Investigation of built-up box columns composed of four cold-formed steel channels

A three part study was conducted to investigate the compression behavior of a built-up box column made from four cold-formed steel (CFS) channels, including an experimental program, finite element study, and evaluation of building code prediction approaches. The experimental study included fifteen box column specimens with various effective lengths and varied eccentricity. Twist and warping of the ends of the specimen were restrained. The test results indicated that the dominant failure modes were global flexural buckling, local buckling, and concentrated localized deformation caused by local buckling. A finite element (FE) study was then conducted to broaden the investigation beyond the tested configurations and it was found that varying the screw spacing from 150 mm to 450 mm has little influence on the axial compressive behavior (less than 4%) and that web height-to-thickness ratio was a key parameter affecting axial compression strength. Finally, the accuracy and applicability of the available design methods for predicting the strength of the box columns were evaluated including the "effective width method " in which the modified slenderness ratio was not considered in AISI S100-16, the "direct strength method " in AISI S100-16, the method in the Chinese building code, and the method in EN 1993-1-3. For the box columns considered in this study, the four design methods produced predictions of the concentric axial compression strength that were between -33% to +6% of the expected actual strength. The selected design methods produced the average results of eccentric axial compression strengths that were between 8% to 25% conservative as compared to the test and FEA results.