Behaviour and design of cold-formed steel built-up sections under concentrated bearing loads

Abstract

Cold-formed steel built-up sections are increasingly utilized in the building industry because of their higher torsional rigidity and favourable load-bearing capacity compared with those of singly symmetric open sections. Cold-formed steel built-up sections are frequently subjected to concentrated bearing loads, such as in applications of floor joist members, primary girders at bearing supports and beams under roof purlins. In the existing international design specifications for cold-formed steel structures, the web crippling design rules for built-up sections are empirical in nature, which were developed based on experiments conducted on built-up I-sections only. Moreover, the current codified design equations do not consider the reduction in web crippling capacity due to web perforations and the effects of screw arrangements along the web height, owing to the limited research on built-up sections subjected to concentrated bearing loads. In this study, the structural behaviours and design of cold-formed steel built-up sections with various cross-sectional configurations under concentrated bearing loads were investigated. A total of 213 web crippling tests were conducted, involving 29 web-stiffened channels, 54 built-up I-sections without web holes, 38 built-up I-sections with web holes, 35 built-up box sections and 57 built-up sections with longitudinal stiffeners. The material properties of the test specimens were determined through a series of tensile coupon tests. The four codified web crippling loading conditions and the floor joists loading conditions were considered. The test setup and procedures, ultimate web crippling capacities, failure modes and load–deformation curves were discussed. In addition, numerical investigations were conducted to replicate the web crippling tests. Nonlinear finite element models were developed and validated against the experimental results. A comprehensive parametric investigation involving 2,312 numerical specimens was conducted using the validated finite element models to provide sufficient data to derive design rules. In the parametric study, a wide range of key parameters were considered, such as web slenderness, web hole dimensions, screw arrangement along the web height and the geometry of the longitudinal stiffeners of the sections. Finally, the existing North American Specification, Australian/New Zealand Standard and European Code were evaluated using the experimental and numerical data. The comparison results indicate that improving the existing design rules is necessary; hence, new design formulae were developed for the web crippling design of cold-formed steel web-stiffened channels, built-up I-sections with and without web holes, built-up box sections and built-up sections with longitudinal stiffeners. Furthermore, the reliability of the existing codified design rules and the newly developed design rules was analysed.