Flexural behaviour and design of cold-formed steel beams with rectangular hollow flanges

Until recently, the hot-rolled steel members have been recognized as the most popular and widely used steel group, but in recent times, the use of cold-formed high strength steel members has rapidly increased. However, the structural behavior of light gauge high strength cold-formed steel members characterized by various buckling modes is not yet fully understood. The current cold-formed steel sections such as C- and Z-sections are commonly used because of their simple forming procedures and easy connections, but they suffer from certain buckling modes. It is therefore important that these buckling modes are either delayed or eliminated to increase the ultimate capacity of these members. This research is therefore aimed at developing a new cold-formed steel beam with two torsionally rigid rectangular hollow flanges and a slender web formed using intermittent screw fastening to enhance the flexural capacity while maintaining a minimum fabrication cost. This thesis describes a detailed investigation into the structural behavior of this new Rectangular Hollow Flange Beam (RHFB), subjected to flexural action The first phase of this research included experimental investigations using thirty full scale lateral buckling tests and twenty two section moment capacity tests using specially designed test rigs to simulate the required loading and support conditions. A detailed description of the experimental methods, RHFB failure modes including local, lateral distortional and lateral torsional buckling modes, and moment capacity results is presented. A comparison of experimental results with the predictions from the current design rules and other design methods is also given. The second phase of this research involved a methodical and comprehensive investigation aimed at widening the scope of finite element analysis to investigate the buckling and ultimate failure behaviours of RHFBs subjected to flexural actions. Accurate finite element models simulating the physical conditions of both lateral buckling and section moment capacity tests were developed. Comparison of experimental and finite element analysis results showed that the buckling and ultimate failure behaviour of RHFBs can be simulated well using appropriate finite element models. Finite element models simulating ideal simply supported boundary conditions and a uniform moment loading were also developed in order to use in a detailed parametric study. The parametric study results were used to review the current design rules and to develop new design formulae for RHFBs subjected to local, lateral distortional and lateral torsional buckling effects. Finite element analysis results indicate that the discontinuity due to screw fastening has a noticeable influence only for members in the intermediate slenderness region. Investigations into different combinations of thicknesses in the flange and web indicate that increasing the flange thickness is more effective than web thickness in enhancing the flexural capacity of RHFBs. The current steel design standards, AS 4100 (1998) and AS/NZS 4600 (1996) are found sufficient to predict the section moment capacity of RHFBs. However, the results indicate that the AS/NZS 4600 is more accurate for slender sections whereas AS 4100 is more accurate for compact sections. The finite element analysis results further indicate that the current design rules given in AS/NZS 4600 is adequate in predicting the member moment capacity of RHFBs subject to lateral torsional buckling effects. However, they were inadequate in predicting the capacities of RHFBs subject to lateral distortional buckling effects. This thesis has therefore developed a new design formula to predict the lateral distortional buckling strength of RHFBs. Overall, this thesis has demonstrated that the innovative RHFB sections can perform well as economically and structurally efficient flexural members. Structural engineers and designers should make use of the new design rules and the validated existing design rules to design the most optimum RHFB sections depending on the type of applications. Intermittent screw fastening method has also been shown to be structurally adequate that also minimises the fabrication cost. Product manufacturers and builders should be able to make use of this in their applications.

Elastic lateral buckling of simply supported LiteSteel beams subject to transverse loading

The buckling strength of a new cold-formed hollow flange channel section known as LiteSteel beam (LSB) is governed by lateral distortional buckling characterised by simultaneous lateral deflection, twist and web distortion for its intermediate spans. Recent research has developed a modified elastic lateral buckling moment equation to allow for lateral distortional buckling effects. However, it is limited to a uniform moment distribution condition that rarely exists in practice. Transverse loading introduces a non-uniform bending moment distribution, which is also often applied above or below the shear centre (load height). These loading conditions are known to have significant effects on the lateral buckling strength of beams. Many steel design codes have adopted equivalent uniform moment distribution and load height factors to allow for these effects. But they were derived mostly based on data for conventional hot-rolled, doubly symmetric I-beams subject to lateral torsional buckling. The moment distribution and load height effects of transverse loading for LSBs, and the suitability of the current design modification factors to accommodate these effects for LSBs is not known. This paper presents the details of a research study based on finite element analyses on the elastic lateral buckling strength of simply supported LSBs subject to transverse loading. It discusses the suitability of the current steel design code modification factors, and provides suitable recommendations for simply supported LSBs subject to transverse loading.

