The modeling of individual seismic isolation bearings advanced quickly in the last two decades (Buckle, 1993). Exploratory testing and analytical modeling of elastomeric bearings enabled the use of such bearings in simple structures. More devices were subsequently suggested, tested and modeled. The need to integrate such devices in building structures, bridges, and other special structures, and to predict the behavior of such structures in the presence of isolation systems motivated the development of a series of specialized computer programs, 3D-BASIS. This series was developed to analyze groups of isolation bearings, and damping devices modeled individually by nonlinear mathematical rules, attached to structural models, which in turn are affected by the seismic isolation system.
As the structure also exerts forces on the isolation system, the interaction between the isolation system and structure is essential in the analysis or the design. This interaction cannot be captured by analyzing the components alone. The program series 3D-BASIS allows for versatile modeling of isolators and superstructures using simple models and general solutions that permit analysis of very complex systems. Moreover, the flexible design of the program allows more isolation components to be easily added without restructuring the program or the solutions.
The research effort was directed toward modeling nonlinear isolation components (such as elastomeric bearings, high or low damping rubber bearings, sliding bearings, linear and nonlinear viscoelastic devices) calibrated by experimental data and by the latest analytical formulations. Moreover, groups or clusters of components were modeled analytically while calibrating them by experiments [such as triaxial interaction in isolation bearings, (Mokha et al.)]. Furthermore, the resulting assemblages of structures and isolators were validated by shaking table testing to emphasize the accuracy and the sensitivity of the program (Mokha, et al., Constantinou, et al., Nagarajaiah et al., Hisano et al.). The performance of this program was also compared to the performance of other commercially available programs, which can analyze some of the configurations of isolation systems, but in a less efficient way. The numerical schemes selected for the 3D-BASIS series, which employ static and dynamic condensations before a step-by-step time analysis, complemented by the pseudo-force technique, produce extremely fast and accurate solutions suitable for verification of design assumptions.
The resulting product is a versatile tool for analysis and design of complex structures with modern isolation. As such, it provides support in the research and development of individual isolation devices that can be immediately verified for practicality in an integrated system. Moreover, the resulting program can and did provide support for the design of isolated structures such as large hospitals, emergency response structures, hazardous materials storage structure, and historic buildings, to name a few. The program has been and is used in verification and development of new standards for design of base isolated structures.