No description available.

Seismic isolation systems are widely recognized as beneficial for protecting both acceleration- and displacement-sensitive nonstructural systems and components. Furthermore, adaptive isolation systems have been shown to enable engineers to achieve various performance goals under multiple hazard levels. These systems have been implemented for horizontal excitation, but there has been very limited research on isolation for vertical excitation. Thus, this paper seeks to evaluate the benefit of adaptive vertical isolation systems for component isolation, specifically for nuclear plants. To do this, three vertical isolation systems are designed to achieve multiple goals: a linear spring and a linear damper (LSLD), a linear spring and a nonlinear damper (LSND) and a nonlinear spring and a linear damper (NSLD). To investigate the effectiveness of the proposed systems, a stiff piece of equipment is considered at an elevated floor within a power plant. A set of 30 triaxial ground motions is used to investigate the seismic response of the equipment. The maximum isolation displacement and equipment acceleration are used to assess the effectiveness of the three isolation systems. While all systems significantly reduce the seismic accelerations on the equipment compared to the fixed-base case, a LSND system is shown to exhibit superior seismic performance across multiple hazard levels. / Thesis / Master of Applied Science (MASc)

abstract: This study focused on investigating the ability of a polymeric-enhanced high-tenacity fabric composite called CarbonFlex to mitigate damages from multi-natural hazards, which are earthquakes and tornadoes, in wood-framed structures. Typically, wood-framed shear wall is a seismic protection system used in low-rise wood structures. It is well-known that the main energy dissipation of the system is its fasteners (nails) which are not enough to dissipate energy leading to decreasing of structure's integrity. Moreover, wood shear walls could not sustain their stiffness after experiencing moderate wall drift which made them susceptible to strong aftershocks. Therefore, CarbonFlex shear wall system was proposed to be used in the wood-framed structures. Seven full-size CarbonFlex shear walls and a CarbonFlex wrapped structures were tested. The results were compared to those of conventional wood-framed shear walls and a wood structure. The comparisons indicated that CarbonFlex specimens could sustain their strength and fully recover their initial stiffness although they experienced four percent story drift while the stiffness of the conventional structure dramatically degraded. This indicated that CarbonFlex shear wall systems provided a better seismic protection to wood-framed structures. To evaluate capability of CarbonFlex to resist impact damages from wind-borne debris in tornadoes, several debris impact tests of CarbonFlex and a carbon fiber reinforced storm shelter's wall panels were conducted. The results showed that three CarbonFlex wall panels passed the test at the highest debris impact speed and the other two passed the test at the second highest speed while the carbon fiber panel failed both impact speeds. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2013

No description available.

A new seismic protection strategy called the hybrid passive control device (HPCD) has been developed which combines typical passive energy dissipation devices. It consists of a high damping rubber (HDR) sandwich damper in series with a buckling restrained brace (BRB). The HPCD provides energy dissipation at small deformations without significantly decreasing the structural period. The significant energy dissipation capacity of a BRB is provided for significant seismic events in the second phase. The transition between these two phases consists of an increasing stiffness as the device transitions from rubber damper to BRB. The HPCD reduces deformations, forces and accelerations from seismic events. The hyperelastic or stiffening effect also prevents resonant build-up and aids in collapse prevention due to p-delta effects. The first phase of this work included characterization of high damping rubber compounds and analytical modeling of the HPCD concept. Experimental testing was completed to measure both the static and dynamic material properties of six different rubber compounds. The two most promising rubber compounds were selected for possible inclusion in the device. Analytical models of these selected materials were developed for nonlinear solid finite element analysis. The most promising configuration of the device was selected from several options. The selected configuration was analyzed using the commercial finite element program ABAQUS. These models were used to confirm the validity of the theoretical behavior of the device. Additionally these tests were used to determine which of the rubber compounds performed best. Experimental testing of a half-scale HPCD specimen was carried out in the Structures and Materials Research Laboratory at Virginia Tech. The prototype was tested under cyclic and static loads. The experimental tests confirmed the potential of the hybrid device while highlighting minor issues with the design of the prototype. The final component in the research was an analytical study using hybrid devices in a 9-story steel moment frame structure. The devices were found to provide improved response over a special steel moment frame and a moment frame combined with a buckling restrained brace frame. / Ph. D.

