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A New Analytical Approach to Flutter Analysis in Long-span Suspension Bridges
Full text linkL'abstract è presente nell'allegato / the abstract is in the attachmen
Linear aeroelastic analysis of suspension bridges with second-order effects
No full textAeroelastic instability induced by wind loads is one of the main concerns in the design of long-span suspension bridges. The increasing length of the main span - the recent 1915 Canakkale Bridge in Turkey established a new World record with its central span of 2023 m - and the consequent increase in flexibility makes these bridges highly sensitive to the wind action. Since the Tacoma Narrows Bridge failure of 1940, several semi-analytic, semi-empirical methods have been proposed to describe the aeroelastic behaviour of suspension bridges in order to predict the stability threshold.
In this talk, an enhancement of a linear semi-analytic continuum model previously proposed by two of the authors for the dynamic aeroelastic analysis of suspension bridges will be presented. In compliance with the linearised deflection theory of suspension bridges, the kinematics of the bridge cross-section is described by means of two displacement parameters: the vertical deflection of the deck cross-section and its torsion rotation. The latter parameters are functions of the centroidal longitudinal coordinate of the bridge deck (coinciding with the elastic axis), allowing to describe the deck deformation by a one-dimensional structural model. The Scanlan’s definition of the aeroelastic lift and moment by means of flutter derivatives is adopted, whereas the contribution of the mean steady drag force is embedded as a Prandtl-like second-order effect. The additional second-order terms introduced, depict distributed vertical and torsion loads arising from a change in the geometric configuration of the system, thus, a form of geometric nonlinearity.
A multi-degree-of-freedom model is thus obtained from the two partial integro-differential equations governing the flexural-torsional motion by Galerkin’s method, defining the vertical and torsional kinematic parameters as weighted sums of sinusoidal functions. An iterative procedure is introduced to carry out the eigenvalue analysis of the self-excited system for increasing wind speeds. The bridge modal frequencies are depicted by the imaginary parts of the complex eigenvalues while the real parts represent the modal damping factors. The variation of the eigenvalues with the wind load is investigated, as well as the evolution of the corresponding modal shapes, these latter being described by the real and imaginary components of the eigenvectors. The effect of the steady drag force on the flexural-torsional aeroelastic behaviour is highlighted and shown to be an affective coupling feature between the degrees of freedom. Divergence and flutter aeroelastic instabilities are identified by the appearance of eigenvalues having a non-negative real part and zero or positive imaginary part, respectively.
Numerical examples will be presented, providing a comparison of the results with those of the literature and of Finite Element analyses
Multimodal aeroelastic analysis of suspension bridges with aerostatic nonlinearities
Full text linkA one-dimensional (1D) continuum model for the multimodal aeroelastic analysis of
suspension bridges is presented which refines a previous model by some of the authors. The
classical linearized equations governing the (self-excited) bridge vertical and torsional
oscillations are enhanced to include geometric stiffness contributions related to the steady lift
and drag forces. A multi-degree-of-freedom (MDOF) system is obtained by Galerkin method
and, for increasing values of wind speed, the damped linear dynamics, modulated by both the
steady and the self-excited aerodynamic forces, is studied in the complex field. Stability
thresholds for divergence and flutter are determined by the classical Lyapunov dynamic
criterion. Selected engineering case studies are considered to demonstrate the capabilities and
the validity of the proposed model and of the relevant numerical code. In a comprehensive
dynamic framework, the method allows for: (i) determining the variation of the bridge mode
shapes and frequencies for increasing values of wind speed, including an effective description
of the coupling between the two displacement components as well as the interaction between
different vibration modes; (ii) detecting both single-degree-of-freedom (i.e. damping-driven)
and coupled (i.e. stiffness-driven) flutter instability; (iii) detecting static divergence instability;
and (iv) investigating geometric nonlinearities associated with the aerostatic load
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
Preliminary Flutter Stability Assessment of the Double-Deck George Washington Bridge
Full text linkWe deal with the flutter analysis of the George Washington bridge, in both the single- and double-deck configurations of 1931 and 1962, respectively. The influence of the additional lower deck on the aerodynamic behavior is investigated. To overcome the lack of aerodynamic data, a simplified approach is followed based on Fung's formulation, in which the flutter derivatives are expressed in terms of the real and imaginary parts of the Theodorsen function and of the steady-state aerodynamic coefficients of the deck cross-section. The latter are obtained by Computational Fluid Dynamics simulations conducted in ANSYS FLUENT, whereas the ANSYS Mechanical APDL finite element package is used to perform the flutter analyses. Two different methods for the application of the aeroelastic forces are employed for the double-deck configuration: (i) self-excited forces, based on flutter derivatives related to the whole cross-section, acting on the upper deck; and (ii) self-excited forces, based on flutter derivatives related to the single deck, simultaneously applied to the upper and lower decks. The obtained results are critically compared with theoretical predictions of simple formulas available from the literature; it is suggested that laboratory tests are needed since no experimental results seem to be available
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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