Compressive behavior of built-up double-box columns consisting of four cold-formed steel channels

Abstract

Thirty specimens of built-up double-box columns consisting of four cold-formed steel channels were experimentally tested under concentric and eccentric axial compression. The test specimens had varied cross-sectional dimensions (outside dimensions equal to 89 mm × 147 mm or 86 mm × 96 mm), different effective lengths (3200 mm, 1700 mm, 210 mm, or 135 mm), and different thicknesses (1.2 mm or 1.5 mm). The load vs. displacement curves and the measured compression strength of specimens were obtained and the failure modes were analyzed. The test results showed that the failure mode of the long specimens (L/r between 103 and 108, where L is the calculated length of the column, r is the minimum radius of gyration) was flexural buckling about the weak axis. The typical failure modes of medium length specimens (L/r between 53 and 57) and short specimens (L/r between 4 and 8) subjected to concentric axial compression were deformation concentration caused by large local buckling. The failure mode of the medium length specimens under eccentric axial compression was flexural buckling, except for the specimens whose web height was 147 mm and the eccentricity direction was along the minor axis, for which the failure mode was deformation concentration. Then, finite element (FE) models considering geometric, material, and contact nonlinearity were developed, compared to experimental results, and used to explore a wider design space. The FE analysis results showed that the slenderness ratio of the column and the web height-to-thickness ratio have an important influence on the strength of these columns. Finally, the applicability and effectiveness of calculating compression strength for this type of built-up column using methods such as “Effective Ratio of Width-to-Thickness Method” in Chinese code, “Effective Width Method” in AISI and “Direct Strength Method” in AISI, were evaluated by comparison with the test and FE results. For all the columns analyzed with slenderness ratio greater than 105, it is reasonable to use these methods to calculate the concentric axial compression strength of columns. The methods in China and AISI S100-16 were found to be conservative for calculating the eccentric axial compression strengths of these built-up double box columns.