine the design strengths and were compared against the test strengths for assessing their adequacy. Furthermore, as discussed in the beginning, the test strengths were used to verify the accuracy of the different relevant proposed design rules in the literature.

A Study on the Effect of Hollow Tubular Flange Sections on the Behavior of Cold-Formed Steel Built-Up Beams

This paper deals with a study conducted on flexural behavior of cold-formed steel built-up I-beams with hollow tubular flange sections. There were two types of test sections, namely, built-up sections that were assembled with either stiffened or unstiffened channels coupling back-to-back at the web and a hollow tubular rectangular flange at the top and bottom of the web to form built-up I-beam. The flexural behavior along with the strength and failure modes of the built-up sections was examined using the four-point loading system. Nonlinear finite element (FE) models were formulated and validated with the experimental test results. It was observed that the developed FE models had precisely predicted the behavior of built-up I-beams. Further, the verified FE models were used to conduct a detailed parametric study on cold-formed steel built-up beam sections with respect to thickness, depth, and yield stress of the material. The flexural strength of the beams was designed using the direct strength method as specified in American Iron and Steel Institute (AISI) for the design of cold-formed steel structural members and was compared with the experimental results and the failure loads predicted from FE models. Since the results were not conservative, a new customized design equation had been mooted and delineated in the study for determining the flexural strength of cold-formed steel built-up beams with hollow tubular flange sections.

Sheathing-fastener connection strength based design method for sheathed CFS point-symmetric wall frame studs

Cold-formed steel (CFS) in construction delivers cost effectiveness and design flexibility. The instability failures of CFS Structural members significantly influences the failures of sheathing boards attached as an external cover. Therefore, present study experimentally examines the structural behavior of sheathing fastener connection in cold-formed steel wall frame. As a first attempt, strength and stiffness of the sheathing fastener connection is determined against the failure modes of point symmetric CFS wall frame studs. A new test setup has been developed to investigate the individual sheathing fastener failure modes. Parameters investigated include sheathing board types (varying thickness) and different dimensions of point-symmetric CFS studs. A total of forty-one individual fastener connection tests were carried out. The experimental investigation revealed that the fastener connection strength varies depending on the geometric dimensions of CFS stud and material composition of sheathing board. The result indicated that the bracing effect provided by plain gypsum board (fiber less) is insignificant and should not be considered for design. For sheathing boards with fiber composition, a generic parameter [ratio to design moment (Mx or My) to CFS stud depth (h)] is introduced to consider the structural bracing effect in CFS wall panel design. The limitations to consider the bracing effect of sheathing in the design of CFS wall frame studs are suggested for minor axis buckling and major axis buckling.

Hysteretic behavior comparison of austenitic and lean duplex stainless steel square hollow section members under cyclic axial loading

In this study, cyclic loading tests for a total of 10 specimens were performed in order to study the cyclic response of square hollow section bracing members with three types of cold-formed stainless steel materials; two austenitic stainless steels and one lean duplex stainless steel. The buckling behaviors, tensile strength, and energy dissipation capacity of the specimens were investigated according to the difference of material properties, width-thickness ratio and slenderness ratio. This study indicated that the number of cycles for grade 304 stainless steel tubular specimens at ultimate state was lower than that of grade 316 stainless steel specimens due to tensile fracture in the weld heat-affected zone. The ultimate strength and energy dissipation capacity of the lean duplex stainless steel specimens was also lower than those of austenitic stainless steel specimens due to initial local buckling. It is found that the grade 316 specimens showed the higher energy dissipation capacity and energy index by 1.78, 2.72 times on average, respectively, compared to the grade 304 members despite their higher material strength. This study also showed that European code tended to underestimate the compressive buckling strength of the bracing members, while Specification for the Design of Cold-formed Stainless Steel Structural Members by American Society of Civil Engineers provided unconservative the buckling strength prediction in the long columns and duplex stainless steel members.

