This work analyses the behaviour of in-plane timber floors which are differently refurbished. The task is to ascertain the stiffness of the different solutions and to study the influence on the global behaviour of the building. The first type analysed is a floor with simple boards to which different reinforcing techniques have been applied. These are double boards, steel plates, diagonally set FRP strips, glued plywood panels and concrete slabs. For each of these types of reinforcement experimental displacement control tests were carried out. They were monotonic and cyclic tests of specimens with dimensions 2x1 m and 5x4 m, with and without perimeter tie-beams. The same tests were numerically reproduced and a numeric model of simple implementation was created able to simulate non-linear behaviour of floor and tie-beam. On the end, the floor model was used in order to analyse a traditional building. Experimental analysis and numeric modelling confirmed the need to guarantee efficient floor-masonry connections and showed the notable contribution offered by perimeter tie-beam in terms of in-plane floor stiffness. The comparison between different techniques of reinforcement showed the inadequacy of simple boards to stand up to seismic action.

Whenever two bodies are in contact due to a normal load and one is sliding against the other, a tangential force arises, as opposed to the motion. This force is called friction force and involves different mechanisms, such as asperity interactions, energy dissipation, chemical and physical alterations of the surface topography and wear. The friction coefficient is defined as the ratio between the friction force and the applied normal load. Despite this apparently simple definition, friction appears to be a very complex phenomenon, which also involves several aspects at both the micro- and nano-scale, including adhesion and phase transformation. Moreover, it plays a key role in a variety of systems, and must be either enhanced (e.g. for locomotion) or minimized (e.g. in bearings), depending on the application. Considering friction as a multiscale problem, an analytical model has been proposed, starting from the literature, to describe friction in the presence of anisotropy, adhesion and wear between surfaces with hierarchical structures, e.g. self-similar. This model has been implemented in a MATLAB code for the design of the tribological properties of hierarchical surfaces and has been applied to study the ice friction, comparing analytical predictions with experimental tests. Furthermore, particular isotropic or anisotropic surface morphologies (e.g., microholes of different shapes and sizes) has been investigated for their influence to the static and dynamic friction coefficients with respect to a flat counterpart. In particular, it has been proved that the presence of grooves on surfaces could decrease the friction coefficients and thus reduce wear and energy dissipation. Experimental tests were performed with a setup realized ad hoc and the results were compared with full numerical simulations. If patterned surfaces showed that they can reduce sliding friction, other applications could require an increase in energy dissipation, e.g. to enhance the toughness of microfibers. Specifically, the applied method consists of introducing sliding frictional elements (sliding knots) in biological (silkworm silk, natural or degummed) and synthetic fibres, reproducing the concept of molecules, where the sacrificial bonds provide higher toughness to the molecular backbone, with a hidden length, which occurs after their breakage. A variety of slip knot topologies with different unfastening mechanisms have been investigated, including even complex knots usually adopted in the textile industry. The knots were made by manipulation of fibres with tweezers and the resulting knotted fibres were characterized through nanotensile tests to obtain their stress-strain curve until failure. The presence of sliding knots strongly increases the dissipated energy per unit mass, without compromising the structural integrity of the fibre itself.

