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External Dissipative Rocking System
Full text linkQuesta tesi ha l'obiettivo di indagare il comportamento dinamico e sismico di edifici esistenti protetti con un sistema esterno di dissipazione basato sul rocking; esso è costituito da una struttura reticolare in acciaio, incernierata a livello di fondazione, il cui movimento oscillante attiva la dissipazione di energia, per mezzo di smorzatori viscosi situati alla base. Nell'ipotesi di comportamento elastico lineare sia del telaio, che della struttura dissipativa, vengono presentate l'equazione del moto del sistema accoppiato ed una generalizzazione del sistema ad un grado di libertà. Per risolvere il problema dinamico è possibile condurre analisi sia nel dominio del tempo, che in quello delle frequenze.
La soluzione nel dominio del tempo si basa su una formulazione alle variabili di stato che, tramite analisi modale complessa del sistema telaio da proteggere-struttura esterna di protezione passiva, permette di valutare l'influenza, sia sul comportamento dinamico che in termini di risposta sismica, dello smorzamento e della linearizzazione degli spostamenti forniti dalla struttura dissipativa. In particolare, la formulazione proposta, consente di valutare separatamente il contributo di ciascuno dei modi di vibrazione complessi del sistema sulla risposta sismica globale. Le analisi armoniche nel dominio delle frequenze, invece, sono particolarmente utili in quanto consentono di lavorare con un sistema algebrico piuttosto che differenziale. Inoltre, scegliendo di rappresentare l'input sismico come un processo stocastico stazionario, è possibile stabilire una relazione tra la densità spettrale di potenza (PSD) dell'input esterno e quella dei parametri di risposta di interesse, tramite analisi armonica del sistema.
Le prestazioni di due casi di studio sono valutate mediante analisi parametriche, riguardanti diversi livelli di smorzamento e rigidezza del sistema di protezione, sia con analisi nel tempo che in frequenza. Infine vengono presentati alcuni aspetti che necessitano di ulteriore approfondimento. Tra di essi la valutazione ed il confronto della performance sismica di smorzatori viscosi a comportamento lineare e non, dimensionati attraverso il criterio di uguaglianza dell'energia dissipata. Le prestazioni del sistema di protezione passiva indagato, vengono confrontate con un'altra configurazione esterna, che prevede l'accoppiamento del telaio con una struttura rigida di contrasto, nota in letteratura come sistema di smorzamento proporzionale alle masse. Infine viene condotta una prima analisi degli effetti dell'interazione terreno-struttura (SSI), mediante l'approccio alle sottostrutture.In this thesis the seismic performance of existing buildings frames coupled with an external dissipative rocking system is investigated; the arrangement consists of a steel truss, hinged at the foundation level, whose rocking motion promotes the dissipation of energy via viscous dampers located at the base. Under the assumption of linear elastic behaviour of both the frame and the dissipative structure, the equation of motion of the system are presented, together with a generalized Single Degree of Freedom (S-DOF) approximation of the system. This way, analysis through time-domain and frequency-domain are allowed for the investigation of the system dynamic behaviour.
Time-domain analysis, based on a state space approach, leads to the complex modal analysis of the coupled system, which allows the evaluation of the influence of the added damping and of the displacements linearization, promoted by the external dissipative rocking system, on both the dynamic behaviour and the seismic response. In particular, the proposed formulation permits to evaluate separately the contribution of each of the complex vibration modes of the system to the global seismic response. On the other hand frequency-domain analysis is particularly useful for the problem under investigation because it allows to work with an algebraic system rather than a differential one. Furthermore by representing the seismic input in terms of a stationary stochastic process, a relation can be established between the Power Spectral Density (PSD) of the input and that of the response parameters of interest via harmonic analysis of the system.
