Buckling behaviour of cold-formed steel z-sections under axial compression and bi-axial bending

Abstract

Cold-formed steel (CFS) Z-sections are widely used as purlins, girts, and secondary framing members, yet their response becomes complex when axial compression acts together with bending about both principal axes. This study investigates the buckling capacity and failure behaviour of Z-section members under concentric and eccentric compression and evaluates the influence of member geometry and web perforations. Material properties were established from tensile coupon tests, after which four 500 mm non-perforated specimens were tested under axial, major-axis eccentric, minor-axis eccentric, and bi-axial loading. Finite element models were developed in ABAQUS and were validated against the experimental results. The validated model was then used to quantify the influence of member length, section thickness, and web opening diameter. Parametric analyses showed that increasing member length from 500 mm to 1500 mm reduced capacity by approximately 14% under pure compression and by 27% and 37% for bi-axial eccentricities of 20 mm by 20 mm and 40 mm by 40 mm, respectively; increasing section thickness from 1.96 mm to 3.00 mm enhanced capacity by approximately 76–82%, whereas a 40 mm web perforation reduced capacity by approximately 4–6%. The results demonstrate that eccentricity, slenderness, section thickness, and opening size are important parameters governing the stability of CFS Z-section beam-columns and provide a validated basis for further design-oriented studies.