Direct stiffness-strength method design for sheathed cold-formed steel structural members - Recommendations for the AISI S100

This paper presents a new design procedure for considering the bracing effect of sheathing boards that are attached to the cold-formed steel (CFS) structural members. The previous investigations show that the current AISI design specification for sheathing bracing design of CFS wall panels is unconservative (i.e. design standard predict a larger failure load than the experimental test results) due to exaggerated sheathing stiffnesses calculated from ideal loading conditions rather than worst-case loading conditions. Therefore, a new design procedure is suggested based on the performance (strength and stiffness) of the individual sheathing fastener connections. A new and simplified test setup is also introduced to simulate the realistic failure modes of the sheathing fastener connections using a conventional universal testing machine. A total of 67 individual sheathing fastener connection tests were carried out, including parameters such as ten different dimensions of the CFS stud (to account for the slenderness) and seven various sheathing board types (to account for the performance of each sheathing type). Based on the individual sheathing fastener connection test results, new expressions are formulated to predict the stiffness and strength of the individual sheathing-fastener connections. A statistical and application-oriented merit-based assessment indicated that the proposed expressions are appropriate for the use of sheathing braced design. In particular, the individual sheathing fastener-connection strength and stiffness arrived by the proposed expressions are conservative in predicting the failure mode and design strength of the sheathed CFS panel. Finally, this paper demonstrates the effectiveness of the proposed simplified design method by illustrating with the help of a design example, which quantifies the benefit of adopting this method in AISI design guidelines.

Fire resistance behaviour of LSF floor-ceiling configurations

Light gauge Steel Frame (LSF) floor-ceiling systems made of thin-walled cold-formed steel structural members deliver innovative, lightweight and cost effective solutions for many floor assemblies. However, the mechanical properties of cold-formed steel structural members deteriorate in fire. Hence fire rated gypsum plasterboard ceilings are required to protect them and avoid premature failures of floor assemblies. However, the behaviour of LSF floor-ceiling systems in fire is not well understood. Hence a series of small-scale standard fire tests was undertaken to investigate the fire resistance of cold-formed LSF floor-ceiling systems of varying configurations. Configurations included structural plywood as the subfloor, gypsum plasterboard ceilings, thin steel sheathing, different joist sections such as lipped channel beam and rivet fastened rectangular hollow flange channel beam and rockwool cavity insulation. The effects of these parameters on the fire resistance of LSF floor-ceiling assemblies are discussed in this paper. Fire resistance improvements of 21–32% were observed when steel sheathing was used in varying configurations. This shows the potential of using thin steel sheathing below the gypsum plasterboard that enhanced the insulation failure times by resisting gypsum plasterboard fall-off. However, cavity insulation led to reduced fire resistance times while plywood subfloors exhibited rapid decomposition and burning when the temperature exceeded 234°C. This paper presents the details of the small-scale standard fire tests of LSF floor-ceiling systems of varying configurations and the results in terms of time-temperature curves and fire resistance times.

Recommendations for design of sheathing bracing systems for slender cold-formed steel structural members

The use of sheathing material as a structural component in the Cold-formed steel (CFS) construction holds great potential for stability of the structure and savings in the construction cost. However, there is no robust design method to account the structural contribution of sheathing boards. This paper presents the experimental results of 107 (including unsheathed and sheathed) CFS wall frame studs subjected to out-of-plane loading to study the feasibility of using gypsum board as a structural bracing sheathing material. The test parameters include various shapes and slendernesses of the CFS frame stud, thickness of the sheathing board and spacing between the sheathing bracing connections. The out-of-plane loading is applied as it causes lateral torsional buckling in the CFS structural members thereby creates pull-through failure at the sheathing bracing connections. Moreover, the suitability of the current AISI and Eurocode specification for the design of sheathing braced CFS structural member is studied. The experimental results indicate that the lateral buckling of the symmetric shaped (against the loading axis) CFS wall frame studs can be inhibited by gypsum sheathing. Whereas most of the singly symmetric and point symmetric CFS studs exhibit lateral torsional buckling and biaxial bending, respectively, due to the inadequate bracing effect of gypsum sheathing resulting in pull-through failure at the sheathing bracing connections. Therefore, a set of limitations for the use of gypsum sheathing as a structural bracing is suggested in the form of a generalized design parameter. Finally, the experimental results indicates that the design strength of the CFS wall frame stud can be increased in the range of 39% to 595% based on the shape and slenderness.

Experimental Investigation on Flexural Behavior of Cold Formed Beams with Lightweight Concrete

In recent years, thin-walled, cold-formed steel (CFS) structural members have gained expanding use in building construction and various sorts of structural systems [1,2,3].The utilization Cold-Formed Steel (CFS) structures has become progressively popular in different fields of building technology. The reasons behind the developing popularity of these products include their ease of fabrication, high strength/weight ratio and suitability for a wide range of applications. These advantages can result in more economic designs, as compared with hot-rolled steel, especially in short-span applications. In this project work attempt has been made to use Cold formed steel section as replacement to conventional steel reinforcement bar.