Cold-formed Channel Sections Research Articles

Effect of thickness on the behaviour of axially loaded back-to-back cold-formed steel built-up channel sections - Experimental and numerical investigation

In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of back-to-back built-up cold-formed steel channel sections are becoming increasingly popular. In such an arrangement, intermediate fasteners are required at discrete points along the length, preventing the channel-sections from buckling independently. Current guidance in the AISI &AS/NZS for such back-to-back built-up cold-formed steel channel sections requires the use of modified slenderness in order to take into account the spacing of the fasteners. Limited research has been done on back-to-back built-up cold-formed steel columns to understand the effect of column thickness and slenderness's on axial capacity. This issue is addressed herein. This paper presents the results of 60 experimental tests performed on back-to-back built-up cold-formed steel channel sections under compression. Detailed observations on different failure modes and column strengths were made through varying thickness, length and cross section of columns. A non-linear finite element model was developed which includes material non-linearity, geometric imperfections and explicit modelling of web fasteners. The finite element model was validated against experimental results. A comprehensive parametric study consisting of 204 models has been carried out covering a wide range of thickness and slenderness for the considered back-to-back built-up columns. Axial capacities obtained from the numerical study were used to assess the performance of the current AISI& AS/NZS standards when applied to cold-formed back-to-back built up columns; obtained comparisons showed that AISI& AS/NZS standards are un-conservative for stub and short columns which were failed by local buckling whereas standards were over-conservative for the strength of intermediate and slender columns which were failed mainly by overall member buckling. This paper has therefore proposed improved design rules and verified their accuracy using finite element analysis and test results of back-to-back built-up cold-formed channel sections, subjected to axial compression.

Nonlinear behavior of axially loaded back-to-back built-up cold-formed steel un-lipped channel sections

Back-to-back built-up cold-formed steel un-lipped channel sections are used in cold-formed steel structures; such as trusses, wall frames and portal frames. In such built-up columns, intermediate fasteners resist the buckling of individual channelsections. No experimental tests or finite element analyses have been reported in the literature for back-to-back built-up coldformed steel un-lipped channel sections and specially investigated the effect of screw spacing on axial strength of such columns. The issue is addressed in this paper. The results of 95 finite element analyses are presented covering stub to slender columns. The finite element model is validated against the experimental tests recently conducted by authors for back-to-back built-up cold-formed steel lipped channel sections. The verified finite element model is then used for the purposes of a parametric study to investigate the effect of screw spacing on axial strength of back-to-back built-up cold-formed steel un-lipped channel sections. Results are compared against the built-up lipped channel sections and it is shown that the axial strength of un-lipped built-up sections are 31% lesser on average than the built-up lipped channel sections. It was also found that the American Iron and Steel Institute (AISI) and the Australian and New Zealand Standards were over-conservative by around 15% for built-up columns failed through overall buckling, however AISI and AS/NZS were un-conservative by around 8% for built-up columns mainly failed by local buckling.

Post-fire mechanical properties of cold-formed steel hollow sections

Cold-formed steel hollow sections are extensively used in building construction due to their potential benefits over hot-rolled and open cold-formed steel sections. Their performance in fire is important for successful structural applications while their post-fire residual capacities are also important to assess the extent of their fire damage and reusability with and without strength enhancing techniques. This paper presents the details of an experimental investigation on the post-fire mechanical properties of cold-formed steel hollow sections. Tensile tests were performed on coupons exposed to elevated temperatures varying from 100 to 800 °C and then cooled down to room temperature. These coupons were cut from cold-formed steel square and rectangular hollow sections with varying thicknesses and grades. The results from this investigation provided post-fire stress-strain curves, yield strengths, ultimate strengths and elastic modulus, and their reduction pattern. They show that the post-fire mechanical properties of cold-formed steel hollow sections are different to those of open cold-formed channel and hot-rolled sections. New predictive equations were proposed to determine the post-fire mechanical property reduction factors. Post-fire mechanical properties were also compared with their elevated temperature mechanical properties to evaluate their strength gain after cooling down. This paper also evaluates various strength enhancement techniques for use with fire damaged cold-formed steel hollow sections.

