Losanno, Daniele (2015) Optimization of supplemental damping in civil engineering structures. [Tesi di dottorato]

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Tipologia del documento: Tesi di dottorato
Lingua: English
Titolo: Optimization of supplemental damping in civil engineering structures
Autori:
AutoreEmail
Losanno, Danieledani.los@libero.it
Data: 31 Marzo 2015
Numero di pagine: 215
Istituzione: Università degli Studi di Napoli Federico II
Dipartimento: Ingegneria Civile, Edile e Ambientale
Scuola di dottorato: Ingegneria civile
Dottorato: Ingegneria delle costruzioni
Ciclo di dottorato: 27
Coordinatore del Corso di dottorato:
nomeemail
Rosati, Lucianorosati@unina.it
Tutor:
nomeemail
Serino, Giorgio[non definito]
Data: 31 Marzo 2015
Numero di pagine: 215
Parole chiave: optimal damping; optimal design; damper brace assembly; seismic isolation damping.
Settori scientifico-disciplinari del MIUR: Area 08 - Ingegneria civile e Architettura > ICAR/09 - Tecnica delle costruzioni
Depositato il: 07 Apr 2015 21:38
Ultima modifica: 24 Set 2015 10:16
URI: http://www.fedoa.unina.it/id/eprint/10124

Abstract

In the last decades the number of applications in civil engineering of special anti-seismic devices such as isolators and supplemental damping systems has been continuously growing thanks to their capability to provide higher safety levels to both new mission critical and existing structures. Passive control systems still appear very attractive with respect to active or semi-active control systems thanks to their capability to work without any external power source. The design of such systems, i.e. supplemental damping systems or seismic isolation ones, usually involves a trial and error process for the achievement of a satisfactory performance of the structural system. To improve competitiveness and effectiveness of passive control systems, their design should be tuned to an optimal value corresponding to a target performance. Aim of the thesis is the investigation of the effects of supplemental damping on the definition of its optimal value in typical passively controlled civil engineering structures, such as damper braced frames or isolated bridges. In case of supplemental damping systems, these are usually inserted in a bracing configuration into new or existing structures, thus being activated by interstory drifts. Structural dynamics of the damping-braced frame may be strongly affected not only in terms of damping but also eigenvalues and eigenvectors. In addition to this, the effect of the brace stiffness in energy dissipation mechanism of supplemental dampers is still not fully addressed in most design and optimization procedures. Even if it is well known that stiffer braces improve damping capacity, the exact value of the brace stiffness is usually neglected, while in practice brace dimensions have to be limited for functional or aesthetic requirements. This thesis properly addresses the effects of the frame to brace relative stiffness parameter on the dynamic behavior and the optimization procedure of single story and multistory frames. In case of seismic isolation, large displacements are usually introduced at the level of the system and supplemental damping is needed for their mitigation. Also in this case an optimal damping level of the isolation system may be defined, since very high damping is not beneficial. Chapter 1 introduces the concept of passive control against earthquake induced vibrations. An overview of most commonly adopted supplemental damping and seismic isolation systems with their characteristics is provided. Chapter 2 is devoted to modeling of damping systems. Concepts of equivalent damping and stiffness are introduced, due to their common use in simplified nonlinear analysis methods. Different kinds of hysteresis, viscous and viscoelastic models are described and mathematical laws for representation of corresponding behavior are suggested. In addition to this, a specific section is devoted to show the effects of the damper supporting brace stiffness on the brace-damper assembly force-displacement behavior. In the final section, elastomeric and friction isolators' behavior is presented. Chapter 3 deals with the general problem of designing supplemental damping systems. After showing effects of damping on structural dynamics of a simple system, general considerations for structures with passive energy dissipation systems are provided. Difference between traditional and supplementally damped structures are highlighted. In a following section, code provisions for supplementally damped structures are provided. Due to deficient European seismic regulations for this kind of structures, American FEMA provisions represent the most acknowledged references. According to those, the different analysis methods and mechanical models, with their applicability and their approximations, together with the definition of equivalent damping are illustrated in detail. A hint to yielding frames with supplemental damping systems is also provided. Chapter 4 deals with an optimal design problem for a simple linear-elastic frame equipped with dissipative braces (steel diagonal brace in series with a dissipative viscous or friction device). Aim of the chapter is the definition of the optimal device parameter (the viscous damping coefficient or the yielding force) able to provide the minimum frame displacement or base shear. An analytical approach is suggested for determining the theoretical optimal value of the viscous damping or the yielding force parameter, able to minimize the maximum displacements, clearly accounting for the influence of the supporting brace stiffness. Properly defined design spectra are provided in the first part of the chapter. It is proved that extremely varying damping coefficients are able to vary the dynamic properties (frequency and mode shapes) of the structure between two limit cases, namely those corresponding to the bare frame (zero value of the damping parameter) and the elastically braced frame (theoretically infinite value of the damping parameter). The proposed analytical method is validated by means of numerical analyses carried out on a simple frame subjected to seven spectrum-compatible earthquake records, according to the Italian Code for Constructions. Therefore, an effective design method is delivered: proposed optimal values can be assumed as a starting point of the optimization operative procedure; then, an iterative analysis is needed in seismic perspective in order to determine the effective optimal values of the design parameters for the minimization of the desired response. Chapter 5 represents a further development of the work presented in Chapter 4 in the case of a multi degree of freedom system. In the first part of the work, a theoretical study in frequency domain has been developed in order to detect the dynamic behavior of a MDOFs frame equipped with viscous dissipative braces. Each brace mounted in series with a damper is modeled by a Maxwell element having a complex stiffness acting in parallel with the stiffness of the bare frame. The proposed approach allows to take into account the effect of the brace stiffness on the optimal value of the viscous damping coefficient and the effectiveness of the supplemental damping system. With the aim of defining an optimal damping parameter, assumed as the one able to yield the minimum resonance peak in the overall range of frequencies, a numerical solution for different MDOFs systems is provided. In the second part of the work, a wide numerical investigation is carried out on different 3 DOFs frames under different recorded earthquakes, assumed to be subjected to a retrofit intervention by means of viscous dissipative braces. The analysis is devoted to prove the combined effect of the brace-damper stiffness on the dynamic behavior of the frame structures, and to validate the optimization procedure. Time history analysis demonstrate the effectiveness of the theoretically obtained design parameter. Chapter 6 presents the definition of optimal design parameters characterizing the isolation system of a bridge, both in case of elastomeric and sliding bearings, having viscoelastic or rigid-plastic behavior, respectively, installed between the piers and the deck. In this case the isolation period is usually defined a priori, than objective of the design becomes the definition of the optimum damping level of the system. Using frequency response analysis, a simple procedure is proposed to determine the optimal value of the viscous coefficient or the yield displacement of the isolators. The adequacy of the proposed procedure is finally verified through time-history analyses performed on a practical case under natural earthquakes. Chapter 7 depicts the main conclusion of the work, with an insight to future developments.

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