Nonlinear dynamic analysis of three-dimensional base isolated structures.

摘要

Structures can be designed to withstand severe earthquake forces by providing ductility and energy dissipation capacity to the structural elements, thus allowing damage in the structural elements and invariably in the nonstructural elements. Another approach which is being rapidly adopted all-around the world is the concept of base isolation, wherein the flexibility and the energy dissipation capacity are provided by a specially designed isolation system that is placed between the superstructure and the foundation. These isolation systems can be designed to essentially limit the nonlinear behavior to the isolation level, imposing little or no ductility demand on the superstructure.; Three dimensional behavior such as: (i) lateral-torsional response (with biaxial effects in isolation bearings); (ii) vertical-rocking response; (iii) effect of biaxial interaction in isolation bearings on the response of base isolated structures; and (iv) response due to rocking input (caused due to Rayleigh and P waves) is of concern.; The study of three dimensional behavior of base isolated structures such as lateral-torsional response and vertical-rocking response requires a comprehensive analytical model. The analytical model should be capable of addressing highly nonlinear behavior of isolation systems such as sliding systems and elastomeric bearing systems (with biaxial effects). The existing analytical models and solution algorithms are not capable of accurately analyzing sliding systems with near rigid plastic behavior or systems with abrupt changes in stiffness.; This dissertation deals with the nonlinear dynamic analysis of three dimensional base isolated structures and the seismic behavior of base isolated structures.; New biaxial and uniaxial models of isolation elements are developed. A new generalized analytical model and solution algorithm involving the pseudo-force method is developed. It is shown that the solution algorithm with pseudo-force method is accurate and efficient, and that the pseudo-force method converges to the correct solution in the presence of severe nonlinearities. The analytical model is used to study the three dimensional behavior of base isolated structures.; Some of the main conclusions of the study are: (i) base isolation is very effective in reducing the lateral-torsional response; (ii) both elastomeric and sliding isolation systems are effective in reducing the coupled lateral-torsional response of base isolated structures, even when three dimensional effects are included; (iii) influence of biaxial interaction between lateral forces that develop in bearings is significant in sliding isolation system and not as important in elastomeric isolation system; and (iv) Sliding systems reduce the lateral-torsional response and maintain the bearing displacements within acceptable limits for a variety of input motions with significantly different frequency content.