Seismic Rehabilitation of “Pre-Northridge” Steel Moment Frame Building to Achieve Immediate Occupancy

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Kurt R. Lindorfer, Melissa Henkel

This paper discusses the seismic retrofit of a steel moment frame building possessing “Pre-Northridge” beam column connections with the goal of achieving an “Immediate Occupancy” performance objective.

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, we designed a rehabilitation scheme which included Fluid Viscous Dampers. Our initial task was to rehabilitate the 2 story building, then address similar deficiencies in the other buildings and identify potential deficiencies. Several scenarios for mitigating the beam column joint deficiencies were presented which would improve the beam column joint performance, however, they would not significantly improve or enhance the buildings beyond mitigating the deficiency of the joints. Upon understanding the client’s expectation of the level of post earthquake performance desired for the 2 story building, we believed it to most closely resemble the “Immediate Occupancy” performance objective.

Although the building possesses “Pre-Northridge” moment frame connections, there is no code mandated seismic retrofit for the deficiencies, and thus, identification of the client’s desired “Performance Objective” was required. The 2 story building consists of 65,000 gsf floor plates for a total of approximately 130,000 gsf and incorporates open web steel joists and girders with non-concrete filled metal deck for the gravity system at the roof with similar framing at the second floor, however, concrete filled metal deck is used as at this level. Steel moment frames comprise the Lateral Force Resisting System (LFRS) which possess “Pre-Northridge” beam-column connections.