This thesis focuses on fatigue problem on riveted structures and the correlated experimental tests. The term “fatigue” denotes the cracking of metals under repeated loading. The technique of riveting structures is obsolete due to the low level of standardization in the construction process. Knowledge concerning riveted structure’s ability to withstand fatigue has not been investigated to the same extent as for modern structures assembled by welding. Nevertheless, many riveted structures are still in service after over 100 years. Clamping force originates when the hot rivet is placed into the hole of the plates and the rivet shorten in length due to cooling. In Europe, a large number of railway bridges are riveted. Moreover, all over the world, the rivet assembly technique has been largely used for different types of buildings. The riveted structures are subjected, evidently, to fatigue degradation as all the other steel structures. The riveted joint complexity and the non-uniform realization lead to a structure-specific consideration. In Europe, current regulations recognise only one fatigue class for riveted structures neglecting the clamping force effect. This approach is characterized by an elevated standard deviation for the assessed results. This research aims to improve the riveted structures fatigue comprehension and to propose appropriate tests. Some elements have been extracted from a dismantled railway bridge located near San Stino di Livenza (Venice). An innovative clamping test (TCT) has been conceived: applying a torsional moment to the rivets and evaluating the sliding friction, the clamping force has been estimated. Contrary to traditional clamping tests, this setup, at an affordable price, provides good precision. Moreover, the test is in-site and does not provoke damages to the examined structure. Fem models of rail bearer have been realized taking into account rivets, holes and multilayer plates section inertia. A specific laboratory set up has been designed with the intention to calibrate the fem models. Frictions and clamping force have been taken into account in the rail bearer models. There is a close correspondence between the non-linear models and the experimental tests. The clamping assumptions, derived from the TCT, have been verified. A full scale fatigue test has been prepared and the first cycles have been monitored. Full scale fatigue tests are sophisticated and involve many parameters. For this test, a specific metallic frame has been designed and realized using advanced fatigue models. An advanced analysis (hot spot method) has been carried out, for the frame, in order to evaluate the stress into the welds. This frame will be used by the DICEA laboratory to test, principally, specimens extracted from bridges.

The interaction between frames and infills in case of earthquake is a topic of great significance in structural engineering. Infills, often considered non-structural elements, in fact behave as they were. Sometimes they make low engineering buildings behave better than one would expect, other times they are responsible of rather bad seismic performances even in recently built constructions. Infills, often stiff and brittle if constructed with common fired bricks and strong mortar, are able to change to a great extent the seismic structural response, invalidating many of the basic design assumptions. Nevertheless, it is not possible to think of buildings without infills, due to the requirement of controlling energy consumption for residential climate control. To overcome this problem, the possibility of assembly ductile infills was investigated in the current thesis. The basic idea was to use weakness as the main tool, pursued both through the choice of materials and the proposal of a new, simple though effective building technique. Particular attention was devoted to adobe (earthen bricks) infills, specifically chosen in a structural perspective to take advantage of the intrinsic weakness, often unacceptable for other applications. Sustainability and thermo-hygrometric performances are considered important characteristics, even though background themes in comparison with weakness, according to the main perspective of this research work. The aim of the research was to evaluate the possibility of forcing a predefined damage pattern, consistent with reuse requirements, by means of ductile mechanisms. The modus operandi was to lay weak (horizontal) surfaces in the panels, breaking their continuity to allow a frictional, stable and ductile sliding to take place. The idea was investigated both numerically and experimentally. During a first phase, shear behavior of mud mortar joints was tested. Then, four full scale panels, confined by a steel frame, were subjected to lateral cyclic load. The four tests differed both in building technique, traditional or with internal partitions, and in material, hollow clay bricks or adobes. The experimental work was then extended numerically by means of FE analysis (through two modeling techniques) to different geometrical and mechanical situations. Attention was focused on the role of partitions in the structural response and on the possibility of capturing global behavior through a local calibration (on small assemblies) of model parameters. As a conclusion, the use of partitioned adobe infills was proposed in association with ductile timber frames because of the compatibility of materials. The solution may provide adequate stiffness and energy dissipation in case of earthquake. Moreover, this kind of buildings usually suffer for a lack of summer thermal insulation: the addition of mud infilling would improve such a performance.

Lo scopo di questo lavoro è stato di sviluppare un programma ad elementi finiti tridimensionale, scritto in Fortran 90, in grado di rappresentare il comportamento di interfaccia di travi in calcestruzzo rinforzate da fogli di FRP, sia in fase elastica, che durante il fenomeno di delaminazione utilizzando la teoria del danno. Si è inoltre indagato sul comportamento del collegamento FRP-cls a causa di carichi a lunga durata, mediante una sperimentazione e attraverso simulazioni numeriche. Il modello FEM si basa sulla teoria del contatto [1] legata al danneggiamento per poter simulare tutto il processo di delaminazione partendo dall’innesco dovuto al superamento del limite di resistenza del collegamento, fino al distacco completo del rinforzo. La modellazione tridimensionale ha consentito ancora, di poter ottenere lo stato tensionale su tutta l’interfaccia, riuscendo quindi a rappresentare il comportamento “di bordo” bidimensionale delle tensioni di taglio riscontrato da numerosi autori in fase di sperimentazione[2],[3], nonché gli effetti di peeling. I test a lungo termine atti a caratterizzare i parametri viscosi dei materiali, sono stati effettuati attraverso prove a trazioni dei materiali costituenti le matrici polimeriche dell’FRP (resine epossidiche), e attraverso prove su travetti rinforzati da fogli di fibre in carbonio monodirezionali, soggetti a carichi concentrati. Numericamente i fenomeni differiti sono stati studiati implementando materiali di tipo elastoviscosi sia per il calcestruzzo, utilizzando il modello B3 proposto da Z. Bazant [4], che per l’FRP impiegando un modello micromeccanico proposto in [5]. BIBLIOGRAFIA [1] P. Wrigger, (2002), “Computational Contact Mechanics”, Springer. [2] C. Pellegrino, D. Tinazzi, C. Modena, (2008), “Experimental study on Bond Behavior between Concrete and FRP reinforcement”, J. Compos. for Constr.,vol 12(2), 180-189. [3] K. V. Subramaiam, C. Carloni, L. Nobile, (2007), “Width effect in the interface fracture during shear debonding of FRP sheets from concrete”, Eng. Frac. Mech., vol. 74, 578-594. [4] Z. Bazant and S. Baweja, Creep and shrinkage prediction model for analysis and design of concrete structures: Model B3, in Adam Neville Symposium: Creep and Shrinkage – Structural Design Effects, ACI SP-194, A. Al-Manaseer Ed., Am. Concrete Institute, Farmington Hills, Michigan, 1–83, 2000. [5] L. Ascione, V.P. Berardi, G. Mancusi, (2003), “Il Comportamento a lungo termine sotto carichi statici di travi in c.a. placcate con lamine di FRP”, XXXII Congresso AIAS, Salerno. [6] S. T. Smith, J. G. Teng, (2001), “Interfacial stress in plated beams”, Eng. Struc., vol. 23, 857-871.

Localized deformation patterns are experimentally observed to prelude failure in many ductile materials (such as metal, alloy, granular material and plastic) and in quasi–brittle materials (such as concrete and rock). Moreover, the presence of a second phase in composites may promote failure due to stress concentrations at the inclusion boundaries. In order to investigate shear bands nucleation, propagation and interaction with a second phase or a defect, analytical solutions for an infinite nonlinear elastic solid subject to a uniform far–field deformation increment are obtained for the following types of inclusion: i) A crack, revealing features related to the interaction between shear bands and crack tip fields. This solution is also fundamental to the understanding of the shear band problem; ii) A rigid line inclusion, the so-called ‘stiffener’, showing the emergence of shear bands at the stiffener tips in highly deformed ductile materials. For null prestress the solution is shown to match correctly with photoelastic experiments and to predict the fracture patterns for a brittle material containing a stiffener; iii) A pre–existing shear band, showing that the deformation is highly focussed and aligned coaxial to the shear band and the energy release rate to blow up to infinity, for incremental loading occurring when the prestress approaches the elliptic boundary, so that the propagation becomes ‘unrestrainable’. All these analytical results substantiate the experimental observations that shear bands emerge at the inclusion tips and they are preferential near-failure deformation modes.

When hybrid simulation (HS) with substructures is employed for assessing the seismic behavior of a large complex structure, it is unrealistic to test all the components that may exhibit strong nonlinearity. Hence, the accuracy of the numerical substructure (NS) faces an increased challenge. To this end, this paper will emphasize on improving the accuracy of the NS in hybrid simulation based on the model updating approach. Most hybrid simulations with model updating (UHS) focus on updating the parameters of the component constitutive model (story shear model) leading to large modeling errors and the unknown detail responses. Moreover, the most extensively used component constitutive models, such as the Bouc-Wen model, are the models in a narrow sense because they are different for various RC members when the size of the component, boundary conditions, axial compression ratio, and the volumetric stirrup ratio are different from one component to the other. Thus, numerous parameters are needed to describe the displacement-force relations of different components, which leads to the huge computational burden. With this respect, this paper proposes a novel hybrid simulation approach based on identifying and updating the parameters of the material constitutive model. The main work and results are concluded as follows, 1. The unified constitutive model of unconfined and confined concrete is derived from the existing uniaxial concrete constitutive models by introducing the volumetric stirrup ratio. 2. To solve the problem that the relation of the measurements (force of the specimen of a RC member) and the identified parameters (concrete constitutive parameters) are difficult to analytical expressed, an OpenSees embedded unscented Kalman filter is proposed for parameter identification. To this end, several parts of the OpenSees source codes are developed and modified. 3. The proposed identification method and hybrid simulation based on updating the concrete constitutive parameters are respectively validated through a monotonic loading test on a RC column and a UHS on a RC frame. The results show that the convergence values of each parameter under various experimental cases are close to each other with a small variance, which indicates that the proposed identification method is robust and reliable. Comparing to the standard HS, the accuracy of the NS, hence the UHS, is significantly improved. 4. Apply the proposed UHS to a RC continuous rigid bridge with tall thin-walled piers. It concludes that the performance of the identification method is still quite good. Contrasting to the simpler structure, the improved accuracy of a large complex structure is even greater. Moreover, the accuracy of the NS can be greatly increased even though the model error is increased. By tuning the initial values of constitutive parameters, the negative influence of model error is decreased to further improve the precision of the NS. By observing the specimen, it is found that the thin-walled hollow section specimen is a bending-shear type damage mode and finally damaged due to the tensile rupture of the stirrup.

The integral abutment bridge (IAB) constituted by the superstructure and the substructure can achieve a composite action responding as a single structural unit by eliminating or reducing expansion joints and bearings. Accordingly, the construction and maintenance costs can be reduced. Therefore, the IAB concept has recently become a topic of remarkable interest among bridge engineers, not only for newly built bridges but also during refurbishment processes. The research topic concerns the retrofit of existing bridges with the IAB concept. In order to investigate the retrofitting technique with the IAB concept, the literature survey on the practical applications of this approach in worldwide was carried out firstly, including retrofitting motivations, detailed processes and structural performance after retrofitting. Besides, another literature review on the critical issues of analysis on the IAB, such as soil-structure interactions, modelling approaches and plastic hinge simulations, was conducted in order to find out the most suitable method in modelling. The case study of a simply supported prestressed concrete bridge (named Viadotto Serrone) with three spans constructed in 1972 was analyzed, which has some durability problems nowadays. The finite element model was built, involving soil-structure interactions, non-linear behaviors and retrofitting processes. The original and updated Italian design codes are compared through static analysis and seismic analysis. Another investigation was conducted to prove the necessity of considering soil-structure interactions in the IAB. Based on the appropriate finite element model, a large number of static sensitive analyses were carried out, taking thermal actions; bridge types; soil conditions and substructure heights as parameters. Through analysing the responses of girders, piers, abutment stems and piles, some important factors and the corresponding influence were found, which could be adopted to guide the retrofitting technique with the IAB concept. Then, the verification was conducted in order to check if the existing sections could be reused without any changes and point out the most critical components, which need to be repaired or replaced. Moreover, the dynamic performance of bridge before and after retrofitting was investigated preliminarily through modal analysis and response spectrum analysis.

