1,169 research outputs found
Stiffness and Slip in Multi-dowel Timber Connections with Slotted-in Steel Plates
Large multi-dowel connections can provide the strength and ductility required for large, highly-loaded timber structures, but their slip under load is not well understood. This is an important gap in knowledge, because accumulated local displacements at connections represent a large part of the deformation of a timber structure. The empirical relationships used in design codes commonly scale a single-dowel stiffness by the number of dowels, so do not capture the dowel interaction effects of the multi-dowel connections used in larger structures. We present the results of an experimental test series, elastic model and probabilistic numerical analysis investigating the development of stiffness in multi-dowel timber connections with slotted-in steel plates. Novel test methods record the development of stiffness due to each individual connector to show that the stiffness of the complete connection is not proportional to the number of dowels. An elastic stress-function model shows that this is partly due to interaction of the stress field around the dowels. For the first time, this work quantitatively considers the influence of misalignment of dowels due to manufacturing tolerances, and it is shown that this may greatly reduce the overall stiffness of a multi-dowel connection. The test series is used to validate a probabilistic model of this misalignment for the stiffness of such a connection. The model incorporates the nonlinear stiffness and hole opening observed in single-dowel connections to predict the behaviour of the group. The study shows that the random misalignment of dowels in multi-dowel connections reduces the range of displacements over which the connection displays zero stiffness slightly, but that this zone is not eliminated as a result of irreversible hole opening under load, even for a connection with 35 dowels and three steel plates. We conclude that two parameters are important for the design of these connections: the unload-reload stiffness and the zero-stiffness region measured between the zero load intercept of the unload-reload linear fit. With these, a reasonable estimate can be made of the displacement at any serviceability load level in either tension or compression
PÓS-GRADUAÇÃO EM CIÊNCIAS CONTÁBEIS NO BRASIL: CONTEXTO E PROCESSO DE SELEÇÃO
A Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES - possibilita a cada Instituição de Ensino Superior determinar os critérios dos processos seletivos para ingresso em programas de pós-graduação Stricto sensu de mestrado e doutorado. Os programas de pós-graduação Stricto sensu de Contabilidade no país estão distribuídos em 19 instituições públicas e privadas, e ofertaram, em 2010, um total de 343 vagas de mestrado e 39 vagas de doutorado. O objetivo desta pesquisa é mapear os critérios utilizados por estes programas para admissão de alunos no mestrado e doutorado. Esta pesquisa foi classificada como descritiva, qualitativa, documental e bibliográfica. Como resultado, verificou-se que é comum, nos processos seletivos das IES no Brasil, a adesão aos processos de análise curricular, entrevista e arguição de projeto de pesquisa, aplicação de provas de conhecimentos específicos, e utilização dos resultados do teste da Anpad. Por meio da leitura completa do artigo é possível verificar: a assimetria existente entre o número de vagas disponibilizadas nos programas de pós-graduação em Contabilidade e em outras áreas afins; as divergências existentes entre os processos seletivos; e a necessidade de discussão e regulamentação mais completa sobre a admissão e sobre os próprios objetivos da pósgraduação no Brasil
Observational Constraints on Visser's Cosmological Model
Theories of gravity for which gravitons can be treated as massive particles
have presently been studied as realistic modifications of General Relativity,
and can be tested with cosmological observations. In this work, we study the
ability of a recently proposed theory with massive gravitons, the so-called
Visser theory, to explain the measurements of luminosity distance from the
Union2 compilation, the most recent Type-Ia Supernovae (SNe Ia) dataset,
adopting the current ratio of the total density of non-relativistic matter to
the critical density () as a free parameter. We also combine the SNe
Ia data with constraints from Baryon Acoustic Oscillations (BAO) and CMB
measurements. We find that, for the allowed interval of values for ,
a model based on Visser's theory can produce an accelerated expansion period
without any dark energy component, but the combined analysis (SNe Ia + BAO +
CMB) shows that the model is disfavored when compared with CDM model.Comment: 6 pages, 4 figure
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High-Speed Empire
Will Doig, author of High Speed Empire, traveled through Laos, Thailand, Malaysia, and Singapore to chronicle the dramatic transformations taking place. Do ordinary people have a voice in this moment of economic, political, and cultural collision
Extra polarization states of cosmological gravitational waves in alternative theories of gravity
Cosmological Gravitational Waves (GWs) are usually associated with the
transverse-traceless part of the metric perturbations in the context of the
theory of cosmological perturbations. These modes are just the usual
polarizations `+' and `x' which appear in the general relativity theory.
However, in the majority of the alternative theories of gravity, GWs can
present more than these two polarization states. In this context, the
Newman-Penrose formalism is particularly suitable for evaluating the number of
non-null GW modes. In the present work we intend to take into account these
extra polarization states for cosmological GWs in alternative theories of
gravity. As an application, we derive the dynamical equations for cosmological
GWs for two specific theories, namely, a general scalar-tensor theory which
presents four polarization states and a massive bimetric theory which is in the
most general case with six polarization states for GWs. The mathematical tool
presented here is quite general, so it can be used to study cosmological
perturbations in all metric theories of gravity.Comment: 26 pages, 1 figure. Accepted for publication in Classical and Quantum
Gravity
Probing the f(R) formalism through gravitational wave polarizations
The direct observation of gravitational waves (GWs) in the near future, and
the corresponding determination of the number of independent polarizations, is
a powerful tool to test general relativity and alternative theories of gravity.
