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One-dimensional models for the spacial behaviour of tapered thin-walled bars with open cross sections : static, dynamic and buckling analyses

Português
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Tapered thin-walled bars are extensively used in the fields of civil, mechanical and aeronautical engineering. The competitiveness of tapered structural members is hindered by the fact that their spatial behaviour is still poorly understood and by the lack of rational and efficient methods for their analysis and design. The present thesis aims at providing a contribution to overcome these drawbacks, by (i) developing one-dimensional models (i.e., models having a single independent spatial variable) to perform linear static, dynamic and lateral-torsional buckling analyses of tapered thin-walled bars with open cross-sections, (ii) supplying physical interpretations for the key behavioural features implied by these models and (iii) offering a detailed examination of several illustrative examples that will be useful for benchmarking purposes. The first part of the thesis is devoted to bars whose shape allows them to resist biaxial bending by the membrane action of their walls (I-section or C-section bars, for instance). It starts with the development, based on the induced-constraint approach, of a linear onedimensional model for the stretching, bending and twisting of tapered thin-walled bars with arbitrary open cross-sections under general static loading conditions. A two-dimensional linearly elastic membrane shell model is adopted as parent theory. The Vlasov assumptions...

Aprimoramento de formulação do MEF para barra geral laminada tridimensional pela consideração da cinemática de empenamento para seção qualquer; An improved finite element formulation for the analysis of general three-dimensional laminated bars with consideration of warping for any cross-section

Santos, Ana Paula Ferreira
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
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Varões embebidos para o reforço ao corte de vigas de betão armado; Embedded bars for shear strengthening of RC beams

Dalfré, Gláucia; Barros, Joaquim A. O.; Machado, Daniel
Fonte: IBRACON 2011 Publicador: IBRACON 2011
Tipo: Conferência ou Objeto de Conferência
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367.18594%

Structure and evolution trends of the external load in uneven bars. Analysis of the flight elements in competition routines.

Ferreirinha, José; Silva, António; Marques, António
Tipo: Conferência ou Objeto de Conferência
Português
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276.09926%
Introduction Specific literature in Artistic Gymnastics is rich in considerations about evolution trends of flight elements in uneven bars routines. Based on the analyses of total participants in world championships and Olympic Games we can observe that gymnasts performed more flight elements in last Olympic cycle compared with preceding cycles. In this way, the purpose of the present study was to analyse the trends in number, difficulty and direct combinations of flight elements in uneven bars routines in elite gymnasts. Methods Through the observational methodology, we constructed and validated an observation category comprising seven variables considered as indicators of the external load concerning flight elements in uneven bars. We observed 83 uneven bars routines from world championships and Olympic Games finals between 1989 and 2004. 12 competitions were framed in 4 Olympic cycles with 2 world championships and 1 Olympic Games each one. It was observed the total number of flight elements, direct combination of 2 and 3 flight elements, dificulty, number of preparation elements and body position in this kind of elements. Data was analysed with descriptive statistics (mean and standard deviation). Kruskal Wallis test was used to compare general significant differences in all cycles and Mann-Whitney teste with Bonferroni correction to analyse differences between each cycle. The level of significance was established in 5% or 1.25% depending of the tests used. Results As main results we observed significant decrease in number of preparation elements from 0.15 in first cycle to 0.00 in last one. Concerning all other variables observed...

