Building tomorrow’s society / Bâtir la société de demain

Abstract: Power transmission lines predominantly involve lattice towers, which are typically composed of steel angle members connected together by means of bolted joints. An effective design of such structures requires the consideration of a range of complex phenomena likely to affect either the carrying capacity or the failure mode. In practice, simple numerical models are combined with standard design equations to consider these effects. A few advanced numerical models reported in the literature deal with eccentricities, stiffness of the connections, and joint slippage. However, the impact of residual stresses on the global behavior of lattice towers is not addressed in prior works. In this work, the influence of residual stresses is studied numerically using the finite element software Code_Aster. The proposed model employs multi-fibre beam elements to model the elastoplastic angle members, and discrete elements to represent the bolted connections. Both the connection eccentricity and the rotational stiffness of connections are modeled. The associated problem is solved in an incremental way, so as to deal with geometric and material nonlinearities, and the results are compared with experimental tests. Considering residual stresses in advanced models is an important step for the numerical evaluation of the failure of lattice towers

Contribution à l'analyse du flambement et post-flambement de poutres sandwich élastoplastiques

Les structures sandwich connaissent actuellement un essor sans précédent dans un large panel d'applications industrielles, du fait du compromis réalisé entre rigidité et légèreté. La présence d'éléments minces ou élancés tels que les peaux rend cependant ces matériaux très vulnérables face à des chargements de compression prédominante, et les instabilités géométriques dont ils font l'objet représentent même leur principale source de ruine. Ce travail est donc voué à l'étude analytique et numérique du flambement et du post-flambement de poutres sandwich sous compression longitudinale. L'approche analytique développée vise à formuler des expressions explicites et précises des différents modes de flambement (globaux/locaux) observés en pratique et des chargements critiques associés. Un élément fini 1D enrichi est ensuite mis en oeuvre, dont la formulation s'inspire des prédictions modales analytiques. Le programme numérique qui en découle est pourvu de techniques robustes de longueur d'arc et de branchement pour assurer un suivi de courbe incrémental efficace en présence de fortes non-linéarités et une gestion automatique de la bifurcation (sans l'introduction de défauts). À des fins de validation, des calculs EF 2D sont enfin réalisés en utilisant un code de calcul spécifique mettant en jeu les méthodes numériques sus-citées. Ces outils permettent l'analyse du post-flambement et la mise en valeur de divers phénomènes de localisation et d'interaction modale, tant en élasticité qu'en plasticité

Time-domain numerical model of substation conductor span subjected to short-circuit loading

Abstract: "Short-circuit faults in electrical power networks result in substantial electromagnetic forces on conductor cables as well as supporting structures. These dynamic forces often have a leading impact on the mechanical response of such elements, particularly in the case of short spans commonly encountered in substation structures. In this regard, most of the available design tools, such as international standard IEC 60865 [1], provide simplified equations to estimate the maximum forces induced during a short-circuit event. Besides the fact that such methods may be either too conservative or completely unsafe, the computed forces are imparted to the support structures as equivalent static loadings with safety margins [2], thus incurring unnecessarily prohibitive costs. Accounting for the dynamic effects in substation structures has shown to be essential for accurate and optimal designs [3]. However, ideal numerical modeling is usually time-consuming and requires large computational resources. In the present paper, a time-domain finite element model devoted to the dynamic analysis of conductors and support structures is presented for short flexible substation spans. The numerical model, developed using the open source software Code_Aster, employs one-dimensional elements accounting for large displacements to model a study case subjected to two consecutive short-circuit conditions. Three levels of modeling are analyzed and the resulting efforts in the structure are compared with analogous full-scale experimental results. Further parametric analysis is carried out numerically with a view to studying the effect of the short-circuits most influential parameters [...].

Building tomorrow’s society / Bâtir la société de demain

Abstract: Lattice towers are extensively used in overhead transmission lines, owing primarily to their lightness and cost-effectiveness. The modeling of such structures is usually laborious due to various complex factors including connection eccentricities, rotational stiffness of connections, bolt slippage, among others. Therefore, full-scale tests are usually performed for the qualification of new overhead line supports, which is a time-consuming and expensive process. Numerical models used in practice rely on simplified hypotheses, using linear truss/beam elements assumed to be pin-connected at both ends. Such models are combined with standard design equations to only evaluate the members’ axial capacities. Finite element models involving solid/shell elements are generally more accurate, though, the computational cost of the resulting problems makes it very difficult to evaluate the response of a complete tower. This paper presents an advanced numerical approach using beam elements aimed at predicting the load-bearing resistance of steel lattice towers under static load cases. Such approach will serve not only to verify the design of new towers, but also to understand various phenomena leading to the collapse of towers in the case of premature failures. The proposed model is developed using the finite element package Code_Aster, wherein lattice towers are modeled using spatial beams. The highly nonlinear problem is solved in an incremental way using advanced features to deal with both geometric and material nonlinearities. An example of a lattice tower loaded until failure is presented and compared with analogous experimental test. The effect of different geometric imperfections on the failure is particularly highlighted

