Cold-formed Steel Compression Members Research Articles

Member capacity of cold-rolled aluminium alloy channel columns – Part II: Numerical investigation and design

The paper first presents the development of detailed finite element (FE) models for investigating the member capacity of cold-rolled aluminium alloy 5052-36 channel columns. The numerical non-linear FE models are developed using the commercial FE software package ABAQUS to simulate laboratory experiments and calibrated against the test results. The experimental program was performed by the authors at the University of Sydney and the detailed test results are fully described in a companion paper. The calibrated numerical FE models with actual mechanical properties and measured geometric imperfections in analyses accurately predict the experimental results and behaviours, including flexural and flexural-torsional buckling as well as local-global interaction buckling failure modes. A parametric study is subsequently performed to extend the data range by varying cross-section dimensions and member lengths. The effects of loading eccentricity positions and initial geometric imperfections are also considered in the parametric study. The experimental and numerical results are compared with design strength predictions from current Australian/New Zealand, American and European specifications for aluminium structures in the companion paper. In this paper, the Direct Strength Method (DSM) for the design of compression members as per the Australian/New Zealand standard AS/NZS 4600 for cold-formed steel structures is adopted for comparison. The DSM design rules for cold-formed steel compression members are found to provide more accurate predictions than those of current specifications for aluminium structures and therefore used to propose improved design equations for cold-rolled aluminium alloy columns. Finally, a reliability analysis is performed to evaluate the safety level of current design specifications and the proposed design rules.

Strength of Cold-Formed Stainless-Steel Corrugated Rectangular Sections under Compression

Provision of intermediate stiffening is an effective method to improve the strength of cold-formed steel compression members without having to increase the outer dimensions of the section. As cold-formed sections can be easily produced in various cross-section profiles, the effect of intermediate stiffening on ultimate load carrying capacity and behavior of structural elements need to be determined. In this study columns with Cold-Formed Stainless-Steel and Corrugated Rectangular Hollow Sections are studied numerically using Finite Element analysis. The column simulation is conducted using ABAQUS software including material nonlinearity and performed with the Static Riks procedure. The objective is to study how providing stiffening in the form of corrugations influences the buckling behavior of these columns. Columns for a range of slenderness ratio with varying cross-section parameters were modelled and compared to identify the influencing parameters. The study showed that buckling strength of columns can be significantly increased by providing corrugations. Corrugations effectively resisted local buckling in columns even with low slenderness ratio. The parameters influencing the buckling behavior are identified as cross-section aspect ratio, corrugation height, corrugation width and minimum radius of gyration of the cross-section. Buckling curves are placed for the corrugated sections based on the numerical analysis, and in relation to the buckling curves provided in design standards. On comparing them with the predictions by Euro Code (EN 1993-1-4 (2006)), it was observed that Euro Code predictions are conservative, which is also seen from the buckling curves shown for the corrugated sections.

Test investigation and direct strength method on cold-formed steel compression members with web holes of different widths

This paper presents a series of test investigations and finite element (FE) analyses on the compressive behaviors of cold-formed steel (CFS) compression members with web holes of different widths and the corresponding direct strength design method is proposed. In the test investigations, a total of twenty specimens were tested under axial compression loading with five different hole width-to-web width ratios of 0, 0.2, 0.4, 0.6, and 0.8. It is shown that the local buckling deformations for specimens with web holes occur in the vicinity of web holes and decrease as the hole widens. For the axial capacities of specimens, they are decreased by 11.7% on average with the increase of hole width-to-web width ratio from 0 to 0.4 while they are decreased by 31.5% on average with the increase of hole width-to-web width ratio from 0.4 to 0.8. In the FE studies, the FE models for CFS compression members with web holes of different widths were developed using the ANSYS program. The simulation results show that the compressive behaviours obtained from FE analyses are in good agreement with test results. Then, combined with test data, a total of 412 FE parametric analyses were conducted to predict the proportional relationships between the axial capacities of compression members with web holes and those without web holes, and the hole impact coefficient formulas were correspondingly derived which differ from the previous researcher's contribution. Based on the hole impact coefficient formulas and the direct strength method (DSM) of CFS compression members without web holes in AISI S100-2016, the DSM formulas of CFS compression members with web holes were developed, in which the elastic critical buckling stress of CFS compression members with web holes was not necessary to be calculated.