Development of a Brief Ataxia Rating Scale (BARS) Based on a Modified Form of the ICARS

Schmahmann, Jeremy D.; Gardner, Raquel; MacMore, Jason; Vangel, Mark G.
Tipo: Artigo de Revista Científica
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277.62105%
To develop a brief ataxia rating scale (BARS) for use by movement disorder specialists and general neurologists. Current ataxia rating scales are cumbersome and not designed for clinical practice. We first modified the International Cooperative Ataxia Rating Scale (ICARS) by adding seven ataxia tests (modified ICARS, or MICARS), and observed only minimally increased scores. We then used the statistics package R to find a five-test subset in MICARS that would correlate best with the total MICARS score. This was accomplished first without constraints and then with the clinical constraint requiring one test each of Gait, Kinetic Function-Arm, Kinetic Function-Leg, Speech, and Eye Movements. We validated these clinical constraints by factor analysis. We then validated the results in a second cohort of patients; evaluated inter-rater reliability in a third cohort; and used the same data set to compare BARS with the Scale for the Assessment and Rating of Ataxia (SARA). Correlation of ICARS with the seven additional tests that when added to ICARS form MICARS was 0.88. There were 31,481 five-test subtests (48% of possible combinations) that had a correlation with total MICARS score of ≥0.90. The strongest correlation of an unconstrained five-test subset was 0.963. The clinically constrained subtest validated by factor analysis...

Journey Beyond the Schwarzschild Black Hole Singularity

Araya, Ignacio J.; Bars, Itzhak; James, Albin
Tipo: Artigo de Revista Científica
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352.87836%
We present the geodesical completion of the Schwarzschild black hole in four dimensions which covers the entire space in (u,v) Kruskal-Szekeres coordinates, including the spacetime behind the black and white hole singularities. The gravitational constant switches sign abruptly at the singularity, thus we interpret the other side of the singularity as a region of antigravity. The presence of such sign flips is a prediction of local (Weyl) scale invariant geodesically complete spacetimes which improve classical general relativity and string theory. We compute the geodesics for our new black hole and show that all geodesics of a test particle are complete. Hence, an ideal observer, that starts its journey in the usual space of gravity, can reach the other side of the singularity in a finite amount of proper time. As usual, an observer outside of the horizon cannot verify that such phenomena exist. However, the fact that there exist proper observers that can see this, is of fundamental significance for the construction of the correct theory and the interpretation of phenomena pertaining to black holes and cosmology close to and beyond the singularities.; Comment: 5 pages, 4 figures

Golden Bars of Consensus and the Truth Quark

Smith, Frank D. Tony; jr
Tipo: Artigo de Revista Científica
Português
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360.50094%
Scientists are imprisoned by Golden Bars of Consensus, says Burton Richter (hep-ex/0001012). A case in point is the mass of the Truth Quark. The consensus analysis of the experimental data indicates that the mass of the Truth Quark is about 170 GeV. On the other hand, an alternative analysis of the same data indicates that the mass of the Truth Quark is about 130 GeV. If the design of future experiments, including trigger, event selection, data analysis procedures, error analysis, etc., takes into account only the consensus value, and if the consensus value happens to be incorrect, then results of future experiments might be compromised.; Comment: 14 pages, pdf format, note added 25 Feb 2002 on pp. 11-13, put two CDF dilepton figures in proper order

Physical Interpretation of Antigravity

Bars, Itzhak; James, Albin
Tipo: Artigo de Revista Científica
Português
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357.2274%
Geodesic incompleteness is a problem in both general relativity and string theory. The Weyl invariant Standard Model coupled to General Relativity (SM+GR), and a similar treatment of string theory, are improved theories that are geodesically complete. A notable prediction of this approach is that there must be antigravity regions of spacetime connected to gravity regions through gravitational singularities such as those that occur in black holes and cosmological bang/crunch. Antigravity regions introduce apparent problems of ghosts that raise several questions of physical interpretation. It was shown that unitarity is not violated but there may be an instability associated with negative kinetic energies in the antigravity regions. In this paper we show that the apparent problems can be resolved with the interpretation of the theory from the perspective of observers strictly in the gravity region. Such observers cannot experience the negative kinetic energy in antigravity directly, but can only detect in and out signals that interact with the antigravity region. This is no different than a spacetime black box for which the information about its interior is encoded in scattering amplitudes for in/out states at its exterior. Through examples we show that negative kinetic energy in antigravity presents no problems of principles but is an interesting topic for physical investigations of fundamental significance.; Comment: 39 pages...