Experimental Study of Axially Tension Cold Formed Steel Channel Members

Experimental testing is commonly used as one of the steps to determine the cause of the collapse of a building structure. The collapse of structures can be due to low quality materials. Although material samples have passed laboratory tests and the existing technical specifications have been met but there may be undetected defects and known material after failure. In this paper will be presented Experimental Testing of Axially Tension Cold Formed Steel Channel Members to determine the cause of the collapse of a building roof truss x in Pekanbaru. Test of tensile strength material cold formed channel sections was performed to obtain the main characteristics of Cold Formed steel material, namely ultimate tensile strength loads that can be held by members and the yield stress possessed by channel sections used in construction. Analysis of axially tension cold formed steel channel section presents in this paper was conducted through experimental study based on specificationsAnnualBook of ASTM Standards: Metal Test methods and Analitical Procedures, Section 3 (1991). The result of capacity loads experimental test was compared with design based on SNI 03-7971-2013standard of Indonesia for the design of cold formed steel structural members. The results of the yield stress of the material will be seen against the minimum allowable allowable stress range. After the test, the percentace of ultimate axial tension capacity theory has a result that is 16.46% larger than the ultimate axial tension capacity experimental. When compared with the load that must be borne 5.673 kN/m it can be concluded that 2 specimens do not meet. Yield stress of member has fulfilled requirement that wass bigger than 550 MPa. Based on the curve obtained ultimate axial tension capacity theory, results greater than experimental. The greatest voltage value (fu) is achieved under the same conditions as its yield stress. For this specimen with a melting voltage value fy = 571.5068 MPa has fulfilled the minimum melting point value of 550 MPa required for standard mild steel materials in accordance with the code SNI 03-7971-2013 about Cold formed steel.

Experimental Study of Axially Tension Cold Formed Steel Channel Members

Experimental testing is commonly used as one of the steps to determine the cause of the collapse of a building structure. The collapse of structures can be due to low quality materials. Although material samples have passed laboratory tests and the existing technical specifications have been met but there may be undetected defects and known material after failure. In this paper will be presented Experimental Testing of Axially Tension Cold Formed Steel Channel Members to determine the cause of the collapse of a building roof truss x in Pekanbaru. Test of tensile strength material cold formed channel sections was performed to obtain the main characteristics of Cold Formed steel material, namely ultimate tensile strength loads that can be held by members and the yield stress possessed by channel sections used in construction. Analysis of axially tension cold formed steel channel section presents in this paper was conducted through experimental study based on specificationsAnnualBook of ASTM Standards: Metal Test methods and Analitical Procedures, Section 3 (1991). The result of capacity loads experimental test was compared with design based on SNI 03-7971- 2013standard of Indonesia for the design of cold formed steel structural members. The results of the yield stress of the material will be seen against the minimum allowable allowable stress range. After the test, the percentace of ultimate axial tension capacity theory has a result that is 16.46% larger than the ultimate axial tension capacity experimental. When compared with the load that must be borne 5.673 kN/m it can be concluded that 2 specimens do not meet. Yield stress of member has fulfilled requirement that wass bigger than 550 MPa. Based on the curve obtained ultimate axial tension capacity theory, results greater than experimental. The greatest voltage value (fu) is achieved under the same conditions as its yield stress. For this specimen with a melting voltage value fy = 571.5068 MPa has fulfilled the minimum melting point value of 550 MPa required for standard mild steel materials in accordance with the code SNI 03- 7971-2013 about Cold formed steel.

Effect of screw spacing on behavior of axially loaded back-to-back cold-formed steel built-up channel sections

In cold-formed steel structures, such as trusses, wall frames, and portal frames, the use of back-to-back built-up cold-formed steel channel sections for the column members is becoming increasingly popular. In such an arrangement, intermediate fasteners at discrete points along the length prevent the individual channel sections from buckling independently. Current guidance by the American Iron and Steel Institute and the Australian and New Zealand Standards for built-up sections describes a modified slenderness approach, to take into account the spacing of the screws. Limited experimental tests or finite element analyses, however, have been reported in the literature for such sections to understand the effect of screw spacing. This issue is addressed herein. The results of 30 experimental tests are reported, conducted on back-to-back built-up cold-formed steel channel sections covering stub columns to slender columns. A finite element model is then described which shows good agreement with the experimental test results. The finite element model is then used for the purposes of a parametric study comprising 144 models. It is shown that while the modified slenderness approach is in general conservative, for stub columns it can be unconservative by around 10%.

Section compression capacity of high strength cold-formed hollow flange channels

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.

