Throughout the past decades, seismic isolation of structures has been studied rigorously as an approach to reduce seismic demand and mitigate structural damage. Research in this field has progressed significantly, starting from the use of simple elastomeric bearings for the decoupling of the superstructure from the base, to more complex devices that incorporate the use of an additional oscillating mass and negative stiffness elements. Characteristic examples of these devices are the Tuned Mass Damper (TMD) and the Quasi-Zero oscillators (QZSs). The KDamper is a novel passive vibration absorption concept, based essentially on the optimal combination of appropriate stiffness and mass elements, which include a negative stiffness element. The concept can be implemented using a system of prestressed elements in order to obtain the negative stiffness effects and might prove to achieve a significant advantage for seismic retrofitting.In this paper, the extended KDamper system is placed at the ground level and the soilstructure interaction (SSI) between the building foundations and superstructure is utilized as a means to effectively transfer the dynamic forces of the KDamper to the building and to distribute the required displacements between the structural elements and foundation, thus leading to absorption of the vibration energy. The configuration of the KDamper properties and tuning of its stiffness elements are studied herein. The effectiveness of the KDamper is subsequently investigated in view of the performance of a 4-storey building. A series of artificial accelerograms and real earthquake time histories are used as excitation. Results indicate the beneficial role of SSI on the reduction of the spectral acceleration and displacements, hence placing the concept as a compelling alternative to existing seismic protection technologies.

In this study, a negative stiffness-based passive vibration absorber is developed and implemented as a seismic retrofitting measure for typical reinforced concrete (RC) residential buildings. The device, namely, the extended KDamper for retrofitting (EKD-R), is introduced at the base of the structure, between the foundation level and the first story of the building. The design of the EKD-R device and the selection of its properties are undertaken by incorporating a harmony search (HS) algorithm that provides optimized parameters for the mechanism, following constraints and limitations imposed by the examined structural system. Nonlinearities due to the plastic behavior of the structural members and soil-structure interaction (SSI) effects are modeled and taken into consideration during the process. Subsequently, a realistic case study of a benchmark three-story RC building is examined, and the performance of the EKD-R system is assessed. The building superstructure is designed according to Eurocodes. The structure-foundation system, along with the EKD-R, is explicitly modeled using finite elements (FE) that may realistically capture structural nonlinearities and SSI effects. The HS algorithm is employed, and optimized EKD-R components are obtained and implemented in the benchmark structure. Finally, a series of recorded real ground motions are selected, and nonlinear time-history dynamic analyses are conducted aiming to assess the behavior of the controlled system. Results indicate the beneficial role of the novel dynamic absorber, hence rendering the concept a compelling seismic retrofitting technology.

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