Industrial process plants and power plants are crucial components of modern infrastructures, playing a pivotal role in sustaining communities through the provision of essential goods and services. These facilities contain critical equipment and structures vulnerable to damage or failure when exposed to seismic excitations without protection measures. Robust seismic protection ensures the preservation of structural integrity and functionality, guaranteeing the uninterrupted delivery of vital services like electricity generation, water treatment, and manufacturing. Consequently, seismic protection measures for these plants enhance overall community resilience and safety, mitigating potential disruptions and harm in the face of earthquakes. Based on these insights, a novel seismic protection system, known as metafoundation (MF), is revisited and enhanced. Geared towards offering cost-effective, modular, and multidirectional protection to critical infrastructure components, the investigation delves into the exploration and implementation of novel nonlinearities and mechanisms to finite locally resonant MFs in order to obtain further improvements. The thesis commences by elucidating the principles of periodic lattices and lattice-based acoustic metamaterials, the foundational concept of finite locally resonant MFs, using both analytical and numerical met-hodologies, followed by some experimental validation. Theoretical exploration encompasses one-dimensional linear and nonlinear metamaterials featuring local resonance properties, via resonators. By incorporating nonlinear mechanisms, such as the Bouc-Wen and Duffing oscillators, the study delves into unique nonlinear wave dynamics and evolving dispersion characteristics. Notably, the focus intensifies on bistable Duffing oscillators as the primary cell spring, and the reliability of numerical simulations is confirmed through experimental validation. Transitioning from theoretical frameworks to practical applications, the finite lattices has been analysed. The effectiveness of MFs under seismic loading relies significantly on the seismic input. Consequently, both natural and synthetic three-directional accelerograms — where the former were generated through a physics-based ground motion model — were utilized for performance assessment and, crucially, for optimization purposes. The inherent locally resonant property of MFs necessitates careful tuning of resonator parameters, as mistuning can lead to notable performance degradation. Optimization strategies encompassed frequency domain and time domain approaches for linear and nonlinear MFs, respectively. In the frequency domain, the Power Spectrum Density (PSD) functions of ground motions were considered alongside the transfer function of MF-superstructure coupled systems to quantify responses. The optimization was achieved through solving the multi-variable, multiobjective optimization problem, facilitated by a specialized algorithm based on sensitivity analysis. In the time domain, conversely, optimization focused on energy dissipation through time history analyses. To streamline computational efficiency, experimental design methods and Kriging models were employed. The pursuit of enhanced performance and novelty, required intricate nonlinear mechanisms in conjunction with MFs to be considered. The columns of MF were substituted with bistable ones. In another application instead, to enhance vertical seismic protection, unit cells featuring vertical quasi-zero-stiffness mechanisms were interconnected in series with locally resonant unit cells. Moreover, to improve performance and minimize MFs’ voluminous size —needed for low-frequency attenuation — the incorporation of novel inerters to resonators was considered. The implemented strategies based on 3D modelled MFs, were applied to a storage tank of a process plant and two in-design stage Small Modular Reactor (SMR) buildings. Detailed time history analyses revealed that MFs can effectively meet targeted performance objectives of the coupled systems. This includes achieving performance levels comparable to conventional isolation solutions for horizontal seismic excitation, while also safeguarding the superstructure against vertical actions and resultant rocking motions. Finally, it was shown that MFs offer a viable solution aligned with the primary development objective of SMRs, facilitating modular standardization for deployment in beyond-design earthquake locations without additional resistance of the superstructure.

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Il presente lavoro è dedicato all’analisi semplificata della vulnerabilità sismica delle torri in muratura, in particolare alla definizione della geometria del cinematismo di collasso. Dopo un’analisi preliminare dei danni e dei meccanismi innescati dal sisma sulle torri, e una rassegna dei metodi di analisi presenti in letteratura, si è elaborato un metodo per determinare la geometria del piano di frattura che individua i blocchi di un meccanismo di ribaltamento globale, a partire da semplici considerazioni di equilibrio. Secondo le Norme Tecniche Nazionali, infatti, le torri (campanarie), vengono classificate come uno dei macroelementi in cui vengono schematizzate le chiese, caratterizzato da propri meccanismi di collasso; a differenza tuttavia di altri macroelementi, per le torri, considerate le masse e le altezze in gioco, lievi variazioni nella geometria del meccanismo comportano sensibili modifiche nel moltiplicatore di collasso; è quindi importante una corretta definizione della geometria del cinematismo. Il metodo proposto è stato applicato anche rimuovendo l’ipotesi, tipica nell’analisi limite di strutture murarie, di resistenza a compressione infinita della muratura. Al fine di rendere di immediato utilizzo pratico i risultati, l’andamento della frattura è stato determinato tramite analisi parametriche per diverse configurazioni geometriche a cui facilmente si possono ricondurre le strutture a torre esistenti. Infine, oltre a un confronto con meccanismi reali rilevati a seguito di terremoti avvenuti in passato, si è applicato il metodo proposto alla analisi di vulnerabilità di una torre medievale, la Ghirlandina del Duomo di Modena.

