VALUTAZIONE DELLA SICUREZZA STRUTTURALE DI GRANDI DIGHE A GRAVITÀ: APPLICAZIONE AD UN CASO STUDIO

Si analizzano i metodi per la valutazione strutturale per dighe a gravità a partire dai primi del '900 ad oggi e si effettua la scelta dei modelli più opportuni per le analisi strutturali. Nel caso studio affrontato si individuano i modelli tramite una procedura di identificazione basata su dati sperimentali. L'esempio studiato consente di effettuare valutazioni sulla scelta del modello da utilizzare nell'analisi strutturale di grandi dighe a gravità

Shedding Light on the Effect of Uncertainties in the Seismic Fragility Analysis of Existing Concrete Dams

The seismic risk assessment of existing concrete gravity dams is of primary importance for our society because of the fundamental role of these infrastructures in the sustainability of a country. The seismic risk assessment of dams is a challenging task due to the lack of case histories, such as gravity dams’ seismic collapses, which hinders the definition of limit states, thus making the application of any conventional safety assessment approach difficult. Numerical models are then fundamental to predict the seismic behaviour of the complex dam-soil-reservoir interacting system, even though uncertainties strongly affect the results. These uncertainties, mainly related to mechanical parameters and variability of the seismic motion, are among the reasons that, so far, prevented the performance-based earthquake engineering approach from being applied to concrete dams. This paper discusses the main issues behind the application of the performance-based earthquake engineering to existing concrete dams, with particular emphasis on the fragility analysis. After a critical review of the most relevant studies on this topic, the analysis of an Italian concrete gravity dam is presented to show the effect of epistemic uncertainties on the calculation of seismic fragility curves. Finally, practical conclusions are derived to guide professionals to the reduction of epistemic uncertainties, and to the definition of reliable numerical models

Die seismische Bewertung bestehender Betonschwerkraftdämme: Quantifizierung und Reduktion der FE-Modellunsicherheit

The implementation of resilience-enhancing strategies on existing concrete gravity dams is a task of primary importance for the society. This aim can be achieved by estimating the risk of concrete dams against multi-hazards and by improving the structural control. Focusing the attention only on the seismic hazard, numerical models assume great importance due to the lack of case studies. However, for the same reason, numerical models are characterised by a high level of uncertainty which must be reduced by exploiting all available information. In this way reliable predictive models of the structural behaviour can be built, thus improving the seismic fragility estimation and the dam control. In this context, the observations recorded by the monitoring systems are a powerful source of information. In this thesis two Bayesian frameworks for Structural Health Monitoring (SHM) of existing concrete gravity dams are proposed. On the one hand, the first proposed framework is defined for static SHM, so the dam displacements are considered as Quantity of Interest (QI). On the other hand, a dynamic SHM framework is defined by assuming the modal characteristics of the system as QI. In this second case an innovative numerical algorithm is proposed to solve the well-known mode matching problem without using the concept of system mode shapes or objective functions. Finally, a procedure based on the Optimal Bayesian Experimental Design is proposed in order to design the devices layout by optimizing the probability of damage detection. In all the three procedures the general Polynomial Chaos Expansion (gPCE) is widely used in order to strongly reduce the computational burden, thus making possible the application of the proposed procedure even without High Performance Computing (HPC). Two real large concrete gravity dams are analysed in order to show the effectiveness of the proposed procedures in the real world. In the first part of the thesis an extended literature review on the fragility assessment of concrete gravity dams and the application of SHM is presented. Afterwards, the statistical tools used for the definition of the proposed procedures are introduced. Finally, before the presentation of SHM frameworks, the main sources of uncertainties in the numerical analysis of concrete gravity dams are discussed in order to quantify their effects on the model outputs.Die Implementation von Strategien zur Erhöhung der Belastbarkeit existierender Schwergewichtsstaumauern ist von großer Bedeutung für eine Gesellschaft. Um dies zu erreichen, muß das Risiko für Betonstaumauern für viele Gefahren eingeschätzt und die Überwachung der Struktur verbesserrt werden. Stehen nur seismische Gefahren im Fokus, werden numerische Modelle sehr wichtig, da es an Fallstudien mangelt. Aus dem gleichen Grunde jedoch werden numerische Modelle durch einen hohen Grad an Unsicherheit charakterisiert, die durch eine Auswertung aller verfügbarer Infromationen reduziert werden muß. Auf diese Art können verläßliche Vorhersagemodelle strukturellen Verhaltens erstellt werden, womit die Einschätzung seismischer Brüchigkeit und die Dammkontrolle verbessert werden. In diesem Zusammenhang sind durch Beobachtungssysteme aufgezeichnete Messungen eine leistungsfähige Informationsquelle. In dieser Doktorarbeit werden zwei Bayes´sche Rahmen für eine strukturelle Zustandskontrolle (SHM) existierender Betonschwergewichtsstaumauern vorgeschlagen. Zum einen für statische strukturelle Zustandskontrolle, indem Dammverschiebungen interessierende Größe betrachtet werden. Zum anderen wird ein dynamischer SHM Rahmen definiert, indem Modelleigenschaften des Systems als interessierende Größe betrachtet werden. Für diesen zweiten Fall wird ein innovativer numerischer Algorithmus vorgeschlagen, um das bekannte Problem beim Anpassen des Modus zu lösen, ohne das Konzept einer Änderung des Systemmodus oder objektiver Funktionen zu nutzen. Schließlich wird ein Verfahren basierend auf optimaler Bayes´scher Versuchsplanung vorgeschlagen, um das Probenlayout zu entwerfen, indem die Wahrscheinlichkeit der Entdeckung von Schäden optimiert wird. Für alle drei Verfahren wird die Polynome Chaos Expansion häufig angewandt, um die Rechneranfordungen stark zu reduzieren, womit die Anwendung dieser vorgeschlagenen Verfahren sogar ohne Hochleistungsrechnen möglich wird. Der erste Teil der Doktorarbeit widmet sich einer umfangreichen Literaturübersicht über die Einschätzung der Brüchigkeit von Betonschwergewichtsdämmen und der Anwendung struktureller Zustandskontrolle. Darauf folgend werden die statistischen Werkzeuge für die Definition der vorgeschlagenen Verfahren eingeführt. Vor der Präsentation des SHM-Rahmens werden schließlich die Hauptquellen der Unsicherheit in der numerischen Analyse von Schwergewichtsdämmen diskutiert, um ihre Effekte auf die Modellergebnisse zu quantifizieren

