Optimal design of a passive nonlinear isolation system for the seismic protection of equipment

For critical facilities having high-risk (e.g. thermal or nuclear power plants) or high-value contents (e.g. high-tech facilities, data centers), the need to protect nonstructural components and equipment, in particular, from the earthquake hazard may be equally important as the seismic design of the structure itself. An effective way to protect seismic vulnerable equipment consists in implementing an isolation system between the internal apparatus and the supporting structure. As a result, the absolute accelerations transmitted to the equipment are reduced and the damages due to excessive inertial loads are prevented. In this work, an innovative passive isolation system with nonlinear hysteretic behaviour is proposed for the seismic protection of equipment. First, the problem of formulating a constitutive relationship for the system is addressed resorting to an integrable model derived from the analytical Duhem hysteresis operator. Second, an optimal design methodology is specifically developed to account for both the reduction of the equipment acceleration response and the maximization of a properly defined energy performance index. The methodology is suitable to either a coupled or a decoupled dynamic analysis approach, depending on the significance of the dynamic interaction between the isolated equipment and its supporting structure.