Seismic Rehabilitation of “Pre-Northridge” Steel Moment Frame Buildings Incorporating Fluid Viscous Dampers

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Kurt R. Lindorfer, Melissa Henkel

This paper discusses the seismic retrofit of 2-5 story steel moment frame buildings possessing Pre-Northridge beam-column connections. As part of a complete campus building remodel and renovation project in the San Francisco Bay Area, which included one 2 story, one 3 story and two 5 story steel moment frame buildings, our task was to review the seismic vulnerability of the existing buildings relative to the known deficiency of the Pre-Northridge beam-column connections, and develop a campus-wide rehabilitation scheme for mitigating the deficiencies.

As part of the campus expansion, the client desired to add approximately 75,000 gsf between two existing buildings, hereinafter referred to as the connector building. Initial discussions with the architect and owner indicated the desire to have the connector building possess an extremely high volume space between the 1st and 3rd floors. Additionally, diagonal bracing was not desired in the connector, thus representing a likely LFRS for the connector consisting of either interior hidden shear walls or very heavy steel moment frames along the building perimeter. With either system, it would be necessary to have very large seismic separations between the existing buildings and the new connector building. Additionally, both scenarios were cost-estimated and, with such high floor-to-floor heights, deemed too costly.

Rather than constructing a building with both large seismic separations and a costly LFRS, the three buildings were attached, thus eliminating the seismic joints, and possibly reducing the cost of the LFRS associated with the connector. The concept of incorporating Fluid Viscous Dampers (FVD’s) as part of the selected seismic rehabilitation of the existing buildings was introduced, which would mitigate building drift, and therefore joint rotation. By adding the connector building and attaching it to the existing buildings, we increased the mass beyond that of the original two buildings by approximately 45% and reduced overall building drifts by approximately 40-60%.