The bar-mode instability in differentially rotating neutron stars: Simulations in full general relativity

Shibata, Masaru; Baumgarte, Thomas W.; Shapiro, Stuart L.
Tipo: Artigo de Revista Científica
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352.87836%
We study the dynamical stability against bar-mode deformation of rapidly spinning neutron stars with differential rotation. We perform fully relativistic 3D simulations of compact stars with $M/R \geq 0.1$, where $M$ is the total gravitational mass and $R$ the equatorial circumferential radius. We adopt an adiabatic equation of state with adiabatic index $\Gamma=2$. As in Newtonian theory, we find that stars above a critical value of $\beta \equiv T/W$ (where $T$ is the rotational kinetic energy and $W$ the gravitational binding energy) are dynamically unstable to bar formation. For our adopted choices of stellar compaction and rotation profile, the critical value of $\beta = \beta_{dGR}$ is $\sim 0.24-0.25$, only slightly smaller than the well-known Newtonian value $\sim 0.27$ for incompressible Maclaurin spheroids. The critical value depends only very weakly on the degree of differential rotation for the moderate range we surveyed. All unstable stars form bars on a dynamical timescale. Models with sufficiently large $\beta$ subsequently form spiral arms and eject mass, driving the remnant to a dynamically stable state. Models with moderately large $\beta \gtrsim \beta_{dGR}$ do not develop spiral arms or eject mass but adjust to form dynamically stable ellipsoidal-like configurations. If the bar-mode instability is triggered in supernovae collapse or binary neutron star mergers...

Stability of general relativistic Miyamoto-Nagai galaxies

Ujevic, Maximiliano; Letelier, Patricio S.
Tipo: Artigo de Revista Científica
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358.3497%
The stability of a recently proposed general relativistic model of galaxies is studied in some detail. This model is a general relativistic version of the well known Miyamoto-Nagai model that represents well a thick galactic disk. The stability of the disk is investigated under a general first order perturbation keeping the spacetime metric frozen (no gravitational radiation is taken into account). We find that the stability is associated with the thickness of the disk. We have that flat galaxies have more not-stable modes than the thick ones i.e., flat galaxies have a tendency to form more complex structures like rings, bars and spiral arms.; Comment: 11 pages, 5 figures, accepted for publication in MNRAS

Constraints on nested bars - implications for gas inflow

Maciejewski, Witold
Tipo: Artigo de Revista Científica
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276.09926%
A wide-spread belief that nested bars enhance gas inflow to the galactic centre has recently been contradicted by dynamical models in which inner bars seem to prohibit such inflow. Can the existing models of dynamically possible double bars be modified to enable strong inflow in the secondary bar? I present here simple dynamical arguments which imply that in general, double bars in resonant coupling do not enhance gas inflow. However, stronger inflow with straight shocks in the inner bar can occur if there is no resonant coupling of the commonly assumed form between the bars.; Comment: 4 pages, 1 figure, to appear in "Disks of Galaxies : Kinematics, Dynamics and Perturbations" (ASP Conference Series), eds E.Athanassoula and A.Bosma

Gravity in 2T-Physics

Bars, Itzhak
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
357.2274%
The field theoretic action for gravitational interactions in d+2 dimensions is constructed in the formalism of 2T-physics. General Relativity in d dimensions emerges as a shadow of this theory with one less time and one less space dimensions. The gravitational constant turns out to be a shadow of a dilaton field in d+2 dimensions that appears as a constant to observers stuck in d dimensions. If elementary scalar fields play a role in the fundamental theory (such as Higgs fields in the Standard Model coupled to gravity), then their shadows in d dimensions must necessarily be conformal scalars. This has the physical consequence that the gravitational constant changes at each phase transition (inflation, grand unification, electro-weak, etc.), implying interesting new scenarios in cosmological applications. The fundamental action for pure gravity, which includes the spacetime metric, the dilaton and an additional auxiliary scalar field all in d+2 dimensions with two times, has a mix of gauge symmetries to produce appropriate constraints that remove all ghosts or redundant degrees of freedom. The action produces on-shell classical field equations of motion in d+2 dimensions, with enough constraints for the theory to be in agreement with classical General Relativity in d dimensions. Therefore this action describes the correct classical gravitational physics directly in d+2 dimensions. Taken together with previous similar work on the Standard Model of particles and forces...

