Behaviour of cold-formed steel portal frames in fire

Behaviour of cold-formed steel portal frames in fire

Portal frames composed entirely of cold-formed steel can be a viable alternative to conventional hot-rolled steel for single storey structures, having sustainable and economic benefits for modest spans. However, despite the increasing popularity of this form of construction, there is limited research on the collapse behaviour at elevated temperatures. Further research is required in order to prevent fire spread as well as to protect the safety of occupants escaping and fire fighters who may be in close proximity to the building.

 

Outline of work

With the financial support of an Institution of Structural Engineers Research Award, a full-scale fire test on a cold-formed steel portal frame building was carried out where displacement and temperature were measured around the structure. An advanced structural analysis model of the frame, using finite element analysis, was validated against the site test results. It was then used as part of a parametric study to investigate the performance of such frames with varying geometries, base fixity and joint stiffness under different fire scenarios.

Findings

The test frame collapsed at a temperature of 715ºC, with the finite element model showing good agreement. Collapse was due to member buckling, rather than failure of connections. Secondary structural elements had a beneficial effect on the structure, helping to prevent an undesirable outwards collapse mechanism. Further details can be found in The Structural Engineer (Vol. 92, Issue 10).

Practical Relevance

Current trends in structural fire engineering are moving away from traditional prescriptive-based design towards performance-based design. This can provide cost savings to the client in addition to improved structural response of buildings subject to elevated temperatures. This research is an example of such an approach. It will be used to form the basis of design guidance for cold-formed steel portal frames in fire boundary conditions.

Details of the Institution’s Research Award scheme are available at: http://www.istructe.org/events-awards/people-and-papers-awards/research-award.