Effect of column base strength on steel portal frames in fire

In the UK, the design of steel portal frame buildings in fire is based on the Steel Construction Institute (SCI) design method, in which fire protection needs only be provided to the columns, provided that the column bases are designed to resist an overturning moment, M_OTM, calculated in accordance with the SCI design method. In this paper, a non-linear elastic-plastic implicit dynamic finite element model of a steel portal frame building in fire is described and used to assess the adequacy of the SCI design method. Both 2-D and 3-D models are used to analyse a building similar to the Exemplar frame described in the SCI design guide. Using the 2-D model, a parametric study comprising 27 frames is conducted. It is shown that the value of the overturning moment, calculated in accordance with the SCI design method, may not be sufficient to prevent collapse of the frame before 890 °C

Cold-formed steel sections with web openings subjected to web crippling under two-flange loading conditions — Part II : Parametric study and proposed design equations

A parametric study of cold-formed steel sections with web openings subjected to web crippling was undertaken using finite element analysis, to investigate the effects of web holes and cross-section sizes on the web crippling strengths of channel sections subjected to web crippling under both interior-two-flange (ITF) and end-two-flange (ETF) loading conditions. In both loading conditions, the hole was centred beneath the bearing plate. It was demonstrated that the main factors influencing the web crippling strength are the ratio of the hole depth to the flat depth of the web, and the ratio of the length of bearing plates to the flat depth of the web. In this paper, design recommendations in the form of web crippling strength reduction factors are proposed, that are conservative to both the experimental and finite element results

Cold-formed steel channel sections under end-two-flange loading condition:design for edge-stiffened holes, unstiffened holes and plain webs

In cold-formed steel (CFS) channel sections, web holes are becoming increasingly popular. Such holes, however, result in the sections becoming more susceptible to web crippling, especially under concentrated loads applied near the web holes. Traditional web holes are normally punched or bored and are unstiffened. Recently, a new generation of CFS channel sections with edge-stiffened web holes has been developed by the CFS industry and is being widely used. However, no research is available in the literature which investigated the web crippling strength of CFS channel sections with edge-stiffened circular web holes under the end-two-flange (ETF) loading conditions. A combination of experimental tests and non-linear FEA were used to investigate the effect of such stiffened holes on web crippling behaviour under ETF loading conditions. The results of 30 web crippling tests are presented. Non-linear FE models are described, and the results are compared against the laboratory test results; a good agreement was obtained in terms of both the strength and failure modes. The results indicate that the stiffened holes can significantly improve the web crippling strength of CFS channel sections. A parametric study involving 1116 FEA was then performed, covering the effect of different hole sizes, edge-stiffener lengths and fillet radii, length of the bearing plates and position of holes in the web. Finally, design recommendations in the form of web crippling strength reduction factors are proposed, that are conservative to both the experimental and FE results

Optimum design of cold-formed steel portal frame buildings including joint effects and secondary members

In steel portal frames, cold-formed steel channel sections are increasingly used as the primary framing components, in addition to the secondary members e.g. purlins and side rails. For such framing systems, the stiffness of the joints at the eaves and apex affects the bending moment distribution, as well as the frame deflections. This paper investigates the influence of two joint configurations having full rigidity and semirigidity, respectively, on the optimum design of cold-formed steel portal frames. A real-coded genetic algorithm is used to search for the most cost-effective design. It is shown that through incorporating joint effects explicitly into the design process, a more appropriate balance between the joints and the member properties can be obtained, thus optimizing material use. The study then investigates the effect of secondary members on the optimum design. It is shown that incorporating the secondary members is important for portal frames having spans less than 12 m. For example, for a frame spacing less than 6 m, the material cost of the primary members can be reduced by up to 15%

Design of top-hat purlins for cold-formed steel portal frames

This paper considers the use of cold-formed steel top-hat sections for purlins in the UK, as an alternative to conventional zed-sections. The use of such top-hat sections could be viable for cold-formed steel portal framing systems, where both the frame spacing and purlin span may be smaller than that of conventional hot-rolled steel portal frames. Furthermore, such sections are torsionally stiffer than zed-sections, and so have a greater resistance to lateral-torsional buckling. They also do not require the installation of anti-sag rods. The paper describes a combination of full-scale laboratory tests and non-linear elasto plastic finite element analyses. The results of twenty-seven tests on four different top-hat sections are presented. In terms of stiffness, good agreement between the experimental and finite element results is shown. The finite element model is then used for a parametric study to investigate the effect of different thicknesses and steel grades. Design recommendations are provided in the form of charts. The use of the finite element method in this way exploits modern computational techniques for an otherwise difficult structural design problem and reduces the need for an expensive and time consuming full laboratory study, whilst maintaining realistic and safe coverage of the important structural design issues

Numerical study of web crippling strength in cold-formed austenitic stainless steel lipped channels with web openings subjected to interior-two-flange loading

In cold-formed stainless steel lipped channel-sections, use of web openings for service purposes are becoming increasingly popular. Web openings, however, result in the sections becoming more susceptible to web crippling. This paper presents a finite element investigation into the web crippling strength of cold-formed austenitic stainless steel lipped channel-sections with circular web openings under the interior-two-flange (ITF) loading condition. The cases of web openings located centred and offset to the bearing plates are considered in this study. In order to take into account the influence of the circular web openings, a parametric study involving 740 non-linear elasto-plastic finite element analyses was performed, covering austenitic EN1.4404 stainless steel grade. From the results of the parametric study, the effect of the size of the web opening, length of bearing plate and location of the web opening is investigated. Strength reduction factor equations are then proposed, that can be used to take into account such web openings in design

The structural behaviour in fire of a cold-formed steel portal frame having semi-rigid joints

This paper describes a non-linear finite element study into the effects of elevated temperature on a cold-formed steel portal frame having semi-rigid joints. Numerical modelling was carried out using ABAQUS finite element analysis software with shell elements used to capture localised buckling effects. Results for the ambient shell models are compared against previous full-scale tests. Material properties are taken from the literature, in order to predict the behaviour of the frame at elevated temperature. The results of finite element beam models are compared against those of shell models to enable comparison. At elevated temperature, shell models are shown to detect failure much earlier within the fire. Therefore shell models are recommended for such studies, for a conservative approach