Marin County Civic Center

Seismic Retrofit Analysis
 

A global finite element model was created using the ADINA computer program primarily using shell elements to represent the floor diaphragms, roof shell, concrete arches, as well as elevator shafts and stairwells. Beam elements were used to model the columns. Because the building is considered to have minimal lateral load-resisting elements, considerable attention was paid to connection details. Refinements were made to reflect openings for doorways and mechanical systems, and structural connections at expansion joints were also carefully modeled.

Unique geometry and topography created major differences in stiffness between segments from one construction joint to another. In some regions, the building is two stories tall; other segments can be as much as six stories tall. The roof shell provides enormous stiffness and mass at the top of the building, further contributing to the difficulties in designing for dynamic loads. Designing the retrofit, therefore required the use of sophisticated analysis to help determine regions of high stress and seismic vulnerablity.

Because of aesthetic and functional concerns, the retrofit design must retain the architectural feel of the original structure as well as accommodate the day-to-day operation of the municipal courtrooms, jail and county offices which are housed in the Hall of Justice. The proposed design addresses these concerns through the use of such non-intrusive energy dissipation devices as eccentrically braced frames (EBF's) and dampers. These frames are strategically placed based on analysis to provide the greatest benefit structurally, while retaining the building's architectural legacy. Effects of column plastic hinging will be investigated by retrofitting existing columns such that they provide more ductility. These retrofit measures will be implemented in the ADINA model using nonlinear moment-curvature beam elements and nonlinear dampers.