A study on elastic stability of steel scaffolding

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A study on elastic stability of steel scaffolding

 

Author: Chu, Yu-tin Albert
Title: A study on elastic stability of steel scaffolding
Degree: M.Sc.
Year: 1994
Subject: Scaffolding, Metal -- Testing
Elastic analysis (Engineering)
Buckling (Mechanics)
Hong Kong Polytechnic -- Dissertations
Department: Multi-disciplinary Studies
Pages: viii, 187 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1183019
URI: http://theses.lib.polyu.edu.hk/handle/200/468
Abstract: A number of steel scaffolding collapsed during the past few years. Unlike other redundant structures, steel scaffolding fails in brittle mode in which the buckling of a member will shed its load to adjacent members quantitatively, which will finally result in a series of member failure and thus the collapse of the structure. This dissertation selects several typical steel scaffolding models commonly used in the industry for investigation. The collapse loads, which are close to the elastic buckling loads, of these scaffolding will then be studied and the factors of safety will then be computed for several loading radius. The dynamic load effect will be considered by using the normal load lifting acceleration. Possible causes that lead to the excess of the calculated factors of safety will be attempted to identify for reference of the designers for steel scaffolding. It is reckoned that when any joint or member fails, the complete structure will become unstable since this is a brittle form of failure. This brittle failure is in contrast to the more common ductile yielding in typical beam-column frames which permit the formation of several plastic hinges before a collapse mechanism develops. Steel scaffolding is a highly indeterminate three - dimensional structure. It is a current practice to model a steel scaffolding as an equivalent plane frame. One assumption usually made in the analysis is that the members meet at a single point. However, it is rare that a steel scaffolding is fabricated so. When the joint is not pin connected or the member is not perfectly straight, there will arise secondary stresses in the members that may lead to premature failure of a portion of the steel scaffolding. Whether these secondary stresses become important from the design point of view for the whole scaffolding, especially in the main members, which carry the maximum load - is the subject of the investigation in this dissertation. It is expected that secondary bending stresses may have very little influence on the design of steel scaffolding, when cross-arm proportions are adequate. The lowermost panel where vertical, transverse, and longitudinal loads are maximum will be the critical panel. Near ultimate load, the members that are subjected to both axial load and moments generated by second-order effects are likely to behave non-linearly. Though it is possible to take into account by an exact computer analysis allowing for both material and geometric non-linearities, for a practical design of the steel scaffolding, the ultimate capacity is the only parameter that is important. This ultimate capacity of the overall steel scaffolding depends mostly on the ultimate capacity of the main members. It can be stated explicitly that the main leg member subjected to compression will fail first before the other members. Invariably this compressive leg member is subjected to bending moments along both the principal axes because of the secondary members. Thus it is prudent to investigate theoretically the capacity of the main member under the combined influence of axial load and bending moments about both the principal axes. The methodology for investigating the ultimate capacity of the section used will be adopted from the work of Chen and Atsuta (1977) and is part of the PhD thesis of Knight (1989). Based on this work, the failure surface, when the main member is subjected to axial load and moments in two directions, has been theoretically worked out. In this investigation, it is assumed that the section and the member are geometrically perfect with no initial crookedness. The imperfection can be simulated by equivalent disturbing lateral loads. The behavior line accounts for joint eccentricity (P, Mx, My) as it indicates the actual P - Mx - My combination. Hence, the section will buckle under the combination of a particular P - Mx - My.

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