Advanced analysis and modeling of strategic infrastructures subjected to extreme loads
Asprone, Domenico (2009) Advanced analysis and modeling of strategic infrastructures subjected to extreme loads. [Tesi di dottorato] (Inedito)
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Recent terrorist acts have contributed to change the design approach to critical infrastructures; in fact, malicious disruptions, blasts, or impacts have unfortunately become part of the possible load scenarios that could act on constructed facilities during their life spans. Hence, a sustainable design aims to ensure the satisfactory performance of the structure during its entire lifetime considering all the possible critical actions, which the structure could be subjected to, including severe dynamic load conditions. The evaluation of the actions on the structure in case of such events is fundamental but represents a critical concern, since uncertainty related to loads definition is often quite high, especially for blast actions. Furthermore, structural response in case of such severe dynamic actions represents a critical issue, since both mechanical properties of materials and dynamic behavior of structural elements under severe dynamic loads can be very different from that exhibited under static actions. Moreover, numerical procedures used to simulate high dynamic loading conditions on structures can suffer of lack of accuracy, due to the rate and the intensity of deformations occurring on structural elements. Hence specific investigations become necessary for all these concerns. In particular, the present work addresses the assessment and design of strategic structures which are to be subjected to multiple hazards during its lifetime, including severe dynamic events, especially blast. At this aim, the most critical issues related to the assessment and design of strategic infrastructures potentially subjected to high dynamic conditions, are discussed and analyzed. Given the uncertainty involved in characterizing the load conditions, it seems inevitable to address the design based on a probabilistic framework. The design can be addressed by limiting the probability of failure below a certain de-minimis risk level that is deemed acceptable by the society. Inevitably, evaluation of the probability of failure requires taking into account possible actions or hazards that the structure could be subjected to; in other words, it needs to be evaluated based on a multi-hazard approach. In details, a multi-hazard framework is proposed and implemented for a strategic reinforced concrete buildings subjected to both seismic and blast hazard. The methodology is described in Chapter I and applied to a case study. Then, a deep investigation is presented on mechanical properties of construction materials in case of dynamic loading conditions. In particular, the strain rate sensitiveness of such material is investigated through a wide experimental activity conducted at Dynamat Laboatory at University of Lugano, Switzerland. In details, results of research activities are presented for: • concrete, in Chapter II, • steel for concrete internal reinforcement, in Chapter III, • Neapolitan yellow tuff, a natural stone widely used in Neapolitan area for masonry structures, Chapter IV, • GFRP (glass fiber reinforced polymer), in Chapter V. A further critical issue related to numerical simulations in case of high dynamic loading conditions on structures. In fact, to address dynamic loading conditions on structural elements, a variety of numerical methods have been recently proposed in the literature; the objective is to address advanced mechanical problems, such as those involving rapid deformations, high intensity forces, large displacement fields. In many of these cases, in fact, classical finite element methods (FEM) suffer from mesh distortion, numerical spurious errors and, above all, mesh sensitiveness. Hence, to overcome such issues, a number of numerical methods, belonging to the family of the so-called meshless techniques, have been widely investigated and applied. The objective of employing these methods is to avoid the introduction of a mesh for the continuum, preferring a particle discretization, with the goal of obtaining an easier treatment of large and rapid displacements. Recently, a number of researchers have tried to extend meshless methods also to solid mechanics problems. Among the several meshless numerical methods proposed, particle methods and in particular Smoothed Particle Hydrodynamics (SPH) has been widely implemented and investigated. A revision of the most common SPH methods is presented in Chapter VI and a rigorous analysis of the error is conducted, focusing on 1D problems. A novel second-order accurate formulation is also proposed for 2D and 3D applications. A further issue is addressed in Chapter VII and is related to protection interventions to be introduced in structural design to minimize disruptive effects in case of malicious blast actions and guarantee the safety of the occupants. In particular, a GFRP porous barrier is developed as fencing structure to prevent malicious disruptions, provide a standoff distance in case of blast actions, and reduce the consequences of an impact. The proposed barrier provides protection through two contributions. First, its geometrical and mechanical characteristics ensure protection against intrusions and blast loads. Second, its shape provides a disruption of the blast shock wave, adding additional protection for structures and facilities located beyond it. The efficacy of the proposed barrier under blast loads is presented by showing the results of the blast tests conducted on full-size specimens with a focus on the reduction of the blast shock wave induced by the barrier. A simplified model is also proposed to predict the reduction of the blast pressure due to the porous barrier, providing a procedure to design the geometrical characteristics of the barrier.
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