The performance of two case studies is evaluated by means of parametric analysis, involving added damping and stiffness, either through time- and frequency-domain. Finally, some aspects which need a deeper investigation are presented. Among them the evaluation and comparison between linear and non-linear fluid viscous dampers, dimensioned through dissipated energy criterion. The performance of the analyzed dissipative system, is compared with an alternative retrofitting configuration, consisting in the coupling with external stiff contrasting structure, known in literature as mass proportional damping system. At the end a first insight on the effects of Soil-Structure-Interaction (SSI), through the substructure approach, is provided
Operational Modal Analysis on r.c. building for the evaluation of the dynamic changes due to retrofitting
No full textThe paper deals with the changes of the modal parameters of a r.c frame building sited in Camerino, Italy. Operational modal analysis were performed before and after the seismic retrofitting of the building achieved with an innovative system that uses external steel towers equipped with dissipative devices. Before retrofitting, ambient vibration tests were carried out with the aim of evaluating the actual linear dynamic behavior of the building including the contribution of non-structural components (e.g. external and internal walls); modal parameters, i.e. natural frequencies and mode shapes determined by means of experimental modal analysis, allowed the calibration of a predictive f.e. model of the building which was crucial for the final design of the retrofitting system. Tests were repeated after the retrofitting works, with the main objective to verify that changes of modal parameters are in agreement with those predicted by the f.e. model. Results and comparisons between the modal parameters obtained before and after the retrofitting works demonstrate that the ambient vibration tests and the data processing techniques used allow the identification of the dynamic behavior of r.c. few-storeys buildings under operating conditions and, then, the identification of dynamic changes due to seismic retrofitting works performed with external steel “dissipative towers”. A model updating procedure is carried out to identify the value of the elastic modulus of the different materials constituting structural and non-structural elements
Modal properties of existing building retrofitted by external bracings with viscous dampers
No full textThe paper deals with the seismic protection of existing buildings using external viscous damper systems to increase energy dissipation capacity. Dampers and bracings can be arranged in very different configurations and the possible solutions can be grouped into different categories, depending on the specific kinematic behavior. In this work, the formulation of the problem involving the coupling of the existing frame with an external damping system is presented in general terms and some issues concerning the influence of the bracing properties on the dynamic response of the coupled system are investigated. Presented results concern the so called "dissipative tower", a recent solution which exploits the rocking motion of a stiff steel truss hinged at the foundation level for the dampers activation; the influence of the external dissipative bracings on the most important modal properties of the system, such as the mode displacement profile, relevant internal action distribution and relations between damper dimensions and effective modal damping are shown. Finally the global effect on the seismic response, in terms of displacements and base shear, is presented by solving the dynamic problem with the modal decomposition method in order to investigate the contribution of the higher modes on the dynamic response
An external bracing system for the retrofit of r.c. frame buildings: modal properties and seismic response
No full textThe paper deals with the seismic protection of existing buildings using external viscous damper systems to increase energy dissipation capacity. Usually the addition of dissipative diagonals in existing frames has some drawbacks as increment of internal actions in the columns, need of intervention at foundation level, feasibility limits and indirect costs related to the interruption of the building utilization. These problems can be efficiently avoided by placing the dissipative bracings and the relevant foundations outside the existing building. Dampers and bracings can be arranged in very different configurations and the possible solutions can be grouped into different categories, depending on the specific kinematic behavior, but all permitting the control of both the total amount of the dissipated energy and the frame deformation at the various storeys. In this work, the formulation of the problem involving the coupling of the existing frame with an external damping system is presented in general terms and is employed to investigate both the influence of the external bracing properties on the overall dynamic properties of the coupled system (such as the mode displacement profile, the relevant internal action distribution and the modal damping ratios) and the global effect of the retrofitting on the seismic response. Presented results concern the so called "dissipative tower", a recent solution which exploits the rocking motion of a stiff steel truss hinged at the foundation level for the dampers activation. The influence of the external dissipative bracings on the most important modal properties of the system are shown and it is observed that the bracing system notably influences the stiffness and damping properties while it modifies only marginally the mass properties of the existing frame. Finally the global effect on the seismic response, in terms of both displacements and base shear, is presented by solving the dynamic problem with the modal decomposition method by also investigating the contribution of the higher modes on the dynamic response
