Reliability analysis in the design of cold-formed steel built-up I sections by modified Direct Strength Method

Resumo

Cold-formed steel (CFS) framing is an economical and efficient structural solution, as it provides high strength and low self-weight. Built-up sections, formed by combining two or more CFS members, can reduce instabilities and obtain more versatility. An approach adopted in standards from several countries for the design of CFS bars is the direct strength method (DSM), which allows calculation of axial force from elastic buckling loads, considering global, local and distortional buckling modes. Currently, there are proposals to modify the DSM for the design of built-up sections, aiming to better fit experimental and numerical data. This study aimed to investigate these modifications of original DSM formulae by applying them to a database of experimental compression tests on built-up I, or back-to-back, sections. Using the database results, it was possible to calculate the professional factor (P), obtained from the ratio between experimental and theoretical results. To obtain reliability indices (β) related to theoretical methods, the professional factor was evaluated as a continuous random variable, according to data grouping by section type, failure type, and all one. The reliability index (β) was obtained using FOSM, FORM and Monte Carlo Simulation (MCS) reliability methods for load combinations from AISI S100 (LRFD and LSD) and NBR 14762.