Le travail présent est axé sur la simulation numérique et l'analyse de l'interaction des ondes de surface de Rayleigh avec des barrières sismiques verticales (murs souterrains, écrans, tranchées, etc.) ainsi que des champs de pieux dans des modèles de matériaux mécaniques élastiques et plastiques linéaires. Le but de la recherche est d'estimer le degré de protection que les barrières verticales et les champs de pieux fournissent contre les vibrations transférées par les ondes de surface de Rayleigh et générées par diverses sources. L'idée principale de ce type de protection est d'éviter que les ondes sismiques ne transmettent l'énergie des vagues dans la zone protégée, diminuant les amplitudes de déplacements, les vitesses et les accélérations aux points situés derrière la barrière (champ de pieux). Les principaux complexes sans dimension sont formulés. L'attention est portée sur les ondes de Rayleigh car elles peuvent être générées à la fois par des sources de vibrations externes (situées à la surface de la Terre) et internes (situées sous la surface de la Terre) et ses ondes peuvent transmettre une portion significative de l’énergie de source de la vibration.Premièrement, des simulations numériques de l'interaction des ondes de Rayleigh avec les barrières sismiques verticales et les champs de pieux sont effectuées en supposant que le sol et les matériaux de barrière se comportent conformément à la loi de comportement linéaire élastique. Cela concerne les vibrations qui induisent des contraintes de cisaillement dans le sol n'excédant pas 0.00001 lors de leur propagation. Les principaux complexes sans dimension sont formulés sur cette base. Des paramètres géométriques et mécaniques de la barrière (champ de pieux) déterminant l'effet de réduction de vibration sont identifiés. Les résultats obtenus révèlent la validité de cette onde de protection contre les vibrations. En outre, l’approche de l’optimisation de la barrière sismique verticale (qui peut également être étendue au champ de pieux) est adoptée sous forme de différences finies pour des conditions de sol particulières et une fréquence de vibration de conception.Plusieurs modèles de comportement du sol sont analysés et leur validité, ainsi que l'applicabilité à l'approximation du comportement dynamique réel du sol, ainsi que le mécanisme de dissipation d'énergie des vibrations, sont identifiés. Sur la base de cette analyse, modèle de Mohr-Coulomb a été choisie car elle dispose d’une base de données expérimentale étendue pour divers sols et reflète de manière appropriée la réduction du module de cisaillement avec l’augmentation de la contrainte de cisaillement ainsi que les effets de dissipation d’énergie. Par la suite, ce modèle est utilisé dans l'analyse de l'interaction des ondes de Rayleigh avec les barrières verticales et les champs de pieux, en tenant compte du caractère non linéaire de la déformation du sol à différents niveaux de déformation de cisaillement. En conséquence, l'influence du niveau de contrainte de cisaillement sur l'efficacité des moyens de protection contre les vibrations considérés est démontrée et les conditions appropriées pour utiliser ces méthodes sont identifiées dans le cadre de cette recherche. / The present work is focused on numerical simulation (FEM) and analysis of surface Rayleigh wave interaction with vertical seismic barriers (underground walls, screens, trenches, etc.) as well as pile fields within the framework of linear elastic and plastic mechanical material models. The aim of the research is to estimate the degree of protection that vertical barriers and pile fields provide against vibrations transferred by surface Rayleigh waves and generated by various sources. The main idea behind this type of protection is to prevent seismic waves form transmitting wave energy into the protected zone, decreasing the amplitude of displacements, velocities and accelerations at the points behind the barrier (pile field). The attention is paid to Rayleigh waves as they can be generated by both external (located on the Earth's surface) and internal (located beneath the Earth's surface) vibration sources and this type of waves can transfer a significant portion of vibration source energy.First, numerical simulations of Rayleigh wave interaction with vertical seismic barriers and pile fields are performed assuming the soil and barrier materials to behave according to the linearly-elastic constitutive law. This regards the vibrations that induce shear strains in the soil not exceeding 0.00001 during their propagation. Based on this, the principal dimensionless complexes are formulated. Geometrical along with mechanical parameters of the barrier (pile field), that determine vibration reduction effect, are identified. The obtained results reveal the validity of this way of vibration protection. In addition to that, the approach towards vertical seismic barrier optimization (which can also be extended to the pile field) is adopted in finite difference form to use for particular soil conditions and design vibration frequency.Several models of soil behaviour are analysed and their validity as well the applicability to approximate real dynamic soil behaviour along with the mechanism of vibration energy dissipation are identified. Based on this analysis, Mohr-Coulomb constitutive model is selected as it has a broad experimental database for various soils and appropriately reflects shear modulus reduction with an increase in the shear strain as well as energy dissipation effects. Afterwards, this model is used in the analysis of Rayleigh wave interaction with the vertical barriers and pile fields accounting for non-linear character of soil deformation at different shear strain level. As a results, the influence of shear strain level on the effectiveness of the considered ways of vibration protections is shown and the appropriate conditions to use these methods are identified within the scope of this research.

