Structural Weight Optimization of Frames Using Tabu Search. II: Evaluation and Seismic Performance

In a companion paper, a structural optimization computer program based on tabu search (TS) was developed and used to optimize three two-dimensional moment resisting frames. All three frames were representative of current design practice for steel structures in the Los Angeles area and were part of an SAC program of study following the Northridge earthquake. The weight reduction obtained for the three-story/three-bay, the nine-story/five-bay, and the twenty-story/five-bay frames in the initial study were 26.4, 18.3, and 25.5%, respectively. In this paper, the seismic performance of the optimized structures is evaluated by calculating the rotation demands in the plastic hinges that form in the structural elements during nonlinear time history analyses. Twenty ground motion records having the return probability of 10% in 50 years are considered in the study. The frames were expected to achieve the Life Safety performance level as outlined in the FEMA-273 guidelines, and in general, they were able to achieve this level of performance. Next, the same frames are optimized using the optimization module of a commercially available program, with the same assumptions (starting point sections, loading, etc.). The TS optimization program was able to achieve a larger weight reduction than the commercially available program. The performance of the final commercial program designs is then briefly evaluated by one nonlinear time history analysis for each building using the ground motion record that produced the most severe responses for the TS designs. Performance of these frames was very similar to the TS design frames. Finally, the performance of the three TS designed frames is compared to that of the original design frames, which were designed under the same code lateral forces. The original frame designs are analyzed for the ground motion record (or records) that resulted in the largest plastic hinge rotations in the TS design frames. The values of the maximum story plastic hinge rotations, story displacements, and interstory drift ratios are compared. The response of the TS optimized frames was comparable to the original frames.