Theoretical modelling of challenging multiscale problems arising in complex (and sometimes bioinspired) solids are presented. Such activities are supported by analytical, numerical and experimental studies. For instance, this is the case for studying the response of hierarchical and nano-composites, nanostructured solid/semi-fluid membranes, polymeric nanocomposites, to electromagnetic, mechanical, thermal, and sometimes biological, electrical, and chemical agents. Such actions are notoriously important for sensors, polymeric films, artificial muscles, cell membranes, metamaterials, hierarchical composite interfaces and other novel class of materials. The main purpose of this project is to make significant advancements in the study of such composites, with a focus on the electromagnetic and mechanical performances of the mentioned structures, with particular regards to novel concept devices for sensing. These latter ones have been studied with different configuration, from 3D colloidal to 2D quasi-hemispherical micro voids elastomeric grating as strain sensors. Exhibited time-rate dependent behavior and structural phenomena induced by the nano/micro-structure and their adaptation to the applied actions, have been explored. Such, and similar, ordered submicroscopic systems undergoing thermal and mechanical stimuli often exhibit an anomalous response. Indeed, they neither follow Fourier’s law for heat transport nor their mechanical time-dependent behavior exhibiting classical hereditariness. Such features are known both for natural and artificial materials, such as bone, lipid membranes, metallic and polymeric “spongy” composites (like foams) and many others. Strong efforts have been made in the last years to scale-up the thermal, mechanical and micro-fluidic properties of such solids, to the extent of understanding their effective bulk and interface features. The analysis of the physical grounds highlighted above has led to findings that allow the describing of those materials’ effective characteristics through their fractional-order response. Fractional-order frameworks have also been employed in analyzing heat transfer to the extent of generalizing the classical Fourier and Cattaneo transport equations and also for studying consolidation phenomenon. Overall, the research outcomes have fulfilled all the research objectives of this thesis thanks to the strong interconnection between several disciplines, ranging from mechanics to physics, from structural health monitoring to chemistry, both from an analytical and numerical point of view to the experimental one.

This thesis focuses on the study of the creep deformations exhibited by concrete structures, with a particular attention to long-span prestressed box girders. During their service life, such structures can experience excessive multidecade deflections mainly due to the creep phenomenon and the large difference in shrinkage between the top and bottom slabs, sometimes causing damages of structural elements and huge economic losses. In order to prevent such consequences, the multidecade deflections of this class of structures need to be carefully predicted; therefore, very refined creep constitutive laws are required for relevant creep analyses. The most widely used creep model for the prediction of the time-dependent behavior of highly creep-sensitive structures is Model B3, which was calibrated through a data bank comprising results coming from different laboratories spread throughout the world. In this thesis, an already existing viscoelastic formulation, conceived for any viscous kernel, is integrated with Model B3 and the resulting finite element scheme is successfully applied to study the long-term behavior of a realistic structure, the Colle Isarco viaduct in Italy. Another contribution to this research work concerns the prediction of multidecade deflections exhibited by concrete structures through a novel creep constitutive law based on variable-order fractional calculus, resulting in an excellent feature with respect to classical creep models. Indeed, the creep deformations obtained through the proposed model are very close to the deformations evaluated by means of Model B3. Moreover, the suggested creep law is characterized by less aging terms than Model B3, with the consequent advantage to exactly derive the relevant relaxation function from the fundamental relationship of linear viscoelasticity. In order to perform creep analyses with the suggested fractional-order law, a numerical integration scheme characterized by a fractional-order viscous kernel is also developed and verified on realistic concrete structures subjected to multiple load histories. To the best of the author's knowledge, this research work presents the first creep constitutive lawavailable in literature that, through fractional operators, explores the time-dependent behavior of aging materials. Furthermore, a suitable numerical integration scheme is introduced and successfully applied to representative concrete structures.