Geodesically Complete Universe

Bars, Itzhak
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
352.87836%
This talk is about solving cosmological equations analytically without approximations, and discovering new phenomena that could not be noticed with approximate solutions. We found all the solutions of the Friedmann equations for a specific model, including all the zero-size-bounce solutions that do not violate the null energy condition, as well as all the finite-size-bounce solutions, and then discovered model independent phenomena. Among them is the notion of geodesic completeness for the geometry of the universe. From this we learned a few new general lessons for cosmology. Among them is that anisotropy provides a model independent attractor mechanism to some specific initial values for cosmological fields, and that there is a period of antigravity in the history of the universe. The results are obtained only at the classical gravity level. Effects of quantum gravity or string theory are unknown, they are not even formulated, so there are new theoretical challenges.; Comment: 11 pages, 9 figures, LaTeX

On Dark Matter, Spiral Galaxies, and the Axioms of General Relativity

Bray, Hubert L.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
355.57062%
Beginning with a geometric motivation for dark matter going back to the axioms of general relativity, we show how scalar field dark matter, which naturally forms dark matter density waves due to its wave nature, may cause the observed barred spiral pattern density waves in many disk galaxies and triaxial shapes with plausible brightness profiles in many elliptical galaxies. If correct, this would provide a unified explanation for spirals and bars in spiral galaxies and for the brightness profiles of elliptical galaxies. We compare the results of preliminary computer simulations with photos of actual galaxies.; Comment: 64 pages, 30 figures

Pattern Speeds of Bars and Spiral Arms From H-alpha Velocity Fields

Fathi, Kambiz; Beckman, John; Piñol-Ferrer, Nuria; Hernandez, Olivier; Martinez-Valpuesta, Inma; Carignan, Claude
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
276.4395%
We have applied the Tremaine-Weinberg method to 10 late-type barred spiral galaxies using data cubes, in H-alpha emission, from the GHAFAS and FANTOMM Fabry-Perot spectrometers. We have combined the derived bar (and/or spiral) pattern speeds with angular frequency plots to measure the corotation radii for the bars in these galaxies. We base our results on a combination of this method with a morphological analysis designed to estimate the corotation radius to bar-length ratio using two independent techniques on archival near infrared images, and although we are aware of the limitation of the application of the Tremaine-Weinberg method using ionised gas observations, we find consistently excellent agreement between bar and spiral arm parameters derived using different methods. In general, the corotation radius, measured using the Tremaine-Weinberg method, is closely related to the bar length, measured independently from photometry and consistent with previous studies. Our corotation/bar-length ratios and pattern speed values are in good agreement with general results from numerical simulations of bars. In systems with identified secondary bars, we measure higher H-alpha velocity dispersion in the circumnuclear regions, whereas in all the other galaxies...

