1,721,071 research outputs found
Optimal design of viscoelastic tuned mass dampers for structures exposed to coloured excitations
No full textDynamic interaction between primary and secondary structures can alter the response of buildings, bridges and other civil engineering structures to external stressors such as earthquakes and windstorms. TMDs (tuned mass dampers) are a well-known example of passive control devices that exploit this concept. ATMD consists of a secondary mass attached to the primary structure through a linear or nonlinear link. Various formulations exist to optimize the performance of TMDs, depending on the chosen criterion. Typically, the TMD is optimized considering the steady-state amplitude of motion of the primary structure, e.g., when subjected to monochromatic harmonic excitation (H∞ criterion) or white noise input (H2 criterion). Several closed-form analytical solutions have been formulated for linear TMDs with elastic and viscoelastic damping. However, the available expressions only cover the case of constitutive laws described by two or three parameters, e.g., the Kelvin-Voight model (an elastic spring in parallel with a viscous dashpot) and the standard linear solid (SLS) model (an elastic spring in parallel with a single Maxwell element). Furthermore, the inherent damping of the primary system is usually neglected, even though it can drastically affect the performance of the TMD. Similarly, the effects of coloured excitations, e.g., the Kanai-Tajimi model of ground shaking, are often disregarded, leading to sub-optimal designs. Aimed at overcoming these limitations, a new stochastic approach is proposed. The only assumptions are: I) the salient dynamic features of the TMD-controlled structure can be captured with a linear 2-DoF (degree of freedom) system; ii) the dynamic action can be represented as a stationary Gaussian process
Improving the Seismic Response of Tall Buildings: From Diagrid to Megastructures and Mega-Subcontrol Systems
Full text linkBackground:
Diagrid structures, widely used for the tall buildings of the third millennium, are characterized by a very effective behaviour in the elastic field due
to the grid triangulation. In particular, under horizontal actions, axial forces and deformations mainly arise in the structural members of the diagrid,
thus resulting in the reduction of the shear lag effect and racking deformations. The response to incremental horizontal actions beyond the plastic
threshold, however, shows a poor plastic redistribution capacity, with consequent low values of global ductility, in spite of a significant design
overstrength.
Objective:
In this paper, it is proposed to exploit the high elastic efficiency of the diagrid type and use a vibration control system, based on mass damping
mechanism with large mass ratios, to reduce a priori the inelastic demands due to seismic actions.
Methods:
Starting from the analysis of the seismic behavior of archetype diagrid buildings, a case study is selected to assess the effectiveness of the proposed
motion-based design approach. For this purpose, the diagrid is first transformed into a megastructure (MS) configuration by densifying the
diagonal elements at the most stressed corner areas and transfer floors, suitably chosen. Then, the exterior mega-frame is detached from interior
sub-structures, thus allowing for a relative motion between the two structural portions according to a “mega-sub-structure control system” (MSCS),
which activates the mass damping mechanism.
Results:
Time-history analyses carried out on simplified lumped-mass models confirm the effectiveness of the proposed strategy in reducing the seismic
response.
Conclusion:
Finally, the practical feasibility of the MSCS and engineering solutions for the relevant structural organization are discussed
Innovative mass-damping-based approaches for seismic design of tall buildings
Full text linkMass damping is a well known principle for the reduction of structural vibrations and applied in tall building design in a variety of configurations. With mass usually small (around 1% of building mass), the properly “tuned” mass damper (TMD) shows great effectiveness in reducing wind vibrations, but minor advantages under earthquake excitations. The above limitation can be surpassed by utilizing relatively large mass TMD. For this purpose, two different solutions are here proposed. In both cases, the idea is to separate the building into two or more parts, thus allowing for a relative motion between them, and activating the mass damping mechanism. In the first solution, the building is subdivided along elevation into an upper and a lower structure, separated by means of an intermediate isolation system (IIS). In the second solution, by revisiting the classical mega-frame typology, the exterior full-height structure provides the global strength and stiffness, and secondary structures, extending between two transfer levels, are physically detached from the main structure at each floor and isolated at transfer level. Simplified lumped-mass models are developed for illustrating the dynamic behaviour of the two solutions and carrying out parametric analyses. Procedures for deriving optimum values of design parameters are also proposed and compared to the parametric study
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Optimal design of viscoelastic tuned mass dampers for structures exposed to coloured excitations
No full textDynamic interaction between primary and secondary structures can alter the response of buildings,
bridges and other civil engineering structures to external stressors such as earthquakes and windstorms.
TMDs (tuned mass dampers) are a well-known example of passive control devices that exploit this concept. A
TMD consists of a secondary mass attached to the primary structure through a linear or nonlinear link. Various
formulations exist to optimize the performance of TMDs, depending on the chosen criterion. Typically, the TMD
is optimized considering the steady-state amplitude of motion of the primary structure, e.g., when subjected to
monochromatic harmonic excitation (H∞ criterion) or white noise input (H2 criterion). Several closed-form
analytical solutions have been formulated for linear TMDs with elastic and viscoelastic damping. However, the
available expressions only cover the case of constitutive laws described by two or three parameters, e.g., the
Kelvin-Voight model (an elastic spring in parallel with a viscous dashpot) and the standard linear solid (SLS)
model (an elastic spring in parallel with a single Maxwell element). Furthermore, the inherent damping of the
primary system is usually neglected, even though it can drastically affect the performance of the TMD. Similarly,
the effects of coloured excitations, e.g., the Kanai-Tajimi model of ground shaking, are often disregarded,
leading to sub-optimal designs. Aimed at overcoming these limitations, a new stochastic approach is proposed.
The only assumptions are: i) the salient dynamic features of the TMD-controlled structure can be captured with
a linear 2-DoF (degree of freedom) system; ii) the dynamic action can be represented as a stationary Gaussian
process
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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