As it is well known in the field of structural dynamics, by designing a ductile structure and letting the structure yield under strong earthquakes, the forces acting on the structure can be reduced to the level dictated by the yield level. However, the structure undergoes permanent displacement. In this study yielding is emulated in a structural system by adding a "adaptive negative stiffness device" and shifting the "yielding" away from the main structural system-leading to the new idea of "apparent softening and weakening" that occurs ensuring structural stability at all displacement amplitudes. For this purpose a novel adaptive negative stiffness device, NSD, that is capable of changing the stiffness as a function of device displacement, is developed. By engaging the adaptive negative stiffness device (NSD) at an appropriate displacement (simulated yield displacement), which is well below the actual yield displacement of the structural system, a composite structure-device assembly, behaves like a yielding structure is achieved. The NSD has a re-centering mechanism thereby avoids permanent deformation in the composite structure-device assembly unless, the main structure itself yields. Essentially, a yielding-structure is "mimicked" without any or minimal permanent deformation or yielding in the main structure. Due to the addition of NSD the stiffness of the combined structural system is reduced substantially beyond simulated yield point resulting in increased structural deformations. Addition of a nonlinear passive damper reduces and controls these deformations without any considerable increase in the base shear.

The proposed NSD does not rely on structural-response feedback and external power supply-unlike previously reported pseudo-negative stiffness devices that do depend on active control-hence, is passive, and exhibits adaptive negative stiffness behavior by possessing predesigned variations of stiffness as a function of structural displacement amplitude. The system is called adaptive because it is predesigned to undergo a desired adaptive stiffness changes at various displacement amplitudes. The adaptive negative stiffness system (ANSS) proposed in this study consists of two elements: 1) a negative stiffness device (NSD) and 2) a passive damper (PD). Upon the addition of NSD to the structural system, predesigned reductions of stiffness occur in the combined system or "apparent softening and weakening" occurs; however, it is important to note that the stiffness and the strength of the main structural system remains unchanged in this study (hence, "apparent")-unlike the concept of weakening proposed earlier wherein the strength and implicitly stiffness of the main structural system itself are reduced. In summary, the main structural system suffers less accelerations, less displacements and less base shear or force at the foundation level, while the ANSS "absorbs" them.

Through numerical simulations it has been found that the concept of ANSS/NSD is very effective in elastic and inelastic structural systems. The effectiveness and the superior performance of the ANSS/NSD as compared to a structural system with supplemental passive dampers when subjected periodic and random input ground motions is studied. The corresponding development of an actual NSD device and experimental/analytical study is in progress in the NEESR-Adapt-Struct (www.ruf.rice.edu/~dsg/) project. The results of the experimental/analytical study will be reported upon its completion in the near future.