This doctoral thesis presents technical strategies for the rational maintenance of the building heritage directed at the integrated retrofit of social housing stocks. The study comprised the analysis of recovered residential buildings in order to develop new sceneries to adopt in critical situations, leading to the definition of a new experimental practice called “adaptive exoskeleton”. This strategy involves the wrapping of the entire original building with a three-dimensional structural envelope, the exoskeleton, using a construction process able to limit interferences on the use of the building and on the life of its inhabitants. The exoskeleton is an independent frame, carefully designed at the joint-scale to achieve awareness of the constructive sequence and of the optimization of the resources. Dry construction technologies resulted to be the most effective, because based on the principles of lightness and reversibility, and because they allow to realize a structural grid able to satisfy different standards in relation to the changing user conditions. The strategy of the adaptive exoskeleton, which exploits optimized and industrialized components, appears convenient in relation to large-scale interventions on the built heritage and, at the same time, it is architectonically versatile, with many possible options adaptable to different cultural contexts. The structural frame can be adjusted to different dimensions, extensions, typologies and technologies, maintaining the same basic characteristics. Passive dissipative devices realized with shape memory alloys, strategically located as connectors with the existing building, are used in order to reduce the lateral displacements during earthquakes. A key idea is the separation between the long lasting elements of the construction, such as the structural systems, and the parts that can be updated progressively in relation to the requirements of the user or to the technological innovations. This principle is convenient in large-scale campaigns, where it is necessary to create a solid base structure without renouncing to the individualization and the variety of the demand, which stimulates the introduction of architectural components with a shorter use-life. The structural characteristics of this construction and its ability to dissipate the seismic input, were analysed during a research period of twelve months undertaken at the Eindhoven University of Technology (Netherlands) at the unit of Innovative Structural Design of the Built environment department. The verification phase considered two building typologies, due to their high diffusion in Europe: the use of the finite element software SAP2000 required the application of a “frame model” for masonry buildings and of a “strut model” for the concrete frame with masonry infill. The seismic behaviour of the buildings was analysed before any intervention and after the introduction of the adaptive exoskeleton implemented with shape memory alloys-based devices. The experimental phase was also undertaken with reference to San Bartolomeo estate in Brescia, Italy. Summarizing, the research underlined the convenience of applying retrofit processes in opposition with demolitions and reconstructions, above all in terms of social and environmental costs. The adaptive exoskeleton, in particular, provides an integrated and synergic solution because while improving the seismic behaviour of the structure, offers additional space for services and functions, increasing the economic value of the building and improving its energy performances and its architectural characteristics.

The mechanical and structural engineering community are increasingly resorting to the use of periodic metamaterials and metastructures to mitigate high amplitude vibrations; and nonlinearities are also an active area of research because they potentially provide different methods for controlling elastic waves. While the theory of propagation of linear elastic waves seems to be fairly complete and has led to remarkable discoveries in a variety of disciplines, there is still much to investigate about nonlinear waves, both in terms of their dispersion analytical description and their numerical characterization. This thesis mainly relies on the latter aspect and focuses on the analysis of nonlinear metamaterials and metastructures for both the mitigation and control of elastic waves. In particular, the thesis covers four main topics, each associated with a different nonlinearity: i) dispersion curves and mechanical parameters identification of a weakly nonlinear cubic 1D locally resonant metamaterial; ii) manipulation of surface acoustic waves (SAWs) through a postbuckling-based switching mechanism; iii) seismic vibration mitigation of a multiple-degrees-of-freedom (MDoF) system, the so-called metafoundation, by means of hysteretic nonlinear lattices; iv) seismic vibration mitigation of a periodic coupled system pipeline-pipe rack (PPR), by means of a vibro-impact system (VIS). To identify the dispersion curves of a cubic nonlinear 1D locally resonant metamaterial, a simple experimentally-informed reference subsystem (RS) which embodies the unit cell is employed. The system identification relies on the Floquet--Bloch (FB) periodic conditions applied to the RS. Instead, the parametric identification is carried out with a revised application of the subspace identification (SSI) method involving harmonic, non-persistent