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Excessive long-time deflections of prestressed box girders. I / Bažant, Zdeněk P. in Journal of structural engineering, Vol. 138 N° 6 (Juin 2012) 

Public ISBD [article]  in Journal of structural engineering > Vol. 138 N° 6 (Juin 2012) . - pp. 676–686 Titre : Excessive long-time deflections of prestressed box girders. I : Record-span bridge in palau and other paradigms Type de document : texte imprimé Auteurs : Bažant, Zdeněk P., Auteur ; Yu, Qiang, Auteur ; Guang-Hua Li, Auteur Année de publication : 2012 Article en page(s) : pp. 676–686 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Prestressed  box  girder  Bridges  Segmental  erection  Shear  lag  Design  standards  Concrete  Relaxation Résumé : The segmental prestressed concrete box girder of Koror-Babeldaob (KB) Bridge in Palau, which had a record span of 241 m (791 ft), presents a striking paradigm of serviceability loss because of excessive multidecade deflections. The data required for analysis have recently been released and are here exploited to show how the analysis and design could be improved. Erected segmentally in 1977, this girder developed a midspan deflection of 1.61 m (5.3 ft) compared with the design camber after 18 years, and it collapsed in 1996 as a consequence of remedial prestressing, after a 3-month delay. Compared with three-dimensional analysis, the traditional beam-type analysis of box girder deflections is found to have errors up to 20%, although greater errors are likely for bridges with higher box-width-to-span ratios than the KB Bridge. However, even three-dimensional finite-element analysis with step-by-step time integration cannot explain the observed deflections when the current American Concrete Institute, Japan Society of Civil Engineers, Comité Euro-International du Béton (or Comité Euro-International du Béton—Fédération internationale de la précontrainte), and Gardner and Lockman prediction models for creep and shrinkage are used. These models give 18-year deflection estimates that are 50–77% lower than measured and yield unrealistic shapes of the deflection history. They also predict the 18-year prestress loss to be 46–56% lower than the measured mean prestress loss, which was 50%. Model B3, which is the only theoretically based model, underestimates the 18-year deflection by 42% and gives a prestress loss of 40% when the default parameter values are used. However, in Model B3, several input parameters are adjustable and if they are adjusted according to the long-time laboratory tests of Brooks, a close fit of all the measurements is obtained. For early deflections and their extrapolation, it is important that Model B3 can capture realistically the differences in the rates of shrinkage and drying creep caused by the differences in the thickness of the walls of the cross section. The differences in temperature and possible cracking of the top slab also need to be taken into account. Other paradigms on which data have recently been released are four bridges in Japan and one in the Czech Republic. Their excessive deflections can also be explained. The detailed method of analysis and the lessons learned are presented in Part II. ISSN : 0733-9445 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0000487 [article] Excessive long-time deflections of prestressed box girders. I : Record-span bridge in palau and other paradigms [texte imprimé] / Bažant, Zdeněk P., Auteur ; Yu, Qiang, Auteur ; Guang-Hua Li, Auteur . - 2012 . - pp. 676–686. Génie Civil Langues : Anglais (eng) in Journal of structural engineering > Vol. 138 N° 6 (Juin 2012) . - pp. 676–686 Mots-clés : Prestressed  box  girder  Bridges  Segmental  erection  Shear  lag  Design  standards  Concrete  Relaxation Résumé : The segmental prestressed concrete box girder of Koror-Babeldaob (KB) Bridge in Palau, which had a record span of 241 m (791 ft), presents a striking paradigm of serviceability loss because of excessive multidecade deflections. The data required for analysis have recently been released and are here exploited to show how the analysis and design could be improved. Erected segmentally in 1977, this girder developed a midspan deflection of 1.61 m (5.3 ft) compared with the design camber after 18 years, and it collapsed in 1996 as a consequence of remedial prestressing, after a 3-month delay. Compared with three-dimensional analysis, the traditional beam-type analysis of box girder deflections is found to have errors up to 20%, although greater errors are likely for bridges with higher box-width-to-span ratios than the KB Bridge. However, even three-dimensional finite-element analysis with step-by-step time integration cannot explain the observed deflections when the current American Concrete Institute, Japan Society of Civil Engineers, Comité Euro-International du Béton (or Comité Euro-International du