Evaluation of phase-to-phase clearances of transmission line conductors under turbulent wind

Abstract: "Wind loads govern the design of a large number of overhead transmission lines worldwide. The spatial and temporal variation of wind speed and the buffeting response of cables may induce swings not fully synchronized between two parallel cables, thus raising serious concerns regarding phase-to-phase clearances. Accurate assessment of the minimum mid-span clearances to be considered in the design of transmission lines is essential to avoid repetitive flashover episodes and hence ensure the reliability of the electrical system. In practice, a wide range of empirical formulae is proposed to evaluate the critical phase-to-phase clearance distances for various transmission line configurations. For instance, the European standard CENELEC EN-50341 [1] provides a simple equation for the minimum horizontal spacing in the case of standard line heights not exceeding 60 m [2]. However, such formulae do not consider all the geometric and wind parameters affecting the phase-to-phase clearances, which is of key importance for efficient and economic design. This paper seeks at studying the most influential parameters affecting the phase-to-phase clearances in extreme wind events. A time-history numerical model is specifically developed using the finite element open software Code_Aster, where the behavior of both the conductors and insulator strings is represented using one-dimensional elements accounting for large displacements. The applied turbulent wind signals are also generated numerically, as functions of time and space coordinates. A two-span transmission line section including two phase conductors is considered herein to carry out a parametric study for various geometric and wind conditions. The results reveal that an empirical formula, considering only the conductor sag and insulator length, often yields misleading results [...].

Growing with youth / Croître avec les jeunes

Abstract: Lattice towers are the most commonly used structures in the field of overhead power transmission lines. In the design process of transmission lines, there are several methods for the evaluation of their capacity. The most common design method involves the use of three-dimensional linear elastic truss analyses to evaluate the axial forces in the pin-ended members. The resulting design is normally validated by full-scale experimental tests. These tests are very expensive and time-consuming. Moreover, the rarity of the testing facilities represents an additional difficulty. Hence there is an interest of using substructured pseudo-dynamic testing methods in which the experimental substructure, tested in a laboratory environment, interacts with a numerical model to emulate the structural behaviour of a complete structure. This method has several advantages but requires several preliminary analyses and planning for defining the critical substructure, dynamic parameters, and the setup’s flexibility. This work aims to develop a completely numerical substructuring strategy using the finite element software Code_Aster to ensure relevance and simplify the preparation and planning of pseudo-dynamic tests on lattice towers. An example of a lattice tower, under quasi-static load case is presented and compared with reference numerical analyses’ results. The effect of dynamic parameters (time step, damping ratio and load rate) on the emulated structure’s behaviour is analyzed in detail. Finally, the effects due to the flexibility of a simplified test set-up on the accuracy of the test results is studied

An enriched finite element model for the global and local buckling and post-buckling analysis of elastoplastic sandwich beam-columns

Ces travaux de thèse contribuent de manière générale au développement d’outils de modélisationanalytiques et numériques robustes et efficaces destinés à l’analyse des phénomènes de flambementet de post-flambement de poutres sandwich élastoplastiques sous divers cas de chargement. Lesdéveloppements analytiques effectués ont abouti à différentes formules explicites et précises pourles modes de flambement (globaux/locaux) observés en pratique et les chargements critiquesassociés. En s’inspirant des prédictions analytiques précédentes, un modèle élément fini 1D enrichide "poutre sandwich" a été mis en œuvre avec un choix déterminant pour la cinématique de l’âme.Le programme numérique qui en découle est pourvu de techniques robustes de longueur d’arc,assurant le suivi incrémental des courbes d’équilibre en présence de fortes non-linéarités à la foisgéométriques et matérielles, et de branchement, pour une gestion automatique de la bifurcationsans l’introduction de défauts. Un tel outil de calcul permet l’analyse du flambement et du post-flambement (jusqu’à des stades post-critiques avancés) de poutres sandwich élastiques ou plastiqueset met en évidence, de manière efficace, les phénomènes d’interaction modale, associés à l’obtentionde modes secondaires, spécifiques aux sandwichs.This Ph.D. thesis generally aims to establish accurate and efficient analytical and numerical toolsdevoted to the analysis of buckling and post-buckling phenomena likely to appear in sandwichstructures under various loading cases. The achieved analytical developments give rise to numerousaccurate closed-form expressions for the (global/local) buckling modes observed in practice and theassociated critical loadings. A 1D "sandwich beam" enriched finite element model is then developedbased on the previous analytical solutions with a particular focus on the core kinematics. Theresulting numerical program includes robust arc-length/path-following as well as branch-switchingtechniques, which allow one to deal efficiently with the possible strong geometric and materialnon-linearities encountered during the incremental calculations and handle the possible bifurcationpoints in a systematic way without introducing any imperfection. Such a numerical tool enablesthe buckling and post-buckling analysis (until advanced post-critical states) of elastic or plasticsandwich beam-columns and highlights, in an efficient way, the modal interaction phenomenaspecific to sandwich materials and associated to secondary modes

