Buckling Behavior of Open Web Steel Joists and Joist Girders

To efficiently support loading, open web steel joists are often fabricated from components with relatively high slenderness ratios. As a result, joists are susceptible to buckling instabilities in the webs and the double angle chords. The current SJI design requirements for steel joists require the consideration of flexural buckling but exclude any consideration of flexural-torsional buckling modes for double angle members, which differs from AISC's design requirements. This study evaluates the underlying SJI assumption that flexural-torsional buckling can be neglected in joist design. The paper reports on the theoretical inelastic buckling behavior of double angles and the details of the finite element model required for accurately simulating the stability behavior. The theoretical investigation applied the methodology behind AISC's treatment of single angle flexural-torsional buckling to verify the expected response for double angles in joist and joist girder applications. The results identified several double angle geometries that appear to be susceptible to flexural-torsional buckling. Next, finite element models of the joist and joist girders, which are validated by making comparisons with full steel joists experimental results, are presented and include web-to-chord connection variations, support and bracing conditions, and loading application. These models were able to accurately predict the bending stiffness of the joist and the different buckling failure modes of the webs. Various modeling parameters and their effects on stability behavior are discussed.

Learning Objectives:
Identify one of the main concepts behind SJI's exclusion of flexural-torsional buckling in double angle design.

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