Lateral buckling strength of simply supported LiteSteel beams subject to moment gradient effects

The flexural capacity of of a new cold-formed hollow flange channel section known as LiteSteel beam (LSB) is limited by lateral distortional buckling for intermediate spans, which is characterised by simultaneous lateral deflection, twist and web distortion. Recent research has developed suitable design rules for the member capacity of LSBs. However, they are limited to a uniform moment distribution that rarely exists in practice. Many steel design codes have adopted equivalent uniform moment distribution factors to accommodate the effect of non-uniform moment distributions in design. But they were derived mostly based on the data for conventional hot-rolled, doubly symmetric I-beams subject to lateral torsional buckling. The effect of moment distribution for LSBs, and the suitability of the current steel design code rules to include this effect for LSBs are not yet known. This paper presents the details of a research study based on finite element analyses of the lateral buckling strength of simply supported LSBs subject to moment gradient effects. It also presents the details of a number of LSB lateral buckling experiments undertaken to validate the results of finite element analyses. Finally, it discusses the suitability of the current design methods, and provides design recommendations for simply supported LSBs subject to moment gradient effects.

Experimental studies of the shear behavior of LiteSteel Beams

This paper presents the details of experimental studies on the shear behaviour of a recently developed, cold-formed steel beam known as LiteSteel Beam (LSB). The LSB section has a unique shape of a channel beam with two rectangular hollow flanges and is produced by a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. To date, no research has been undertaken on the shear behaviour of LiteSteel beams with torsionally rigid, rectangular hollow flanges. In the present investigation, experimental studies involving more than 30 shear tests were carried out to investigate the shear behaviour of 13 different LSB sections. It was found that the current design rules in cold-formed steel structures design codes are very conservative for the shear design of LiteSteel beams. Significant improvements to web shear buckling occurred due to the presence of rectangular hollow flanges while considerable post-buckling strength was also observed. Experimental results are presented and compared with corresponding predictions from the current design codes in this paper. Appropriate improvements have been proposed for the shear strength of LSBs based on AS/NZS 4600 design equations.

Post-buckling Strength of LiteSteel Beams in Shear

This paper presents the details of experimental and numerical studies on the shear behaviour of a recently developed, cold-formed steel beam known as LiteSteel Beam (LSB). The LSB sections are produced by a patented manufacturing process involving simultaneous cold-forming and electric resistance welding. It has a unique shape of a channel beam with two rectangular hollow flanges. Recent research has demonstrated the presence of increased shear capacity of LSBs due to the additional fixity along the web to flange juncture, but the current design rules ignore this effect. Therefore they were modified by including a higher elastic shear buckling coefficient. In the present study, the ultimate shear capacity results obtained from the experimental and numerical studies of 10 different LSB sections were compared with the modified shear capacity design rules. It was found that they are still conservative as they ignore the presence of post-buckling strength. Therefore the design rules were further modified to include the available post-buckling strength. Suitable design rules were also developed under the direct strength method format. This paper presents the details of this study and the results including the final design rules for the shear capacity of LSBs.

Effect of section geometry on the lateral distortional buckling of LiteSteel beams

The LiteSteel Beam (LSB) is a new hollow flange section developed by OneSteel Australian Tube Mills using their patented dual electric resistance welding and automated continuous roll-forming technologies. It has a unique geometry consisting of torsionally rigid rectangular hollow flanges and a relatively slender web. It has found increasing popularity in residential, industrial and commercial buildings as flexural members. The LSB is considerably lighter than traditional hot-rolled steel beams and provides both structural and construction efficiencies. However, the LSB flexural members are subjected to a relatively new lateral distortional buckling mode, which reduces their member moment capacities. Unlike the commonly observed lateral torsional buckling of steel beams, the lateral distortional buckling of LSBs is characterised by simultaneous lateral defection, twist and cross sectional change due to web distortion. The current design rules in AS/NZS 4600 (SA, 2005) for flexural members subject to lateral distortional buckling were found to be conservative by about 8% in the inelastic buckling region. Therefore, a new design rule was developed for LSBs subject to lateral distortional buckling based on finite element analyses of LSBs. The effect of section geometry was then considered and several geometrical parameters were used to develop an advanced set of design rules. This paper presents the details of the finite element analyses and the design curve development for hollow flange sections subject to lateral distortional buckling.

