Throughout recent history, famous records of building failures may be found, unfortunately accompanied by great human loss and major economic consequences. One of the mechanisms of failure is referred to as ‘progressive collapse’: one or several structural members suddenly fail, whatever the cause (accident or attack). The building then collapses progressively, every load redistribution causing the failure of other structural elements, until the complete failure of the building or of a major part of it. The civil engineering community’s attention to this type of event was first drawn by the progressive collapse of the building called Ronan Point, following a gas explosion in one of the last floors. Different simplified procedures for simulating the effects of progressive collapse can now be found in the literature, some of them described in detail. However, no extensive study can be found, in which these procedures are compared to more complete approaches for progressive collapse simulation, aiming at the comparison of the assumptions underlying them. To further contribute to the elaboration of design codes for progressive collapse, such a study would therefore be of great interest for practitioners.<p>All parties involved with the subject of progressive collapse are currently attempting to bridge the gap between the work done on the research front on the one hand, what can be considered as a fitting numerical model for regular industrial use on the other, and finally, the normalisation committees. The present research work aims at providing insight as to how the gaps between these poles may be reduced. The approach consists in studying the various hypotheses one by one, and gradually adding complexities to the numerical model, if they prove to be warranted by the need for sufficient accuracy. One of the contributions of the present work stems from this approach, in that it provides insight regarding the validity of the various simplifying assumptions. It also leads to the development of procedures which are kept as simple as possible, in an attempt to design them as best as possible for regular industrial use.<p>The objective of simplifying assumptions validation is pursued in Chapter 2. This chapter consists of the text of a paper entitled “Comparison and study of different progressive collapse simulation techniques for RC structures”, in which the main simplifying assumptions of the progressive collapse guidelines are detailed and assessed. The DoD [1] and GSA [2] static linear and non-linear procedures are investigated, and compared to more complete approaches in order to assess their validity.<p>In the next two chapters, two new procedures for design against progressive collapse are developed. They are based on quasi-static computations, their main objective being to account accurately for dynamic inertial effects. The first of these chapters consists in the text of a paper entitled “A new pushover analysis procedure for structural progressive collapse based on a kinetic energy criterion”, in which energetic considerations allow for the development of a static equivalent pushover procedure. The second chapter consists of the text of a paper entitled “A new pushover analysis procedure for structural progressive collapse based on optimised load amplification factors”, which uses load amplification factors resulting from optimisation procedures in order to account for dynamic inertial effects. The contributions of these two papers lie in the fact that they offer an improved accuracy on the results, when compared with other procedure available in the literature, which follow the same general principles. The two proposed procedures are thoroughly validated by systematic comparisons with results obtained with the more costly dynamic non-linear computations.<p>Finally, an additional chapter focuses on the various approaches that can be adopted for the simulation of reinforced concrete beams and columns. Because a rather simple model for reinforced concrete is used in Chapter 2, the bulk of this chapter consists in the implementation of a more complex fibre-based non-linear beam element. Comparisons performed with this model provide insight to the limitations of the simpler model, which is based on the use of lumped plastic hinges, but show this simpler model to be valid for the purposes of the present work.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

This doctoral research work focuses on the simulation of progressive collapse of reinforced concrete structures. It aims at contributing to the ‘alternate load path’ design approach suggested by the General Services Administration (GSA) and the Department of Defense (DoD) of the United States, by providing a detailed yet flexible numerical modelling tool. <p><p>The finite element formulation adopted here is based on a multilevel approach where the response at the structural level is naturally deduced from the behaviour of the constituents (concrete and steel) at the material level. One-dimensional nonlinear constitutive laws are used to model the material response of concrete and steel. These constitutive equations are introduced in a layered beam approach, where the cross-sections of the structural members are discretised through a finite number of layers. This modelling strategy allows deriving physically motivated relationships between generalised stresses and strains at the sectional level. Additionally, a gradual sectional strength degradation can be obtained as a consequence of the progressive failure of the constitutive layers. This means that complex nonlinear sectional responses exhibiting softening can be obtained even for simplified one dimensional constitutive laws for the constituents.<p><p>This numerical formulation is used in dynamic progressive collapse simulations to study the structural response of a multi-storey planar frame subject to a sudden column loss. The versatility of the proposed methodology allows assessing the influence of the main material and design parameters in the structural failure. Furthermore, the effect of particular modelling options of the progressive collapse simulation technique, such as the column removal time or the strategy adopted for the structural verification, can be evaluated.<p><p>The potential strain rate effects on the structural response of reinforced concrete frames are also investigated. To this end, a strain rate dependent material formulation is developed, where the rate effects are introduced in both the concrete and steel constitutive response. These effects are incorporated at the structural level through the multilayered beam approach. In order to assess the degree of rate dependence in progressive collapse, the results of rate dependent simulations are presented and compared to those obtained via the rate independent approach. The influence of certain parameters on the rate dependent structural failure is also studied.<p><p>The differences obtained in terms of progressive failure degree for the considered parametric variations and modelling options are analysed and discussed. The parameters observed to have a major influence on the structural response in a progressive collapse scenario are the ductility of the steel bars, the degree of symmetry and/or continuity of the reinforcement and the column removal time. The results also depend on the strategy considered (GSA vs DoD). The strain rate effects are confirmed to play a significant role in the failure pattern. Based on these observations, general recommendations for the design of progressive collapse resisting structures are finally derived.