In the last decades, powder technology has become one of the most important technological processes for the production of metallic and ceramics components; free sintering, hot isostatic pressing and hot forging are different ways to realize a key-phase in which the primary mechanical properties of the final material are obtained. A theory of sintering is necessary in order to be able to predict the final structure of a body undergoing such a kind of process. In this respect, it is crucial to be able to follow the evolution of the mechanical properties of the material (determined by this structure) during sintering and to get the final features of the compound at the end of this process. In this thesis, the influence of the pressure called “sintering stress” or “Laplace pressure” produced by the gas employed during the process and which gets trapped into the pores is analyzed. This is done for pre-compacted (micro/nano)-powdered axially-symmetric samples undergoing (i) isostatic pressing (also covering the case of free sintering), (ii) "free" forging (i.e. axial compressive load acting at the top and bottom faces of the specimens, with no lateral confinement) and (iii) constrained forging (i.e. transverse compression of the samples in a rigid die). Such cases are among the ones suggested in Olevsky, E.A., Molinari, A., “Kinetics and stability in compressive and tensile loading of porous bodies”. The role of the Laplace pressure in all of the mentioned cases is twofold. First of all, such a pressure influences the evolution of the porosity and, for instance, its residual value for a given time duration of the process. It is worth emphasizing that threshold pressures below which the sintering stress is actually not negligible are determined in this thesis; the duration of the process is indeed heavily affected by such a stress. In turn, such a duration would be underestimated otherwise. Furthermore, industrial processes often entail loading pressures lower than the thresholds mentioned above, especially of "small" grain sizes. The second aspect is based on a common feature exhibited by the two modes mentioned above: the loading parameter may be tuned in such a way that, at some stage of the sintering process, its value may equate the Laplace pressure, leading to a constant value of the porosity. Whenever this is the case, for (i) there exists a whole range of the loading parameter for which the process is actually unstable. Henceforth, in order to have stability of sintering either the loading parameter must be high enough with respect to the Laplace pressure or zero, leading to (stable) free sintering. For (ii), the stability analysis shows that the results obtained by using the different models for the shear and bulk moduli do not agree for a restricted range of external load. This is of course an intrinsic pathology of his specific loading mode. Moreover, it is worth noting that large strains occur in such a mode. Thus, both the stress and the (infinitesimal) strain employed in this analysis should be replaced by appropriate (possibly work-conjugate) choices of the stress and strain measures, although this goes beyond the aim of this work. For (iii), a stability analysis allows us to conclude that such a value represents a critical threshold, below which the sintering process cannot proceed. In the second part of the present work, the mechanical behavior of sintered specimens are investigated. Such a behavior is strongly influenced by the stress state at the end of the process, which depends on the final value of the interstitial pressure and of the loading mode used during the process. For the sake of simplicity, only the two "realistic" cases of isostatic pressing (also covering free sintering) and constrained forging are considered. For such components, isostatic pressing may induce isotropy, whereas constrained forging processes may enforce a transverse isotropic behavior in the direction of forging. Although for prestresses isotropic material the explicit constitutive law is given by Man in “Hartig's law and linear elasticity with initial stress”, the analog for the case of transversely isotropic material is deduced here, for the first time, through a method, suggested by Weiyi , “Derivation of the general form of elasticity tensor of the transverse isotropic material by tensor derivate”, based upon the partial differentiation of the strain energy with respect to both the strain tensor and the residual stress. Finally, the residual stress tensor for specimens sintered through (i) and (iii) is obtained and the correspondent stress response is deduced. Equivalent material constants (two constant in the case of isotropy, five in the case of transverse isotropy) arising in the presence of prestress may be introduced; such constants take the place of the classical material moduli characterizing the response in the absence of residual stresses. Finally, an experimental procedure to determine the values of such constants is proposed.

The research hereinafter describes aims at mechanically characterize the behavior of timber framed buildings, with particular focus on their behaviour in seismic zones. An extensive experimental campaign divided into three phases has been completed in order to achieve this objective. In the first phase tests on the connection between sheating panels and timber studs were conducted. Subsequently the behavior of the connections used for anchoring the walls to the foundation were investigated. In the second stage the full scale timber framed walls were subjected to tests. The walls were realized using different materials and different type of connections to the ground. The results have allowed the study of the individual structural components which constitute an entire building, from the single connector to the entire wall. In order to understand the behavior of the components themselves in a real building, and thus their interaction with each other in case of an earthquake, in the third step a shaking table test of a three-story building has been done. All tests have permitted to collect a wealth of data with which to populate a database to be used both to understand in detail the structural behavior of the timber framed building system and in also to validate the proposed formulations. From the analytical point of view, an equation able to provide the horizontal displacement of a wall subject to a horizontal force has been developed. This equation was then successfully validated by comparing the results with those obtained from laboratory tests. For the execution of the tests have been created procedures and set-up ad hoc, that could be used also in the future to carry out similar tests. An important work has been done to achieve a configuration suitable to test full scale timber framed walls through well controlling the boundary conditions.