Cold-formed steel built-up compression members with longitudinal stiffeners

Abstract

In this study, the structural behaviour of cold-formed steel built-up compression members with longitudinal stiffeners was investigated by experiments and numerical simulation. The influence of different screw spacings on the ultimate strengths of built-up compression members was also investigated. In current design specifications, generally there are no direct and explicit design rules for cold-formed steel built-up sections. Therefore, design rules for built-up open sections and closed sections have been proposed.

The test program was conducted for cold-formed steel built-up sections, which were compressed axially between fixed ends with various column lengths ranged from 300 to 3200 mm. Two identical open sections were connected by self-tapping screws to form a built-up section. The material properties and initial geometric imperfections of the test specimens were measured. The experimental investigation consisted of built-up open sections with edge and web stiffeners, and built-up closed sections with inward or outward web stiffeners. In total, 47 specimens were tested with a screw spacing of 100 mm, and another 25 specimens were tested with screw spacings of 300 and 600 mm.

A further investigation on the structural behaviour of cold-formed steel built-up compression members was performed using finite element analysis (FEA). Firstly, accurate and reliable finite element models (FEMs) were developed and verified against the test results. Secondly, extensive parametric studies of cross-section geometry and the screw spacing were carried out by the verified FEMs. The parametric study contained 442 specimens for built-up open sections, built-up closed sections, and built-up sections with different screw spacings. Therefore, the complex structural behaviour of built-up compression members with edge and web stiffeners, together with different spacings of self-tapping screws were studied thoroughly, as reported in this thesis.

The current direct strength method (DSM) is a favored design method for cold-formed steel columns and beams due to its ease of calculation. However, DSM does not cover the design of cold-formed steel built-up sections. The appropriateness of DSM for cold-formed steel built-up compression members was accessed by both experimental and numerical investigation. It was found that the current DSM is not quite suitable for the design of built-up compression members. Therefore, the current DSM was modified for built-up open and closed sections. The results show that the modified design rules are generally conservative and reliable for the design of built-up compression members. In addition, design recommendations are proposed for built-up sections with different screw spacings. The reliability of the current and modified design rules was evaluated using reliability analysis.