Béton—Fédération internationale de la précontrainte), and Gardner and Lockman prediction models for creep and shrinkage are used. These models give 18-year deflection estimates that are 50–77% lower than measured and yield unrealistic shapes of the deflection history. They also predict the 18-year prestress loss to be 46–56% lower than the measured mean prestress loss, which was 50%. Model B3, which is the only theoretically based model, underestimates the 18-year deflection by 42% and gives a prestress loss of 40% when the default parameter values are used. However, in Model B3, several input parameters are adjustable and if they are adjusted according to the long-time laboratory tests of Brooks, a close fit of all the measurements is obtained. For early deflections and their extrapolation, it is important that Model B3 can capture realistically the differences in the rates of shrinkage and drying creep caused by the differences in the thickness of the walls of the cross section. The differences in temperature and possible cracking of the top slab also need to be taken into account. Other paradigms on which data have recently been released are four bridges in Japan and one in the Czech Republic. Their excessive deflections can also be explained. The detailed method of analysis and the lessons learned are presented in Part II. ISSN : 0733-9445 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0000487

Excessive long-time deflections of prestressed box girders. II / Bažant, Zdeněk P. in Journal of structural engineering, Vol. 138 N° 6 (Juin 2012) 

Public ISBD [article]  in Journal of structural engineering > Vol. 138 N° 6 (Juin 2012) . - pp. 687–696 Titre : Excessive long-time deflections of prestressed box girders. II : Numerical analysis and lessons learned Type de document : texte imprimé Auteurs : Bažant, Zdeněk P., Auteur ; Yu, Qiang, Auteur ; Guang-Hua Li, Auteur Année de publication : 2012 Article en page(s) : pp. 687–696 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Kelvin  chain  Design  standards  Segmental  erection  Bridges  Shear  lag  Prestressed  concrete  Relaxation Résumé : As a sequel to Part I, which clarified the causes of the unexpectedly large deflections of the Koror-Babeldaob Bridge in the Pacific island nation of Palau, Part II presents the numerical procedure and reviews the lessons learned. The box girder represents a thick shell that is discretized by eight-node, three-dimensional (3D) finite elements. Except for corrections due to cracking, concrete creep is assumed to follow aging linear viscoelasticity and is modeled by a rate-type law based on the Kelvin chain, the properties of which are adjusted for humidity conditions and temperature. In each time step and at each integration point, Widder’s formula is used to convert the aging compliance function to a continuous retardation spectrum for the current age of concrete, and discretization of the spectrum yields the current elastic moduli of the Kelvin units. The shrinkage strains depend on the environmental humidity and the thickness of each plate in the cross section. The computations proceed according to Bažant’s exponential algorithm, which is unconditionally stable and reduces the problem to a sequence of elasticity problems with an orthotropic effective stiffness of material and nonisotropic inelastic strains, different for each integration point in each time step. These problems are solved by commercial software ABAQUS. The segmental construction sequence is also modeled. The computer results reported in Part I explain the excessive deflections and compare the performance of various material models for creep and shrinkage. Part II formulates the lessons learned and makes recommendations for implementation. ISSN : 0733-9445 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0000375 [article] Excessive long-time deflections of prestressed box girders. II : Numerical analysis and lessons learned [texte imprimé] / Bažant, Zdeněk P., Auteur ; Yu, Qiang, Auteur ; Guang-Hua Li, Auteur . - 2012 . - pp. 687–696. Génie Civil Langues : Anglais (eng) in Journal of structural engineering > Vol. 138 N° 6 (Juin 2012) . - pp. 687–696 Mots-clés : Kelvin  chain  Design  standards  Segmental  erection  Bridges  Shear  lag  Prestressed  concrete  Relaxation Résumé : As a sequel to Part I, which clarified the causes of the unexpectedly large deflections of the Koror-Babeldaob Bridge in the Pacific island nation of Palau, Part II presents the numerical procedure and reviews the lessons learned. The box girder represents a thick shell that is discretized by eight-node, three-dimensional (3D) finite elements. Except for corrections due to cracking, concrete creep is assumed to follow aging linear viscoelasticity and is modeled by a rate-type law based on the Kelvin chain, the properties of which are adjusted for humidity conditions and