Mass damping-based solutions for reducing the seismic response of new and existing buildings: analytical methods and optimization criteria

In the recent years, the need of improving safety standards for both existingand new building structures against earthquake and wind excitations hascreated a growing interest in conceiving new energy absorption or dissipation mechanisms, capable to control or redirect, in active or passive way, thedynamic response of structures. In this perspective, mass-damping systemsare employed with the aim to control and reduce excessive vibrations in civilstructures. Inspired by other engineering fields, the concept of the tuned massdamper (TMD) has been applied to tall building applications for the reductionof lateral deformation caused by wind loads since 1976, with the first examplebeing the John Hancock Tower in Boston, USA. Classical TMDs are conceivedas simple mass-spring-damper systems attached to the so-called primarystructure to control the response. It should be pointed out that conventionalTMDs which have an absorber mass that’s much smaller than the primarybuilding mass (in most cases, less than 1%) are quite successful at reducingthe dynamic response of buildings where wind excitation is prevalent. However, two main concerns arise for the case of seismic input: the TMD’s highsensitivity to the design parameters and the impact of the earthquake frequency content on the system’s efficiency. To solve these issues and improvethe robustness of the overall structure, non-conventional TMD systems withlarge mass ratios should be employed. In this light, two engineering solutionsbased on the basic idea of utilizing a part of the building mass as a giantabsorber have been proposed in literature: the intermediate isolation system(IIS) and the mega-subcontrol system (MSCS). The IIS is realized by placingthe isolation system at an intermediate level, thus dividing the building intothree portions: a lower structure, an isolation layer, an upper structure. Interms of dynamics, this system combines both the effects of isolation andmass damping. On the one hand the isolation interface acts as a filter for theinertial forces rising to the upper structure, on the other hand, the relativedisplacement between the structural portions leads to a reduction of the globalseismic response. Moreover, in the MSCS, the building is divided in a mainsystem (mega frame) and several secondary systems (sub-configurations),disconnected from the primary structure by isolation layers. The reduction ofthe global seismic response is achieved by the mass damping effect exerted on the mega frame.Within this framework, after introducing the mathematical fundamentals forapproaching the differential problem governing the dynamic response ofn-DOF structural systems, the present Thesis investigates how to properlyidentify geometrical and mechanical parameters in order to formally set theoptimization of tuned mass dampers considering both the main visco-elasticconfigurations, i.e. the Kelvin-Voigt and the Standard Linear Solid models,and thus deriving general criteria for designing such systems. In order toassess the analytical and numerical optimization methods previously defined,ad hoc designed 3D-printed structures modelling a 2-DOF TMD system arealso realized in scale and tested under several dynamic actions, highlightingsome asymptotic behaviours and providing insights into cases of presence ofnonlinear elastic elements.Finally, the analytical and numerical optimization criteria defined for simplified 2-DOF TMD models have been first adopted for the preliminary designphase of some civil engineering structures and then validated, through finiteelement approaches, to applications involving the design of new tall buildings, through MSCS and IIS configurations. Furthermore, by exploiting themass-damping principles, a novel strategy for seismically retrofitting existingmasonry buildings is also proposed and suggested first for a wide-scale application of the architectural heritage of a historical city and then applied to thedesign of the interventions on an exemplar case-study building.