A general reconstruction of the recent expansion history of the universe

Vitenti, S. D. P.; Penna-Lima, M.
Tipo: Artigo de Revista Científica
Português
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352.87836%
Distance measurements are currently the most powerful tool to study the expansion history of the universe without specifying its matter content nor any theory of gravitation. Assuming only an isotropic, homogeneous and flat universe, in this work we introduce a model-independent method to reconstruct directly the deceleration function via a piecewise function. Including a penalty factor, we are able to vary continuously the complexity of the deceleration function from a linear case to an arbitrary $(n+1)$-knots spline interpolation. We carry out a Monte Carlo (MC) analysis to determine the best penalty factor, evaluating the bias-variance trade-off, given the uncertainties of the SDSS-II and SNLS supernova combined sample (JLA), compilations of baryon acoustic oscillation (BAO) and $H(z)$ data. The bias-variance analysis is done for three fiducial models with different features in the deceleration curve. For each fiducial model, we test different reconstructions using, in each case, more than $10^4$ catalogs in a total of about $5\times 10^5$. This investigation proved to be essential in determining the best reconstruction to study these data. We show that, evaluating a single fiducial model, the conclusions about the bias-variance ratio are misleading. We determine the reconstruction method in which the bias represents at most $10\%$ of the total uncertainty. Finally...

Traversing Cosmological Singularities, Complete Journeys Through Spacetime Including Antigravity

Bars, Itzhak
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
352.87836%
A unique description of the Big Crunch-Big Bang transition is given at the classical gravity level, along with a complete set of homogeneous, isotropic, analytic solutions in scalar-tensor cosmology, with radiation and curvature. All solutions repeat cyclically; they have been obtained by using conformal gauge symmetry (Weyl symmetry) as a powerful tool in cosmology, and more generally in gravity. The significance of the Big Crunch-Big Bang transition is that it provides a model independent analytic resolution of the singularity, as an unambiguous and unavoidable solution of the equations at the classical gravitational physics level. It is controlled only by geometry (including anisotropy) and only very general features of matter coupled to gravity, such as kinetic energy of a scalar field, and radiation due to all forms of relativistic matter. This analytic resolution of the singularity is due to an attractor mechanism created by the leading terms in the cosmological equation. It is unique, and it is unavoidable in classical relativity in a geodesically complete geometry. Its counterpart in quantum gravity remains to be understood.; Comment: 14 pages, 9 figures, minor corrections in v2

Local Conformal Symmetry in Physics and Cosmology

Bars, Itzhak; Steinhardt, Paul; Turok, Neil
Tipo: Artigo de Revista Científica
Português
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352.87836%
We show how to lift a generic non-scale invariant action in Einstein frame into a locally conformally-invariant (or Weyl-invariant) theory and present a new general form for Lagrangians consistent with Weyl symmetry. Advantages of such a conformally invariant formulation of particle physics and gravity include the possibility of constructing geodesically complete cosmologies. We present a conformal-invariant version of the standard model coupled to gravity, and show how Weyl symmetry may be used to obtain unprecedented analytic control over its cosmological solutions. Within this new framework, generic FRW cosmologies are geodesically complete through a series of big crunch - big bang transitions. We discuss a new scenario of cosmic evolution driven by the Higgs field in a \textquotedblleft minimal\textquotedblright% \ conformal standard model, in which there is no new physics beyond the standard model at low energies, and the current Higgs vacuum is metastable as indicated by the latest LHC data.; Comment: 34 pages, 1 figure, v2 improves the presentation

Constraints on Interacting Scalars in 2T Field Theory and No Scale Models in 1T Field Theory

Bars, Itzhak
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
355.57062%
In this paper I determine the general form of the physical and mathematical restrictions that arise on the interactions of gravity and scalar fields in the 2T field theory setting, in d+2 dimensions, as well as in the emerging shadows in d dimensions. These constraints on scalar fields follow from an underlying Sp(2,R) gauge symmetry in phase space. Determining these general constraints provides a basis for the construction of 2T supergravity, as well as physical applications in 1T-field theory, that are discussed briefly here, and more detail elsewhere. In particular, no scale models that lead to a vanishing cosmological constant at the classical level emerge naturally in this setting.; Comment: 22 pages. Footnote 14 added in v2

Solution of the inverse problem in spherical gravitational wave detectors using a model with independent bars

Lenzi, César H.; Magalhães, Nadja S.; Marinho Jr., Rubens M.; Costa, César A.; Araújo, Helmo A. B.; Aguiar, Odylio D.