Design of cold-formed CHS braces for steel roof structures

Abstract

Thin-walled cold-formed circular hollow sections (CHSs) are widely used as the primary and secondary structural members in the construction of roof structures often used for high rise buildings in the Middle East. The British steel design code BS 5950 is the main code used in the Middle East for the design of such structural elements. Existing experimental static axial tests performed by the second author on CHSs were used to assess BS 5950. This code was shown to over-estimate the axial capacity of compression members under static axial load in the middle member slenderness range of 0.35–0.75. This is because most of the braces in such range failed by the interaction of local buckling and overall flexural buckling. Newly derived design curve termed “Curve e” is proposed for design purposes. The newly derived curve is compared against other design codes such as the North American Steel Design code ANSI AISC 360-10, the European Steel Design Specification Eurocode 3, and the Australian Steel Design Code AS 4100. One case study for a newly constructed roof structure in the Middle East is presented. It was found that the existing curve “c” over-predicts the capacities of the most stressed secondary and primary braces by 37% and 15% respectively. Finite element models were constructed and found to provide good agreement against the experimental load-defection curves. Once validation is completed, additional FE models were created to expand the available database for strength of different CHS braces up to a member slenderness of 2.0. Newly derived design curve ‘‘e” within BS5950 platform was found to be a suitable lower bound for the experimental and numerical results. Additionally, a modified version for the AISC design model is suggested in this paper by reducing the strengths by 15%; applying a form factor of 0.85.