Influence of torsional stiffness in double-angle open-web joist and joist girder chords

Abstract

The double angle chords of open-web steel joists are often designed to efficiently support loading with relatively slender sections, which introduces buckling instabilities that are treated differently by various design standards. This study investigates the buckling behavior of double angles, focusing on the importance of flexural–torsional buckling. A theoretical inelastic buckling investigation considering current design practices and known steel behavior demonstrates how flexural–torsional buckling does not control for all double angle configurations. Subsequent investigation focuses on double angles theoretically susceptible to flexural–torsional buckling, while implementing finite element modeling of full joist structures and isolated chord segments. The resulting elastic eigenbuckling analysis and geometrically and materially nonlinear analysis with imperfections captures significant twisting of individual angles in the built-up section; however, global flexural–torsional buckling was not observed to control. Most double angle configurations are controlled by flexural buckling due to an increased flexural–torsional buckling capacity. This result is attributed to the observed increased torsional stiffness of the chords, which is typically not considered in design. A supplementary investigation indicated this torsional stiffness was primarily a result of torsion being transferred through the stiff chord-to-chord connections, which do not exist in all double angle members. Accounting for the additional torsional stiffness in a theoretical inelastic buckling calculation leads to flexural–torsional buckling not controlling the design of most double angle cross sections considered in this study. Overall, this study indicates that designing double angles in joist structures by only considering flexural buckling with local buckling is reasonable with current industry practices.