Impact of Fabrication Tolerances on Cold-Formed Steel Section Properties, Stability, and Strength

摘要

The objective of this paper is to examine the impact of typical fabrication tolerances employed in the United States on the developed properties, stability, and strength, of cold-formed steel structural members. For a typical lipped channel cold-formed steel member the manufacturing process results in variations in the dimensions of the web, flanges, and lips. These dimensional variations lead to variations in the cross-sectional properties, the cross-sectional stability modes, and the resulting strength the section can develop. In the past, variations on the out-of-plane imperfection of member flats and out-of-straightness of member folds, and material strength properties have been studied; however, less attention has been paid to the impact of dimensional tolerances. Decisions to change current dimensional tolerances have significant financial impact for the manufacturers, but little in terms of the potential benefit for the performance is fully known. Using current United States dimensional tolerances and employing Monte Carlo simulation expected fabrication variation in a subset of common cold-formed steel lipped channel members are generated. The impact of dimensional tolerances on stability is studied, using the finite strip method (CUFSM) through generation of the axial and major-axis bending buckling load factors for focal, distortional, and global buckling. The Direct Strength Method is used to track the impact of cross-section properties and stability onto the final predicted strength. This strength prediction provides means to investigate the fabrication factor currently used in Load and Resistance Factor Design in the United States. Sensitivity of the solution to correlation in the dimensional tolerances is also studied. Further, the impact of changes in current dimensional tolerances, are also studied. In the future, direct data on measured dimensional variations from as-formed sections can be employed to further understand expected variations in properties, stability, and strength. Understanding the importance of the dimensional tolerances and their impact on the structural performance provides crucial knowledge for future decisions on setting appropriate tolerances for cold-formed steel fabrication.