Terremotos recientes han demostrado la gran vulnerabilidad de algunos puentes ante movimiento fuerte. Los de tipo atirantado constituyen una tipología estructural muy atractiva, y que actualmente es empleada para muchos fines prácticos, por lo que es necesaria su protección sísmica. Entre las actuales estrategias de protección, el uso de dispositivos pasivos es la más robusta, económica y apropiada opción para mejorar el desempeño sísmico de estructuras, de entre los que destacan los sistemas de disipación de energía adicional como una buena alternativa. Debido a sus capacidades, fácil recambio y mantención, así como su buen comportamiento mecánico, los amortiguadores de fluidos viscosos son un excelente sistema de disipación de energía para proteger grandes estructuras contra eventos sísmicos intensos. Es por ello que el análisis, evaluación y comparación de la respuesta sísmica no lineal de puentes atirantados de hormigón, con y sin la incorporación de amortiguamiento viscoso suplementario, con el propósito de investigar su efectividad ante eventos sísmicos, es el principal objetivo de esta investigación aplicada. Para alcanzar lo antes expuesto, se definieron previamente ocho modelos teóricos de puentes atirantados basados en los internacionalmente conocidos puentes de Walter [Walter, 1999], considerando variaciones del esquema de atirantamiento, nivel del tablero, tipo de tablero y espaciamiento de los cables. Como punto de partida para el análisis dinámico no lineal, se realizó un análisis estático no lineal para todos los casos. Luego, se llevó a cabo una caracterización dinámica de los puentes mediante un análisis modal. Como primera aproximación a la respuesta sísmica de los modelos, se ejecutó un análisis mediante espectros de respuesta para cada caso, con el propósito de comparar el comportamiento sísmico en función de las principales variaciones consideradas, y para seleccionar los dos modelos más representativos para ser analizados usando análisis no lineal paso-a-paso. En seguida, se analizaron las estructuras elegidas en el paso previo mediante uso de análisis temporal no lineal por integración directa, sin la consideración de amortiguamiento viscoso suplementario, y tomando en cuenta sismos de campo lejano y campo cercano. En este sentido, se aplicaron cinco eventos sísmicos artificiales para el análisis de campo lejano, y cinco eventos reales que incorporasen pulsos de velocidad de período largo para el análisis de campo cercano, según el Capítulo 3. Finalmente, el análisis de la ubicación óptima de los amortiguadores, un estudio paramétrico tendiente a seleccionar los parámetros óptimos de los mismos, y el análisis paso-a-paso no lineal considerando los amortiguadores viscosos definitivos, fueron investigados con la idea de comparar las respuestas en función de la naturaleza del evento sísmico y el tipo de atirantamiento de los cables, considerando los mismos eventos sísmicos antes expuestos. Los resultados de la investigación muestran que la aplicación de amortiguamiento viscoso suplementario es una eficiente estrategia para incrementar el amortiguamiento de una estructura, absorbiendo una gran cantidad de la energía de entrada, y controlando la respuesta de estructuras de período largo, sobre todo en la dirección longitudinal, en donde se manifiestan las mayores respuestas. Más de un 55% de la energía de entrada puede ser disipada usando éstos dispositivos, los cuales resultan ser igualmente efectivos para sismos de campo lejano y campo cercano, con independencia del esquema de atirantamiento empleado, por lo que constituyen una excelente estrategia de protección pasiva. Debido a la gran no linealidad de éstas estructuras, el método del espectro de respuesta debe ser considerado sólo como primera aproximación al problema, y para propósitos comparativos. Para resultados más precisos, y para aplicaciones de diseño, el análisis no lineal paso-a-paso es siempre la mejor opción. Por otro lado, ésta investigación prueba el despreciable efecto del esquema de atirantamiento en la respuesta sísmica, así como el importante aumento de la respuesta cuando son tomados en cuenta los efectos