Post-buckling analysis of elastoplastic sandwich columns by means of an enriched 1D finite element model

International audienceAn advantageous 1D finite element model is designed in the present paper so as to analyze in an efficient way the post-buckling behavior of sandwich beams, known to be commonly responsible for the final collapse of such structures. An enriched beam formulation is defined, where the relatively thin skin layers are modeled as Timoshenko-Reissner beams, as they may undergo large rotations at advanced post-buckled stages. As for the homogeneous core layer, its complex behavior is represented by specific kinematics involving hyperbolic functions. The numerical model is developed within a total Lagrangian formulation framework, considering purely elastic behavior for the skins and an elastoplastic core material. The 1D finite element program incorporates effective incremental control techniques, namely arc length methods and branch-switching procedures, in order to cope with limit and bifurcation points due to material and geometric non-linearities. A series of incremental calculations is performed in the case of axially compressed columns, exhibiting both global and local buckling modes depending on the geometric and material features. Secondary bifurcations, giving rise to unstable post-buckled solutions, are encountered in most cases due to the operating modal interaction phenomena. The results are compared with reference numerical computations achieved using a 2D finite element customized program. (C) 2017 Elsevier Ltd. All rights reserved

Global/local analytical and numerical free vibration analysis of sandwich columns

International audienceSandwich structures are increasingly applied in many industrial fields of application, due to their lightweight combined with favorable mechanical properties. Despite this, such structures are subject to specific failure modes, such as buckling or vibratory resonance. In both cases, due to the presence of thin and stiff skins, global but also local modes may be of great interest when dimensioning such composite structures, which makes it impossible to use classical models. In this paper, the free vibration problem of classical sandwich columns (with homogeneous core materials) is investigated, using special kinematic models, so as to deal with both global and local eigenmodes in an effective and precise way. First, the problem is addressed analytically, where the two faces are represented by Euler-Bernoulli beams and the core material is considered as a 2D continuous solid, in small strain elasticity. Then, an enriched 1D finite element formulation is developed, so as to handle efficiently more general configurations encountered in practice. The homogeneous core layer is here described using hyperbolic functions, in accordance with the modal displacement fields obtained analytically. The present analytical and numerical solutions (natural frequencies and vibration modes) are contrasted against each other and compared to 2D reference numerical results

Time-domain numerical model of substation conductor span subjected to short-circuit loading

Abstract: "Short-circuit faults in electrical power networks result in substantial electromagnetic forces on conductor cables as well as supporting structures. These dynamic forces often have a leading impact on the mechanical response of such elements, particularly in the case of short spans commonly encountered in substation structures. In this regard, most of the available design tools, such as international standard IEC 60865 [1], provide simplified equations to estimate the maximum forces induced during a short-circuit event. Besides the fact that such methods may be either too conservative or completely unsafe, the computed forces are imparted to the support structures as equivalent static loadings with safety margins [2], thus incurring unnecessarily prohibitive costs. Accounting for the dynamic effects in substation structures has shown to be essential for accurate and optimal designs [3]. However, ideal numerical modeling is usually time-consuming and requires large computational resources. In the present paper, a time-domain finite element model devoted to the dynamic analysis of conductors and support structures is presented for short flexible substation spans. The numerical model, developed using the open source software Code_Aster, employs one-dimensional elements accounting for large displacements to model a study case subjected to two consecutive short-circuit conditions. Three levels of modeling are analyzed and the resulting efforts in the structure are compared with analogous full-scale experimental results. Further parametric analysis is carried out numerically with a view to studying the effect of the short-circuits most influential parameters [...].