Structural Design of Cold-formed Steel face-to-face Connected Built-up beams using Direct Strength Method

Abstract

The structural behavior of cold-formed steel (CFS) built-up beams that are gaining interest in the construction industry is investigated in this study. As per the current American Iron and Steel Institute's (AISI) CFS structural members specifications (direct strength method), there are no explicit guidelines or procedures for the types of built-up sections being evaluated in this research. The present study, therefore, examines the suitability of using the current AISI design specifications for the design of CFS built-up beams. The built-up beams investigated in this study are assembled using two identical sigma sections, and interconnected face-to-face by discrete spot welding. The test parameters studied include the length of the member (L) and spacing (sl) between the interconnecting spot welds. Based on the failure modes observed in experiments, new limitations for the interconnection spacing (smax) of the face-to-face connected built-up beams that failed in distortional buckling is suggested. The design strength prediction (MDSM) for the tested built-up beams is calculated using current AISI design procedure for the purpose of verification and to extend the current design procedure for the built-up beams. The comparison between experimental tests and design predictions (MEXP vs. MDSM) indicates that the design results are unconservative and unreliable due to the incorrect failure mode prediction and overestimation of the critical elastic buckling stresses in the elastic buckling analysis which is a key input to the direct strength method of design calculations. Hence, modified design procedures and design equations were suggested for the CFS built-up beams after carrying out a comprehensive investigation on failure modes and numerical study that are vulnerable to fail in distortional buckling. It was also shown that the newly suggested procedures and equations are reliable for the design of CFS built-up beams.