Semi-Active Structural Control for the Seismic Protection of Cultural Heritage

A number of natural hazards, such as earthquakes, flood, landslides, wildfires and cyclones can threaten the integrity of the Cultural Heritage with potentially devastating effects. The reduction of the seismic vulnerability of the cultural legacy constitutes a question of utmost importance especially in countries where vast Cultural Heritage combines with a medium/high seismic risk, such in Italy. From the second half of the 20th century the scientific community conceived a series of resolutions and recommendations to the aim of mitigating the seismic vulnerability of the Cultural Heritage. For many years most efforts focused on the protection of historical buildings. Only recently non-structural objects with recognised historical, artistic or expressive significance started to gain prominence in earthquake engineering studies. The damage suffered by these artworks can either derive from their direct exposure to earthquakes (inherent vulnerability) or it can be a consequence of the vulnerability of their container (indirect vulnerability). Within this framework the present thesis investigates the markedly non-linear seismic dynamic behaviour of art objects in 2D and proposes a novel strategy to reduce the inherent vulnerability to overturning. This novel mitigation strategy uses a bang-bang control to regulate the adjustable stiffness of a semi-active anchorage. More specifically, feedback and feedback-feedforward strategies are proposed and investigated in order to set in real-time the stiffness of the anchorage. Numerical simulations under different excitation conditions were conducted to evaluate and compare the effectiveness of the proposed control algorithms. The simulations also assess the robustness of the algorithms with respect to noises and timing issues. Art objects controlled by the proposed control strategies present a higher stability and a lower vulnerability compared to uncontrolled art objects. The topics summarised above are developed in the thesis in the following order. Chapter 1 introduces the reader to the seismic protection of art objects. The sources of both inherent and indirect vulnerability are described together with current methodologies for their evaluation. Chapter 2 presents an extensive review on the rocking motion of rigid bodies starting from the rocking model proposed by Housner in 1963. Chapter 3 reports on the seismic retrofitting strategies currently in use to reduce the overturning vulnerability of art objects. Limitations of the existing strategies are also pointed out. Chapter 4 describes a novel semi-active control system that reduces the inherent vulnerability of art objects to overturning. This chapter also describes the analytical model and the Matlab® code of the rocking motion, the two control strategies investigated to stabilize art objects and two performance indices to evaluate the benefit of the proposed control are defined. Possible solutions are examined for the practical realization of anchorages with adjustable stiffness. Chapter 5 investigates numerically the performances afforded by the two control strategies described in Chapter 4. In particular, the efficacy of the proposed control is validated with respect to synthetic ground motions. Chapter 6 extends the validation of the proposed control to real seismic accelerograms. To this aim, 100 registrations with diverse spectral characteristics were selected. Chapter 7 comments on the main results of the thesis, draws conclusions and presents some directions for future research.