Numerical study on restrained thermal elongation of cold-formed steel Column subjected to non-uniform thermal loading under transient state

Cold-formed steel (CFS) is extensively used as non-load bearing and load-bearing structural members because of its less weight to strength ratio, transportation economy, and handling ease. Since CFS members are treated as slender sections, they are highly susceptible to buckling under the compression loads at ambient and elevated temperatures. The codes IS801, EN1993–1.3, AS/NZS4600, and BS5950 are used to design the cold-formed steel compression members at ambient temperature, and the code EN1993–1.2 is used to design the CFS member by treating it as a slender member with a limiting temperature of 350 °C. Many researchers have attempted to study the behaviour of CFS members at elevated temperatures, both numerically and experimentally, under uniform thermal loading conditions at steady-state. However, studies on non-uniform thermal loading under transient states are minimal. The present study focuses on the numerical modelling of buckling behaviour of CFS members with restrained thermal elongation under non-uniform temperature conditions. The numerical modelling of the CFS column subjected to non-uniform temperature under a transient state is validated with the past results. The parametric study is carried out by varying the initial load, the member's slenderness ratio, the stiffness of the supporting structure, the width-thickness ratio(w/t), and the temperature difference between flanges.

DSM design of fixed-ended slender built-up back-to-back cold-formed steel compression members

Built-up back-to-back cold-formed steel channel (BC) sections have gained increased application as compression members in building structures. Although many researchers have studied the behaviour and design of BC compression members, the design equations in the current standards are still inadequate for them, while the new design equations proposed in recent studies are inconsistent and lead to predictions with varying accuracy. This study focuses on slender BC members with fixed ends, which are sensitive to local-global interaction or global buckling. Many tests of fixed-ended slender channel and BC compression members, which failed mostly by local-global interaction buckling, were conducted first, followed by developing validated finite element models. The models were then used in a parametric study of BC compression members with varying sections, screw arrangements and lengths, to investigate their failures by local-global interaction and global buckling. The outcomes from tests and finite element analyses provided improved understanding of the behaviour of BC members, and were used to compare their compression capacities against twice the individual member capacities, and show the limitations of the current design equations available in the standards and research publications. Finally, reliable design guidelines were proposed using the Direct Strength Method (DSM).

Interaction of Stiffened and Unstiffened Element Buckling Modes in CFS Plain Channel Compression Members

Plain channel compression members are composed of stiffened (web) and unstiffened (flanges) plate elements. Effective width method (EWM) for the design of cold-formed steel (CFS) compression members accounts for unstiffened and stiffened plate elements through plate buckling coefficient, k of 0.425 and 4.0, respectively in the elastic local buckling stress (fcrl) calculation. The more recent direct strength method (DSM) recommends the local buckling strength equations for the design of plain channel compression members. A lipped stub channel compression member undergoes interaction of local buckling initiated by the stiffened elements and distortional buckling initiated by the partially stiffened flange-lip assembly. Based on finite element analysis (FEA) results, it is demonstrated that such an interaction between buckling modes initiated by the stiffened web and unstiffened flanges is possible in plain channel compression members also. The behavior of plain channels can be represented as an interaction of buckling of the unstiffened flange and stiffened web elements, which is equivalent to local-distortional interaction in lip stiffened channels. The deformation plots are also provided to understand these mode interactions. The present study focuses on stub columns and does not consider the interaction of global buckling mode.

Behaviour and design of slender built-up nested cold-formed steel compression members

Built-up nested channel (NC) members, which are made of asymmetric cold-formed steel (CFS) lipped channel sections are commonly used in various building applications in Australia. The aim of this research study is to provide suitable design guidelines for NC compression members, which fail by global or local–global interaction buckling. Firstly, the design rules in the current CFS design standards and recently published research studies were reviewed, and their limitations for NC members were discussed. A test series was then conducted on single channel and NC compression members, which failed by local–global interaction buckling. Based on the test results, validated finite element (FE) models were developed, followed by a detailed parametric study of NC members failing in global or local–global interaction buckling for different sections, lengths, end support boundary conditions as well as screw sizes and arrangements. The test and FE analysis results were used to: (1) compare the compression capacities of NC members with the sum of their individual member capacities, (2) evaluate the effect of screw characteristics and arrangements, and (3) investigate the design methods in the current design standards and past research studies. Finally, based on the Direct Strength Method in the current design standards, a simple and reliable design method was proposed. This paper presents the details of this research study and the results including its findings.

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

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

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

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

Unstiffened Elements as Limiting Case of Distortional Buckling of Partially Stiffened Elements

AbstractLip stiffened flanges in cold-formed steel compression members may undergo local or distortional buckling depending on the slenderness of the flange and rigidity of the lip. In distortional...