tipo pulso de la directividad de la falla, característicos de sismos de fuente cercana. / Recent seismic events have demonstrated the vulnerability of some bridges under strong ground motions. Cable-stayed bridges are an attractive bridge typology currently used for many practical purposes, constituting important structural systems to be protected against earthquakes. Amongst the current seismic protection strategies, the use of passive devices is the most robust, economic and well-suited option to improve the seismic performance of structures, in which additional energy dissipation systems is good choice. Because of their capacities, easy replacement and maintenance, as well as their interesting mechanical properties, fluid viscous dampers could be an excellent additional energy dissipation system to protect large structural systems against strong earthquakes. For that reason, the analysis, assessment and comparison of the nonlinear seismic response of concrete cable-stayed bridges, with and without the incorporation of nonlinear fluid viscous dampers in order to investigate their effectiveness for seismic protection purposes, is the main objective of this applied research. To reach the proposed objectives, firstly, eight theoretical cable-stayed bridge models based on the well-known Walter's Bridges [Walter, 1999] were defined; considering variations of the stay cable layout, deck level, deck type and stay spacing. As a starting point of the nonlinear dynamic analysis, a nonlinear static analysis was performed for all the cases. After that, the dynamic characterization of the models was carried out by means of a modal analysis. As a first approach of the seismic response of the bridges, response spectrum analysis was performed in order to compare the seismic behaviour as function of the main variations considered, and to select the two most representative bridges to be analyzed using nonlinear time history analysis. The following stage was the seismic analysis of the selected bridge models from the previous step, applying nonlinear direct integration time history analysis, without additional energy dissipation devices, and considering both far-fault and near-fault ground motions. In these sense, five artificially generated earthquake events were considered for the far-fault analysis, as long as five real earthquake events containing long-period velocity pulses were included for the near-fault analysis, according to Chapter 3. Finally, the analysis of the optimal layout of the dampers, a parametric study to select the optimal damper parameters and the nonlinear step-by-step analysis considering the incorporation of the definitive fluid viscous dampers were investigated in order to compare the seismic responses as a function of the earthquake nature and stay cable layout, taking into account the same earthquake events before mentioned. Results of this investigation show that application of fluid viscous dampers as additional passive energy dissipation systems is a very efficient strategy to increase the damping of a structure, absorbing a significant amount of the seismic input energy, and controlling the seismic response of long-period structures, mainly in the longitudinal direction, where the main responses occur. More than 55% of the input energy can be dissipated with these devices, being equally efficient for far-fault and near-fault ground motions, independent on the stay cable layout, which constitutes a very promising strategy to protect cable-stayed bridges against earthquakes. Because of the highly nonlinear behaviour of those structures, response spectrum analysis must be considered only as first approach to the seismic response and for comparative purposes. For more accurate analysis results, and for design applications, nonlinear time-history analysis is a necessary choice. Likewise, it is demonstrated that the effect of the stay cable layout on the nonlinear seismic response of the bridges is not very important, as well as an important increase of the seismic response when forward rupture directivity pulse effects are considered, a characteristic of near-source ground motions.

