Andrzej M. Wrzesien

Optimum joint detail for a general cold-formed steel portal frame

In cold-formed steel portal framing systems that use bolted moment connections, formed through brackets, for the eaves and apex joints, it is well-known that the joints are semi-rigid, have finite connection-lengths and limited moment capacity. For such frames, it is therefore necessary for these joint effects to be taken into account when conducting frame design and analysis. However, as the semi-rigidity and the finite connection-lengths of each joint influence the bending moment distribution as well as the deflected profile of the frame, the joint detail for the eaves and the apex should not be designed independently of the frame. In this paper, a method of determining the optimum joint detail is described. It is demonstrated that careful selection of the joint detail can result in as much as a 25% increase in efficiency of the frame. Including joint effects explicitly into the design process provides better opportunities to devise the most appropriate balance between joints and member properties and thus reduce material use and construction costs.

The Collapse Behaviour of Cold-formed Steel Portal Frames at Elevated Temperatures

This paper describes the results of non-linear elasto-plastic implicit dynamic finite element analyses that are used to predict the collapse behaviour of cold-formed steel portal frames at elevated temperatures. The collapse behaviour of a simple rigid-jointed beam idealisation and a more accurate semi-rigid jointed shell element idealisation are compared for two different fire scenarios. For the case of the shell element idealisation, the semi-rigidity of the cold-formed steel joints is explicitly taken into account through modelling of the bolt-hole elongation stiffness. In addition, the shell element idealisation is able to capture buckling of the cold-formed steel sections in the vicinity of the joints. The shell element idealisation is validated at ambient temperature against the results of full-scale tests reported in the literature. The behaviour at elevated temperatures is then considered for both the semi-rigid jointed shell and rigid-jointed beam idealisations. The inclusion of accurate joint rigidity and geometric non-linearity (second order analysis) are shown to affect the collapse behaviour at elevated temperatures. For each fire scenario considered, the importance of base fixity in preventing an undesirable outwards collapse mechanism is demonstrated. The results demonstrate that joint rigidity and varying fire scenarios should be considered in order to allow for conservative design.

Sustainable applications of cold-formed steel structures: Portal frames

This chapter considers the analysis and design of cold-formed steel portal frames. A literature review of tests on cold-formed steel joints is first presented. The requirement in beam idealization for cold-formed steel portal frames to take into account both the semirigidity and the finite connection length is shown through comparisons against full-scale test results. In the case of joints formed through the web, a reduced strength as a result of the bimoment in the section is demonstrated. For practical small-sized frames, however, serviceability design is more important, to avoid tearing of the cladding at the gable end frames caused by load returned to the gables as a result of stressed skin. The effect of stressed-skin action for portal frames with small spans is shown. For such frames, a three-dimensional stressed-skin frame analysis should be used for design purposes. Finally, performance in fire conditions is described, suggesting the potential advantages of fire protection to the side rails.