Local Buckling Limit States in Rod-Braced Metal Building Frames

Flange local buckling in metal building primary frames can initiate as rod brace anchor loads are carried through the closest steel frame flange to a girt, purlin, or to flange bracing collectors. Anchor rods are typically placed eccentrically in a steel frame, adjacent to one flange of the built-up steel cross-section to limit local web deformation, and this eccentricity initiates weak axis bending and torsion that is accompanied by axial force from the rod inclination. An experimental program was conducted to study frame-anchor interaction and document strength limit states. One of these limit states was flange local buckling, which resulted in a sudden primary frame lateral stiffness reduction that was mitigated after buckling by high warping restraint provided by frame continuity, leading to a stiff post-buckling path at large lateral frame deformations. The experimental results inspired a primary frame structural model, developed with insight from thin-shell finite element simulations, that allows mapping of rod anchor forces to flange demand, critical elastic buckling, and yielding parameters that define flange slenderness. With this model in hand, possible approaches for calculating metal building primary frame capacity including flange local buckling are explored, and possible pathways for future research are identified.

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