Experimental Study on the Flexural-Torsional Behavior of Cold-Formed Steel Channel Section Connected Back to Back

Cold-formed steels are made by rolling or pressing steel into semi-finished or finished goods at room temperatures. Cold-formed steel products are made by the working of steel billet, bar or sheet using stamping, rolling and including roll forming or presses to deform it into a usable product. Objective: By flexural-torsional behaviour channel section connected back to back. Four specimen of built up I section formed by using symmetric channel section connected back to back with the help of bolts provided in the web part of section. All the specimens were tested for flexural-torsional strength under two point loading. For this analytical study of cold-formed channel section four specimen of dimension of a 230 mm × 100 mm × 3 mm with a lip of 30 mm and 4 specimens of dimension of 180 mm x 50 mm x 1.6 mm with a lip of 20 mm and 1.8 m length were loaded vertically were unrestrained to allow flexural buckling. Method: The analytical investigation of the built up I section will be carried out using FEM modelling ANSYS. The theoretical values are calculated using IS 801:1975 and BS-5:5950. Findings: For the selected channel section with 230 mm and 180 mm depth the difference in theoretical, analytical and experimental results are found to be 2.5% and 4%. Application: Further this study can be carried out for steel section with increased or decreased depth of web and lip. It can also be used as a roofing sheet in warehouses etc.

Fire design rules for LSF walls made of hollow flange channel sections

Cold-formed Hollow Flange Channel (HFC) sections can be used in Light gauge Steel Frame (LSF) wall systems due to their structural efficiency. Recent experimental and finite element analysis based investigations conducted by the authors have demonstrated the superior fire performance of LSF walls made of welded HFC sections. The authors have developed a wide range of fire performance data of LSF walls through a finite element analysis (FEA) based extensive parametric study. This paper investigates the applicability of the available fire design rules to predict their structural capacities. Since fire design rules were not available for HFC sections, the latest design rules for LCS studs subjected to non-uniform temperature distributions were selected for evaluation from the pool of various design rules given in standards and previous studies. Suitable modifications were then incorporated for simplification and improved accuracy. Two improved design methods based on AS/NZS 4600 and Eurocode 3 were proposed. The structural capacity of HFC section stud found from the design rule predictions was converted into load ratio which is the ratio between the structural capacities under fire and ambient conditions, and the load ratio versus FRR curve were produced for different LSF walls. These were then compared with the FEA results, to verify the accuracy of the proposed design rules. This paper also presents suitable DSM based design method proposed for HFC section studs subject to non-uniform temperature distributions in LSF walls, and verifies its accuracy.

Optimization of lip length and aspect ratio of thin channel sections under minor axes bending

Cold-formed steel members are used as structural members in a wide range of applications because of their simpler fabrications and high strength-to-weight ratios. Though high width-to-thickness ratio of such sections make them vulnerable to local buckling under compressive stresses, past studies have shown that these members possess inelastic reserve strengths at the ultimate stage under flexural loading. This study explore the influence of width-to-thickness ratio, lip lengths and depth-to-width (aspect) ratio on the ultimate flexural strengths of cold-formed channel sections bending about their minor axes. Finite element (FE) analyses have been conducted on 216 numbers of channel sections with varying geometric properties to investigate their ultimate resistance and mode of failure under flexural loading. The ultimate flexural strengths of these sections are theoretically predicted using the current code provisions and are compared with the exact values obtained from FE analyses. An experimental investigation has also been performed on five specimens of channel sections with and without lips to validate the FE analyses results. Based on the analysis and experimental results, the limiting values of lip lengths, aspect ratios, and width-to-thickness ratios of channel sections are discussed.

Bearing capacity of cold‐formed unlipped channels with restrained flanges under EOF and IOF load cases

AbstractBearing failure is a form of localized failure that occurs when thin‐walled cold‐formed steel sections are subjected to concentrated loads or support reactions. To determine the bearing capacity of cold‐formed channel sections, a unified design equation with different bearing coefficients is given in the current North American specification AISI S100 and the Australian/New Zealand standard AS/NZS 4600. However, coefficients are not available for unlipped channel sections that are normally fastened to supports through their flanges. Eurocode 3 Part 1.3 includes bearing capacity equations for different load cases, but does not distinguish between fastened and unfastened support conditions. Therefore, an experimental study was conducted to determine the bearing capacities of these sections as used in floor systems. Twenty‐eight web bearing tests on unlipped channel sections with restrained flanges were conducted under End One Flange (EOF) and Interior One Flange (IOF) load cases. Using the results from this study, a new equation was proposed within the AISI S100 and AS/NZS 4600 guidelines to determine the bearing capacities of cold‐formed unlipped channels with flanges fastened to supports. A new design rule was also proposed based on the direct strength method.

