Experimental investigation of cold formed high strength steel tubular joints subjected to moment loading

In recent years, simple design methods for high strength steel tubular joints have been proposed by several design standards. According to these standards, the design resistance of high strength steel joints is obtained from the same design equations used for mild steel joints, but the resistance should be reduced properly by applying the material factor and/or the design yield stress. The material factor is a direct reduction factor to design resistance which generally corresponds to 0.9 for 460-grade and 0.8 for 700-grade steels, while the design yield stress (or f(u)/f(y) limit) is a measure to account for the low f(u)/f(y) ratios of high strength steels. In this study, a recent experimental program on moment-loaded high strength steel tubular joints is reported, which covered both circular (CHS) and rectangular (RHS) hollow section joints and both in-plane bending (IPB) and out-of-plane bending (OPB) moments. Based on the test results and additional test data collected from available database, the design equations, material factor, and design yield stress recommended by the revised draft of EN 1993-1-8 (prEN 1993-1-8) are evaluated for moment-loaded tubular joints made of grade 700 steels. It is shown that the prEN 1993-1-8 equations, combined with the material factor and design yield stress, generally provide conservative moment resistances for 700-grade tubular X and T-joints. However, based on the experimental implication that nonuniform stress distribution may cause an early fracture in CHS joints, it is suggested that the effective width, which is usually considered only for RHS joints, be considered also for CHS joints subjected to OPB.