Papers

Evaluating horizontal gradients in three-dimensional shallow water models that use bottom-following sigma coordinate can lead to large errors near steep bathymetry. A technique that has been proposed to minimize this problem involves computing horizontal gradients in cartesian coordinates, while treating all other terms in a sigma-coordinate framework. We study this technique through both truncation error analysis and numerical experimentation, and compare it to the direct application of sigma coordinates. While the cartesian coordinate method can be more attractive when horizontal gradients are zero, the sigma coordinate method can be more accurate in other physically relevant situations. Also, the cartesian coordinate method introduces significant numerical diffusion of variable sign near the bottom (where physical diffusion is particularly small), thus potentially leading to instabilities. Overall, we consider the sigma coordinates to be the best approach.

The model presented on this article is a semi-analytical method, developed for the study of two dimensional steel structures made of linear members. It allies the lightness and rapidity of the "cross section models" with the accuracy of the numerical methods, fulfilling the exigencies mentioned above. The control variables are the global deformations of the cross section. A new representation of the domains of evolution of the cross section behaviour, based on these control variables, is presented. The calculation of the section efforts and stiffness is developed analytically for all possible combinations of global deformations evolution. The resulting expressions, written in a non dimensional form, enable a fast computation of these variables, which makes the application of this model specially suitable for very heavy calculations, as in reliability studies for example.The formulation of this model is introduced, and an example of its application to the study of the influence of the material hardening and spreading of the yielding, on the cross section behaviour, is shown for two different types of section shapes.

L'objectif principal de ce travail est l'élaboration d'un modèle non linéaire géométrique et matériel d'analyse des structures métalliques planes à barres. Il permet de tenir compte des principaux facteurs physiques affectant leur comportement non linéaire : évolution progressive de la plasticité suivant les sections transversales et le long des éléments, influence des contraintes résiduelles et effets de l'écrouissage.

O trabalho apresenta o estudo de uma escavação escorada num maciço de argila mole simulada numericamente com recurso a um modelo de cálculo elastoplástico baseado no método dos elementos finitos. Na análise dos resultados são postas em destaque as redistribuições de tensões por efeito de arco no maciço interessado pela escavação e identificados os parâmetros que contribuem para tal fenómeno. É proposta uma analogia estrutural simples para interpretar a interacção solo-estrutura.

We propose an empirical law for vertical nodal placement in tidal simulations that depends on a single parameter, p. The influence of dimensionless numbers on the optimal value of p is analysed through a series of numerical experiments for an individual vertical, and a single value of p is found to be adequate for all cases. The proposed law can lead to gains in accuracy of over two orders of magnitude relative to a uniform grid, and compares favorably with non-uniform grids previously used in the literature. In practical applications, the most effective use of this law may require each vertical to have a different number of nodes. Criteria for the distribution of the total number of nodes among different verticals are also proposed, based on the concept of equalizing errors across the domain. The usefulness of the overall approach is demonstrated through a two-dimensional laterally-averaged application to a synthetic estuary. Key words: Vertical discretization. Sigma coordinates.Localized sigma coordinates. Tidal flow. Numerical experimentation.

Selected interpolation and integration finite element Eulerian-Lagrangian methods for the solution of the transport equation are systematically compared, through a combination of formal analysis and numerical experimentation. Methods that approach conditions at the feet of the characteristic lines with the perspective of "integration" rather than "interpolation" tend to have superior accuracy. However, integration methods that track quadrature points are only conditionally stable. Stability criteria are provided for these methods, and are only mildly restrictive for most applications. A method that performs an exact integration is proposed and shown to have the best accuracy among the studied methods. Key Words: Transport equation. Eulerian-Lagrangian methods. Stability analysis. Accuracy analysis. Numerical experimentation.

Descrevem-se os aspectos técnicos relacionados com a concepção, instalação e sistema de contrôle de estanqueidade da impermeabilização do aterro sanitário para deposição de cinzas de Sines.

Solução corrente, de há longa data, nos países mais industrializados, só agora a construção metálica se começa a implantar no nosso país, devido a uma série de factores que, conjugados, têm contribuído para uma maior preferência pelo betão armado ao longo do século XX. Confinada, por enquanto, a segmentos muito específicos, a construção metálica tem tendência para ver progredir a sua utilização e alargadas as suas aplicações, desde que os projectistas e arquitectos estejam atentos às vantagens das estruturas metálicas.

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