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Conventional lateral force-resisting systems can provide a stable, ductile response but also experience significant inelastic demands, rendering repairs impractical or uneconomical. Thus, there is a need for novel structural systems that protect structural and nonstructural components to reduce post-earthquake repairs and downtime. A U.S.-Japan research team – including three U.S. universities, two Japanese universities, and two major experimental research labs – is developing a structural solution to reduce peak drift and acceleration demands, thereby protecting buildings, their contents, and occupants during major earthquakes. The primary components of the system are: (1) steel base moment-resisting frames designed and detailed to behave in the inelastic range and dissipate energy, (2) stiff and strong elastic spines designed to remain essentially elastic to redistribute seismic demands more uniformly over the building height, and (3) force-limiting connections (FLC) that connect the frame to the spines to provide a yielding mechanism that limits acceleration demands. This economical earthquake-resilient system is intended to be used in essential facilities, such as hospitals, where damage to the buildings and contents and occupant injuries must be prevented and where continuity of operation is imperative. The system was recently tested at full scale at the E-Defense shake-table facility in Miki, Japan. This paper provides an overview of pre-test numerical simulations, shake-table test setup and instrumentation, and preliminary test results. 

Steel moment-resisting frames (MRFs) are widely used in the United States to resist seismic forces. MRFs have many advantages, including high ductility, architectural versatility, and vetted member and connection detailing requirements. However, MRFs require large members to meet story drift criteria. Moreover, strong-column-weak-beam requirements can result in significant member sizes, and – even in the cases where strong-column-weak-beam requirements are satisfied – MRFs can still be vulnerable to story mechanisms in one or a few stories. Recently, the concept of a strongback has been utilized successfully to delay or prevent story mechanism behavior in braced frames. The strongback is represented by a steel truss or column that is designed to remain essentially elastic, thus allowing the system to transfer inelastic demands across stories. Although systems including strongbacks exhibit more uniform story drift demands with building height and reduced peak drift response, the elastic nature of the strongback can also result in near-elastic higher-mode force demands. This study compares the dynamic response of a baseline MRF to that of a retrofit using a strongback column. Several ground motions are considered to determine which cases produce the largest drift, acceleration, and force demands.