Behaviour, optimisation and design of cold-formed steel purlins

Almatrafi, Meshal (2021). Behaviour, optimisation and design of cold-formed steel purlins. University of Birmingham. Ph.D.

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Abstract

Purlins are arguably the most widely used cold-formed steel members in construction. They are employed as secondary steelwork to support roof cladding, transfer loads and offer effective restraint to the primary steel members. Owing to their thin-walled nature, they are susceptible to a variety of instabilities including local, distortional, lateral torsional buckling or interactions thereof. Currently, most cold-formed producers try to optimise their cross-section geometries to achieve maximum strength with minimum material usage and hence gain a competitive advantage in the construction market. Furthermore, most producers can customise their cross-sections on demand to suit a particular application and hence expand the scope of application for their products. Therefore, determining optimal cross-section shapes as well as the pertinent dimensions of the various parts of a cross-section for a given shape for cold-formed steel members, is a topic of great interest.

The aim of this thesis is to assess current and propose novel design approaches for cold-formed steel purlins. To this end a series of experimental tests and numerical analyses is performed focusing on the structural response of purlins subjected to loads likely to occur in practical applications. Since purlins are commonly restrained by cladding, only their cross-sectional response will be considered in this project. The failure modes considered include local and distortional buckling as well as web crippling, whilst the studied cross-section shapes are Z and Sigma purlins. Z purlins are particularly susceptible to local and distortional buckling due to their deep flat web; hence these failure modes are studied in great detail. In addition to generating structural performance data, an attempt to optimise the sections is also reported. To this end, a series of 8 tests followed by numerical investigations for a wide range of Z-sections have been conducted. It was found that for most of sections, small lip size to flange width ratios (i.e. d/b ≤ 0.28) leads to decreased efficiency by about 8%, whilst lip size to flange width ratios above 0.4 leads to improved efficiencies particularly for deeper sections, for which gains in the region of 5% can be achieved. Furthermore, the obtained experimental and numerical results are used to assess the accuracy of the design predictions of Eurocode 3 and the Direct strength method (DSM). On the other hand, Sigma sections employ a non-flat web which offers greater strength against local and distortional buckling at the expense of reduced resistance to web crippling. Furthermore, Eurocode 3 does not currently cover the design resistance of sigma sections to web crippling, thereby necessitating the development of a novel design approach. A series of six tests was performed on sigma purlins subjected to interior one flange loading. A numerical model was generated and validated against experimental results. Upon validation of the numerical model, parametric studies were conducted to generate additional data considering a wide range of cross-section geometries. Using the obtained results, the North American specifications was assessed and found to be unsafe for sigma sections, thus a modification was proposed. A recently proposed design framework based on cross-section slenderness was successfully extended to sigma sections.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):