The Behaviour of Built-up Thin-Walled Steel Structural Elements

Cold-formed steel structural members are produced by rolling or press forming a metal sheet into a structural shape at room temperature. They are typically light-weight and possess a high strength-to-weight ratio, facilitating more sustainable construction solutions. Whilst traditionally cold-formed steel members have been used for structural elements of secondary importance, the current trend is to increasingly use cold-formed steel products as primary members, for instance in portal frames for industrial halls or in multi-storey residential buildings. This evolution puts increasing demands on cold-formed steel structural members in terms of load carrying capacity and span lengths. Built-up members, created by bolting, riveting or screwing individual shapes together in structurally efficient configurations, are therefore often used in practice. However, our fundamental understanding of the mode of response of these built-up structural elements is lagging behind practice, and appropriate design rules are completely lacking in the structural design Eurocodes.
The aim of the proposed research is to investigate the behaviour, strength and stability of cold-formed built-up sections. Particular attention will be paid to coupled instabilities resulting from an interaction between buckling of the individual cross-section components in between connector points and the various instabilities of the full built-up member. It is thereby expected that the connector spacing will have a crucial effect on the load carrying capacity. A first stage of the investigation will consist of experimental investigations on innovative built-up cross-sectional shapes. A total of 55 laboratory tests on built-up columns and beams are envisaged. The resulting dataset will be augmented by carrying out detailed numerical studies using finite element simulations, varying relevant parameters such as the cross-section geometry, the connector spacing, the connector type and the material properties. In a final step, the increased understanding of the behaviour of built-up cold-formed steel members gained in the previous stages of the research is expected to culminate in the development of a safe an accurate design procedure, with design equations that are verified against the available data through statistical reliability analysis.

The research will benefit design engineers, practitioners, consultants, contractors and producers working in the cold-formed steel industry. The UK is home to a substantial cold-formed steel industry, including major international companies, as well as small and medium scale businesses.
An increased understanding of the behaviour of built-up cold-formed steel members, reflected in a comprehensive set of safe and reliable design rules, will encourage the development of new innovative and efficient structural members. It will allow for the custom development and optimization of new cross-sections and result in potential cost savings. Studies commissioned by the European Commission Directorate General for Enterprise and Industry have identified innovative products meeting custom demands as a key to the competitiveness of the European steel sector. Built-up sections thereby address a definite need in the market resulting from an increasing trend to construct structures (such as buildings or large open space halls) entirely out of cold-formed steel.
The research will also benefit society in general by the provision of alternative, more sustainable structural components for the built environment with lower embodied carbon. Consequently, the proposed research on cold-formed steel structures fits within the EPSRC priority areas of sustainability and energy use. The steel industry and the construction industry in general have both been identified as major producers of greenhouse gases and are thus under pressure to reduce their emissions. Replacing hot-rolled steel by cold-formed solutions has the ability to reduce the self-weight of structures and the associated carbon emissions by 15%-40% due to their higher strength-to-weight ratio. As an example, replacing hot-rolled steel beams by cold-formed alternatives alone could reduce the annual global carbon emissions by 15-40 million tonnes. Moreover, cold-formed members are easy to handle, transport, stack and install, thus reducing energy demands. They are completely recyclable and even reusable. Built-up sections thereby have the added advantage that they lend themselves to a modular approach in which they can be taken apart and re-assembled in different configurations.
The project will also train the Research Associate and provide him/her with high-level skills and knowledge, as well as with a network of industry and academic contacts. Such training will have a direct economic impact via the provision of a skilled worker who may subsequently be employed by a UK organization.
Finally, both the University and the industry will benefit from closer collaboration and the exchange of expertise through this project.