Lateral distortional buckling design rules for back to back LiteSteel beams

A new cold-formed steel beam, known as the LiteSteel Beam (LSB), has the potential to transform the low-rise building industry. The new beam is effectively a channel section with two rectangular hollow flanges and a slender web, and is manufactured using a simultaneous cold-forming and electric resistance welding process. Research into the flexural behaviour of single LSB members showed that the LSBs are susceptible to lateral distortional buckling effects and their moment capacities are significantly reduced for intermediate spans. Build-up LSB sections are expected to improve their flexural capacity and to enhance their applications. They are also likely to mitigate the detrimental effects of lateral distortional buckling observed with single LSB members of intermediate spans. However, the behaviour of build up beams is not well understood. Currently available design rules were found to be inadequate to predict the member moment capacities of back to back LSBs. Therefore a research project based on both experimental and numerical studies was undertaken to investigate the flexural behaviour of back to back LSBs with various longitudinal connection spacings under a uniform moment. New design rules were developed using the moment capacity data obtained using finite element analyses and experimental tests. This paper presents the details of the development of design rules for the back to back LSB sections.

Ultimate shear strength of LiteSteel beams

This paper presents the details of an investigation on the shear behaviour of a recently developed, cold-formed steel beam known as LiteSteel Beam (LSB).The LSB section has a unique shape of a channel beam with two rectangular hollow flanges and is produced by a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. In the present investigation, a series of numerical analyses based on three-dimensional finite element modeling and an experimental study were carried out to investigate the shear behaviour of 10 different LSB sections. It was found that the current design rules in cold-formed steel structures design codes are very conservative for the shear design of LiteSteel beams. Significant improvements to web shear buckling occurred due to the presence of rectangular hollow flanges while considerable post-buckling strength was also observed. Therefore the design rules were further modified to include the available post-buckling strength. Suitable design rules were also developed under the direct strength method format. This paper presents the details of this investigation and the results including the final design rules for the shear capacity of LSBs. It also presents new shear strength formulae for lipped channel beams based on the current design equations for shear strength given in AISI (2007) using the same approach used for LSBs.

Lateral-torsional buckling of cold-formed steel beams at ambient temperature

Cold-formed steel beams are increasingly used as floor joists and bearers in residential, industrial and commercial buildings. Their structural behaviour and moment capacities are influenced by lateral-torsional buckling and hence a research study was undertaken to investigate the lateral-torsional buckling behaviour of cold-formed steel lipped channel beams at ambient and elevated temperatures. For this purpose a finite element model of a simply supported cold-formed steel lipped channel beam under uniform bending was developed first and validated using available numberical and experimental results. It was then used in a detailed parametric study to simulate the lateral-torsional behaviour of cold-formed steel beams under varying conditions. The moment capacity results were then compared with the predictions from the current ambient temperature design rules in Australia, New Zealand, American and European codes for cold-formed steel structures. Some very interesting results have been obtained. European design rules are found to be conservative while Australian and American design rules are unsafe. This paper presents the results of finite element analyses for ambient temperature conditions, and the comparison with the current design rules.

Structural behaviour and design of cold-formed steel beams at elevated temperatures

