Seismic Performance of Fixed‐Base and Base‐Isolated Steel Frames

The seismic performance of fixed‐base and base‐isolated concentrically braced and special moment‐resisting steel frames are determined. The fixed‐base frames are designed in accordance with the 1990 Structural Engineers Association of California (SEAOC) Recommended Lateral Force Requirements and Commentary, while the base‐isolated frames are designed to 100%, 50%, and 25% of the SEAOC‐recommended lateral force levels. Nonlinear time‐history analyses, for 54 ground motion records, are then conducted. The number of yielded or collapsed frames, number of yielded elements, total and relative roof displacement, and isolation‐bearing displacement are statistically evaluated. Comparable or better performance is achieved for the base‐isolated braced frame designed to 50% of the SEAOC‐recommended lateral force. Comparable performance is achieved for the base‐isolated moment frame at various design force levels, depending on the performance criteria.