###### Abstract  
A case study where a tuned mass damper (TMD) was installed at the top of a five-story reinforced concrete (RC) building is presented. The aim of the study was to investigate the effectiveness of the TMD in changing the structural behavior of medium-rise existing buildings from dissipative to non-dissipative in order to eliminate reparation or demolition costs resulting from damages caused by strong earthquakes. The TMD mass is made by a RC slab lying on flat surface sliders. Horizontal stiffness and damping of the TMD are both provided by lead rubber isolators in the first proposed solution and by low-damping rubber isolators and viscous linear dampers, respectively, in the second. The improvement in the building's structural behavior attained with the installation of the TMD was assessed by considering the flexural demand over capacity ratios of structural elements and the energy dissipated by the TMD. These results are compared with those of the same building retrofitted with a base isolation system. In both of the proposed solutions, TMD remarkably changed the modal behavior of the considered building, improved the flexural verifications, and dissipated most of the input seismic energy. It is therefore demonstrated that a TMD is a valid solution for the retrofit of medium-rise existing buildings. t +
Footnote †: journal: Applied sciences  
building. The most recent design methods either provide formulas coming from closed-form solutions of mechanical models [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24] or consist of genetic algorithms that output the best design parameters [25; 26; 27; 28; 29; 30; 31]. All these methods focus on the optimization of one or more control parameters characterizing the seismic response of the building equipped with the TMD, e.g., the minimization of the highest floor acceleration or displacement or the maximization of energy dissipated by the TMD. The effectiveness of the design method in reducing the internal forces in structural members is fundamental and depends both on the loads considered in the design process and on the chosen control parameter. Actually, an inadequate design due to, for example, neglecting the soil-structure interaction effects on the building in the TMD design [21], can cause the plasticization of some structural members, leading to a decrease in the vibration mode frequencies. As a consequence, the resonance between the TMD and the building's main vibrational modes vanishes, and a detuning of the TMD occurs. Detuning conditions can also arise during the building's lifespan due to variations in the parameters affecting the vibration frequencies. This occurs when an increase in the building mass or a change in the stiffness occurs due to degradation of the rubber layers in seismic isolators. Since a correctly designed TMD can remarkably improve the behavior of the structure under horizontal loads, several applications to different types of structures, such as tall buildings, skyscrapers, bridges and wind turbines [35; 36], have been studied by researchers. However, only a few applications of TMD to existing buildings for their seismic retrofit have been found. These include: the installation of TMD on the top of multi-story buildings sited in Armenia ([PERSON] and [PERSON]. [37]) to reduce the base shear and the top floor displacements; the roof substitution of the city tower in Rieti, Italy, including a TMD, to increase the vibrational periods and limit the displacements' amplitude of the tower ([PERSON]. [38]); the application of a TMD on the top of a 13-story building to reduce structural and non-structural damage ([PERSON]. [39]); the modification of the penthouses of ten buildings in order to make them act as masses of TMDs to reduce the overall seismic demand on structural members ([PERSON] et al. [40]). Such a limited use of TMDs in the retrofit of existing buildings is probably due to the increase in the building's total mass and, thus, gravity loads.