Seismic performance enhancement of steel moment frame structures using fluid viscous dampers

Seismic performance of a multi-story hospital structure located in Southern California was evaluated using performance-based earthquake engineering. The uppermost eight stories of the superstructure use steel framed construction, whereas reinforced concrete framing is used for the lowest floor (L2) and the four sub-grade parking stories. The building is rectangular in plan, has a typical story height of 3.6 m (11.8 ft) and a total floor area of 12,000 m(2) (130,000 ft(2)). Steel moment-resisting frames using Pre-Northridge connection details were used to resist lateral loading. Reinforced concrete framing comprise the lateral load resisting system for L2 and parking levels. Provisions of FEMA 273 (FEMA 1997), FEMA 351 (FEMA 2000), and OSHPD-approved design procedure (NYA&MI 2003) were used to evaluate the expected seismic performance of the structure in the existing configuration when subjected to a 475-year return period seismic event. A comprehensive three-dimensional mathematical model of the structure was prepared and subjected to acceleration histories generated to match the site-specific response spectrum. Response was evaluated using nonlinear analyses. Structural performance of the existing building was not satisfactory and both story drifts and member nonlinear flexural rotations exceeded allowable limits. To mitigate seismic deficiencies, the analytical model was modified by adding fluid viscous dampers attached to new exterior frames. Analyses of the structure in the upgraded configuration indicated that the performance requirements were satisfied. Parametric studies were conducted to investigate the effects of variation in key parameters (for example: damper coefficients, nonlinear hinge properties, and soil-structure interaction) on the response of the retrofitted structure.