Behaviour and design of cold-formed steel built-up section structural members

Abstract

Cold-formed steel (CFS) is popularly used as construction material due to various of merits such as the convenience of prefabrication, ease of quality control and economy in transportation. The steel strips, sheets and plates are feasibly manufactured into versatile CFS cross-sectional shapes at ambient temperature through the fabrication process of cold-rolling or brake-pressing. By assembling individual CFS elements with discrete fasteners along the longitudinal direction, CFS built-up components with closed and doubly-symmetric open cross-sections are handily composed. With the great potential in optimization of loading capacity and torsional stability, CFS built-up section members have aroused widespread attention in the past years. To explore advanced sectional profiles, the CFS built-up open and closed sections with stiffened flanges and longitudinal stiffeners were designed in this study. The objectives of this thesis are to investigate the structural performance of innovative CFS built-up section members subjected to various loading conditions and develop appropriate design rules. First of all, the thorough experimental study was conducted on the CFS built-up sections of steel grades G500 and G550. A total of 18 four-point bending tests, 22 pin-ended column tests, 63 eccentric compression tests and 35 non-uniform bending beam-column tests were carried out to observe the buckling behaviour of beams, columns, as well as beam-columns under uniform bending and moment gradients. The experimental loading capacities, failure modes and full-history responses were obtained for the validation of finite element models and the assessment of design provisions. In addition, the actual material properties considering the effect of cold-forming were acquired by the tensile coupon tests and were utilized in the numerical simulation as well as the calculation of nominal strengths. The finite element (FE) models with prudent treatments of material nonlinearity, complicate contact and restraint situations as well as initial geometric imperfections and solution schemes to overcome convergence problems were developed for the thin-walled built-up section structural members. The extensive parametric study was performed using the calibrated FE models to extend the scope of cross-section configurations, geometric parameters, member slenderness and moment distribution. The numerical predictions of additional 300 beams, 224 pin-ended columns, 560 beam-columns under uniform bending and 280 beam-columns subjected to moment gradients were determined to examine the influence of different parameters on the performance of CFS built-up section components, which can provide guidelines for future applications. Underpinned by the experimental and numerical database, the applicability of the existing design provisions was evaluated for the novel CFS built-up section structural members. Based on the assessment results, the codified direct strength method equations are modified to provide more suitable strength predictions for the CFS built-up section columns and beams subjected to uniform as well as non-uniform minor axis bending. In addition, a bi-linear axial load-moment interactive expression is proposed to consider the effects of global buckling slenderness and moment resistance slenderness for the CFS built-up section beam-columns experiencing uniform bending and moment gradients. Moreover, the recommended design rules are summarized accordingly in this thesis for the CFS built-up section structural members.