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.

New design rules for the shear strength of LiteSteel Beams

Abstract: The LiteSteel Beam (LSB) is a new cold-formed hollow flange channel section produced using dual electric resistance welding and automated continuous roll-forming technologies. The innovative LSB sections have many beneficial characteristics and are commonly used as flexural members in building construction. However, limited research has been undertaken on the shear behaviour of LSBs. Therefore a detailed investigation including both numerical and experimental studies was undertaken to investigate the shear behaviour of LSBs. Finite element models of LSBs in shear were developed to simulate the nonlinear ultimate strength behaviour of LSBs including their elastic buckling characteristics, and were validated by comparing their results with experimental test results. Validated finite element models were then used in a detailed parametric study into the shear behaviour of LSBs. The parametric study results showed that the current design rules in cold-formed steel structures design codes are very conservative for the shear design of LSBs. Significant improvements to web shear buckling occurred due to the presence of torsionally rigid rectangular hollow flanges while considerable post-buckling strength was also observed. This paper therefore proposes improved shear strength design rules for LSBs within the current cold-formed steel code guidelines. It presents the details of the parametric study and the new shear strength equations. The new equations were also developed based on the direct strength method. The proposed shear strength equations have the potential to be used with other conventional cold-formed steel sections such as lipped channel sections.

New design rules for the shear strength of LiteSteel Beams with web openings

Abstract: LiteSteel beam (LSB) is a new cold-formed steel hollow flange channel section produced using a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. The LSBs are commonly used as floor joists and bearers with web openings in residential, industrial and commercial buildings. Their shear strengths are considerably reduced when web openings are included for the purpose of locating building services. However, no research has been undertaken on the shear behaviour and strength of LSBs with web openings. Therefore experimental and numerical studies were undertaken to investigate the shear behaviour and strength of LSBs with web openings. In this research, finite element models of LSBs with web openings in shear were developed to simulate the shear behaviour and strength of LSBs including their buckling characteristics. They were then validated by comparing their results with available experimental test results and used in a detailed parametric study. The results showed that the current design rules in cold-formed steel structures design codes are very conservative for the shear design of LSBs with web openings. Improved design equations have been proposed for the shear capacity of LSBs with web openings based on both experimental and parametric study results. An alternative shear design method based on an equivalent reduced web thickness was also proposed. It was found that the same shear strength design rules developed for LSBs without web openings can be used for LSBs with web openings provided the equivalent reduced web thickness equation developed in this paper is used. This is a significant advancement as it simplifies the shear design methods of LSBs with web openings considerably.

Improved design rules for hollow flange sections subject to lateral distortional buckling

The LiteSteel Beam (LSB) is a new hollow flange section with a unique geometry consisting of torsionally rigid rectangular hollow flanges and a relatively slender web. It is subjected to lateral distortional buckling when used as flexural members, which reduces its member moment capacity. An investigation into the flexural behaviour of LSBs using experiments and numerical analyses led to the development of new design rules for LSBs subject to lateral distortional buckling. However, the comparison of moment capacity results with the new design rules showed that they were conservative for some LSB sections while slightly unconservative for others due to the effects of section geometry. It is also unknown whether these design rules are applicable to other hollow flange sections such as hollow flange beams (HFB). This paper presents the details of a study into the lateral distortional buckling behaviour of hollow flange sections such as LSBs, HFBs and their variations. A geometrical parameter defined as the ratio of flange torsional rigidity to the major axis flexural rigidity of the web (GJf/EIxweb) was found to be a critical parameter in evaluating the lateral distortional buckling behaviour and moment capacities of hollow flange sections. New design rules were therefore developed by using a member slenderness parameter modified by K, where K is a function of GJf/EIxweb. The new design rules based on the modified slenderness parameter were found to be accurate in calculating the moment capacities of not only LSBs and HFBs, but also other types of hollow flange sections.

Review of current fire design rules for cold-formed steel wall systems

Light gauge steel frame wall systems are commonly used in industrial and commercial buildings, and there is a need for simple fire design rules to predict their load capacities and fire resistance ratings. During fire events, the light gauge steel frame wall studs are subjected to non-uniform temperature distributions that cause thermal bowing, neutral axis shift and magnification effects and thus resulting in a combined axial compression and bending action on the studs. In this research, a series of full-scale fire tests was conducted first to evaluate the performance of light gauge steel frame wall systems with eight different wall configurations under standard fire conditions. Finite element models of light gauge steel frame walls were then developed, analysed under transient and steady-state conditions and validated using full-scale fire tests. Using the results from fire tests and finite element analyses, a detailed investigation was undertaken into the prediction of axial compression strength and failure times of light gauge steel frame wall studs in standard fires using the available fire design rules based on Australian, American and European standards. The results from both fire tests and finite element analyses were used to investigate the ability of these fire design rules to include the complex effects of non-uniform temperature distributions and their accuracy in predicting the axial compression strength of wall studs and the failure times. Suitable modifications were then proposed to the fire design rules. This article presents the details of this investigation on the fire design rules of light gauge steel frame walls and the results.

Development of improved fire design rules for cold-formed steel wall systems

Traditionally the fire resistance rating of LSF wall systems is based on approximate prescriptive methods developed using limited fire tests. Therefore a detailed research study into the performance of load bearing LSF wall systems under standard fire conditions was undertaken to develop improved fire design rules. It used the extensive fire performance results of eight different LSF wall systems from a series of full scale fire tests and numerical studies for this purpose. The use of previous fire design rules developed for LSF walls subjected to non-uniform elevated temperature distributions based on AISI design manual and Eurocode3 Parts 1.2 and 1.3 was investigated first. New simplified fire design rules based on AS/NZS 4600, North American Specification and Eurocode 3 Part 1.3 were then proposed in this study with suitable allowances for the interaction effects of compression and bending actions. The importance of considering thermal bowing, magnified thermal bowing and neutral axis shift in the fire design was also investigated. A spread sheet based design tool was developed based on the new design rules to predict the failure load ratio versus time and temperature curves for varying LSF wall configurations. The accuracy of the proposed design rules was verified using the test and FEA results for different wall configurations, steel grades, thicknesses and load ratios. This paper presents the details and results of this study including the improved fire design rules for predicting the load capacity of LSF wall studs and the failure times of LSF walls under standard fire conditions.

Improved design rules for cold-formed steel columns

Current design rules for determining the member strength of cold-formed steel columns are based on the effective length of the member and a single column capacity curve for both pin-ended and fixed-ended columns. This research has reviewed the use of AS/NZS 4600 design rules for their accuracy in determining the member compression capacities of slender cold-formed steel columns using detailed numerical studies. It has shown that AS/NZS 4600 design rules accurately predicted the capacities of pinned and fixed ended columns undergoing flexural buckling. However, for fixed ended columns undergoing flexural-torsional buckling, it was found that current AS/NZS 4600 design rules did not include the beneficial effect of warping fixity. Therefore AS/NZS 4600 design rules were found to be excessively conservative and hence uneconomical in predicting the failure loads obtained from tests and finite element analyses of fixed-ended lipped channel columns. Based on this finding, suitable recommendations have been made to modify the current AS/NZS 4600 design rules to more accurately reflect the results obtained from the numerical and experimental studies conducted in this research. This paper presents the details of this research on cold-formed steel columns and the results.

Improved design rules for fixed ended cold-formed steel columns subject to flexural-torsional buckling

This paper has presented the details of an investigation into the flexural and flexuraltorsional buckling behaviour of cold-formed structural steel columns with pinned and fixed ends. 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. This research has reviewed the accuracy of the current design rules in AS/NZS 4600 and the North American Specification in determining the member capacities of cold-formed steel columns using the results from detailed finite element analyses and an experimental study of lipped channel columns. It was found that the current Australian and American design rules accurately predicted the member capacities of pin ended lipped channel columns undergoing flexural and flexural torsional buckling. However, for fixed ended columns with warping fixity undergoing flexural-torsional buckling, it was found that the current design rules significantly underestimated the column capacities as they disregard the beneficial effect of warping fixity. This paper has therefore proposed improved design rules and verified their accuracy using finite element analysis and test results of cold-formed lipped channel columns made of three cross-sections and five different steel grades and thicknesses.

Improved shear design rules for lipped channel beams with web openings

Cold-formed steel Lipped Channel Beams (LCB) with web openings are commonly used as floor joists and bearers in building structures. The shear behaviour of these beams is more complicated and their shear capacities are considerably reduced by the presence of web openings. However, limited research has been undertaken on the shear behaviour and strength of LCBs with web openings. Hence a detailed numerical study was undertaken to investigate the shear behaviour and strength of LCBs with web openings. Finite element models of simply supported LCBs under a mid-span load with aspect ratios of 1.0 and 1.5 were developed and validated by comparing their results with test results. They were then used in a detailed parametric study to investigate the effects of various influential parameters. Experimental and numerical results showed that the current design rules in cold-formed steel structures design codes are very conservative. Improved design equations were therefore proposed for the shear strength of LCBs with web openings based on both experimental and numerical results. This paper presents the details of finite element modelling of LCBs with web openings, validation of finite element models, and the development of improved shear design rules. The proposed shear design rules in this paper can be considered for inclusion in the future versions of cold-formed steel design codes.

