Semi-active control of the rocking motion of monolithic art objects

The seismic behaviour of many art objects and obelisks can be analysed in the context of the seismic response of rigid blocks. Starting from the pioneering works by Housner, a large number of analytical studies of the rigid block dynamics were proposed. In fact, despite its apparent simplicity, the motion of a rigid block involves a number of complex dynamic phenomena such as impacts, sliding, uplift and geometric nonlinearities. While most of the current strategies to avoid toppling consist in preventing rocking motion, in this paper a novel semi-active on–off control strategy for protecting monolithic art objects was investigated. The control procedure under study follows a feedback–feedforward scheme that is realised by switching the stiffness of the anchorages located at the two lower corner of the block between two values. Overturning spectra have been calculated in order to clarify the benefits of applying a semi-active control instead of a passive control strategy. In accordance with similar studies, the numerical investigation took into account the dynamic response of blocks with different slenderness and size subject to one-sine pulse excitation

Dynamic investigation on the Mirandola bell tower in post-earthquake scenarios

After the seismic events of the 20th and 29th of May 2012 in Emilia (Italy), most of the monumental and historic buildings of the area were severely damaged. In a few structures, partial collapse mechanisms were observed (e.g. façade tilting, out-of-plane overturning of panels…). This paper presents the case-study of the bell tower of the Santa Maria Maggiore cathedral, located in Mirandola (Italy). The dynamic response of the structure was evaluated through operational modal analysis using ambient vibrations, a consolidated non-destructive procedure that estimates the dynamic parameters of the bell-tower. The dynamic tests were carried out in pre-intervention and post-intervention conditions in order to understand the sensitivity of dynamic measurements to safety interventions. Furthermore, a comparative study is made with similar cases of undamaged masonry towers up to the 6th mode. Finally, an investigation on the state of connections and of the building itself is carried out via FE model updating

Sensitivity analysis of damaged monumental structures: the example of S. Maria del Suffragio in L'Aquila

In Italy, which accounts for an impressive number of architectural heritage sites, a large part of the territory is subject to seismic risk. Nonetheless, also the two recent examples of the 2009 L'Aquila earthquake and 2012 Emilia earthquake confirmed and highlighted the vulnerability of cultural heritage structures to these types of events. In this paper the church of Santa Maria del Suffragio (Anime Sante) in L'Aquila is used as a benchmark for the experimental validation of a finite element model on the basis of the data gathered by the permanent structural health monitoring system installed on the building by IUAV in 2009. Structural health monitoring techniques have been largely applied to cultural Heritage buildings in recent times, mostly because of their non-destructive nature, and they have proven to be a valid tool in assessing the damage evolution and in characterising the global dynamic behaviour of the structure. In particular, a global sensitivity analysis technique has been applied to a finite element model. The model underwent a model updating procedure on the parameters chosen in the sensitivity analysis. The calibrated model is an invaluable tool in assessing the dynamic behaviour of the structure and may serve for several purpose

" Sensitivity analysis of damaged monumental structures: the example of S. Maria del Suffragio in L'Aquila "

In Italy, which accounts for an impressive number of architectural heritage sites, a large part of the territory is subject to seismic risk. Nonetheless, also the two recent examples of the 2009 L'Aquila earthquake and 2012 Emilia earthquake confirmed and highlighted the vulnerability of cultural heritage structures to these types of events. In this paper the church of Santa Maria del Suffragio (Anime Sante) in L'Aquila is used as a benchmark for the experimental validation of a finite element model on the basis of the data gathered by the permanent structural health monitoring system installed on the building by IUAV in 2009. Structural health monitoring techniques have been largely applied to cultural Heritage buildings in recent times, mostly because of their non-destructive nature, and they have proven to be a valid tool in assessing the damage evolution and in characterising the global dynamic behaviour of the structure. In particular, a global sensitivity analysis technique has been applied to a finite element model. The model underwent a model updating procedure on the parameters chosen in the sensitivity analysis. The calibrated model is an invaluable tool in assessing the dynamic behaviour of the structure and may serve for several purposes