Behaviour of cold-formed steel compression members at sub-zero temperatures

More people are living near the polar regions due to the availability of land and emerging job opportunities in these regions. This has led to increased construction activities in these sub-zero temperature regions for which a suitable cost-effective construction material is needed. Although cold-formed steel (CFS) consists of many good features that are required for cold region construction, the knowledge of the structural behaviour and capacity of CFS members at sub-zero temperatures is limited. In this research, low and high strength CFS lipped channel columns subject to local buckling and yielding failures were tested in the temperature range of +20 to −70 °C to understand their behaviour and determine their sub-zero temperature compression capacities. Tests showed that there were no premature failures caused by brittle fracture, even after significant localised deformations during the post-ultimate phase. In all cases the ultimate capacity increased considerably even at temperatures below −50 °C, especially for low strength steel columns. Finite element models were also developed to simulate the local buckling and yielding failures of tested columns, and the models were validated using the test results. The ambient temperature design methods given in the cold-formed steel standards such as AS/NZS 4600 were shown to be adequate to determine the sub-zero temperature compression capacities of CFS columns when sub-zero temperature mechanical properties are used instead of ambient temperature mechanical properties.

Behaviour of cold-formed steel built-up I-section columns composed of four U-profiles

This work reports numerical results concerning the cold-formed steel built-up I-section columns composed of four U-profiles under axial compression. A finite element model is developed by using the software program ABAQUS. The developed model includes geometric, material nonlinearities and geometric imperfections. The finite element model was verified against the experimental results reported in the cold-formed steel built-up open section columns. In the parametric study, the sections are analysed with several cross-sectional dimension ratios and lengths, in order to assess their influence on the buckling behaviour and ultimate strength of cold-formed steel built-up I-section columns. After presenting and discussing the numerical parametric results, the article shows that the current direct strength method in the North American Specification for cold-formed steel compression members design curve fails to predict adequately the ultimate strength of some of the columns analysed and addresses the modification proposed on current direct strength method curves, providing improved predictions of all the numerical ultimate strength available. The proposed method is also assessed by reliability analysis.

Short-to-intermediate slender pin-ended cold-formed steel equal-leg angle columns: Experimental investigation, numerical simulations and DSM design

Angles exhibit a complex structural behaviour, responsible for the fact that, in the current North American Specification for Cold-Formed Steel Structures, short-to-intermediate equal-leg angle columns are (i) not yet pre-qualified for the Direct Strength Method (DSM) design and (ii) excluded from the application of the LFRD resistance factor ϕ=0.85, valid for all other cold-formed steel compression members. Recently, the specific behavioural features exhibited by the above angle columns were incorporated into the proposal of a novel DSM-based design approach, for both fixed-ended and pin-ended columns, and it was shown that this added rationality goes along with quite accurate and reliable failure load predictions. However, the investigation leading to this design proposal also unveiled that there are no available experimental failure loads of slender pin-ended columns with intermediate-to-high slenderness values, which implied that the design procedure was validated for such columns exclusively on the basis of numerical failure loads. The research work reported in this paper provides a contribution towards filling this gap, since it mainly consists of an experimental study, carried out at the Federal University of Rio de Janeiro, on the behaviour and collapse of short-to-intermediate slender pin-ended cold-formed steel equal-leg angle columns. After addressing the selection of the columns to be tested, the experimental set-up and test procedure are described in detail and the results obtained are presented and discussed. Such results involve (i) initial imperfection measurements, (ii) equilibrium paths relating the applied load to key column displacements, (iii) deformed configurations (including the collapse mode) and (iv) failure loads. Next, those same experimental results are used to validate a shell finite model previously developed by the authors, which is subsequently employed to obtain additional numerical failure load data concerning the pin-ended angle columns under scrutiny. Then, attention is turned to assessing the merits of the novel design approach. The comparison between the experimental and numerical values obtained in this work and their estimates provided by the design equations shows a very good correlation, perfectly in line with that observed in the recent studies available in the literature−this means that the validation and calibration of the above design approach may be deemed (successfully) completed. Finally, the paper closes with the presentation and assessment of small alterations to the existing design expressions, aimed at improving their accuracy and rationality, thus paving the way towards codification in the near future.