Cold-formed steel members are extensively used in the building construction industry, especially in residential, commercial and industrial buildings. In recent times, fire safety has become important in structural design due to increased fire damage to properties and loss of lives. However, past research into the fire performance of cold-formed steel members has been limited, and was confined to compression members. Therefore a research project was undertaken to investigate the structural behaviour of compact cold-formed steel lipped channel beams subject to inelastic local buckling and yielding, and lateral-torsional buckling effects under simulated fire conditions and associated section and member moment capacities. In the first phase of this research, an experimental study based on tensile coupon tests was undertaken to obtain the mechanical properties of elastic modulus and yield strength and the stress-strain relationship of cold-formed steels at uniform ambient and elevated temperatures up to 700oC. The mechanical properties deteriorated with increasing temperature and are likely to reduce the strength of cold-formed beams under fire conditions. Predictive equations were developed for yield strength and elastic modulus reduction factors while a modification was proposed for the stressstrain model at elevated temperatures. These results were used in the numerical modelling phases investigating the section and member moment capacities. The second phase of this research involved the development and validation of two finite element models to simulate the behaviour of compact cold-formed steel lipped channel beams subject to local buckling and yielding, and lateral-torsional buckling effects. Both models were first validated for elastic buckling. Lateral-torsional buckling tests of compact lipped channel beams were conducted at ambient temperature in order to validate the finite element model in predicting the non-linear ultimate strength behaviour. The results from this experimental study did not agree well with those from the developed experimental finite element model due to some unavoidable problems with testing. However, it highlighted the importance of magnitude and direction of initial geometric imperfection as well as the failure direction, and thus led to further enhancement of the finite element model. The finite element model for lateral-torsional buckling was then validated using the available experimental and numerical ultimate moment capacity results from past research. The third phase based on the validated finite element models included detailed parametric studies of section and member moment capacities of compact lipped channel beams at ambient temperature, and provided the basis for similar studies at elevated temperatures. The results showed the existence of inelastic reserve capacity for compact cold-formed steel beams at ambient temperature. However, full plastic capacity was not achieved by the mono-symmetric cold-formed steel beams. Suitable recommendations were made in relation to the accuracy and suitability of current design rules for section moment capacity. Comparison of member capacity results from finite element analyses with current design rules showed that they do not give accurate predictions of lateral-torsional buckling capacities at ambient temperature and hence new design rules were developed. The fourth phase of this research investigated the section and member moment capacities of compact lipped channel beams at uniform elevated temperatures based on detailed parametric studies using the validated finite element models. The results showed the existence of inelastic reserve capacity at elevated temperatures. Suitable recommendations were made in relation to the accuracy and suitability of current design rules for section moment capacity in fire design codes, ambient temperature design codes as well as those proposed by other researchers. The results showed that lateral-torsional buckling capacities are dependent on the ratio of yield strength and elasticity modulus reduction factors and the level of non-linearity in the stress-strain curves at elevated temperatures in addition to the temperature. Current design rules do not include the effects of non-linear stress-strain relationship and therefore their predictions were found to be inaccurate. Therefore a new design rule that uses a nonlinearity factor, which is defined as the ratio of the limit of proportionality to the yield stress at a given temperature, was developed for cold-formed steel beams subject to lateral-torsional buckling at elevated temperatures. This thesis presents the details and results of the experimental and numerical studies conducted in this research including a comparison of results with predictions using available design rules. It also presents the recommendations made regarding the accuracy of current design rules as well as the new developed design rules for coldformed steel beams both at ambient and elevated temperatures.

Elastic shear buckling characteristics of LiteSteel Beams

LiteSteel beam (LSB) is a new cold-formed steel hollow flange channel beam. The unique LSB section is produced by a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. To date, limited research has been undertaken on the shear buckling behaviour of LSBs with torsionally rigid, rectangular hollow flanges. For the shear design of LSB web panels, their elastic shear buckling strength must be determined accurately including the potential post-buckling strength. Currently the elastic shear buckling coefficients of web panels are determined by assuming conservatively that the web panels are simply supported at the junction between the flange and web elements. Therefore finite element analyses were carried out to investigate the elastic shear buckling behaviour of LSB sections including the effect of true support conditions at the junction between their flange and web elements. An improved equation for the higher elastic shear buckling coefficient of LSBs was developed and included in the shear capacity equations of Australian cold-formed steel codes. Predicted ultimate shear capacity results were compared with available experimental results, both of which showed considerable improvement to the shear capacities of LSBs. A study on the shear flow distribution of LSBs was also undertaken prior to the elastic buckling analysis study. This paper presents the details of this investigation and the results including the shear flow distribution of LSBs. Keywords: LiteSteel beam, Elastic shear buckling, Shear flow, Cold-formed steel structures, Slender web, Hollow flanges.

Experimental studies on the shear behaviour and strength of LiteSteel Beams

This paper presents the details of an experimental study on the shear behaviour and strength of a recently developed, cold-formed steel hollow flange channel beam known as LiteSteel Beam (LSB). The new LSB sections with rectangular hollow flanges are produced using a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. They are commonly used as flexural members in buildings. However, no research has been undertaken on the shear behaviour of LSBs. Therefore a detailed experimental study involving 36 shear tests was undertaken to investigate the shear behaviour of 10 different LSB sections. Simply supported test specimens of LSBs with aspect ratios of 1.0 and 1.5 were loaded at midspan until failure using both single and back to back LSB arrangements. Test specimens were chosen such that all three types of shear failure (shear yielding, inelastic and elastic shear buckling) occurred in the tests. Comparison of experimental results with corresponding predictions from the current Australian and North American cold-formed steel design rules showed that the current design rules are very conservative for the shear design of LSBs. Significant improvements to web shear buckling occurred due to the presence of rectangular hollow flanges while considerable post-buckling strength was also observed. Appropriate improvements have been proposed for the shear strength of LSBs based on the design equations in the North American Specification. This paper presents the details of this experimental study and the results. When reduced height web side plates or only one web side plate was used, the shear capacity of LSB was reduced. Details of these tests and the results are also presented in this paper. Keywords: LiteSteel beam, Shear strength, Shear tests, Cold-formed steel structures, Direct strength method, Slender web, Hollow flanges.