A multi‐objective genetic algorithm strategy for robust optimal sensor placement

The performance of a monitoring system for civil buildings and infrastructures or mechanical systems depends mainly on the position of the deployed sensors. At the current state, this arrangement is chosen through optimal sensor placement (OSP) techniques that consider only the initial conditions of the structure. The effects of the potential damage are usually completely neglected during its design. Consequently, this sensor pattern is not guaranteed to remain optimal during the whole lifetime of the structure, especially for complex masonry buildings in high seismic hazard zones. In this paper, a novel approach based on multi‐objective optimization (MO) and genetic algorithms (GAs) is proposed for a damage scenario driven OSP strategy. The aim is to improve the robustness of the sensor configuration for damage detection after a potentially catastrophic event. The performance of this strategy is tested on the case study of the bell tower of the Santa Maria and San Giovenale Cathedral in Fossano (Italy) and compared to other classic OSP strategies and a standard GA approach with a single objective function

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 researc

Semiactive control of rigid blocks under earthquake excitation

This paper investigates the usefulness of a semiactive control to reduce the overturning vulnerability of a rigid block on a rigid plane under earthquake excitation. The proposed feedback law is used to set the stiffness of restraints placed at the 2 lower corners of the block. The performance of the semiactive control is numerically validated by subjecting the block to 100 recorded accelerograms. Specific simulations are performed to study the effect of different anchorage design parameters on the utility of the control. Finally, the robustness of the proposed control is addressed with respect to typical issues of the real-world implementation

Use of overturning spectra in the performance evaluation of on-off control strategies for rocking objects

Rocking of rigid bodies induced by seismic events triggers a number of complex dynamic phenomena such as impacts, sliding, uplift, which can potentially result in disastrous outcomes. Typical structures that present a significant seismic vulnerability with respect to overturning are water tanks, electrical and hospital equipment, statues and art objects. Several methods have been investigated in the past years to prevent the overturning or damage, such as rigid anchorages or base isolation devices. This paper presents some numerical investigations about a novel on-off adaptive control strategy for rigid blocks subjected to rocking motion. In more detail, control algorithms were specifically conceived to regulate an adjustable stiffness of two restrainers placed at the lower corners of the block. The control’s laws and the anchorage devices exhibited good performance when excited by simple one-sine pulse excitation, as reported by the authors in a previous study. The present work will instead investigate the performance and the robustness of the controlled system with respect to amplitude modulated harmonic excitations

Adaptive control of the rocking motion of art objects

The seismic behaviour of many art objects and obelisks can be analysed in the context of the seismic response of rigid blocks. Starting from the pioneering works by Housner (1963), a large number of analytical studies of the rigid block dynamics were proposed. In fact, despite its apparent simplicity, the motion of a rigid block involves a number of complex dynamic phenomena such as impacts, sliding, uplift and geometric nonlinearities. Methods that prevent the possible overturning were also presented in the past years, especially to protect statues and art objects with respect to seismic events. In the more general context of control, to date a current strategy is represented by base isolation. In this paper a novel adaptive control strategy for protecting monolithic art objects was investigated. The control under study follows a feedback-feedforward scheme that is realised by adjusting the stiffness of anchorages with adaptive stiffness located at the two corners of the block. A numerical comparison was made between the behaviour of the unanchored block, the anchored block with a system having constant stiffness and the anchored block with adaptive stiffness. Finally, numerical investigations were carried out in order to verify the expected efficiency of the proposed control system, and to validate simple control laws

Comparison of semi-active control strategies for rocking objects under pulse and harmonic excitations

Recently, a considerable literature has grown up around the theme of seismic protection of rigid blocks, with a special focus on strategies to reduce the overturning vulnerability due to rocking motion. The present paper investigates a semi-active control method for rocking blocks and compares different strategies for its implementation. In more detail, a feedback control algorithm was developed to adjust the stiffness of the restraints placed at the two lower corners of the block. The utility of the proposed control was quantified through “ad hoc” indices derived from overturning spectra. The performance of a feedback strategy was numerically investigated and specific simulations were performed to quantify the control method degradation when implemented for a real-world application. Finally, the stability of the block controlled with the proposed strategy is compared with the stability of the block whose anchorage is set according to different control strategies