Failure modes and buckling coefficient of partially stiffened cold-formed sections in bending

Flange edge stiffeners increase the ultimate moment capacity of cold-formed channel sections. At the same time, they cause complexity to the buckling failure mode of the section. There is a lack of experimental research on the failure mode of sections with a partially stiffened element, such as channel sections with edge stiffeners, in which a distortional buckling mode can be observed. The focus of recent studies is mainly on the behaviour of the whole section as one member under bending without any concern about the relationship between the web and flange ratio.In this study, an extensive experimental analysis of 42 cold-formed channel sections was used to explore the failure behaviour of cold-formed channel sections under pure bending. The sections were made from cold-formed G450 steel with a nominal thickness of 1.6mm. The results of the pure bending experimental investigations are used to describe the relationship between the web and flange ratio and the failure deformations. It is also shown that the current international cold-formed steel specifications over-predict the buckling coefficient of partially stiffened elements with high aspect ratio values. The experimental results are used to propose revisions to current international cold-formed steel specifications.

Post-yield capacity of cold-formed channel sections in bending

Flange edge stiffeners can increase the ultimate moment capacity of cold-formed channel sections up to their post-yielding (inelastic) capacity. This paper investigates the post-yielding behaviour under bending of cold-formed channel sections with partially stiffened elements. The relevant literature was reviewed, experimental studies were carried out and a semi-empirical analysis was performed. Experimental and numerical analysis was undertaken of 40 cold-formed channel sections, each with a partially stiffened element. The results were compared with the existing Australian design rules and revisions are proposed to these rules. In addition, by using the test observations and the yield line mechanism model, the ultimate capacity of the tested sections was determined and compared with the test results. The model was found to provide accurate and reliable capacity predictions for slender cold-formed channel sections under bending.

Experimental study of bolted connections using light gauge channel sections and packing plates at the joints

Cold-formed structural members are being used more widely in routine structural design as the world steel industry moves from the production of hot-rolled section and plate to coil and strip, often with galvanized and/or painted coatings. Steel in this form is more easily delivered from the steel mill to the manufacturing plant where it is usually cold-rolled into open and closed section members. In the present experimental study, the use of packing plate at the joints in cold-formed channel sections may increase the load carrying capacity and also reduce the buckling of unconnected cold form channel steel plate at joints. The present study focuses on examining the experimental investigation to use mild steel as a packing plate with cold-formed channel sections by bolted connection at the joints and the connection subjected to axial tension. Series of tests are carried out with increase in the thickness of packing plate and results are observed and analyzed. Total Twelve experimental tests have been carried out on cold-formed channel tension members fastened with single as well as three numbers of bolts at the connection and from the observations the strength of the joint is increased by increasing the various thicknesses of packing plates and also the buckling of unconnected leg of channel specimen is reduced. It is analyzed by plotting the entire load versus elongation path, so that the behavior of the connection is examined.

Section classifications for cold-formed channel steel

The inelastic reserve capacity, which is the additional capacity of a member beyond the first yielding, is very important for limit-state design of frame structures. The aim of this work was to investigate the inelastic bending capacity of cold-formed channel sections according to European standards and to propose revisions to current design rules. An extensive experimental and analytical analysis of 42 cold-formed channel sections was conducted. Material properties of the tested sections were examined using tension tests on metal coupons. The sections were cold-formed from G450 steel with a nominal thicknesses of 1·6 mm, and varying theoretical buckling stresses ranging between elastic and seven times the yield stress. The results from the pure bending experimental investigations and the European design standards for steel structures were compared. It is concluded that the section classifications defined in Eurocode 3 are not accurate for cold-formed channel sections. Therefore, modifications to the section classifications that have been derived for hot-rolled sections are required in the case of cold-formed sections to maintain accuracy (based on the presented test series). Design rules are developed to account for such behaviour.

Effective Shaping of Cold-Formed Thin-Walled Built-up Beams in Pure Bending

Cold-formed thin-walled beams with an open cross section are widely used in different branches of industry. They tend to buckle in a variety of modes, which can be described as local, distortional, lateral-torsional buckling and bending. In this paper, the investigation on built-up I-sections, formed by assembling the stiffened cold-formed channel sections, with or without edge stiffener is presented. Parametric studies are carried out for five different section geometries with 16 specimens, tested under two point loading, to identify factors affecting the flexural strength of stiffened cold-formed steel built-up sections. The section geometries are chosen such that all types of buckling modes are met with. Critical load obtained from experiments is compared with finite element analysis results. Finally, it discusses the suitability of the current design methods (American Iron and Steel Institute) and provides design recommendations for stiffened cold-formed built-up sections.

