Congratulations to the hardworking teams from both VMA & Reid Middleton on their work on the Navy Hospital Twentynine Palms Seismic Upgrades! Awarded for excellence in structural engineering, this project not only took design creativity but technical innovation and close collaboration.
The upgrade design used fluid viscous dampers to reduce floor accelerations, velocities, and drifts. This type of design required a more advanced analysis than is typical of most building designs. While fluid viscous dampers have been used to reduce building movement in moment frame structures before, this application was among the first for a military hospital.
The design team performed the seismic evaluation and subsequent seismic retrofit design for the 3-story, 160,000 SF hospital facility at the Marine Corps Air Ground Combat Center in Twentynine Palms, California. Nonlinear dynamic response history analysis was conducted to determine the building performance limits and to determine the design forces and displacements of the dampers. We were able to conduct parametric studies to determine the optimum damper locations, orientations, and the damper properties (i.e. damping coefficient, velocity exponent constant) that best suit the needs of the design. Nonlinear dynamic response history analysis allowed us to tailor each of these design aspects to reduce construction costs, minimize impact to architectural features, reduce impacts to hallways and doorways, and minimally affect line-of-site where dampers crossed windows.
The ASCE 31 Tier 3 seismic evaluation revealed seismic deficiencies, including the brittle beam-column joints of the pre-Northridge welded steel moment frame system. Three seismic upgrade strategies were considered during the evaluation and planningphase. Option 1 was the installation of fluid viscous dampers, which are like shock absorbers in a car, to absorb earthquake energy. Option 2 was the installation of steel concentric braced frames. Option 3 was the stiffening and strengthening of the existing moment frames throughout the structure.
To mitigate the seismic deficiencies, Option 1, Fluid Viscous Dampers, was selected in order to absorb earthquake forces. This option was chosen because it minimized cost and disruption to the hospital. The fluid viscous dampers also serve to reduce earthquake floor accelerations, velocities, and displacements due to their energy absorption capability. The other upgrade options would have increased floor accelerations and velocities.
The upgrade design included 53 dampers in 28 building bays of the hospital. Since the dampers are only installed in select locations and the connection of the dampers to the structure required minimal modifications to the existing structure, the installation of the dampers caused minimal disruption to the hospital. The hospital was able to remain operational throughout the upgrade construction, and the temporary demolition of existing architectural, mechanical, electrical and plumbing components was limited to relatively small locations in only certain areas.