A NEW METHOD FOR NONLINEAR DYNAMIC ANALYSIS OF BASE ISOLATED STRUCTURES Hamid MOHARRAMI 1 , Navid NICKDOOST 2 ABSTRACT Base isolation has been one of the most effective approaches to structural control in the last few decades. Because of nonlinear behavior of base isolators, analysis of such structures requires efficient nonlinear analysis software that has both characteristics of speed and accuracy. Many classical approaches have been proposed for efficient nonlinear dynamic analyses, however this subject still needs more attention. In this paper a novel nonlinear dynamic analysis is proposed that not only increases the analysis speed significantly, but also gives more accurate results compared to those in the literature. The method is formulated based on release of extra internal forces that are mistakenly stored in linear analysis of nonlinear structures. To explain the method, one should remind that in linear analysis of structures, it is intuitively assumed that structural members have infinite capacities and the internal forces of members are proportional to the external loads. Application of the proposed method to nonlinear analysis of several base isolated structures show that the proposed method results in much less computation time compared to classical methods. To illustrate the power and speed of the proposed method two examples have been included in the paper. It is shown that the proposed method can reach to a certain level of accuracy using larger time intervals compared to traditional classical methods. Keywords: Base Isolation; Nonlinear Dynamic Analysis; 1. INTRODUCTION Seismic control of structures is believed to be one of the most effective approaches in protecting structures against earthquake damages and mitigation of human disasters. As a well-known seismic control system, seismic base isolation has been used in several structures in many locations around the world. The isolation system reduces the interaction between two different surfaces. Isolating a structure is creating an interface that minimizes interaction between the ground and the structure (Connor 2003). Structural seismic isolation was perhaps conceived first by Touaillon in 1870, (US patent 99973). In 1909 Calantarients proposed a Talc layer in structure base level to provide sliding of base of structure on the ground (Bozorgnia et al. 2004). Two groups of isolators including rubber- based and sliding-base are generally addressed in the literature. The philosophy of Seismic Isolation theory is simple. It is increasing the natural period of structure by implementation of a seismic isolator system with small lateral stiffness in the base level of structure. (Bozorgnia et al. 2004). Increasing the natural period of a structure reduces the effect of horizontal ground motions on the superstructure. The isolator in the base of a structure makes the superstructure not sense the ground motions and show somehow rigid behavior in the events of earthquake excitations. The almost rigid behavior will cause little internal forces in superstructure elements. A base isolated structure is normally composed of a base on the ground, the base isolator system, a mat foundation and the superstructure. The behavior of base isolators is usually modeled as bilinear elastic-plastic. However, because of small transmitted lateral excitation, the superstructure behaves 1 Professor, Civil and Environmental Faculty in Tarbiat Modares University, Tehran, Iran, hamid@modares.ac.ir 2 Graduate Student, Civil and Environmental Faculty in Tarbiat Modares University, Tehran, Iran, psi.navid@gmail.com