Nonlinear Dynamic Analysis of a Structure with a Friction-Based Seismic Base Isolation System

The seismic activity in some parts of the world requires that the performance of structures under this type of excitation, is carefully evaluated. In regions where seismicity is insignificant, the conventional design approach aims at the design of structural members in such a way that all static and dynamic loads (such as wind load) are withstood elastically. However, in cases where seismic excitation has to be taken into account, this approach may lead to uneconomic design solutions. Therefore, two alternative design concepts are often employed. In the first design concept, plastic deformation is allowed in special parts of the structure. The second concept consists of the implementation of mechanical devices into the structure. In some of these cases, the mechanical devices are placed underneath the superstructure. This is often referred to as seismic base isolation. In seismic base isolation systems, frequently, nonlinear behaviour is encountered. In many cases, though, these systems are modelled linearly. This may lead to an unrealistic representation of the dynamic response of the isolated structure. In this research, the influence of the dynamic characteristics of the superstructure and the nonlinear modelling of base isolation systems on the total system’s dynamic response, is considered. Furthermore, the dynamic performance of a baseisolated structure is evaluated, based on additional insight in its nonlinear behaviour, obtained using dynamic analyses. After evaluation of the ground motion characteristics (dominant frequency range and correlation between horizontal, vertical and rotational excitation), a superstructure is chosen. A dynamic model of this system is derived with Finite Element Method techniques. With this model, a modal analysis is made. Based on the results of this analysis, model reduction is applied to reduce the number of degrees of freedom of the dynamic model and improve the computational efficiency of (nonlinear) dynamic analyses. Next, base isolation is regarded. An isolation system with both linear and nonlinear elements is chosen and its dynamic model is derived. Here, the nonlinearities are caused by friction elements. With the dynamic model of the base-isolated structure, time simulations are carried out. After ”optimal” design of the base isolation system through a parameter line-search, a comparison is made between fixedand isolated-base simulations for various earthquake records. Next, the influence of three modelling approaches on the dynamic response is analysed. These three modelling effects are the vertical ground excitation, the variation of the normal force for the determination of the friction force and the superstructure flexibility. Subsequently, the dynamic behaviour of the nonlinear isolated structure is regarded by evaluation of periodic solutions. These periodic solutions are identified as a function of the horizontal excitation frequency. As a periodic solution solver, the Shooting Method is utilised in conjunction with stepped frequency sweeping. The influence of the isolator parameters on these periodic solutions is treated and some asymptotic behaviour, observed in the bifurcation diagrams, is clarified. Finally, these steady-state dynamic analyses are correlated to the transient time simulations.