Progressive collapse simulation of reinforced concrete structures: influence of design and material parameters and investigation of the strain rate effects

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. 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.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.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.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.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.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.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.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.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.Doctorat en Sciences de l'ingénieurinfo:eu-repo/semantics/nonPublishe

Modelling of RC Structures for Progressive Collapse Simulation

info:eu-repo/semantics/nonPublishe

Non-linear static and dynamic simulation methods for progressive collapse

Invited plenary lectureinfo:eu-repo/semantics/nonPublishe

Effect of concrete rate dependent behaviour on structural progressive collapse

This contribution focuses on the need to take into account the material rate dependence in the modelling of reinforced concrete structures for progressive collapse analyses. Since progressive collapse is a dynamic phenomenon which depends strongly on stress redistribution, the strain rate effects may play a significant role in the overall structural response. A viscoplastic model is adopted to couple strain rate effects to the plastic response of both concrete and steel. The rate dependence in the elastic domain is also considered. The introduction of 1D constitutive laws for concrete and steel in a layered beam model provides strain rate dependent relations between generalised stresses and strains at the cross-sectional level. This multilevel approach is used in progressive collapse simulations where the structural response of a RC planar frame undergoing a sudden column loss is studied. Inertial effects are also taken into account. The results obtained using rate dependent material laws are compared to those provided by a rate independent approach. The influence of the column removal time on the structural resistance is also assessed. Noticeable differences are observed in terms of the degree of progressive failure. It can be concluded that the material strain rate effects might lead to an increased structural resistance to the sudden loss of a load-bearing member.info:eu-repo/semantics/publishe

On the influence of design and material parameters on the progressive collapse of reinforced concrete structures

Progressive collapse of civil engineering structures is originated by the localized failure of one or more structural member following an abnormal loading event (accidental action, natural hazard, attack). This local failure initiates load redistribution in the structure which, in turn leads to additional collapse and eventually results in a wide-spread catastrophic partial or total structural failure to an extent disproportionate to the initial triggering event.Codes of design in civil engineering (e.g. Eurocodes) require ensuring structural robustness to decrease the destructive impact of accidents or malevolent attacks to structures, without however precisely specifying how to define and quantify structural robustness. A fundamental question therefore arises with regards to the assessment of structural robustness. Considering the complex (dynamic) nature of structural damage and collapse and the nonlinear interplay of materials behaviour (rate-dependent elastic and plastic behavior for instance) and design parameters (e.g. reinforcements position and fraction in reinforced concrete beam sections, connection features in steel structures), this requires the use of advanced computational tools.The prime goal of this research axis of the BATir department of the Univesité Libre de Bruxelles (ULB) is to develop dedicated computational tools for the investigation of the notion of structural robustness via the numerical simulation of structural collapse. The current methodology (multilayered beam formulation and dynamic nonlinear structural framework) uses a direct transition from the nonlinear strain rate dependent behavior of concrete and steel at the constituent level to the response of the structural members at the global scale via a computational homogenization scheme. This allows deriving physically motivated relationships between generalized stresses and strains at the sectional level and conducting sensitivity analyses on the design and material parameters influence on the structural response of multi-storey planar frames subjected to a sudden column loss. The universal demand for developments in structural and civil engineering striving for increased structural safety safeguards the motivation of this research subject.Plenary lectureinfo:eu-repo/semantics/publishe