Shear capacity of lipped channel beams with web openings : design rules and improvements

Cold-formed steel Lipped Channel Beams (LCB) with web openings are commonly used as floor joists and bearers in building structures. Shear behaviour of these beams is more complicated and their shear capacities are considerably reduced by the presence of web openings. Hence detailed numerical and experimental studies of simply supported LCBs under a mid-span load with aspect ratios of 1.0 and 1.5 were undertaken to investigate the shear behaviour and strength of LCBs with web openings. Experimental and numerical results showed that the current design rules in cold-formed steel structures design codes are very conservative. Improved design equations were therefore proposed for the shear strength of LCBs with web openings based on both experimental and numerical results. This research showed a significant reduction in shear capacities of LCBs when large web openings are included for the purpose of locating building services. A cost effective method of eliminating such detrimental effects of large circular web openings was also therefore investigated using experimental and numerical studies. For this purpose LCBS were reinforced using plate, stud, transverse and sleeve stiffeners with varying sizes and thicknesses that were welded and screw-fastened to the web of LCBs. These studies showed that plate stiffeners were the most suitable. Suitable screw-fastened plate stiffener arrangements with optimum thicknesses were then proposed for LCBs with web openings to restore their original shear capacities. This paper presents the details of finite element analyses and experiments of LCBs with web openings in shear, and the development of improved shear design rules. It then describes the experimental and numerical studies to determine the optimum plate stiffener arrangements and the results. The proposed shear design rules in this paper can be considered for inclusion in the future versions of cold-formed steel design codes.

Fire design rules of light gauge steel frame walls – a review

Cold-formed steel members are widely used in load bearing Light gauge steel frame (LSF) wall systems with plasterboard linings on both sides. However, these thin-walled steel sections heat up quickly and lose their strength under fire conditions despite the protection provided by plasterboards. Hence there is a need for simple fire design rules to predict their load capacities and fire resistance ratings. During fire events, the LSF wall studs are subjected to non-uniform temperature distributions that cause thermal bowing, neutral axis shift and magnification effects and thus resulting in a combined axial compression and bending action on the LSF wall studs. In this research a series of full scale fire tests was conducted first to evaluate the performance of LSF wall systems with eight different wall configurations under standard fire conditions. Finite element models of LSF walls were then developed, analysed under transient and steady state conditions, and validated using full scale fire tests. Using the results from fire tests and finite element analyses, a detailed investigation was undertaken into the prediction of axial compression strength and failure times of LSF wall studs in standard fires using the available fire design rules based on Australian, American and European standards. The results from both fire tests and finite element analyses were used to investigate the ability of these fire design rules to include the complex effects of non-uniform temperature distributions and their accuracy in predicting the axial compression strengths of wall studs and the failure times. Suitable modifications were then proposed to the fire design rules. This paper presents the details of this investigation into the accuracy of using currently available fire design rules of LSF walls and the results.

Fire design rules for load bearing cold-formed steel frame walls exposed to realistic design fire curves

Light gauge Steel Frame (LSF) walls are extensively used in the building industry due to the many advantages they provide over other wall systems. Although LSF walls have been used widely, fire design of LSF walls is based on approximate prescriptive methods based on limited fire tests. Also these fire tests were conducted using the standard fire curve [1] and the applicability of available design rules to realistic design fire curves has not been verified. This paper investigates the accuracy of existing fire design rules in the current cold-formed steel standards and the modifications proposed by previous researchers. Of these the recently developed design rules by Gunalan and Mahendran [2] based on Eurocode 3 Part 1.3 [3] and AS/NZS 4600 [4] for standard fire exposure [1] were investigated in detail to determine their applicability to predict the axial compression strengths and fire resistance ratings of LSF walls exposed to realistic design fire curves. This paper also presents the fire performance results of LSF walls exposed to a range of realistic fire curves obtained using a finite element analysis based parametric study. The results from the parametric study were used to develop a simplified design method based on the critical hot flange temperature to predict the fire resistance ratings of LSF walls exposed to realistic fire curves. Finally, the stud failure times (fire resistance rating) obtained from the fire design rules and the simplified design method were compared with parametric study results for LSF walls lined with single and double plasterboards, and externally insulated with rock fibres under realistic fire curves.

Improved shear design rules of cold-formed steel beams

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