CONCRETE GRAVITY DAMS FE MODELS PARAMETERS UPDATING USING AMBIENT VIBRATIONS

Most of the dams around the world were designed before the introduction of seismic regula-tions and without concerns about their dynamic behavior. The failure of a large gravity dam might have catastrophic effects putting at risk a large number of human lives, not counting the considerable economic consequences. Since there are no case histories of concrete gravity dams failed after seismic events, numerical models assume great importance for the evalua-tion of the seismic performance of such structures or to control them within a SHM frame-work. Several different sources of uncertainty are involved in numerical models of concrete gravity dams, their effects can be reduced by exploiting all available information about the structure. Ambient vibrations are an important source of information because they can be used to characterize the dynamic behavior of the structure. In this paper, a procedure, de-fined in the Bayesian framework, which allows calibrating the dynamic model parameters us-ing ambient vibration is presented. Ambient vibrations are used to determine the modal characteristics of the system, by applying the Operational Modal Analysis (OMA), which are used in the updating process. The use of meta models based on the general Polynomial Chaos Expansion (gPCE) and a modified version of Markov Chain Monte Carlo (MCMC) allows both considering the SSI in the numerical model of the dam and solving the problem of coher-ence between experimental and numerical modes. Finally, the proposed procedure is applied to the case of an Italian dam showing the applicability to real cases

Sensitivity analysis of frequency-based tie-rod axial load evaluation methods

Tie-rods are often crucial elements for the safety of cultural heritage assets, having been employed both in new constructions and as a retrofitting intervention along the centuries. Several experimental techniques exist to estimate their axial load, with frequency-based ones being the most common. Axial load evaluation depends on several parameters, related to the geometry, mechanical properties and boundary conditions of tie-rods. In engineering practice, uncertainties affect the measurement of such parameters, and may compromise axial load evaluation. The present research focuses on a sensitivity analysis of the eigenfrequency computing model on which many dynamic tie-rod axial load estimation methods are based. Through the application of general Polynomial Chaos Expansion and the calculation of Sobol’ indices, the influence of relevant parameters on eigenfrequencies is investigated. The results, together with a real-life application on tie-rods of the historical buildings in Pisa (Italy), are then used to derive practical recommendations for practitioners and researchers in the field of cultural heritage preservation

FE MODELS FOR THE EVALUATION OF HYDRODYNAMIC PRESSURE ON CONCRETE GRAVITY DAMS DURING EARTHQUAKES

Earthquakes are a significant factor in the design and safety evaluation of dams and, in this context, an accurate prediction of forces acting on the structure is needed. The hydrodynamic action plays a key role in this regard and its accurate evaluation is a major issue. Most of the earthquake analyses of gravity dams in the past have ignored the interaction between dam and reservoir. The equivalent body of hydrodynamic forces, the “added mass” model, derived from the well-known Westergaard theory, is nowadays the most widespread approach still adopted in modern design codes. Nevertheless, taking into account Fluid Structure Interaction (FSI) is sometimes more appropriate, though more complicated. In this work, hydrodynamic pressures on a large Italian gravity dam have been calculated using different modelling approaches: rigid barrier, deformable dam with added masses and FSI. Frequency domain analyses have been carried out both on plane strain models and on more refined 3D models, by applying a horizontal acceleration to the dam base. The comparison among frequency response curves obtained from 2D and 3D models highlights noticeable differences concerning resonance frequencies and peak response values, even with the same modelling approach. This study demonstrates that simplified 2D added-mass models may be over-conservative compared to 3D FSI simulations. Concluding, it can be argued that full 3D coupled analyses should be preferred to simplified 2D ones to estimate the hydrodynamic pressure acting on the upstream face of the dam during earthquakes

Uncertainty Quantification and reduction in the structural analysis of existing concrete gravity dams

The failure of a large gravity dam might have catastrophic effects putting a large number of human lives at risk, not counting the considerable economic consequences. Most of dams are prone to natural hazards, with particular regard to flood and earthquake, so the structural control and the evaluation of the dam fragility assume great importance both to apply early warning procedure and to enhance the resilience of the system. Numerical models assume great importance in order to predict the seismic behaviour of the complex dam-soil-reservoir interacting system, nevertheless they are affected by different sources of uncertainty. The effects of uncertainties can be reduced by exploiting all available information about the structure in order to calibrate finite element models. In this scenario, measurements acquired by a monitoring system and in-situ tests take on a major role as important sources of information. This paper investigates the effect of the uncertainties in the static and dynamic analysis of existing concrete gravity dams by means of two case studies. The general Polynomial Chaos Expansion (gPCE) technique makes possible the uncertainties propagation through numerical models even without High Performance Computing (HPC). This way, the effects of the uncertainties can be quantified in terms of model output. Hybrid-predictive models based on the gPCE allow reducing the computational burden in the solution of the inverse problem and in the structural control as well