temperature. In each time step and at each integration point, Widder’s formula is used to convert the aging compliance function to a continuous retardation spectrum for the current age of concrete, and discretization of the spectrum yields the current elastic moduli of the Kelvin units. The shrinkage strains depend on the environmental humidity and the thickness of each plate in the cross section. The computations proceed according to Bažant’s exponential algorithm, which is unconditionally stable and reduces the problem to a sequence of elasticity problems with an orthotropic effective stiffness of material and nonisotropic inelastic strains, different for each integration point in each time step. These problems are solved by commercial software ABAQUS. The segmental construction sequence is also modeled. The computer results reported in Part I explain the excessive deflections and compare the performance of various material models for creep and shrinkage. Part II formulates the lessons learned and makes recommendations for implementation. ISSN : 0733-9445 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0000375

Size-effect testing of cohesive fracture parameters and nonuniqueness of work-of-fracture method / Bažant, Zdeněk P. in Journal of engineering mechanics, Vol. 137 N° 8 (Août 2011) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 137 N° 8 (Août 2011) . - pp.580-588 Titre : Size-effect testing of cohesive fracture parameters and nonuniqueness of work-of-fracture method Type de document : texte imprimé Auteurs : Bažant, Zdeněk P., Auteur ; Qiang Yu, Auteur Année de publication : 2011 Article en page(s) : pp.580-588 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Quasibrittle  fracture  Cohesive  crack  model  Fracture  mechanics  Cracks  Quasibrittle  materials  Concrete  Scaling  Testing  Brittleness Résumé : The cohesive crack model has been widely accepted as the best compromise for the analysis of fracture of concrete and other quasibrittle materials. The softening stress-separation law of this model is now believed to be best described as a bilinear curve characterized by four parameters: the initial and total fracture energies Gf and GF, the tensile strength ft′, and the knee-point ordinate σ1. The classical work-of-fracture test of a notched beam of one size can deliver a clear result only for GF. Here it is shown computationally that the same complete load-deflection curve can be closely approximated with stress-separation curves in which the ft′ values differ by 77% and Gf values by 68%. It follows that the work-of-fracture test alone cannot provide an unambiguous basis for quasibrittle fracture analysis. It is found, however, that if this test is supplemented by size-effect testing, all four cohesive crack model parameters can be precisely identified and the fracture analysis of structures becomes unambiguous. It is shown computationally that size-effect tests do not suffice for determining GF and ft′, which indicates that they provide a sufficient basis for computing neither the postpeak softening of fracturing structures nor the peak loads of a very large structure. However, if the size-effect tests are supplemented by one complete softening load-deflection curve of a notched specimen, an unambiguous calculation of peak loads and postpeak response of structures becomes possible. To this end, the notched specimen tests must be conducted in a certain size range, whose optimum is here established by extending a previous analysis. Combination of the work-of-fracture and size-effect testing could be avoided only if the ratios GF/Gf and σ1/ft′ were known a priori, but unfortunately their estimates are far too uncertain. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v137/i8/p580_s1?isAuthorized=no [article] Size-effect testing of cohesive fracture parameters and nonuniqueness of work-of-fracture method [texte imprimé] / Bažant, Zdeněk P., Auteur ; Qiang Yu, Auteur . - 2011 . - pp.580-588. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 137 N° 8 (Août 2011) . - pp.580-588 Mots-clés : Quasibrittle  fracture  Cohesive  crack  model  Fracture  mechanics  Cracks  Quasibrittle  materials  Concrete  Scaling  Testing  Brittleness Résumé : The cohesive crack model has been widely accepted as the best compromise for the analysis of fracture of concrete and other quasibrittle materials. The softening stress-separation law of this model is now believed to be best described as a bilinear curve characterized by four parameters: the initial and total fracture energies Gf and GF, the tensile strength ft′, and the knee-point ordinate σ1. The classical work-of-fracture test of a notched beam of one size can deliver a clear result only for GF. Here it is shown computationally that the same complete load-deflection curve can be closely approximated with stress-separation curves in which the ft′ values differ by 77% and Gf values by 68%. It follows that the work-of-fracture test alone cannot provide an unambiguous basis for quasibrittle fracture analysis. It is found, however, that if this test is supplemented by size-effect testing, all four cohesive crack model parameters can be precisely identified and the fracture analysis of structures becomes unambiguous. It is shown computationally that size-effect tests do not suffice for determining GF and ft′, which indicates that they provide a sufficient basis for computing neither the postpeak softening of fracturing structures nor the peak loads of a very large structure. However, if the size-effect tests are supplemented by one complete softening load-deflection curve of a notched specimen, an unambiguous calculation of peak loads and postpeak response of structures becomes possible. To this end, the notched specimen tests must be conducted in a certain size range, whose optimum is here established by extending a previous analysis. Combination of the work-of-fracture and size-effect testing could be avoided only if the ratios GF/Gf and σ1/ft′ were known a priori, but unfortunately their estimates are far too uncertain. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v137/i8/p580_s1?isAuthorized=no

Temperature Effect on Concrete Creep Modeled by Microprestress-Solidification Theory / Bažant, Zdeněk P. in Journal of engineering mechanics, Vol. 130 N°6 (Juin 2004)

Public ISBD [article]  in Journal of engineering mechanics > Vol. 130 N°6 (Juin 2004) . - 691-699 p. Titre : Temperature Effect on Concrete Creep Modeled by Microprestress-Solidification Theory Titre original : L'Effet de la Température sur le Fluage Concret modelisé par la Théorie de Micro-précontrain de Solidification Type de document : texte imprimé Auteurs : Bažant, Zdeněk P., Auteur ; Gianluca Cusatis, Auteur ; Cedolin, Luigi ; Anandarajah, A. Rajah, Editeur scientifique Article en page(s) : 691-699 p. Note générale : Génie Mécanique Langues : Anglais (eng) Mots-clés : Temperature  effects  Concrete  Creep  Shrincage  Solidification  Aging  Viscoelasticity  Microstructure  Thermodynamics  Adsorption  Effets  de  la  température  Concret  Fluage  Rétrécissement  Solidification  Vieillissement  Visco-élasticité  Micro-structure  Thermodynamique  Adsorption Index. décimale : 621.34/624 Résumé : The previously developed microprestress-solidification theory for concrete creep and shrinkage is generalized for the effect of temperature (not exceeding 100°C). The solidification model separates the viscoelasticity of the solid constituent, the cement gel, from the chemical aging of material caused by solidification of cement and characterized by the growth of volume fraction of hydration products. This permits considering the viscoelastic constituent as non-aging. The temperature dependence of the rates of creep and of volume growth is characterized by two transformed time variables based on the activation energies of hydration and creep. The concept of microprestress achieves a grand unification of theory in which the long-term aging and all transient hygrothermal effects simply become different consequences of one and the same physical phenomenon. The microprestress, which is independent of the applied load, is initially produced by incompatible volume changes in the microstructure during hydration, and later builds up when changes of moisture content and temperature create a thermodynamic imbalance between the chemical potentials of vapor and adsorbed water in the nanopores of cement gel. As recently shown, this simultaneously captures two basic effects: First, the creep decreases with increasing age at loading after the growth of the volume fraction of hydrated cement has ceased; and, second, the drying creep, i.e., the transient creep increases due to drying (Pickett effect) which overpowers the effect of steady-state moisture content (i.e., less moisture—less creep). Now it is demonstrated that the microprestress buildup and relaxation also captures a third effect: The transitional thermal creep, i.e., the transient creep increase due to temperature change. For computations, an efficient (exponential-type) integration algorithm is developed. Finite element simulations, in which the apparent creep due to microcracking is taken into account separately, are used to identify the constitutive parameters and a satisfactory agreement with typical test data is achieved. Précédemment développés micro-précontraint la théorie de solidification pour le fluage concret et le rétrécissement est généralisé pour l'effet de la température (n'excédant pas 100°C). Le modèle de solidification sépare la visco-élasticité du constituant plein, le gel de ciment, du vieillissement chimique du matériel provoqué par solidification de ciment et caractérisé par la croissance de la fraction de volume des produits d'hydration. Ceci laisse