Current seismic design practice promotes inelastic response in order to reduce the design forces. By allowing the structure to yield while increasing the ductility of the structure, the global forces can be kept within the limited bounds dictated by the yield strength. However, during severe earthquakes, the structures undergo significant inelastic deformations leading to stiffness and strength degradation, increased interstory drifts, and damage with residual drift. The research presented in this thesis has three components that seek to address these challenges. To prevent the inelastic effects observed in yielding systems, a new concept “apparent weakening” is proposed and verified through shake table studies in this thesis. “Apparent weakening” is introduced in the structural system using a complementary “adaptive negative stiffness device” (NSD) that mimics "yielding” of the global system thus attracting it away from the main structural system. Unlike the concept of weakening and damping, where the main structural system strength is reduced, the new system does not alter the original structural system, but produces effects compatible with an early yielding. Response reduction using NSD is achieved in a two step sequence. First the NSD, which is capable of exhibiting nonlinear elastic stiffness, is developed based on the properties of the structure. This NSD is added to the structure resulting in reduction of the stiffness of the structure and NSD assembly or “apparent weakening”-thereby resulting in the reduction of the base shear of the assembly. Then a passive damper, designed for the assembly to reduce the displacements that are caused due to the “apparent weakening”, is added to the structure-thereby reducing the base shear, acceleration and displacement in a two step process. The primary focus of this thesis is to analyze and experimentally verify the response reduction attributes of NSD in (a) elastic structural systems (b) yielding systems and (3) multistory structures. Experimental studies on 1:3 scale three-story frame structure have confirmed that consistent reductions in displacements, accelerations and base shear can be achieved in an elastic structure and bilinear inelastic structure by adding the NSD and viscous fluid damper. It has also been demonstrated that the stiffening in NSD will prevent the structure from collapsing. Analogous to the inelastic design, the acceleration and base shear and deformation of the structure and NSD assembly can be reduced by more than 20% for moderate ground motions and the collapse of structure can be prevented for severe ground motions. Simulation studies have been carried on an inelastic multistoried shear building to demonstrate the effectiveness of placing NSDs and dampers at multiple locations along the height of the building; referred to as “distributed isolation”. The results reported in this study have demonstrated that by placing a NSD in a particular story the superstructure above that story can be isolated from the effects of ground motion. Since the NSDs in the bottom floors will undergo large deformations, a generalized scheme to incorporate NSDs with different force deformation behavior in each storey is proposed. The properties of NSD are varied to minimize the localized inter-story deformation and distribute it evenly along the height of the building. Additionally, two semi-active approaches have also been proposed to improve the performance of NSD in yielding structures and also adapt to varying structure properties in real time. The second component of this thesis deals with development of a novel device to control the response of structural system using adaptive length pendulum smart tuned mass damper (ALP-STMD). A mechanism to achieve the variable pendulum length is developed using shape memory alloy wire actuator. ALP-STMD acts as a vibration absorber and since the length is tuned to match the instantaneous frequency, using a STFT algorithm, all the vibrations pertaining to the dominant frequency are absorbed. ALP-STMD is capable of absorbing all the energy pertaining to the tuned-frequency of the system; the performance is experimentally verified for forced vibration (stationary and non-stationary) and free vibration. The third component of this thesis covers the development of an adaptive control algorithm to compensate hysteresis in hysteretic systems. Hysteretic system with variable stiffness hysteresis is represented as a quasi-linear parameter varying (LPV) system and a gain scheduled controller is designed for the quasi-LPV system using linear matrix inequalities approach. Designed controller is scheduled based on two parameters: linear time-varying stiffness (slow varying parameter) and the stiffness of friction hysteresis (fast varying parameter). The effectiveness of the proposed controller is demonstrated through numerical studies by comparing the proposed controller with fixed robust H∞ controller. Superior tracking performance of the LPV-GS over the robust H∞ controller in different displacement ranges and various stiffness switching cases is clearly evident from the results presented in this thesis. The LPV-GS controller is capable of adapting to the parameter changes and is effective over the entire range of parameter variations.