excitation. It is remarkable that the proposed methodology, despite the linearization caused by the FB boundary conditions, is responsive to the amplitude of the excitation that affects the dispersion curves. The FB theorem, in fact, is often adopted to reduce the computational burden in calculating the dispersion curves of metamaterials. In contrast, the experimental dispersion reconstruction requires multiple velocity measurements by means of laser Doppler vibrometers (LDVs), as for the case of SAWs. To manipulate SAWs, a proof-of-concept experiment was performed for a postbuckling-based mechanical switching mechanism. Precompressed beams are periodically arranged on one face of an elastic plate to manipulate the dispersion of the SAWs propagating as edge waves. By compressing the columns over their Euler critical load, in fact, it is possible to manipulate the surface wave dispersion: the dispersion curve’s dispersive branches, originally caused by the beams in the undeformed configuration, are cleared, and the original path of the group velocity is restored. This concept is introduced analytically and numerically in this thesis, and a novel device is proposed for controlling the SAWs. With regard to the mitigation of seismic waves, this thesis presents the application of two nonlinear dissipative devices to periodic components and structures of industrial facilities. Firstly, a finite locally resonant metafoundation of an MDoF fuel storage tank is equipped with fully nonlinear hysteretic devices to mitigate absolute accelerations and displacements in the low-frequency regime. Secondly, for mitigating the vibrations in PPRs, spatial periodicity and internal damping are combined to obtain an enhancement in the attenuation rate of the system. At the same time, the seismic performance of the PPR is improved by means of an external nonlinear VIS. These investigations show the characterization of the structures’ responses due to the stochastic nature of the input; and for the case of the VIS, a chaotic behavior is sometimes observed and demonstrated. In conclusion, this thesis investigates the nonlinear response of different periodic structures and their potential for wave control and mitigation in various applications. The results of this research contribute to the understanding of the nonlinear behavior of these periodic structures and provide insights into the design, the optimization, and the identification of metamaterials and metastructures performance.

This thesis is mainly focused on the seismic behaviour of cross laminated timber (CLT) buildings. The document can be subdivided into three main sections closely related to each other. In the first part, after a short introduction on the state of the art on timber buildings regarding the constructive and legislative issues, the behaviour of CLT buildings is presented. The research is focused on the study on single shear-walls, on the multi-storey single-walls and on the behaviour of the whole buildings. The analyses are performed in order to assess the ductility level achievable by a CLT building as a result of different choices for the ductility of the connections at the foundation level. In order to estimate the ductility level, a large number of non-linear analyses were performed. This was possible thanks to a Matlab code, specifically developed, which allowed to reduce the computational burden. The results are used to evaluate a reliable set of behaviour factors to be applied in the seismic design of CLT buildings. In the second part of thesis, the low-cyclic fatigue strengths for different typologies of dissipative timber connections are presented. The low-cyclic fatigue strength represents a key-parameter in the assessment of the seismic behaviour of timber connections. In fact, high values of ductility associated with low values of strength degradation ensure a remarkable and reliable energy dissipation without a significant loss of strength. Despite the current version of chapter 8 of Eurocode 8 requires specific values of seismic demand for timber connections in terms of low-cyclic fatigue strength, no specific provision is reported to this regard in the European Standard for the cycling testing of timber connections and assemblage in seismic design (EN 12512). In This Standard the ductility capacity and the impairment of strength are calculated as separate mechanical parameters. For this reason, a proposal of revision of European Standard EN12512 is presented and discussed. The third and last part of the thesis describes an innovative connection for CLT buildings. This innovative connection was originally developed in order to absorb both traction and shear actions. Furthermore, a good performance has been obtained in terms of low-cyclic fatigue strength and ductility, with the aim of conceiving a connection able of satisfy the requirements of the current seismic European Standard. The design of this new connection was an iterative process, starting from some simplified numerical models. After some improvements, it was possible to obtain the expected performance levels. The strength and rigidity of the designed connection were initially obtained through numerical analysis, and then compared with the results of physical tests carried out in the Materials and Structures Testing Laboratory (MSTL), that is a part of the Department of Civil, Environmental and Mechanical Engineering (DICAM) of the University of Trento.

