Tesi di Dottorato
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Item Non-conventional approaches to statical and dynamical analysis of geometrically nonlinear structure(2011) La Sala, Gabriella; Casciaro, Raffaele; Lopez, SalvatoreThe purpose of the work presented in this thesis is to implement two innovative approaches in order to analyze elastic instability problems both in statical and in dynamical eld. The proposed formulations have their main feature in the possibility of describing large rotations without the use of rotation matrices in order to overcome complex manipulations required to obtain conservative descriptions and well-posed transformation matrices. The basis is a total Lagrangian description, where the rigid body motions (translations and rotations) are separated from the total deformation that consequently can be treated using the small-deformation linearized theory. The principal di erence between the suggested two models concerns the chosen parameterization of nite rotations, with distances applied to twoand three-dimensional nite elements in the rst and slopes applied to two node nite element beams in the second one. The theoretical features of the the approaches are presented and their main items are discussed referring to some implementations to planar and spatial beam and thin plate models. An incursion into dynamics has also been performed. The present approach is featured by the fact that the formulation is simple, the expressions in the equations of the nonlinear system are explicit and computationally e cient, and the analysis is robust and economical. A large amount of numerical analysis is performed and the good agreement with the results reported in the literature shows the accuracy and capability of the proposed approaches for numerical implementations.Item Koiter's asymptotic numerical methods for shell structures using a corotational formulation(2009-11-30) Zagari, Giuseppe; Aristodemo, Maurizio; Casciaro, Raffaele; Garcea, GiovanniItem La meccanica della frattura nelle applicazioni ai calcestruzzi fibrorinforzati(2014-06-13) Colosimo, Emanuela; Ombres, Luciano; Bruno, DomenicoThe advent of modern computing technology over the last 20 years has permitted to the researchers and engineers to solve very difficult structural problems using sophisticated techniques. The use of finite element method (FEM) in the analysis and design of reinforced concrete structures is one of these various successful techniques. As a matter of fact, the FEM is a powerful computational tool which can be used to simulate the response of structures, structural components and materials when submitted to a specified load. In particular its ability to capture the nonlinear behaviour of RC has made the finite element method a very powerful tool in understanding the behaviour of structures and to quantify its load carrying capacity, stress distribution and cracking path. The fiber reinforced concrete (FRC) have been introduced recently in buildings materials to improve the durability of conventional RC structures. Actually, it is well known that the concrete is a relatively brittle material, that could not support tensile strength. As a consequence, concrete member reinforced with continuous reinforcing bars were used to withstand tensile stresses but they don’t provide a concrete with homogeneous tensile properties. The additional of steel reinforcement significantly increases the strength of concrete but without producing an homogeneous material due to the developments of microcracks and voids in the body. The introduction of fibres randomly distributed throughout the concrete can overcome cracks more effectively. The formation of cracks is undoubtedly one of the most important non linear phenomena which govern the behaviour of concrete structures. Ever since the finite element method has been applied to concrete, the formation of cracks has received much attention. The nonlinear fracture mechanics theory has been used to simulate the quasi-brittle fracture of concrete. The discrete and the smeared crack models are the most used in the literature to model the concrete fracture. The first is especially suitable to simulate the failure in concrete structures where fracture is governed by the occurrence of a small number of cracks with a path that can be predicted. The second crack approach is more appropriate than the first to simulate fracture in concrete structures with a reinforcement ratio that ensures crack stabilization. The main purpose of this research is to analyze short fibres in structural concrete; in particular, according to a nonlinear fracture mechanics approach based on smeared crack models, comparisons in terms of fiber amount between concrete containing no fibres and concrete with fibres are proposed. Starting from an investigation in the fracture mechanics frameworks (LEFM and NLFM), and on fiber reinforced concrete concepts, the work presents the results based on finite methodology, performed consistently with the distributed-crack concept and implemented Diana FE code. In particular the results of simulation of four point bending tests on polymeric and steel fiber reinforced concrete beams were presented in this work of thesis. A 2D plane stress model for analysing the development of cracks is employed; this approach, named rotating crack model, within the smeared crack concept, is based on the total strain crack model. The total strain crack model describes the tensile and compressive behaviour of concrete beams with one stress-strain relationship. With the use of Diana FE program, a standard nonlinear, incremental, iterative approach is performed. The employed theory of the smeared crack concept describes the deterioration of simple