Compression tests of cold-formed channel sections with perforations in the web

This paper describes a series of compression tests performed on cold-formed steel channel sections with perforations in the web (thermal studs) fabricated from a galvanized steel plate whose thickness ranged from 1.0 mm to 1.6 mm and nominal yield stress was 295 MPa. The structural behavior and performance of thermal studs undergoing local, distortional, or flexural-torsional buckling were investigated experimentally and analytically. The compression tests indicate that the slits in the web had significant negative effects on the buckling and ultimate strength of thin-walled channel section columns. The compressive strength of perforated thermal studs was estimated using equivalent solid channel sections of reduced thickness instead of the studs. The direct strength method, a newly developed and adopted alternative to the effective width method for designing cold-formed steel sections in the AISI Standard S100 (2004) and AS/NZS 4600 (Standard Australia 2005), was calibrated to the test results for its application to cold-formed channel sections with slits in the web. The results verify that the DSM can predict the ultimate strength of channel section columns with slits in the web by substituting equivalent solid sections of reduced thickness for them.

Numerical studies and design of hollow flange channel beams subject to combined bending and shear actions

LiteSteel beam (LSB) is a cold-formed steel hollow flange channel section produced using a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. It is commonly used as floor joists and bearers in residential, industrial and commercial buildings. Design of the LSB is governed by the Australian cold-formed steel structures code, AS/NZS 4600. Due to the geometry of the LSB, as well as its unique residual stress characteristics and initial geometric imperfections resultant of manufacturing processes, currently available design equations for common cold-formed sections are not directly applicable to the LSB. Many research studies have been carried out to evaluate the behaviour and design of LSBs subject to pure bending actions and predominant shear actions. To date, however, no investigation has been conducted into the strength of LSB sections under combined bending and shear actions. Hence experimental and numerical studies were conducted to assess the combined bending and shear behaviour of LSBs. Finite element models of LSBs were developed to simulate their combined bending and shear behaviour and strength of LSBs. They were then validated by comparing the results with available experimental test results and used in a detailed parametric study. The results from experimental and finite element analyses were compared with current AS/NZS 4600 and AS 4100 design rules. Both experimental and numerical studies show that the AS/NZS 4600 design rule based on circular interaction equation is conservative in predicting the combined bending and shear capacities of LSBs. This paper presents the details of the numerical studies of LSBs and the results. In response to the inadequacies of current approaches to designing LSBs for combined bending and shear, two lower bound design equations are proposed in this paper.

Experimental studies of the shear behavior and strength of LiteSteel beams with stiffened web openings

LiteSteel beam (LSB) is a new cold-formed steel hollow flange channel section produced using a patented manufacturing process. It is commonly used as flexural members in residential, industrial and commercial buildings. Current practice in flooring systems is to include openings in the web element of floor joists or bearers so that building services can be located within them. Test results have shown that the shear capacity of LSBs can be reduced considerably by the inclusion of web openings. A cost effective method of eliminating the detrimental effects of a large web opening is to attach suitable stiffeners around the web openings of LSBs. A detailed experimental study consisting of 17 shear tests was therefore undertaken to investigate the shear behavior and strength of LSBs with stiffened circular web openings. Both plate and stud stiffeners with varying sizes and thicknesses were attached to the web elements of LSBs using a number of screw-fastening arrangements in order to develop a suitable stiffening arrangement for LSBs. Simply supported test specimens of LSBs with an aspect ratio of 1.5 were loaded at mid-span until failure. This paper presents the details of this experimental study of LSBs with stiffened web openings, and the results of their shear capacities and associated behavioral characteristics. Suitable screw-fastened plate stiffener arrangements have been recommended in order to restore the original shear capacity of LSBs.

Numerical Modelling of Stiffness and Strength Behaviour of Top-Seat Flange-Cleat Connection for Cold-Formed Double Channel Section

Prediction of structural behaviour by numerical modelling can reduce the cost in conducting full-scaled experiments. This paper studies the stiffness and strength behaviour of top-seat flange-cleat connection for cold-formed steel double channel sections using finite element method. In this investigation, cold-formed channel sections are assembled back-to-back to form I-shape beam and column members. The 2 mm cold-formed bracket and 6 mm hot-rolled angle are used to connect the members. The results were collected from different beam depth ranged 150 mm, 200 mm and 250 mm. The rotational stiffness and strength obtained from the numerical modelling are then compared to the design requirements from BS EN 1993-1-8 and experimental data. The comparison of moment-rotation behaviour for top-seat flange-cleat connection has shown not more than 35% difference for strength behaviour and 50% difference for rotational stiffness behaviour between numerical modelling and experimental data. However, there is a noticeable difference between finite element models and analytical calculation. The differences are recorded from 18% to 65% for strength behaviour and between 1% and 153% for stiffness behaviour. The differences obtained between finite element analysis and experimental investigation are caused by edge stiffener while differences from finite element models and analytical models are due to strain hardening.