Στην παρούσα εργασία μελετήθηκε η δυναμική συμπεριφορά μιας τυπικής σφαιρικής δεξαμενής για την εφαρμογή διάφορων συστημάτων σεισμικής προστασίας. Πέντε διαφορετικά συστήματα σεισμικής προστασίας εξετάζονται:Οι αποσβεστήρες ιξώδους απόσβεσης, οι αποσβεστήρες τριβής και οι μη-λυγίζοντες μεταλλικοί σύνδεσμοι που η λειτουργία τους βασίζεται στην κατανάλωση σεισμικής ενέργειας, και τα ελαστομερή εφέδρανα υψηλής απόσβεσης και αυτά με πυρήνα μολύβδου που η λειτουργία τους βασίζεται στην απομόνωση της βάσης της δεξαμενής. Η αριθμητική ανάλυση έγινε με την χρήση ενός λεπτομερούς μοντέλου πεπερασμένων στοιχείων που προσομοιώνει την ακριβή γεωμετρία της σφαιρικής δεξαμενής, την επίδραση του φαινομένου αλληλεπίδρασης υγρού-δεξαμενής για ενδεικτικά ποσοστά πλήρωσης, την αλληλεπίδραση εδάφους-δεξαμενής και την μη-γραμμική συμπεριφορά των συστημάτων σεισμικής προστασίας. Παρουσιάζονται αποτελέσματα της τέμνουσας δύναμης βάσης, της κατακόρυφης μετατόπισης της ελεύθερης επιφάνειας του περιεχόμενου υγρού και οριζόντιες μετατοπίσεις χαρακτηριστικών σημείων της δεξαμενής, για καθε περίπτωση συστήματος σεισμικής προστασίας υπό σεισμική διέγερση, και συγκρίνονται με τα αντίστοιχα που προκύπτουν για την σφαιρική δεξαμενή με τους συμβατούς χιαστούς συνδέσμους. / Various seismic protection systems are used to study numerically the dynamic behavior of a typical spherical liquid storage tank. Five different anti-seismic devices are investigated; nonlinear viscous dampers, buckling restrained braces and friction devices based on the passive energy dissipation technique and lead core and high damping rubber bearings based on base isolation technique. The numerical analysis is performed by means of a detailed finite element model, taking into account the exact geometry of the steel tank, the fluid-structure interaction effects for an arbitrary level of filling, the soil-structure interaction as well as the non-linearities introduced by either the dissipative bracing systems. Representative results for base shear forces, vertical displacements of the fluid content and displacements at characteristic locations of the spherical tank are presented and compared to those corresponding to a tank with a conventional bracing system.

Nuclear power plays a crucial role in energy supply in the world: around 15% of the electricity generated worldwide is provided from nuclear stations avoiding around 2.5 billion tonnes of CO2 emissions. As of January 2016, 442 reactors that generated 380+ GW were in operation and 66 new reactors were under construction. The seismic design of new nuclear power plants (NPPs) has gained much interest after the high-profile Fukushima Dai-ichi accident. In the UK, a tectonically stable continental region that possesses medium-to-low seismic activity, strong earthquakes capable of jeopardising the structural integrity of NPPs, although infrequent, can still occur. Despite that no NPP has been built in Great Britain after 1995, a New Build Programme intended to build 16 GW of new nuclear capacity by 2030 is currently under way. This PhD project provides a state-of-the-art framework for seismic probabilistic safety assessment and risk control of NPPs in Northwest Europe with particular application to the British Isles. It includes three progressive levels: (i) seismic input, (ii) seismic risk analysis, and (iii) seismic risk control. For seismic input, a suitable model to rationally define inputs in the context of risk assessments is proposed. Such a model is based on the stochastic simulation of accelerograms that are compatible with seismic scenarios defined by magnitude 4 < Mw < 6.5, epicentral distance 10 km < Repi < 100 km, and different types of soil (rock, stiff soil and soft soil). It was found to be a rational approach that streamlines the simulation of accelerograms to conduct nonlinear dynamic analyses for safety assessments. The model is a function of a few variables customarily known in structural engineering projects. In terms of PGA, PGV and spectral accelerations, the simulated accelerograms were validated by GMPEs calibrated for the UK, Europe and the Middle East, and other stable continental regions. For seismic risk analysis, a straightforward and logical approach to probabilistically assess the risk of NPPs based on the stochastic simulation of accelerograms is studied. It effectively simplifies traditional approaches: for seismic inputs, it avoids the use of selecting/scaling procedures and GMPEs; for structural outputs, it does not use Monte Carlo algorithms to simulate the damage state. However, it demands more expensive computational resources as a large number of nonlinear dynamic analyses are needed. For seismic risk control, strategies to control the risk using seismic protection systems are analysed. This is based on recent experience reported elsewhere of seismically protected nuclear reactor buildings in other areas of medium-to-low seismic activity. Finally, a scenario-based incremental dynamic analysis (IDA) is proposed aimed at the generation of surfaces for unacceptable performance of NPPs as function of earthquake magnitude and distance. It was found that viscous-based devices are more efficient than hysteretic-based devices in controlling the seismic risk of NPPs in the UK. Finally, using the proposed scenario-based IDA, it was found that when considering all controlling scenarios for a representative UK nuclear site, the risk is significantly reduced ranging from 3 to 5 orders of magnitude when using viscous-based devices.