Hyperstatisme | EPFL Graph Search

Le but de l'assemblage des pièces est de réaliser une ou plusieurs fonctions. Une des fonctions peut consister à être une structure immobile (bâtiment, pont…), ou bien à réaliser un travail (par exemple déplacer une charge). Dans tous les cas, chaque pièce est en contact avec plusieurs autres, ce qui va d'une part limiter les mouvements de chaque pièce, et d'autre part permettre la transmission d'efforts. En mécanique du solide, l'hyperstatisme est la situation d'un assemblage pour lequel le fonctionnement se fait avec plus de contraintes que ce qui est strictement nécessaire pour le maintenir, ce qui signifie qu'au moins un degré de mobilité d'une pièce est supprimé plusieurs fois. À l'inverse, on parle d'isostatisme lorsque le fonctionnement se fait sans contrainte excessive ou pour être plus rigoureux si le principe fondamental de la dynamique suffit à déterminer toutes les inconnues de liaisons du mécanisme. Ainsi, la mobilité d'une pièce d'un assemblage est nécessairement limité

Capacité portante de ponts en arc en maçonnerie de pierre naturelle

Alix Grandjean

Historical masonry arch bridges are an integral part of our cultural heritage and deserve the attention and the efforts required to safeguard their patrimonial value. There is currently a diverse range of methods available to engineers for evaluating the load-bearing capacity of masonry arches. The major drawbacks are that these methods are either too conservative or computationally intensive. The study presented here proposes a model based on the theory of plasticity. The result is a transparent model that is both rapid and easy to use. Moreover, its application does not involve heavy computation tools. The model is validated by comparison with available experimental results and with the results of a widely-recognised analysis software. In order to take into account the material and structural behaviour of an arch in an optimal way, the present thesis provides a new integral model divided into three evaluation levels. At the micro-level, the response of masonry under pure compression is used to characterise the material of an arch. At this level, numerical models are used to describe the behaviour of natural-stone masonry. These models distinguish between the constitutive units and mortar as well as the unit – mortar interface. The shape and arrangement of the units have the most influential role in defining the failure mode in comparison to the properties of each constitutive material. At the meso-level, the material behaviour from the preceding level is used to define the limit for the local plastification of an arch section under combined compression and flexure. This phenomenon is described as the formation of a hinge. The resistance of an arch section can be expressed using the bending moment – normal force interaction diagram based on the stress-strain curve of the masonry material. Each point along the interaction diagram refers to a state of stress causing the local plastification of an arch section due to the formation of a hinge. The macro-level focuses on the structural analysis of a masonry arch – a structure with four degrees of indeterminacy. The arch failure corresponds to the successive formation of four hinges defined by the yielding condition determined at the meso-level. The analytical model developed in this study considers an arch at the state of being a statically determinate structure – after three of the four hinges are developed. Using the principle of equilibrium, the model calculates the load-bearing capacity of the arch at the ultimate limit state. A large number of existing natural-stone masonry arches include deficiencies accumulated throughout years of service without any regular maintenance. Three common types of deficiencies on these bridges are considered. The model is extended to take into account their effects on the load-bearing capacity of arches. Finally, the model developed in this study is incorporated into a global methodology for the examination of masonry arch bridges. This methodology implies two complementary approaches, which have to be applied parallel to one another. The first uses the developed model and gives a quantitative evaluation of structural security. The second includes a qualitative appreciation of the risk, based on the condition survey of the structure. This methodology allows an objective examination of the structure and recommendations for intervention measures.

EPFL2010

Punching strength of actual two-way slabs

Jürgen Einpaul, Miguel Fernández Ruiz, Aurelio Muttoni

Many building codes rely on empirical formulations with respect to the punching shear design provisions. Such empirical equations are typically calibrated on the basis of laboratory tests performed on simplified geometries (usually isolated specimens) with usually statically-determined loading conditions. On the contrary, actual structures, particularly two-way slabs, are continuous and highly redundant systems. The isolated test specimens are thus unfortunately not always suitable to reproduce the actual behaviour of flat slabs and empirical models calibrated on their basis may not be applicable for their design. In this paper, the application of the Critical Shear Crack Theory (theoretical support of Model Code 2010 provisions) is shown for some unusual test specimens aiming at investigating actual inner bays of continuous flat slabs. This analysis shows that moment redistribution between the column region and mid-span develop after concrete cracking and that accounting for this behaviour is required to obtain good predictions of the actual flat slab response. It is also shown that the Critical Shear Crack Theory leads to consistent results in terms of failure mode, punching shear strength and deformation capacity as it allows accounting for this behaviour. On the contrary, empirical design formulas fail to predict the influence of continuous flat slabs on the punching strength.

4th fib Congress, Mumbai2014

Are there "dragon-kings" events (i.e. genuine outliers) among extreme avalanches?

Christophe Ancey

Predicting the occurrence and spatial extent of extreme avalanches is a longstanding issue. Using field data pooled from various sites within the same mountain range, authors showed that the avalanche size distribution can be described using either an extreme value distribution or a thick-tailed distribution, which implies that although they are much larger than common avalanches, extreme avalanches belong to the same population of events as "small" avalanches. Yet, when looking at historical records of catastrophic avalanches, archives reveal that a few avalanches had features that made them "extra-ordinary." Applying avalanche-dynamics or statistical models to simulate these past events runs into considerable difficulty since the model parameters or the statical properties are very different from the values usually set to model extreme avalanches. Were these events genuine outliers (also called "dragon-kings")? What were their distinctive features? This paper reviews some of the concepts in use to model extreme events, gives examples of processes that were at play in extreme avalanches, and shows that the concept of dragon-king avalanches is of particular relevance to describing some extreme avalanches.

Springer-Verlag2012

Capacité portante de ponts en arc en maçonnerie de pierre naturelle

Alix Grandjean

EPFL2010