An innovative seismic protection system for existing buildings: external dissipative towers
No full textA common technique for the retrofit of existing structure is based on the installation of dissipative devices connecting adjacent storeys of buildings in either diagonal or chevron brace configurations. This type of damping system may present some disadvantages like the increment of axial forces in columns, which may leads to premature local failures, or some feasibility limits on the strengthening of the existing foundations at the base of the bracing system. In addition, indirect costs related to the interruption of the building utilization during execution of the retrofit can be very demanding, in particular for strategic buildings, such as hospitals or schools. Most of previous problems can be overcome by placing the dissipative bracings and the relevant foundations outside the building; systems with external dampers can be grouped into three main categories, depending on the kinematic behaviour of the system, which is a function of the arrangement of dampers and bracings. Recently, some applications have been developed by proposing a new configuration exploiting the rocking motion of a stiff external tower. This paper deals with this innovative system for seismic protection of existing buildings, especially strategic ones, patented as “Dissipative Towers”. The protection system is based on the structural coupling of the building with new steel truss towers constructed externally and then rigidly connected to the building floors by means of steel elements; the towers are erected over a rigid r.c. thick base plate that is restrained to the foundation cap with a spherical hinge located in central position of the base slab. The towers are equipped with dissipative devices connecting the corners of the two plates; the effectiveness of the dampers is enhanced by the use of articulated quadrangles which amplify the vertical displacements of the devices. The efficiency of the system is so high that usually it is designed to satisfy the immediate occupancy limit state even for high intensity earthquakes. The above system is applied to the retrofit of an existing school building in Italy, constituted by different blocks made of reinforced concrete frames. The seismic rehabilitation is obtained by suitably positioning external dissipative towers and eliminating expansion joints between adjacent blocks. The towers allow a high level of seismic protection at the ultimate limit state, with a significant reduction of horizontal displacements and accelerations. Moreover also the shear actions resisted by the existing frame are significantly reduced by the "Dissipative Towers". The seismic protection is achieved with a moderate economic impact due to the elimination of indirect costs related to the arrangement of internal spaces, interruption and/or relocation of activities
Influence of viscous dampers ultimate capacity on the seismic reliability of building structures
Full text linkAnti-seismic devices should be designed with proper safety margins against their failure, because the reliability of the structural system where they are installed is strongly influenced by their reliability. Seismic Standards generally prescribe safety factors (reliability factors) amplifying the device responses at the design condition, in order to reach a target safety level. In the case of Fluid Viscous Dampers (FVDs), these factors are applied to the stroke and velocity, and their values are not homogeneous among seismic codes. This paper investigates the influence of the values of the safety factors for FVDs on the reliability of the devices and of the structural systems equipped with them. An advanced FVD model is employed to account for the impact forces arising when the dampers reach the end-stroke and the brittle failure due to the attainment of the maximum force capacity. The effect of damper failure on both the fragility and the seismic risk of the structural system is investigated by performing multiple-stripe analysis and monitoring different global and local demand parameters. In particular, a parametric study has been carried out, considering two case studies consisting of a low-rise and a medium-rise steel building, coupled with a dissipative system with linear and nonlinear properties and studying the consequences of different values of safety factors for stroke and forces. The study results give evidence to the potential brittle behaviour of the coupled system and provide information about the relationships between damper safety factors and effective structural reliability. Some preliminary suggestions are given on possible improvements of current design approaches and on the values of the reliability factors to be considered for future code revision
Analysis and comparison of two different configurations of external dissipative systems
No full textThis paper deals with the seismic protection of existing buildings, especially r.c. frame ones, by means of external passive
dissipative systems. These type of systems provide larger flexibility in controlling the structural behavior, and some feasibility
advantages, but their efficiency in terms of performance still need to be proven. In particular, this study analyzes and compares
the performance of two external solutions using linear fluid viscous dampers (FVDs) for the seismic upgrading of an existing
benchmark structure, the Van Nuys building. The first arrangement is a recent solution, known as "Dissipative Tower", which
exploits the rocking motion of a steel truss hinged at the foundation level for the dampers activation; the second one consists in
coupling the building with an external stiff contrasting structure, where the dampers are located horizontally at the storey level.
