COMPARISON OF LOW-DUCTILITY MOMENT RESISTING FRAMES AND CHEVRON BRACED FRAMES UNDER MODERATE SEISMIC DEMANDS

Non-linear analyses utilizing site specific ground motions were performed on four structural steel frames with geometry and loadings based on the SAC Joint Venture’s Boston structure: Pre-Northridge and Post-Northridge moment resisting frames, and strong and weak connection R=3 chevron braced frames. Although the two types of frames exhibited similar ultimate capacities, they are very different with respect to stiffness. Significant damage was observed in the braced frames under certain earthquakes, while almost no damage was observed in the moment frames. Because of this lack of damage, the behavior of the moment frames was further observed using elastic techniques. Response spectrum analyses on the moment frames revealed that the demands from a series of site specific ground motions for Boston rarely exceeded the demands imposed by the IBC maximum credible earthquake (MCE) response spectrum. The lower natural periods of the stiffer braced frames correspond to higher accelerations on the design response spectrum, resulting in a larger seismic demand than in the moment frames. This helps to explain why the braced frames showed many instances of non-linear behavior, while the moment frames responded almost entirely in the elastic range. Introduction As part of the SAC Joint Venture, moment frames were designed to resist seismic loads according to local building codes for Los Angeles (UBC 1994), Seattle (UBC 1994), and Boston (BOCA 1993). Multiple researchers have discussed the non-linear dynamic behavior of the Los Angeles and Seattle frames (FEMA 2000b; Gupta and Krawinkler 2000; Yun and Foutch 2000). To the authors’ knowledge, however, published discussion of the seismic performance of the Boston frames is not readily available in the open literature. This paper discusses the observed response of low-ductility structural steel frames subjected to site-specific ground motions in Boston, Massachusetts. Graduate Research Assistant, Dept. of Civil and Env. Engineering, Tufts University, Medford, MA 02155, timothy.nelson@tufts.edu Structural Engineer, LeMessurier Consultants, Cambridge, MA 02139, mgryniuk@lemessurier.com Research Assistant Professor, Dept. of Civil and Env. Engineering, Tufts University, Medford, MA 02155; Associate, LeMessurier Consultants, Cambridge, MA 02139, emhines@lemessurier.com Paper No. 1286