considérer le constituant visco-élastique comme non-vieillissement. La dépendance de la température des taux de fluage et de croissance de volume est caractérisée par deux variables transformées de temps basées sur les énergies d'activation de l'hydration et du fluage. Le concept de micro-précontraint réalise une unification grande de théorie dans laquelle le vieillissement à long terme et tous effets hygrothermal passagers deviennent simplement différentes conséquences d'un et le même phénomène physique. Micro-précontraint, qui est indépendant de la charge appliquée, est au commencement produit par les changements de volume incompatibles de la microstructure pendant l'hydration, et s'accumule plus tard quand les variations température le contenu et d'humidité créent un déséquilibre thermo-dynamique entre les potentiels chimiques de la vapeur et l'eau adsorbée dans les nanopores du gel de ciment. Comme récemment montré, ceci capture simultanément deux effets de base : D'abord, le fluage diminue avec l'augmentation de l'âge au chargement après que la croissance de la fraction de volume du ciment hydraté ait cessé ; et, en second lieu, le fluage de séchage, c.-à-d., les augmentations passagères de fluage dues au séchage (effet de Pickett) qui maîtrise l'effet du contenu d'humidité équilibré (c.-à-d., moins de fluage d'humidité-moins). Maintenant on le démontre que l'habillage et la relaxation de micro-précontraint capture également un troisième effet : Le fluage thermique transitoire, c.-à-d., l'augmentation passagère de fluage due au changement de température. Pour des calculs, un algorithme efficace d'intégration (de type exponentiel) est développé. Des simulations finies d'élément, dans lesquelles le fluage apparent dû à la microfissuration est pris en considération séparément, sont employées pour identifier les paramètres constitutifs et un accord satisfaisant avec des essais typiques est réalisées. [article] Temperature Effect on Concrete Creep Modeled by Microprestress-Solidification Theory = L'Effet de la Température sur le Fluage Concret modelisé par la Théorie de Micro-précontrain de Solidification [texte imprimé] / Bažant, Zdeněk P., Auteur ; Gianluca Cusatis, Auteur ; Cedolin, Luigi ; Anandarajah, A. Rajah, Editeur scientifique . - 691-699 p. Génie Mécanique Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 130 N°6 (Juin 2004) . - 691-699 p. Mots-clés : Temperature  effects  Concrete  Creep  Shrincage  Solidification  Aging  Viscoelasticity  Microstructure  Thermodynamics  Adsorption  Effets  de  la  température  Concret  Fluage  Rétrécissement  Solidification  Vieillissement  Visco-élasticité  Micro-structure  Thermodynamique  Adsorption Index. décimale : 621.34/624 Résumé : The previously developed microprestress-solidification theory for concrete creep and shrinkage is generalized for the effect of temperature (not exceeding 100°C). The solidification model separates the viscoelasticity of the solid constituent, the cement gel, from the chemical aging of material caused by solidification of cement and characterized by the growth of volume fraction of hydration products. This permits considering the viscoelastic constituent as non-aging. The temperature dependence of the rates of creep and of volume growth is characterized by two transformed time variables based on the activation energies of hydration and creep. The concept of microprestress achieves a grand unification of theory in which the long-term aging and all transient hygrothermal effects simply become different consequences of one and the same physical phenomenon. The microprestress, which is independent of the applied load, is initially produced by incompatible volume changes in the microstructure during hydration, and later builds up when changes of moisture content and temperature create a thermodynamic imbalance between the chemical potentials of vapor and adsorbed water in the nanopores of cement gel. As recently shown, this simultaneously captures two basic effects: First, the creep decreases with increasing age at loading after the growth of the volume fraction of hydrated cement has ceased; and, second, the drying creep, i.e., the transient creep increases due to drying (Pickett effect) which overpowers the effect of steady-state moisture content (i.e., less moisture—less creep). Now it is demonstrated that the microprestress buildup and relaxation also captures a third effect: The transitional thermal creep, i.e., the transient creep increase due to temperature change. For computations, an efficient (exponential-type) integration algorithm is developed. Finite element simulations, in which the apparent creep due to microcracking is taken into account separately, are used to identify the constitutive parameters and a satisfactory agreement with typical test data is achieved. Précédemment développés micro-précontraint la théorie de