Evaluation of the SEAOC/UBC97 Provisions for the Tall Base-Isolated Structures

The base isolation systems are among the passive control devices that have been used over the last three decades to limit the seismic-induced response of structures. In this regard, the Uniform Building Code provisions (UBC) has incorporated a special section for the seismic design of base isolated structures since its 1991 edition. Due to the importance of the behavior of these structures under seismic loading, and to evaluate the equivalent static analysis method of UBC97, a three dimensional, 4-story base-isolated structural model is selected. Considering 8 different types of the LRB base-isolation systems, the 8 structural models are designed using the UBC97 provisions for these structures. Extensive nonlinear dynamic time history analyses are performed for these structural models, using five pairs of earthquake components recorded on the SA and SB soil profile types according to the UBC97 provisions in the dynamic analyses, both the base isolation systems and the superstructure are allowed to behave nonlinearly (bilinear). The results indicate that the UBC97 suitably predicts the base shear force for the superstructure, but that it does not provide isolation systems. Also, underestimation of elastic force for the higher floors by UBC97, considered to be the main reason for the nonlinear behavior of the columns of these floors, revealed in the dynamic time history analyses. Besides, the corner and the center bearings’ displacement of the base floor predicted by the equivalent static method of the UBC94 were usually larger than their mean dynamic time history response. This issue has been appropriately modified in the UBC97. In addition, to provide the required strength capacity for the columns of the higher floors, an alternative shear force distribution over height for the base isolated structures is proposed. The results of the nonlinear dynamic analysis performed indicates that there is a closer match between the mean response of the structures designed by proposed shear force distribution and those obtained from the equivalent static method of the UBC97 provisions.