Author: Cheung, Chi-shing
Title: Lateral torsional buckling of cold-formed steel purlin member in roof systems
Degree: M.Sc.
Year: 2000
Subject: Roofs
Roofing, Iron and steel
Buckling (Mechanics)
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Department of Civil and Structural Engineering
Pages: vi, 184 leaves : ill. ; 31 cm
Language: English
Abstract: The dissertation presented a theoretical study on the load-carrying capacities of a member of commercially available cold-formed steel purlin systems. A comparison on the design capacities according to BS5950: Part 5(20) with the safe load tables of a number of cold-formed steel purlin systems was conducted. Based on basic structural design principles, the effect of end-fixity of each roof system was quantified by the end fixity parameter, 帢. It was found that through back analyses on the safe load tables, the value of 帢 might be obtained from elastic analysis as modified by the connection details of the roof systems. The values of 帢 for three roof systems with different connection configurations were obtained and presented in the dissertation. It should be noted that the results of the design method recommended in BS5950: Part 5(20) was found to be typically in good agreement with those values given in the safe load tables of the three different roof systems. The design method tended to give conservative results within 70% to 95% of those given in respective safe load tables. However, for long span roofs without sag rods under uplift condition, it was found that the design method gave very conservative results, typically only 30% to 50% to those given in the safe load tables. This was due to the fact that the restraining effect of roof cladding onto purlins was not allowed for in BS5950: Part 5(20), leading to severe underestimation of strength when compared with test results. However, in practice, sag rods were always installed in long span purlins in order to maximize the structural performance of purlin systems. Consequently, the deficiency of the design method in BS5950: Part 5(20) was not significant in practice. Thus, the design method was regarded to be effective in assessing the structural performance of cold-formed steel purlin systems, provided that the value of 帢 was given either from simple connection tests or with reference to typical connection configurations. A new structural form of cold-formed steel purlin was proposed which comprised of two purlin members nested over one third of the member length from supports in a staggered manner. The compound box section was effective in not only resisting vertical shear over supports, but also stable against lateral torsional buckling. An investigation was carried out to assess the structural performance of the proposed roof system with compound sections. It was found that for short span purlins, i.e. for purlins with span over depth ratio not greater than 45, the proposed system was an economical alternative to conventional systems, with a material saving of 4% to 6%. As no sag rod was required, there would be additional saving in the overall construction cost. However, for long span purlins, the design was typically controlled by lateral torsional buckling over the mid-span region of the purlin member, i.e. where there was only a single section. In order to improve the structural performance of the proposed roof systems, the restraining effect of roof cladding was recommended to be incorporated in the design.
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