Failure Modes of Starred Angle Braces

Abstract eng:
One of the most used structural steel systems in industrial structures in Chile is the Concentrically Braced Frame (CBF). Among the shapes used as braces, a common choice before 2010 was the use of starred angles (or XL shapes). This is a built-up shape composed of two angles joined intermittently at the heels, forming a crossshaped section. Starred angles have several advantages: they are simple to fabricate, easy to install on the frame, and their configuration makes clean up and maintenance less complicated. However, a number of these braces failed during the 2010 Maule earthquake in Chile. Failures observed included local buckling, lateral torsional buckling, and connection fractures. These failures, although not yet completely explained, have made engineers steer away from the use of starred angles as braces. This work presents a numerical study on the failure modes of starred angle braces in X configuration, made up from commercially available angle shapes. Finite element models of different brace configurations are built considering initial imperfections, nonlinear material behavior and large deformations. Gusset plates and stitch plates are also modeled and contact conditions established between these elements and the angles. The models are subjected to cycles of increasing displacement amplitude. Results for axial load capacity, hysteresis loops, failure mode, and forces between the gusset plates and stitch plates and the angles are extracted from the analysis. The results indicate that the global buckling failure mode (flexural, torsional, or flexural-torsional) developed by the brace depends on its size. In addition, for some cases the forces developed in the connections between the stitch plates and gusset plates and the angles, are larger than the forces normally used to design these connections, which could explain the failures observed after the earthquake.

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