Shear behaviour and design of LiteSteel beams

OneSteel Australian Tube Mills has recently developed a new hollow flange channel cold-formed section, known as the LiteSteel Beam (LSB). The innovative LSB sections have the beneficial characteristics of torsionally rigid closed rectangular flanges combined with economical fabrication processes from a single strip of high strength steel. They combine the stability of hot-rolled steel sections with the high strength to weight ratio of conventional cold-formed steel sections. The LSB sections are commonly used as flexural members in residential, industrial and commercial buildings. In order to ensure safe and efficient designs of LSBs, many research studies have been undertaken on the flexural behaviour of LSBs. However, no research has been undertaken on the shear behaviour of LSBs. Therefore this thesis investigated the ultimate shear strength behaviour of LSBs with and without web openings including their elastic buckling and post-buckling characteristics using both experimental and finite element analyses, and developed accurate shear design rules. Currently the elastic shear buckling coefficients of web panels are determined by assuming conservatively that the web panels are simply supported at the junction between the web and flange elements. Therefore finite element analyses were conducted first to investigate the elastic shear buckling behaviour of LSBs to determine the true support condition at the junction between their web and flange elements. An equation for the higher elastic shear buckling coefficient of LSBs was developed and included in the shear capacity equations in the cold-formed steel structures code, AS/NZS 4600. Predicted shear capacities from the modified equations and the available experimental results demonstrated the improvements to the shear capacities of LSBs due to the presence of higher level of fixity at the LSB flange to web juncture. A detailed study into the shear flow distribution of LSB was also undertaken prior to the elastic buckling analysis study. The experimental study of ten LSB sections included 42 shear tests of LSBs with aspect ratios of 1.0 and 1.5 that were loaded at midspan until failure. Both single and back to back LSB arrangements were used. Test specimens were chosen such that all three types of shear failure (shear yielding, inelastic and elastic shear buckling) occurred in the tests. Experimental results showed that the current cold-formed steel design rules are very conservative for the shear design of LSBs. Significant improvements to web shear buckling occurred due to the presence of rectangular hollow flanges while considerable post-buckling strength was also observed. Experimental results were presented and compared with corresponding predictions from the current design rules. Appropriate improvements have been proposed for the shear strength of LSBs based on AISI (2007) design equations and test results. Suitable design rules were also developed under the direct strength method (DSM) format. This thesis also includes the shear test results of cold-formed lipped channel beams from LaBoube and Yu (1978a), and the new design rules developed based on them using the same approach used with LSBs. Finite element models of LSBs in shear were also developed to investigate the ultimate shear strength behaviour of LSBs including their elastic and post-buckling characteristics. They were validated by comparing their results with experimental test results. Details of the finite element models of LSBs, the nonlinear analysis results and their comparisons with experimental results are presented in this thesis. Finite element analysis results showed that the current cold-formed steel design rules are very conservative for the shear design of LSBs. They also confirmed other experimental findings relating to elastic and post-buckling shear strength of LSBs. A detailed parametric study based on validated experimental finite element model was undertaken to develop an extensive shear strength data base and was then used to confirm the accuracy of the new shear strength equations proposed in this thesis. Experimental and numerical studies were also undertaken to investigate the shear behaviour of LSBs with web openings. Twenty six shear tests were first undertaken using a three point loading arrangement. It was found that AS/NZS 4600 and Shan et al.'s (1997) design equations are conservative for the shear design of LSBs with web openings while McMahon et al.'s (2008) design equation are unconservative. Experimental finite element models of LSBs with web openings were then developed and validated by comparing their results with experimental test results. The developed nonlinear finite element model was found to predict the shear capacity of LSBs with web opening with very good accuracy. Improved design equations have been proposed for the shear capacity of LSBs with web openings based on both experimental and FEA parametric study results. This thesis presents the details of experimental and numerical studies of the shear behaviour and strength of LSBs with and without web openings and the results including the developed accurate design rules.