concrete and of fiber reinforced concrete characterized by the tensile strength , energy release rate , crack band width , and the shape of the stress-strain curve of the material in the crack band during softening. The smeared crack models are modelled based on concepts using linear and exponential tension-softening constitutive law. tffGh The cracks are defined in the integration points of the elements, i.e., discrete points to compute the elemental mechanical behaviour, are numerically simulated by an adjustment of the compliance matrix at the integration point level and then are modelled according to the rotating model in which the direction of the crack may change during the loading process. The loading process is considered as a sequence of quasi-static loading steps (increments). The theory of linearity is assumed until the maximum local principal stress reached the strength limit of the material, whereafter initiation of mode I cracking in the plane normal to the maximum principal stress will occur. The model is able to simulate multiple crack propagation predicted cracking processes as well as distributed crack pattern, in agreement with experimental observations. Moreover, load-deflection curves are accurately predicted and as well as these corresponding to a linear tension softening assumption of the model. The results show how the proposed approach predicts accurately the maximum loads for the two different class of beam employed as well as it is able to make reasonably good predictions of load and displacement throughout the bending tests. The advantages provided by the short fibers to the properties of concrete improves mainly its post-cracking behaviour (ductility,cracking control and performance under dynamic loading) and can also alter tensile strength. These advantages vary according to the type and volume of fiber added to the matrix. The characteristics of the concrete are strictly dependent from the amount of fiber volume fraction. Fibres have the ability to prevent crack formation and to increase the ultimate load capacity of the concrete structures. KEYWORDS : fracture mechanics, smeared crack models, tension-softening; fiber reinforced concrete, steel fibres, polymeric fibres; four point bending test.Item H-infinity optimal control of active dumpers for automotive suspension systems and civil structures(2014-06-09) Ezzine, Jamal; Vulcano, Alfonso; Aristodemo, MaurizioIn recent years the protection of structures against hazardous vibration has gained special interest. Structures such as buildings, bridges and vehicles are subject to vibrations that may cause malfunctioning, un-comfort or collapse. It is an extended practice to install damping devices in order to mitigate such vibrations. Furthermore, when the dampers are controllable, the structure acts as an adaptronic system. Adaptronic systems are characterized by their abil- ity to respond to external loading conditions and adapt themselves to these changes. These abilities can be exploited to solve the vibration mitigation problems through the installation of controllable dampers and the design of appropriate control laws for an adequate actuation. These modern systems in the case of vehicle suspension systems o®er improved comfort and road hold- ing in varying driving and loading conditions compared to traditional passive devices.Item Post-buckling behaviour of transversely stiffened plate girders(2014-05-28) Presta, Francesco; Aristodemo, Maurizio; Turco, EmilioMany investigations have been carried out to date into the behaviour of transversely stiffened web panels in bending and shear and many different theories have been proposed. Different code rules have been developed based on these theories. The British steel bridge code, BS 5400 Part 3, based its design rules for transverse stiffeners on the work of Rockey, while early drafts of Eurocode prEN 1993-1-5 were based on the work of Höglund. The former's tension field theory places a much greater demand on stiffener strength than does the latter's rotated stress field theory. Due to a lack of European agreement, EN 1993-1-5 was modified late on its drafting to include a stiffener force criterion more closely aligned to that in BS 5400 Part 3. The rules for stiffener design in EN 1993-1-5 are thus no longer consistent with the rotated stress field theory and lead to a greater axial force acting in the stiffener. The rules for the design of the web panels themselves in shear however remain based on Höglund's rotated stress field theory, creating an inconsistency. Recent investigations have suggested that the rules in BS 5400 Part 3 and, to a lesser extent, in the current version of EN 1993-1-5 can be unduly pessimistic. This thesis investigates the behaviour of transversely stiffened plate girders in bending and shear using non-linear finite element analyses. It considers slender symmetrical steel girders with and without axial force and also steel-concrete composite plate girders (which are therefore asymmetric). It discusses the observed web post-buckling behaviour, compares it with the predictions of other current theories and recommends modified design rules. It includes investigation into whether a stiffness-only approach to stiffener design can be justified, rather than a combined stiffness and force approach. The shear-moment interaction behaviour of the girders as a whole are also investigated aItem Rielaborazioni e sviluppi degli approcci per l'analisi di problemi dinamici nonlineari delle strutture(2014-05-19) Russo, Katia; Lopez, Salvatore; Aristodemo, MaurizioItem The implicit corotational method: general theory and FEM implementation(2014-03-12) Madeo, Antonio; Garcea, Giovanni; Casciaro, Raffaele; Aristodemo, Maurizio