Capacité portante de ponts en arc en maçonnerie de pierre naturelle

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.

Model-Free Methodologies for Data-Interpretation during Continuous Monitoring of Structures

Irwanda Laory

Most civil engineering infrastructures, especially bridges worldwide, are approaching the end of their designed lifespan. They are continuously deteriorating due largely to material aging, excessive loads and changing environments. Therefore, it is crucial to evaluate the performance of existing structures to prevent catastrophic events. Structural Health Monitoring (SHM) has the potential to provide a proper assessment of structural performance and to further reduce cost through early damage detection and thus replacement avoidance. SHM integrates technologies to monitor structural behaviour and with current advances in sensor technology and measurement systems, the number of bridges that are continuously monitored is increasing. The bottleneck in SHM is data interpretation and this task is even more challenging in the presence of environmental variations. The main stream of data interpretation in SHM involves physics-based modelling and validation. However, building a model can be expensive, time consuming and difficult due to the structural complexity and uncertain environments. This research focuses on model-free methodologies for continuous monitoring of civil structures under environmental variations. The work involves important aspects of SHM such as damage detection, measurement system configuration and environmental variations. For damage detection, a novel model-free methodology that combines Moving Principal Component Analysis (MPCA) and regression analyses is proposed. Such approach aims to exploit the advantages of both MPCA and regression-analysis methods. The methodology has been tested on numerical and experimental studies including a full-scale bridge application. Results of a comparative study with other model-free methodologies demonstrate the superior performance of the proposed methodology in terms of damage detectability and time to detection. This research work also compares the performance of model-free methodologies for predicting natural frequencies of a continuously monitored bridge under environmental variations. Relative importance of environmental factors and traffic loading is also evaluated. Results of the case study reveal that traffic loading and temperature variations are the most influential parameters. A structural identification strategy that takes advantage of thermal variations is developed. The strategy utilizes thermal variations as load cases to evaluate structural performance. Results exhibit the promising potential of the strategy for enhancing structural identification tasks. As for measurement system configuration, a systematic approach that involves multi-objective optimization and Multi Criteria Decision Making (MCDM) methodologies is proposed. The approach is able to accommodate model-free methodologies for damage detection and provide support for selecting the best compromise configuration. It is also applicable for situations where several model-free methods are used for data interpretation.

EPFL2013

Fire endurance of multicellular panels in an FRP building system

Craig Tracy

Fiber-reinforced synthetic polymers (FRP) are the building materials that may permit both the improvement of long-term building performance and the simplification of the construction process. Thanks to their high specific strength, low thermal conductivity, good environmental resistance, and their ability to be formed into complex shapes, FRP materials are well-suited to fulfilling many building functions. By integrating traditionally separate building systems and layers into single function-integrated components and industrially fabricating those components, the amount of on-site labor can be greatly reduced and overall quality can be improved. In order to profit from the advantageous qualities of FRP, however, it is essential to address the unique weaknesses and disadvantages of the material. Most notably, the problems of poor fire safety and high material costs must be overcome. In response to these challenges, a new multiple-story building system employing FRP materials is proposed. Within this system, fire safety is ensured through the use of an internal liquid cooling system, which circulates a cooling medium through the load-bearing FRP elements to maintain their temperature within a safe operating range. This system is made cost-effective through the integration of the building's heating and cooling system. By controlling the temperature of the circulating liquid, the building's structural elements can serve as heating or cooling emitters (radiators). Further, the addition of the liquid within the cells of the FRP elements helps maintain a more constant interior climate through the "thermal flywheel" effect, which improves energy efficiency and comfort. Experimental investigations were performed to explore the fire safety aspects of the proposed system. An existing FRP cellular bridge deck material was adapted to incorporate an internal liquid cooling system. After several preliminary investigations, large-scale experiments involving structural and fire loading were conducted on both liquid-cooled and non-liquid cooled specimens. The experiments demonstrated the efficacy of the system in protecting load-bearing FRP elements from the weakening effects of high temperatures, especially those that are stressed in compression. Structural fire endurance times were improved from less than one hour to more than two hours (EC1 Part 1.2) through the implementation of the liquid cooling system. Alongside the experimental program, a series of mathematical models were developed. Numerical thermochemical and thermomechanical models simulate the response of loaded liquid-cooled FRP panels in fire, while analytical models predict the post-fire mechanical behavior of fire-damaged sections. All models provide predictions that are within 10% of experimentally measured values.

