Higher-Mode Force Demands in Systems
Employing Elastic Strongback Spines

NHERI Lehigh Seminar Series

March 31, 2021 | 12:00 pm – 1:00 pm EST


Research has identified the potential advantages of pivoting seismic force-resisting systems that employ an elastic spine, or “strongback”. The strongback is represented by a truss or stiff column that is designed to remain essentially elastic, thus providing an elastic load path to re-distribute seismic demands across the building height. As such, strongback systems are expected to result in more uniform drift distributions, reduced peak inelastic demands, and improved design flexibility compared to conventional seismic force-resisting systems.

However, since the strongback is designed to remain elastic in every mode, systems employing strongback spines can be highly influenced by higher- mode force and acceleration demands that are not well-constrained by inelastic response. These higher-mode demands can affect acceleration-sensitive nonstructural components. A series of investigations were aimed at understanding the nonlinear dynamic response of strongback systems, including higher-mode mitigation strategies.

At the E-Defense shake-table in Japan, an international team of U.S. and Japanese researchers tested a moment- resisting frame (MRF) retrofitted with an elastic spine. The specimen included acceleration-sensitive hospital equipment. The MRF-spine system was then subsequently modified with force-limiting connections to alleviate higher- mode accelerations through controlled yielding. Results demonstrate the influence of higher-mode effects on strongback and similar systems.

About the Author

Dr. Simpson is an Assistant Professor at Oregon State University. She received her Ph.D. from UC Berkeley and her Bachelor of Science from the University of Kansas. She uses advanced computational and experimental methods to characterize structural response. Her aim is to develop innovative structural systems that improve building performance and reduce the effects of natural hazards on the built environment. Research areas include resilient design and retrofit of building structures, performance-based earthquake engineering, and next-generation computational modeling and simulation, such as the development of hybrid simulation methods for wave-structure interaction.

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