Eccentric Braced Frame Design for Moderate Seismic Regions: Fid-Bau Portal

Eccentric Braced Frame Design for Moderate Seismic Regions

Hines, Eric M.

Recent discussions related to the seismic performance of low-ductility steel systems designed for moderate seismic regions have generated new interest in the cost-effective design of ductile systems for such regions. Concentrically braced frames (CBFs) are prevalent in moderate seismic regions, both because of their high stiffness-to-weight ratio, and because of the ease with which they can be designed and evaluated by the equivalent lateral force method. Eccentrically braced frames (EBFs) can offer the same advantages as CBFs, while also providing significant ductility capacity, and greater flexibility with architectural openings. The office of LeMessurier Consultants designed several eccentrically braced frames for prominent buildings in the United States from the 1960's through the 1980's. For these structures, designed prior to modern seismic provisions, eccentric bracing was the most efficient means to developing a stiff lateral system that accommodated the architectural program. Since the branding of eccentric bracing as a high-seismic, high-ductility system in the 1990's however, use of these schemes has tapered off to almost non-existent—restricting the potential both for working creatively with architects and for achieving moderate levels of ductility in the lateral force resisting system (LFRS). This paper first discusses the recent transition from the 6^th Edition to the 7^th Edition of the Massachusetts State Building Code and its implications for the relationship between seismic loads and wind loads. Thereafter a theoretical 9-story EBF design is developed for Boston, Massachusetts according to the 2005 AISC Seismic Provisions for the purpose of comparing its tonnage to a lower ductility CBF. Special link details are introduced that allow reduced tonnage without compromising capacity design principles.