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

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

Flexural–torsional buckling behaviour and design of cold-formed steel compression members at elevated temperatures

Current design rules for the member capacities of cold-formed steel columns are based on the same non-dimensional strength curve for both fixed and pinned-ended columns at ambient temperature. This research has investigated the accuracy of using current ambient temperature design rules in Australia/New Zealand (AS/NZS 4600), American (AISI S100) and European (Eurocode 3 Part 1.3) standards in determining the flexural–torsional buckling capacities of cold-formed steel columns at uniform elevated temperatures using appropriately reduced mechanical properties. It was found that these design rules accurately predicted the member capacities of pin ended lipped channel columns undergoing flexural torsional buckling at elevated temperatures. However, for fixed ended columns with warping fixity undergoing flexural–torsional buckling, the current design rules significantly underestimated the column capacities as they disregard the beneficial effect of warping fixity. This paper has therefore recommended the use of improved design rules developed for ambient temperature conditions to predict the axial compression capacities of fixed ended columns subject to flexural–torsional buckling at elevated temperatures within AS/NZS 4600 and AISI S100 design provisions. The accuracy of the proposed fire design rules was verified using finite element analysis and test results of cold-formed lipped channel columns at elevated temperatures except for low strength steel columns with intermediate slenderness whose behaviour was influenced by the increased nonlinearity in the stress–strain curves at elevated temperatures. Further research is required to include these effects within AS/NZS 4600 and AISI S100 design rules. However, Eurocode 3 Part 1.3 design rules can be used for this purpose by using suitable buckling curves as recommended in this paper.

Resistance factor calibration for cold-formed steel compression members

The objective of this paper is to provide a review of the calibration procedure of the resistance factor for a target reliability index. Resistance factors for cold-formed steel columns are calculated with a first order second moment reliability approach. A test database of 323 cold-formed steel columns concentrically loaded with plain and lipped C-section was assembled, and test-to-predicted statistics were obtained for the three design methods available in Brazilian cold-formed steel code: the Effective Width Method (EWM), the Effective Section Method (ESM) and the Direct Strength Method (DSM). For the normal load combination, dead-to-live load ratio of 1/3, and target reliability index <img border=0 width=32 height=32 src="../../../../../img/revistas/rem/v66n2/a15for01.jpg" align=absmiddle > or = 2.5, the calculated resistance factors, g, for the three methods are consistent with current value specified by Brazilian code. For all methods, with target reliability index <img border=0 width=32 height=32 src="../../../../../img/revistas/rem/v66n2/a15for01.jpg" align=absmiddle > or = 3.0, the calculated resistance factors, g, are higher than the current value used in Brazilian code.

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

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

LRFD resistance factor for cold-formed steel compression members

This manuscript summarizes recent work to determine if the LRFD resistance factor for cold-formed steel compression members can be increased above its current value of 0.85. An experimental database of 675 concentrically loaded columns with plain and lipped C-sections, plain and lipped Z-sections, hat sections and angle sections, including members with holes, was compiled. The predicted strength of each specimen was calculated with the American Iron and Steel Institute's Main Specification and the Direct Strength Method, with the Direct Strength Method making more accurate strength predictions, especially for columns with partially effective cross sections. The LRFD resistance factor, calculated with a first order second moment reliability approach, was consistent with that currently specified in the code for both the Main Specification and the Direct Strength Method. Capacity predictions of columns failing by the distortional buckling limit state were more accurate than columns failing by global buckling or local–global buckling interaction. The test-to-predicted strength ratios for single angle columns were highly variable, with capacity predictions becoming excessively conservative with increasing global slenderness.

Numerical modelling of light gauge cold-formed steel compression members subjected to distortional buckling at elevated temperatures

Fire safety design of building structures has received greater attention in recent times due to continuing loss of properties and lives during fires. However, fire performance of light gauge cold-formed steel structures is not well understood despite its increased usage in buildings. Cold-formed steel compression members are susceptible to various buckling modes such as local and distortional buckling and their ultimate strength behaviour is governed by these buckling modes. Therefore a research project based on experimental and numerical studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. Lipped channel sections with and without additional lips were selected with three thicknesses of 0.6, 0.8, and 0.95 mm and both low and high strength steels (G250 and G550 steels). More than 150 compression tests were undertaken first at ambient and elevated temperatures. Finite element models of the tested compression members were then developed by including the degradation of mechanical properties with increasing temperatures. Comparison of finite element analysis and experimental results showed that the developed finite element models were capable of simulating the distortional buckling and strength behaviour at ambient and elevated temperatures up to 800 °C. The validated model was used to determine the effects of mechanical properties, geometric imperfections and residual stresses on the distortional buckling behaviour and strength of cold-formed steel columns. This paper presents the details of the numerical study and the results. It demonstrated the importance of using accurate mechanical properties at elevated temperatures in order to obtain reliable strength characteristics of cold-formed steel columns under fire conditions.

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

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