The interest in structural health monitoring (SHM) has grown considerably in the past half century, due to an explosive growth in the availability of new sensors, the development of powerful data analysis techniques, and the increasing number of civil infrastructure that are approaching or exceeding their initial design life. In SHM, we acquire observation on the behavior of a structure to understand its condition state, based on which we decide how to manage it properly. However, this optimistic view of SHM is in contrast with what happen in real life: infrastructure operators are typically skeptical about the capacity of monitoring to support decisions, and instead of following the suggestions provided by SHM, they often act based on their experience or common sense. The reason is that at present it is not fully clear how in practice to make decisions based on monitoring observation. To fill this gap between theory and practice, I propose to consider SHM as a logical process of making decision based on observation consisting of two steps: judgment, in which the condition state of structures is inferred based on SHM data, and decision, in which the optimal action is identified based on a rational and economic principle. From this perspective, a monitoring system should provide information that can improe he managers knoledge on he srcral condiion sae enough to allow them to make better decision on the structure management. Therefore, in designing a monitoring system, the design target must be the accuracy in the knowledge of structural state achieved analyzing the observations provided by it. However, when an engineer designs a monitoring system, the approach is often heuristic, with performance evaluation based on experience or common sense rather than on quantitative analysis. For this reason, I propose a performance-based monitoring system design, which is a quantitative method for the calculation of the expected performance of a monitoring solution a pre-posteriori and for checking it effectiveness in the design phase. It is based on the calculation of the monitoring capacity and the monitoring demand the counterparts of structural capacity and demand in the semi-probabilistic structural design, and like in structural design, the solution is satisfactory if the capacity is equal or better than the demand. The choice in whether to invest a limited budget on a monitoring system or in a retrofit is another critical choice for infrastructure managers: a retrofit work can increase the capacity and the safety of a structure, while sensors do not change the capacity, nor reduce the loads. Recently, the SHM-community has acknowledged that the benefit of installing a monitoring system can be properly quantified using the concept of Value of Information (VoI). A typical assumption in the VoI estimation is that a single decision-maker is in charge for decisions on both the investment in SHM for a structure, and its management based on SHM data. However, this process is usually more complex in the real world, with more individuals involved in the decision chain. Therefore, I formalize a rational method for quantifying the conditional value of information when two different actors are involved in the decision chain: the manager, who operate the structure based on monitoring data; and the owner, who chooses whether to install the monitoring system or not, before having access to these data. The results are particularly interested, showing that under appropriate conditions, the owner may be willing to pay to prevent the manager to use the monitoring system. Application to case studies are presented for all the research contribution presented in this doctoral thesis.

Industrial steel storage racks are pre-engineered lightweight structures commonly used to store goods from supermarkets to big warehouses. These systems are framed structures, usually made of cold-formed steel profiles and characterised by non-standard details. Their performance is quite complex and the prediction of their global response is more difficult than for the traditional steel frames. This difficulty is due to the racks’ main features: the use of cold-formed thin-walled steel sections which are sensitive to different buckling modes, the presence of regular perforation patterns on the uprights, the highly non-linear behaviour of joints, the influence of the structural imperfections and the significant frame sensitivity to second order effects. The behaviour of racks becomes even more complex when seismic or accidental events induce significant horizontal forces acting on the structures. The complexity and variability that characterise racks make it difficult to identify general design solutions. Hence, racks design is traditionally carried out by using the “design by testing” approach, which requires the experimental characterisation of the main structural components, of the joints and the sub-assemblies. The complexity of the racks also affects their numerical modelling, which results in complex analyses that must take into account all the aforementioned features. The work presented in this thesis focuses on the study of a typical steel pallet rack, identified as case study. The research aims to contribute to building up a comprehensive knowledge of the response of both the main rack components and of the whole structure. The main rack components were first individually studied. The behaviour of the uprights, of the base-plate joints and of the beam-to-column joints was experimentally investigated. The experimental data were then taken as reference for the calibration of FE models that enabled exploring each component’s performance. These models were then incorporated into the whole rack model. The response of the uprights was first investigated through stub column tests. The non-negligible interaction between axial force and bending moment of the upright response was then experimentally and numerically analysed to define the M-N domains. In addition, the rules provided by different European standards for the design of isolated members subjected to combined axial load and bending moment were considered and critically compared, identifying the main critical issues of the different design approaches. Although the contribution of joints on the rack global response is of paramount importance, to date, the knowledge is quite limited. In particular, the experimental studies of the behaviour of base-plate joints are still rather modest, especially for the cyclic range. Therefore, an experimental campaign on the rack base-plate joints was carried out: three levels of axial load were considered and the response in both the down-aisle and the cross-aisle direction was investigated under monotonic and cyclic loadings. Similarly, the beam-to-column joint was tested both monotonically and cyclically, taking into account its non-symmetric behaviour. Numerical models for both joint types were developed and validated enabling the characterisation of joints in the monotonic and cyclic range. This in-depth knowledge of the response of individual components facilitated the evaluation of the global rack behaviour. As a final stage of the research, full-scale tests of four-level two-bay racks were performed taking advantage of an innovative full-scale testing set-up and, on the basis of the experimental outcomes, the racks’ global behaviour was numerically investigated. Critical standards issues and needs for future research were further identified.