Design of Top-hat Purlins for Cold-formed Steel Portal Frames

Abstract

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 hotrolled steel portal frames. Furthermore, such sections are torsionally stiffer than zedsections, 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. NotationCOV Coefficient of variation; E Young's modulus of elasticity; FEA Finite element analysis; M u EXP Experimental ultimate moment capacity; M u FEA Ultimate moment capacity predicted from finite element analysis; M g,EC3 Gross moment capacity predicted from EC3; M eff,EC3 Effective moment capacity predicted from EC3; t Thickness of section; 0.2  Static 0.2% proof stress; u  Tensile ultimate strength; true  True strain eng  Engineering strain true  True stress eng  Engineering stress 1 Introduction In the UK, single-storey steel portal frames account for approximately 50% of the constructional steel used each year and 90% of all single-storey buildings [1, 2].Such buildings typically use conventional hot-rolled steel sections for the primary column and rafter framing members, which in turn support the secondary coldformed steel purlin and side rail members; these secondary members, in turn, support the cladding.For portal frames of modest span (around 12 m), the introduction of higher strength grades of cold-formed steel into the UK in the past decade, has led to coldformed steel sections being used for the both primary members as well as for the secondary members (see Fig. 1). Such cold-formed steel portal framing systems are now a viable alternative to conventional hot-rolled steel portal framing systems [3].However, unlike conventional hot-rolled steel portal frames in the UK, where the frame spacing is typically 6 m, there is scope to vary the frame spacing in the design of cold-formed steel portal frames. This is because cold-formed steel sections are lighter than hot-rolled steel sections, so structural members can be bolted and erected on site by semi-skilled workers, wit...

The analysis of web opening configuration on cold-formed steel hat purlin under torsion behaviour

“…Recently, hat and sigma sections for purlins and grits are mostly used in building design for secondary members. Hat section acts as an alternative to the use of conventional Z sections for purlins (Uzzaman et al , 2014; Wojnar and Sieńkowska, 2020). Compared to Z sections, Hat sections are more resistance to lateral-torsional buckling.…”