| doi:10.3850/978-981-08-6218-3_SS-Fr020 |
 Final Paper PDF
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STRENGTH IMPROVEMENT METHODS FOR BACK TO BACK LITESTEEL BEAMS
S. Jeyaragana and M. Mahendranb
Faculty of Built Environment and Engineering, Queensland University of Technology, Brisbane, Queensland.
as.jeyaragan@student.qut.edu.aul
bm.mahendran@qut.edu.aul
EXTENDED ABSTRACT
LiteSteel Beam (LSB) is a new cold-formed steel beam produced by OneSteel Australian Tube Mills. The new beam is effectively a channel section with two rectangular hollow flanges and a slender web, and is manufactured using patented dual electric resistance welding and automated continuous roll-forming technologies. OneSteel Australian Tube Mills is promoting the use of LSBs as flexural members in a range of applications, such as floor joists and bearers in building construction.
When LSBs are used as back to back built-up sections, they are likely to improve their moment capacities and thus extend their applications further. However, the structural behaviour of builtup beams is not well understood. Many steel design codes include guidelines for connecting two channels to form a back to back built-up I-section including the required longitudinal spacing of connections. But these rules were found to be inadequate in some applications. Flexural behaviour of single and back to back LSBs was investigated using large scale experimental tests and advanced finite element analyses and suitable moment capacity design rules were developed. However, both experiments and finite element analyses showed that the detrimental effects of lateral distortional buckling that was observed in single LSB members remain with back to back built-up LSB members. Hence the ultimate moment capacity of back to back LSB member was also affected by lateral distortional buckling failure. Eliminating the effects of lateral distortional buckling was expected to lead to improved flexural strength of back to back LSBs Therefore an investigation was conducted with an aim to develop suitable strength improvement methods for back to back LSB members, which are likely to mitigate lateral distortional buckling effects and hence improve their flexural strengths.
The back to back LSB members exhibit two types of lateral buckling modes, namely, lateral distortional buckling and lateral torsional buckling. During lateral distortional buckling failure, the web elements of back to back LSBs distort while the top and bottom flanges rotate. Effective strength improvement methods are expected to eliminate web distortion and hence their critical buckling mode will become lateral torsional buckling. In this investigation, the use of many potential connection methods and stiffeners was investigated to determine whether they are capable of eliminating lateral distortional buckling effects in back to back LSBs.
Effects of different connection methods including spot welding of outside flanges, the use of “C” brackets connecting the flanges and the use of web side plates attached to the web elements were first considered in this study. This was followed by different types of web stiffeners including transverse web stiffener, longitudinal web stiffener, “C” brackets and box stiffeners. For this purpose, finite element analyses of LSBs strengthened using the above methods were undertaken. The analysis results showed that the use of transverse web stiffener was the most effective and simple strengthening method. The configuration based on transverse web stiffeners at third span points and supports was finally recommended, which will considerably eliminate the lateral distortional buckling effects in back to back LSBs with minimum cost. It was also recommended to use 3 to 5 mm thick stiffeners depending on the size of LSB sections.
A design equation was developed to calculate the elastic lateral buckling moments of back to back LSBs with the recommended web stiffener configuration while the same design rules developed for unstiffened back to back LSBs were recommended to calculate their ultimate moment capacities. This paper presents the details of this investigation, the results and recommendations for the most suitable and cost-effective method, which significantly improves the flexural strength (moment capacity) of back to back LSB members.