EPFL2005

Assessment of mechanical properties in unirradiated and irradiated zircaloys and steels with non-standard tests and finite element calculations

Emiliano Campitelli

Nuclear materials have been successfully employed in commercial fission reactors for many decades. Research and material development for advanced fission system and future thermo-nuclear fusion reactors have acquired maturity while great challenges remain to be taken up. The goal of the material scientist is to understand the way neutron irradiation and aggressive reactor environment alter the original material properties and to safely manage the operation conditions for each particular reactor component. In the framework of surveillance programme, the need has arisen to design experimental methods to be used with a limited amount of material for Post Irradiation Examination (PIE) of reactor components to test the integrity of those on a regular time basis. Having to deal with radioactive material, it is particularly important to devise experimental testing procedures as simple as possible but from which reliable information can be drawn. Within materials development programme, irradiations have continuously been performed either in reactors or with particle accelerators. In both cases, the irradiation volumes are usually limited. The concern was to optimize the volume of the irradiated samples in order to increase their number to obtain a good statistical representation of their mechanical behavior after irradiation, and to have a very definite and homogeneous damage dose and irradiation temperature. The concerns explained above originated the development of small-scale specimen testing methods. Advantage is taken of the fact that, within size and geometry limitations, some mechanical properties of a sample are independent of its absolute size. In this way it is possible to perform mechanical tests on samples of reduced volume that represents big advantages to face the aforementioned challenges. Non-standard test on very small specimen have also been proposed to extract mechanical properties of the bulk. If these non-standard testing methods are to be employed for PIE, it becomes necessary to develop modeling tool to understand under what circumstances they can provide useful information and specially how to obtain it. Finite element modeling (FEM) of the non-standard tests has played a key role in this sense, providing a link between standard test analysis and that of the non-standard ones. The goal of this work was to implement finite element models for non-standard tests to ultimately study the mechanical properties of Zircaloy cladding tube and tempered martensitic steels in the unirradiated and irradiated condition. One model was developed for ring tensile tests. It was first validated with a quite ductile iron-chromium ferritic alloy, for which ring and uniaxial tensile test were performed at room temperature. Using the constitutive behavior determined from the uniaxial tensile tests, the load – displacement curves of the ring tensile tests were very well reproduced. Another model was developed for the so-called small ball punch test, which consists in deforming a 3 mm diameter disk clamped between two dies with a ball puncher and to record the load – displacement curve. In order to validate the model, a series of ball punch tests and of tensile tests were carried out on the austenitic steel 316L at room temperature. In this last case, the ball punch test curves were also well reconstructed with the model. Based upon a simple analysis and simulation results, we showed that it is possible to obtain, by ring tensile tests, a quick and accurate assessment of the constitutive behavior of an unknown material, simply by processing the experimental force-elongation data in the same way as for uniaxial tests, taking half the ring perimeter as initial length and twice the ring cross section as the sample cross section. The ring tensile test method was used to determine the hoop mechanical properties of Zircaloy tubes used in fission reactors. Tensile tests were also performed in the axial direction of the Zircaloy tubes. Differences between the yield stress as well as the post-yield behaviors in the axial direction and those in the circumferential direction of the tube could be measured reflecting some component of the anisotropy of the tube. Ball punch specimens were cut also out of the tube in the radial direction and tested. Using the FE model for punch test, in which the anisotropy of the mechanical properties are taken into account through the Hill's theory, the values of the Hill's yield function parameters were estimated. The irradiation hardening of neutron irradiated Zircaloy tube was finally determined. The ball punch test model was extensively used for numerical investigations, to determine how the constitutive behavior of the materials (uniaxial true stress – true strain) employed as input acts in combination with other experimental factors (specimen thickness, friction coefficient) in mediating the shape of the experimental curves. In the light of the results, we derived calibrations between parameters of the non-standard and standard tests. For instance we proposed a new approach to determine the yield stress from the ball punch test. We also showed how the slope of the punch tests scales with the average flow stress of the material. The calibrations proposed in the past by authors and published in the open literature have been revisited and critically reviewed. Based upon the findings of the numerical studies, we propose a given methodology to unambiguously evaluate the material constitutive behavior of a material in a case where the only available data comes from various non-standard tests. Experimental ball punch test results obtained on proton-irradiated specimen (0.5 dpa and Tirr = 523 K), of the tempered martensitic steel, EUROFER97, have been obtained. The irradiation hardening was found equal to about 150 MPa, consistently with previous data on similar steels and irradiation conditions. From the simulations of the ball punch test curves of the irradiated specimens, it was concluded that the irradiation does not affect the strain hardening at relatively low fluence.

EPFL2005

Assessment of mechanical properties in unirradiated and irradiated zircaloys and steels with non-standard tests and finite element calculations

Emiliano Campitelli

EPFL2005

Fire endurance of multicellular panels in an FRP building system

Craig Tracy

EPFL2005

Model-Free Methodologies for Data-Interpretation during Continuous Monitoring of Structures

Irwanda Laory

EPFL2013