Dynamic models for ancient heritage structures

Risks to cultural heritage and the related losses should be mitigated before disasters such as earthquakes happen. Risks can be addressed by various means, from raising the cultural attention of authorities to documenting the artistic or historical value of an object. The main contribution of structural engineering to cultural heritage concerns the regular maintenance and monitoring for risk reduction. Risk mitigation of historical buildings, as a part of the more general concept of conservation, involves different disciplines. Teams need to be multidisciplinary and information deriving from historical, metric, stylistic, structural, seismic, geotechnical and physical analysis may contribute to the achievement of an overall comprehension of cultural assets. The synergic action of the characterization and monitoring techniques are essential factors to understand, on one hand the mechanisms and the consequences of degradation and, on the other hand, to provide reliable and well-grounded guidelines for the definition of technical interventions to prevent/ to stop the degradation phenomena, to restore the functionality and the use of the historical building/ artifact, or to predict, mitigate and even control the response to accidental events, including strong motions. In this field, an important role is played not only by the analytical aspects, but also by the development and validation of innovative materials and systems for conservation. International deontological guidelines on conservation of cultural heritage define the structural rehabilitation of heritage structures as the cure of a sick person, hence "the heritage structures require anamnesis, diagnosis, therapy and controls, corresponding respectively to the searches for significant data and information, individuation of the causes of damage and decay, choice of the remedial measures and control of the efficiency of the interventions". Moreover, the same codes state that: "the best therapy is preventive maintenance", which can only be achieved via monitoring of the structure. In this thesis work a few topical issues of the structural modelling, monitoring and assessment of historic masonry buildings were addressed, with particular emphasis on the dynamic testing and identification. The possible connections with other disciplines are analysed and discussed throughout the text. In this framework, the outline of the thesis includes an introductive first chapter in which the context established by the most recent codes and guidelines concerning the architectural heritage conservation is duly reviewed and analysed. The importance of attaining a knowledge of the structure is also discussed. The second chapter sets up the scene, in which it introduces the principal issues of seismic risk and safety assessment of architectural heritage. Firstly, a brief overview is given of the seismic risk and of geological and geotechnical aspects as related to ancient heritage. Successively, the viability of performance-based approaches, for application to the seismic assessment of architectural heritage, is discussed also in the light of a few recent proposals. In this context, the fundamentals of structural health monitoring are also reported. Chapter 3 is intended to stress the importance of modal testing as an effective tool for ancient structures characterisation, so it starts with a state-of-the-art on linear system identification methods with emphasis on output-only techniques. In particular, time domain and joint time-frequency domain identification techniques are introduced and deeply analysed. Model updating is then addressed and its connection with operational modal analysis is underlined. Finally, a few noteworthy examples of linear identification and model updating of architectural heritage structures are reported. Chapter 4 is about the dynamic and seismic behaviour of domes. The coverage focuses on three ideal benchmarks on reconciling geometric survey with dynamic monitoring. The analyses concerned structures with oval shape domes, such as the Sanctuary of Vicoforte, S. Caterina in Casale Monferrato and S. Agostino in L'Aquila. The final products are virtual models which were enabled to predict the linear dynamic response under earthquake excitation. Chapter 5 inspects modelling strategies suited for masonry under intense seismic excitations. The state-of-the-art covers both models for equivalent static analysis and models which operates in dynamics. A model allowing for stiffness degradation, pinching and hysteresis is then proposed, whose formulation admits extensions to multiple degree of freedom systems. The proposal originates from the well-known Bouc-Wen model. Chapter 6 deals with non-linear identification methods. In perspective, also non-linear identification is expected to become a powerful tool in the context of structural and seismic reliability assessment, especially in the light of the increasing levels of knowledge and prediction capabilities which recent standards strive for. Unfortunately, non-linear identification is to date a specialized and challenging matter, and it has been seldom applied to full-scale structures. In this chapter, special emphasis is given to on-line implementations, with several numerical examples showing the potential of non-linear as well as hysteretic system identification. The last chapter presents an experimental application of non-linear identification. A scaled model of a two-span masonry arch bridge has been artificially damaged and monitored at each damage step. A non-linear identification has been performed from shaker tests data. Results of the experimental campaign will be used to corroborate a non-linear and hysteretic model of the bridge endowed with prediction capabilities