System analysis of nonsymmetric cold-formed steel cross sections members

Abstract

Cold-formed steel components exist in a variety of structural systems. Wall systems and floor systems are often composed of channel members, and metal building systems often consist of channels or Z-section purlins to construct the wall girts and roofing systems. A common feature of these cross-sections is that they are often open and singly symmetric or point symmetric. While design requirements for these cross-sections account for the relevant effects resulting from their lack of symmetry, structural analysis programs do not always consider these effects. Engineers will use structural analysis programs to calculate the appropriate load sharing among members in the structural system. Accounting for the appropriate stiffness of each member and the related deformations is a vital component to determining the final distribution of bending moment, forces, and displacements. Many common structural analysis programs treat all sections as doubly symmetric without warping. Removing this assumption and considering asymmetry causes non-symmetric cross-sections to exhibit different displacements and complicates the stability limits. The evaluation of these sections is commonly no longer limited to a single plane. The additional displacements directly affect the stability equations, such as the lateral torsional buckling of a beam through the Wagner section parameters, which are all zero for doubly symmetric sections. The analysis of two structural systems composed of nonsymmetric members was completed with varying member modeling assumptions utilizing multiple finite element software programs. A single channel portal frame was investigated that was subjected to simulated gravity load and wind loading with varying bracing support. Additionally, a roofing system with Z-section purlins and channel bracing was investigated. The finite element analyses results were compared among the various modeling assumptions and existing experiments where applicable. It was observed that as deformations and loading increased, the inclusion of non-symmetric section properties becomes more important to ensure an accurate solution is calculated.