solidification pour le fluage concret et le rétrécissement est généralisé pour l'effet de la température (n'excédant pas 100°C). Le modèle de solidification sépare la visco-élasticité du constituant plein, le gel de ciment, du vieillissement chimique du matériel provoqué par solidification de ciment et caractérisé par la croissance de la fraction de volume des produits d'hydration. Ceci laisse considérer le constituant visco-élastique comme non-vieillissement. La dépendance de la température des taux de fluage et de croissance de volume est caractérisée par deux variables transformées de temps basées sur les énergies d'activation de l'hydration et du fluage. Le concept de micro-précontraint réalise une unification grande de théorie dans laquelle le vieillissement à long terme et tous effets hygrothermal passagers deviennent simplement différentes conséquences d'un et le même phénomène physique. Micro-précontraint, qui est indépendant de la charge appliquée, est au commencement produit par les changements de volume incompatibles de la microstructure pendant l'hydration, et s'accumule plus tard quand les variations température le contenu et d'humidité créent un déséquilibre thermo-dynamique entre les potentiels chimiques de la vapeur et l'eau adsorbée dans les nanopores du gel de ciment. Comme récemment montré, ceci capture simultanément deux effets de base : D'abord, le fluage diminue avec l'augmentation de l'âge au chargement après que la croissance de la fraction de volume du ciment hydraté ait cessé ; et, en second lieu, le fluage de séchage, c.-à-d., les augmentations passagères de fluage dues au séchage (effet de Pickett) qui maîtrise l'effet du contenu d'humidité équilibré (c.-à-d., moins de fluage d'humidité-moins). Maintenant on le démontre que l'habillage et la relaxation de micro-précontraint capture également un troisième effet : Le fluage thermique transitoire, c.-à-d., l'augmentation passagère de fluage due au changement de température. Pour des calculs, un algorithme efficace d'intégration (de type exponentiel) est développé. Des simulations finies d'élément, dans lesquelles le fluage apparent dû à la microfissuration est pris en considération séparément, sont employées pour identifier les paramètres constitutifs et un accord satisfaisant avec des essais typiques est réalisées.

Why the observed motion history of world trade center towers is smooth / Jia-Liang Le in Journal of engineering mechanics, Vol. 137 N° 1 (Janvier 2011) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 137 N° 1 (Janvier 2011) . - pp.82-84 Titre : Why the observed motion history of world trade center towers is smooth Type de document : texte imprimé Auteurs : Jia-Liang Le, Auteur ; Bažant, Zdeněk P., Auteur Année de publication : 2011 Article en page(s) : pp.82-84 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Structural  dynamics  Structural  failures  Damage  Monitoring  Building  high-rise  New  York  City  Velocity  Imaging  techniques. Résumé : The collapse of the World Trade Center towers was initiated by the impact of the upper falling part onto the underlying intact story. At the moment of impact, the velocity of the upper part must have decreased. The fact that no velocity decrease can be discerned in the videos of the early motion of the tower top has been recently exploited to claim that the collapse explanation generally accepted within the structural mechanics community was invalid. This claim is here shown to be groundless. Calculations show that the velocity drop is far too small to be perceptible in amateur video records and is much smaller than the inevitable error of such video records. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v137/i1/p82_s1?isAuthorized=no [article] Why the observed motion history of world trade center towers is smooth [texte imprimé] / Jia-Liang Le, Auteur ; Bažant, Zdeněk P., Auteur . - 2011 . - pp.82-84. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 137 N° 1 (Janvier 2011) . - pp.82-84 Mots-clés : Structural  dynamics  Structural  failures  Damage  Monitoring  Building  high-rise  New  York  City  Velocity  Imaging  techniques. Résumé : The collapse of the World Trade Center towers was initiated by the impact of the upper falling part onto the underlying intact story. At the moment of impact, the velocity of the upper part must have decreased. The fact that no velocity decrease can be discerned in the videos of the early motion of the tower top has been recently exploited to claim that the collapse explanation generally accepted within the structural mechanics community was invalid. This claim is here shown to be groundless. Calculations show that the velocity drop is far too small to be perceptible in amateur video records and is much smaller than the inevitable error of such video records. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v137/i1/p82_s1?isAuthorized=no