Assessment of buckling strength curves in Direct Strength Method for estimating axial strengths of Cold-formed Steel members considering average yield stresses of cross-sections

Abstract

The Direct Strength Method (DSM) is incorporated in current design specifications for designing Cold-formed Steel (CFS) members under compression. The North American design specification AISI S100 allows taking advantage of the increased material yield stress due to cold-forming in the application of the DSM only for members subjected to (i) local buckling and having slenderness ratios (λl) ≤ 0.776 or (ii) global buckling and having strengths not reduced by local buckling. This study aims to explore whether this limit could be released. First, Karren’s model for predicting the yield stresses of corners in CFS members was modified to improve the performance and extend the application limit. Second, Karren’s expression for estimating the average full-section yield stresses of CFS members was refined. Third, an extensive experimental database established previously by the authors was employed to assess the buckling strength curves in the DSM. Nominal axial strengths of the CFS members in the database were calculated using the average cross-section yield stresses as design parameters, which were then compared with the tested axial strengths. The modified Karren’s model and expression were used to estimate the average cross-section yield stresses of these members. Finally the LRFD resistance factors (ϕc) for the DSM buckling strength curves were calculated for a target reliability index of 2.5, and the results were compared with their counterparts obtained previously, considering the yield stress of virgin material as the design parameter. The results show that the design parameter of average cross-section yield stress was safe to be used with the DSM for estimating the axial strengths of solid CFS members subjected to local or global buckling.