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A three-dimensional (3-D) base isolation system to control both the horizontal and vertical components of ground motion is presented in this paper. The system is adopting a negative stiffness device (NSD) that can be considered as an adaptive passive protection system, which can apparently change the stiffness of the structure. This work is focused on studying through numerical simulations the mitigation performance of the NSD against strong earthquakes in the vertical direction. The base isolation arrangement consists of elastomeric bearings acting both in the horizontal and vertical direction and NSDs acting only in the vertical direction. So, a 3-D base isolation is achieved, where it is assumed that the NSDs affect the vertical stiffness of the system only. Numerical analyses show that the presence of NSDs reduces the vertical acceleration in the structure. Nevertheless, accordingly with the passive control theory …