Negative stiffness device for seismic protection of structures: An analytical and experimental study

Structural weakening and damping is an approach previously proposed for the reduction of seismic forces and drifts in structures. While this approach is very efficient, it requires a reduction in strength, which will result in inelastic excursions and permanent deformation of the structural system during the seismic event. This paper describes a true Negative Stiffness Device (NSD) that can emulate weakening of the structural system without inelastic excursions and permanent deformations. The Negative Stiffness Device (NSD) is a self contained device that produces a force which is in the same direction as the imposed displacement, thus the name “negative stiffness”. The device is inherently unstable but when installed in a structure transfers a force at its installation level which in turn reduces all forces, drifts and accelerations above that level. The NSD is therefore capable of limiting the forces developed in a structure without any yielding which could lead to permanent deformations. The NSD was developed at the University at Buffalo (UB), Rice University and Taylor Devices Inc. and tested on a shake table by authors in a three storied structural model isolated with elastomeric bearings. The NSD consists of a highly compressed spring in a double negative stiffness magnification mechanism. In order to ensure a considerable amount of positive stiffness of the global system around small displacements to minimize wind vibrations a so called gap spring assembly (GSA) mechanism is implemented which delays the engagement of the device by a prescribed displacement. Moreover, the device employs double containment chevron braces that contain the large vertical forces needed for the development of negative stiffness without transferring these forces to the structure. This paper describes the development of analytical and computational tools that describe the behavior of the device and presents results for the experimental verification of these tools. Apostolos A. Sarlis, Dharma Theja R Pasala, Michael C. Constantinou, Andrei M. Reinhorn, Satish Nagarajaiah and Douglas Taylor 2