Relative Performance of Fixed‐Base and Base‐Isolated Concrete Frames

Results of a study are presented in which the performance of code-designed fixed-base and base-isolated concrete frames are compared. The purpose of the investigation is to compare in a quantitative manner the relative performance of code-designed frames, and to determine approximately the design force level for an isolated frame that will result in performance comparable to that of the fixed-base frame. The fixed-base frames were designed in accordance with the 1990 Structural Engineers Association of California (SEAOC) \IRecommended Lateral Force Requirements and Commentary.\N The base-isolated frames were designed to varying levels of the SEAOC-recommended base shear. Time-history analyses were conducted for three ensembles of recorded earthquakes. Analysis considered the nonlinear behavior of the isolation system and superstructure. Statistical analysis of peak response quantities demonstrates the superior performance of the code-designed isolated structures. Results illustrate that comparable performance is generally achieved when the isolated frames are designed to between 25% and 50% of the recommended lateral force.