In the present work we use the Newman-Penrose formalism to characterize GWs in
quadratic gravity and in a particular class of f(R) Lagrangians. We find that
both quadratic gravity and the f(R) theory belong to the most general invariant
class of GWs, i.e., they can present up to six independent polarizations of
GWs. For a particular combination of the parameters, we find that quadratic
gravity can present up to five polarizations states. On the other hand, if we
use the Palatini approach for f(R) theories, GWs present only the usual two
transverse-traceless polarizations such as in general relativity. Thus, we
conclude that the observation of GWs can strongly constrain the suitable
formalism for these theories.Comment: 18 pages, 1 figure, accepted for publication in Physics Letters
First Results from Sentinel-1A InSAR over Australia: Application to the Perth Basin
Past ground-based geodetic measurements in the Perth Basin, Australia, record small-magnitude subsidence (up to 7 mm/y), but are limited to discrete points or traverses across parts of the metropolitan area. Here, we investigate deformation over a much larger region by performing the first application of Sentinel-1A InSAR data to Australia. The duration of the study is short (0.7 y), as dictated by the availability of Sentinel-1A data. Despite this limited observation period, verification of Sentinel-1A with continuous GPS and independent TerraSAR-X provides new insights into the deformation field of the Perth Basin. The displacements recorded by each satellite are in agreement, identifying broad (>5 km wide) areas of subsidence at rates up to 15 mm/y. Subsidence at rates greater than 20 mm/y over smaller regions ( 2 km wide) is coincident with wetland areas, where displacements are temporally correlated with changes in groundwater levels in the unconfined aquifer. Longer InSAR time series are required to determine whether these measured displacements are representative of long-term deformation or (more likely) seasonal variations. However, the agreement between datasets demonstrates the ability of Sentinel-1A to detect small-magnitude deformation over different spatial scales (from 2 km–10 s of km) in the Perth Basin. We suggest that, even over short time periods, these data are useful as a reconnaissance tool to identify regions for subsequent targeted studies, particularly given the large swath size of radar acquisitions, which facilitates analysis of a broader portion of the deformation field than ground-based methods or single scenes of TerraSAR-X
CMB Polarization and Theories of Gravitation with Massive Gravitons
We study in this paper three different theories of gravitation with massive
gravitons - the modified Fierz-Pauli (FP) model, Massive Gravity and the
bimetric theory proposed by Visser - in linear perturbation theory around a
Minkowski and a flat FRW background. For the TT tensor perturbations we show
that the three theories give rise to the same dynamical equations and to the
same form of the Boltzmann equations for the radiative transfer in General
Relativity (GR). We then analyze vector perturbations in these theories and
show that they do not give the same results as in the previous case. We first
show that vector perturbations in Massive Gravity present the same form as
found in General Relativity, whereas in the modified FP theory the vector
gravitational-wave (GW) polarization modes ( amplitudes in the
Newman-Penrose (NP) formalism) do not decay too fast as it happens in the
former case. Rather, we show that such polarization modes give rise
to an unusual vector Sachs-Wolfe effect, leaving a signature in the quadrupole
form on the CMB polarization. We then derive the
details for the Thomson scattering of CMB photons for these modes,
and then construct the correspondent Boltzmann equations. Based upon these
results we then qualitatively show that -mode vector signatures - if
they do exist - could clearly be distinguished on the CMB polarization from the
usual tensor modes.Comment: To appear in Classical and Quantum Gravit
Patency of endoscopic ultrasound-guided gastroenterostomy in the treatment of malignant gastric outlet obstruction
Background and study aims Endoscopic ultrasoundguided gastroenterostomy (EUS-GE) with a lumen-apposing metal stent (LAMS) is a novel, minimally invasive technique in the palliative treatment of malignant gastric outlet
obstruction (GOO). Several studies have demonstrated feasibility and safety of EUS-GE, but evidence on long-term
durability is limited. The aim of this study was to evaluate
patency of EUS-GE in treatment of malignant GOO.
Patients and Methods An international multicenter study
was performed in seven centers in four European countries.
Patients who underwent EUS-GE with a LAMS between
March 2015 and March 2019 for palliative treatment of
symptomatic malignant GOO were included retrospectively. Our main outcome was recurrent obstruction due to
LAMS dysfunction; other outcomes of interest were technical success, clinical success, adverse events (AEs), and survival.
Results A total of 45 patients (mean age 69.9 ± 12.3 years
and 48.9% male) were included. Median duration of followup was 59 days (interquartile range [IQR] 41–128). Recurrent obstruction occurred in two patients (6.1 %), after 33
and 283 days of follow-up. Technical success was achieved
in 39 patients (86.7 %). Clinical success was achieved in 33
patients (73.3 %). AEs occurred in 12 patients (26.7 %), of
which five were fatal. Median overall survival was 57 days
(IQR 32–114).
Conclusions EUS-GE showed a low rate of recurrent obstruction. The relatively high number of fatal AEs underscores the importance of careful implementation of EUSGE in clinical practice
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