Seismic Retrofit of Two Steel Moment Frame Steel Structures

Abstract eng:
Two 18-level twin towers with a total of 53,900 m2 of usable covered space located in Sacramento, California were designed using a 1966 code, inadequate to accommodate current seismic code prescriptions resulting from the updated seismic hazard estimated for the site. The building structures include steel gravity frames from a concrete mat foundation to roof, perimeter steel moment frames, and asymmetrically-recessed concrete shear walls from foundation to third floor. Seismic forces must transfer from the perimeter frames to the shear walls through third-floor slabs. Three of the first-story shear walls are discontinued at the first-floor slabs and must transfer horizontal seismic forces to basement walls through the first-floor slabs, and vertical forces onto concrete piers located at the basements. As a result of current-code higher seismic demands, the buildings were found seismically deficient due to: pre-Northridge steel moment-frame joints, first and thirdfloor slab shear capacities, shear transfer connections between slabs and walls, deficient basement concrete piers supporting shear walls, first- and second-story wall shear capacities, and excessive flexibility of the steel frames. A number of conventional and advanced seismic retrofit schemes were studied to provide drift control to: (a) avoid unacceptable damage to the moment frame joints; and (b) limit the higher seismic loads that the structural members would undergo in the case of a destructive earthquake. These studies involved state-of-the-art approaches such as site-specific time-history seismic demands, three-dimensional non-linear time-history analysis, and project-specific software tools developed for time-history FEMA design checks. This latter approach allowed us to confidently accept the seismic demand defined for the structures in a way that enveloped results would not. By strengthening the perimeter frames with diagonal braces and in-line viscous dampers, the seismic safety requirements are met. The final retrofit design, a massive viscousdamper configuration, was distributed from the third floor throughout the height of the buildings. This configuration restricted the extent of retrofitting to sections of the building with easy access, limiting the required strengthening of the seismically-deficient reinforced-concrete elements below the third floor. The diagonal brace with in-line viscous damper configuration included sliding cross braces defining a novel design not used before in this type of applications. The solution reduced the estimated seismic retrofit construction cost from USD 15.2 million to an amount just above USD 3.3 million. The 136 dampers designed, per building, were subjected to a rigorous testing protocol, and subsequently installed in the newly-renovated buildings, currently in use.

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