First, a state space formulation of the problem, based on the assumption of linear elastic behavior for both the existing frame and
the external dissipative structures, is presented in general terms. The proposed formulation, suitable for both the external
arrangements, allows to evaluate the influence of the dissipative solutions on the system modal properties. Successively, the
performance of the two proposed external passive structures, is evaluated and compared with that of the bare existing frame, by
considering important engineering demand parameters (EDPs) such as interstorey drifts, absolute accelerations and shear actions
resisted by the frame and by external systems
Stochastic seismic analysis and comparison of alternative external dissipative systems
Full text linkThis paper deals with the seismic retrofit of existing frames by means of external passive dissipative systems. Available in different configurations, these systems allow high flexibility in controlling the structural behaviour and are characterized by some feasibility advantages with respect to dissipative devices installed within existing frames. In particular, this study analyzes and compares the performances of two external solutions using linear viscous dampers. The first is based on the coupling of the building with an external fixed-based steel braced frame by means of dampers placed horizontally at the floor levels. The second is an innovative one, based on coupling the building with a “dissipative tower,” which is a steel braced frame hinged at the foundation level, and activating the dampers through its rocking motion.The effectiveness of the two solutions is evaluated and compared by considering a benchmark existing reinforced concrete building,employing a stochastic dynamic approach, under the assumption of linear elastic behaviour for the seismic performance evaluation. This allows efficiently estimating the statistics of many response parameters of interest for the performance assessment and thus carrying out extensive parametric analyses for different properties of the external systems. The study results provide useful information regarding the design and the relative efficiency of the proposed retrofit solutions
Analysis and comparison of two different configurations of external dissipative systems
Full text linkThis paper deals with the seismic protection of existing buildings, especially r.c. frame ones, by means of external passive dissipative systems. These type of systems provide larger flexibility in controlling the structural behavior, and some feasibility advantages, but their efficiency in terms of performance still need to be proven. In particular, this study analyzes and compares the performance of two external solutions using linear fluid viscous dampers (FVDs) for the seismic upgrading of an existing benchmark structure, the Van Nuys building. The first arrangement is a recent solution, known as "Dissipative Tower", which exploits the rocking motion of a steel truss hinged at the foundation level for the dampers activation; the second one consists in coupling the building with an external stiff contrasting structure, where the dampers are located horizontally at the storey level. First, a state space formulation of the problem, based on the assumption of linear elastic behavior for both the existing frame and the external dissipative structures, is presented in general terms. The proposed formulation, suitable for both the external arrangements, allows to evaluate the influence of the dissipative solutions on the system modal properties. Successively, the performance of the two proposed external passive structures, is evaluated and compared with that of the bare existing frame, by considering important engineering demand parameters (EDPs) such as interstorey drifts, absolute accelerations and shear actions resisted by the frame and by external systems
MODAL PROPERTIES AND SEISMIC RESPONSE OF EXISTING BUILDING RETROFITTED BY EXTERNAL BRACINGS WITH VISCOUS DAMPERS
No full textThis paper deals with the seismic protection of existing buildings using external viscous damper systems to increase the energy dissipation capacity. Dampers and bracings can be arranged in very different configurations and the possible solutions can be grouped into different categories, depending on the specific kinematic behavior. The study analyzes a recent solution called "dissipative tower", which, for the dampers activation, exploits the rocking motion of a stiff steel truss hinged at the foundation level. A state space formulation of the dynamic problem is presented in general terms and some issues concerning the influence of the bracing properties on the behavior of a case study consisting of a building coupled with a dissipative tower are investigated. The results presented concern both the influence of the external dissipative bracings on the most important modal properties of the system, and the global effect on the seismic response, evaluated via the modal decomposition method. It is shown that the addition of the towers yields a regularization of the drift demand along the building height, but it can also induce significant changes in the distribution of internal actions in the existing frame. Moreover, the contribution of higher order modes can be important for the internal actions evaluation, while it is negligible for the displacement estimation. The results obtained by considering the coupled system are compared with the corresponding results obtained by considering two limit cases: the bare frame, and the frame equipped with an infinitely stiff dissipative tower
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