Evaluating adaptive vertical seismic isolation for equipment in nuclear power plants

Seismic isolation systems are widely recognized as beneficial for protecting both acceleration- and displacement-sensitive nonstructural systems and components. So-called adaptive isolation systems exhibit nonlinear characteristics that enable engineers to achieve various performance goals at different hazard levels. These systems have been implemented to control the horizontal response, but there has been limited research on seismic isolation for controlling the vertical response. Thus, this paper seeks to evaluate the merits of adaptive vertical isolation systems for components, specifically in nuclear power plants (NPPs). To do this, six vertical isolation systems are designed to achieve multiple goals. The systems consist of in-parallel configurations of linear and nonlinear springs and dampers, falling into three groups depending on the elements they combine: (1) linear springs and linear dampers (LSLD), (2) linear springs and nonlinear dampers (LSND), and (3) nonlinear springs and linear dampers (NSLD). To investigate the effectiveness of the systems, a stiff piece of equipment is considered at an elevated floor within an NPP. A set of 30 triaxial ground motions is used to investigate the seismic performance of the equipment. The peak isolation displacement and peak equipment acceleration are used to assess the effectiveness of the vertical isolation systems. While all systems significantly reduce the seismic accelerations on the equipment, the relatively simple LSLD and LSND systems exhibit superior performance over multiple hazard levels.