<p><p><p><p><p>L’effondrement progressif est un sujet de recherche qui a connu un grand développement suite aux événements désastreux qui se sont produits au cours des dernières décennies. Ce phénomène est déclenché par la défaillance soudaine d’un nombre réduit d’éléments porteurs de la structure, qui provoque une propagation en cascade de l’endommagement d’élément en élément jusqu’à affecter une partie importante, voire la totalité de l’ouvrage. Le résultat est donc disproportionné par rapport à la cause. La plupart des codes de construction ont inclus des prescriptions pour le dimensionnement des structures face aux actions accidentelles. Malheureusement, ces procédures se limitent à fournir des ‘règles de bonne pratique’, ou proposent des calculs simplifiés se caractérisant par un manque de détail pour permettre leur mise en oeuvre.<p><p>Cette thèse de doctorat intitulée Simulation de l’Effondrement Progressif des Structures en Béton Armé: Influence des Paramètres Materiaux et de Dimensionnement et Investigation des Effets de Vitesse a pour but de contribuer à la simulation numérique de l’effondrement progressif des structures en béton armé. Une formulation aux éléments finis basée sur une approche multi-échelles a été développée, où la réponse à l’échelle structurale est déduite à partir de la réponse au niveau matériel des constituants (le béton et l’acier). Les sections des éléments structuraux sont divisées en un nombre fini de couches pour lesquelles des lois constitutives unidimensionnelles sont postulées. Cet outil permet une dégradation graduelle de la résistance des sections en béton armé suite à la rupture progressive des couches. Des comportements complexes au niveau des points de Gauss peuvent être ainsi obtenus, et cela même à partir de lois unidimensionnelles pour les constituants.<p><p>Cette formulation est utilisée pour la simulation de l’effondrement progressif d’ossatures 2D, avec prise en compte des effets dynamiques. La versatilité de la présente stratégie numérique permet d’analyser l’influence de différents paramètres matériaux et de dimensionnement, ainsi que d’autres paramètres de modélisation, sur la réponse structurale face à la disparition soudaine d’une colonne.<p><p>Les effets de la vitesse de déformation sur le comportement des matériaux constituants est aussi un sujet d’attention dans ce travail de recherche. Des lois constitutives prenant en compte ces effets sont postulées et incorporées au niveau structural grâce à l’approche multi-couches. Le but est d’étudier l’influence des effets de la vitesse de chargement sur la réponse structurale face à la disparition d’un élément porteur. Les resultats obtenus à l’aide de cette approche avec effets de vitesse sont comparés à ceux obtenus avec des lois indépendantes de la vitesse.<p><p>Les différences dans la réponse à la disparition d’une colonne sont analysées pour les variations paramétriques étudiées. Les paramètres ayant une influence importante sont notamment: la ductilité des matériaux constituants et la disposition et/ou la symétrie des armatures. Les effets de vitesse sont également significatifs. Sur base de ces résultats, des recommandations sont proposées pour le dimensionnement et/ou l’analyse des structures face à l’effondrement progressif.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

<p align="justify">In a context of restoration of historical masonry structures, it is crucial to properly estimate the residual strength and the potential structural failure modes in order to assess the safety of buildings. Due to its mesostructure and the quasi-brittle nature of its constituents, masonry presents preferential damage orientations, strongly localised failure modes and damage-induced anisotropy, which are complex to incorporate in structural computations. Furthermore, masonry structures are generally subjected to complex loading processes including both in-plane and out-of-plane loads which considerably influence the potential failure mechanisms. As a consequence, both the membrane and the flexural behaviours of masonry walls have to be taken into account for a proper estimation of the structural stability.</p><p><p align="justify">Macrosopic models used in structural computations are based on phenomenological laws including a set of parameters which characterises the average behaviour of the material. These parameters need to be identified through experimental tests, which can become costly due to the complexity of the behaviour particularly when cracks appear. The existing macroscopic models are consequently restricted to particular assumptions. Other models based on a detailed mesoscopic description are used to estimate the strength of masonry and its behaviour with failure. This is motivated by the fact that the behaviour of each constituent is a priori easier to identify than the global structural response. These mesoscopic models can however rapidly become unaffordable in terms of computational cost for the case of large-scale three-dimensional structures.</p><p><p align="justify">In order to keep the accuracy of the mesoscopic modelling with a more affordable computational effort for large-scale structures, a multi-scale framework using computational homogenisation is developed to extract the macroscopic constitutive material response from computations performed on a sample of the mesostructure, thereby allowing to bridge the gap between macroscopic and mesoscopic representations. Coarse graining methodologies for the failure of quasi-brittle heterogeneous materials have started to emerge for in-plane problems but remain largely unexplored for shell descriptions. The purpose of this study is to propose a new periodic homogenisation-based multi-scale approach for quasi-brittle thin shell failure.</p><p><p align="justify">For the numerical treatment of damage localisation at the structural scale, an embedded strong discontinuity approach is used to represent the collective behaviour of fine-scale cracks using average cohesive zones including mixed cracking modes and presenting evolving orientation related to fine-scale damage evolutions.</p><p><p align="justify">A first originality of this research work is the definition and analysis of a criterion based on the homogenisation of a fine-scale modelling to detect localisation in a shell description and determine its evolving orientation. Secondly, an enhanced continuous-discontinuous scale transition incorporating strong embedded discontinuities driven by the damaging mesostructure is proposed for the case of in-plane loaded structures. Finally, this continuous-discontinuous homogenisation scheme is extended to a shell description in order to model the localised behaviour of out-of-plane loaded structures. These multi-scale approaches for failure are applied on typical masonry wall tests and verified against three-dimensional full fine-scale computations in which all the bricks and the joints are discretised.</p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished