Special Concentrically Braced Frames (SCBFs) rely on yielding and buckling of the braces to achieve ductility of the system. As a result, AISC 341, “Seismic Provisions for Structural Steel Buildings,” requires designers to evaluate the columns, beams, and connections in SCBFs for the capacity-limited seismic load effect associated with the expected brace strengths. In practice, especially for simple or regularly-shaped buildings, designers often compute these force demands by hand and manually combine them with the demands from other loads. However, for irregularly-shaped or atypical buildings, such as large industrial buildings without diaphragms or structures with unevenly distributed mass or stiffness, this can become an arduous task, especially during an iterative design process.
This presentation will provide an overview of a method to automate a plastic mechanism analysis of an SCBF in an analysis model by imposing artificial self-straining (thermal) loads to emulate the capacity-limited seismic load effect. The method expands upon brief guidance provided in NEHRP Seismic Design Technical Brief No. 8, "Seismic Design of Steel Special Concentrically Braced Frame Systems." We will discuss the relevant AISC 341 provisions as well as the alternative methods for conducting plastic mechanism analyses. We will outline the procedure for employing the automated method and address important considerations including the selection of modified thermal and stiffness parameters, the placement of artificial lateral restraints to provide temporary stability, and the combination of the resulting force demands with the other seismic demands and gravity loads. We will present a case study of a recent project for which we implemented this method and discuss the challenges we encountered.