Documents disponibles écrits par cet auteur

Computation of probability distribution of strength of quasibrittle structures failing at macrocrack initiation / Jia-Liang Le in Journal of engineering mechanics, Vol. 138 N° 7 (Juillet 2012) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 138 N° 7 (Juillet 2012) . - pp.888–899. Titre : Computation of probability distribution of strength of quasibrittle structures failing at macrocrack initiation Type de document : texte imprimé Auteurs : Jia-Liang Le, Auteur ; Jan Elias, Auteur ; Zdenek P. Bazant, Auteur Année de publication : 2012 Article en page(s) : pp.888–899. Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Finite  weakest  link  model  Strength  statistics  Representative  volume  element  Structural  safety  Fracture  Concrete  structures  Composites Résumé : Engineering structures must be designed for an extremely low failure probability, Pf<10−6. To determine the corresponding structural strength, a mechanics-based probability distribution model is required. Recent studies have shown that quasibrittle structures that fail at the macrocrack initiation from a single representative volume element (RVE) can be statistically modeled as a finite chain of RVEs. It has further been demonstrated that, based on atomistic fracture mechanics and a statistical multiscale transition model, the strength distribution of each RVE can be approximately described by a Gaussian distribution, onto which a Weibull tail is grafted at a point of the probability about 10−4 to 10−3. The model implies that the strength distribution of quasibrittle structures depends on the structure size, varying gradually from the Gaussian distribution modified by a far-left Weibull tail applicable for small-size structures, to the Weibull distribution applicable for large-size structures. Compared with the classical Weibull strength distribution, which is limited to perfectly brittle structures, the grafted Weibull-Gaussian distribution of the RVE strength makes the computation of the strength distribution of quasibrittle structures inevitably more complicated. This paper presents two methods to facilitate this computation: (1) for structures with a simple stress field, an approximate closed-form expression for the strength distribution based on the Taylor series expansion of the grafted Weibull-Gaussian distribution; and (2) for structures with a complex stress field, a random RVE placing method based on the centroidal Voronoi tessellation. Numerical examples including three-point and four-point bend beams, and a two-dimensional analysis of the ill-fated Malpasset dam, show that Method 1 agrees well with Method 2 as well as with the previously proposed nonlocal boundary method. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29EM.1943-7889.0000396 [article] Computation of probability distribution of strength of quasibrittle structures failing at macrocrack initiation [texte imprimé] / Jia-Liang Le, Auteur ; Jan Elias, Auteur ; Zdenek P. Bazant, Auteur . - 2012 . - pp.888–899. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 138 N° 7 (Juillet 2012) . - pp.888–899. Mots-clés : Finite  weakest  link  model  Strength  statistics  Representative  volume  element  Structural  safety  Fracture  Concrete  structures  Composites Résumé : Engineering structures must be designed for an extremely low failure probability, Pf<10−6. To determine the corresponding structural strength, a mechanics-based probability distribution model is required. Recent studies have shown that quasibrittle structures that fail at the macrocrack initiation from a single representative volume element (RVE) can be statistically modeled as a finite chain of RVEs. It has further been demonstrated that, based on atomistic fracture mechanics and a statistical multiscale transition model, the strength distribution of each RVE can be approximately described by a Gaussian distribution, onto which a Weibull tail is grafted at a point of the probability about 10−4 to 10−3. The model implies that the strength distribution of quasibrittle structures depends on the structure size, varying gradually from the Gaussian distribution modified by a far-left Weibull tail applicable for small-size structures, to the Weibull distribution applicable for large-size structures. Compared with the classical Weibull strength distribution, which is limited to perfectly brittle structures, the grafted Weibull-Gaussian distribution of the RVE strength makes the computation of the strength distribution of quasibrittle structures inevitably more complicated. This paper presents two methods to facilitate this computation: (1) for structures with a simple stress field, an approximate closed-form expression for the strength distribution based on the Taylor series expansion of the grafted Weibull-Gaussian distribution; and (2) for structures with a complex stress field, a random RVE placing method based on the centroidal Voronoi tessellation. Numerical examples including three-point and four-point bend beams, and a two-dimensional analysis of the ill-fated Malpasset dam, show that Method 1 agrees well with Method 2 as well as with the previously proposed nonlocal boundary method. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29EM.1943-7889.0000396

Problems with hu-duan boundary effect model and Its comparison to size-shape effect law for quasi-brittle fracture / Yu, Qiang in Journal of engineering mechanics, Vol. 136 N° 1 (Janvier 2010) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 136 N° 1 (Janvier 2010) . - pp. 40-50 Titre : Problems with hu-duan boundary effect model and Its comparison to size-shape effect law for quasi-brittle fracture Type de document : texte imprimé Auteurs : Yu, Qiang, Auteur ; Jia-Liang Le, Auteur ; Hoover, Hristian G., Auteur Article en page(s) : pp. 40-50 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Cracking  Concrete  Structural  failures  Data  analysis  Size  effect. Résumé : Recent disagreements on the mathematical modeling of fracture and size effect in concrete and other quasi-brittle materials are obstacles to improvements in design practice, and especially in design codes for concrete structures. In an attempt to overcome this impediment to progress, this paper compares the Hu-Duan boundary effect model (BEM) expounded since 2000 to the size-shape effect law (SEL) proposed at Northwestern University in 1984 and extended to the geometry (or shape) effects in 1990. It is found that within a rather limited part of the range of sizes and shapes, the fracture energy values identified by BEM and SEL from the test data on maximum loads are nearly the same. But in other parts of the range the BEM is either inferior or inapplicable. The material tensile strength values identified by BEM have a much larger error than those obtained from the SEL after calibration by the cohesive crack model. From the theoretical viewpoint, several hypotheses of BEM are shown to be unrealistic. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/dbt/dbt.jsp?KEY=JENMDT&Volume=136&Issue=1 [article] Problems with hu-duan boundary effect model and Its comparison to size-shape effect law for quasi-brittle fracture [texte imprimé] / Yu, Qiang, Auteur ; Jia-Liang Le, Auteur ; Hoover, Hristian G., Auteur . - pp. 40-50. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 136 N° 1 (Janvier 2010) . - pp. 40-50 Mots-clés : Cracking  Concrete  Structural  failures  Data  analysis  Size  effect. Résumé : Recent disagreements on the mathematical modeling of fracture and size effect in concrete and other quasi-brittle materials are obstacles to improvements in design practice, and especially in design codes for concrete structures. In an attempt to overcome this impediment to progress, this paper compares the Hu-Duan boundary effect model (BEM) expounded since 2000 to the size-shape effect law (SEL) proposed at Northwestern University in 1984 and extended to the geometry (or shape) effects in 1990. It is found that within a rather limited part of the range of sizes and shapes, the fracture energy values identified by BEM and SEL from the test data on maximum loads are nearly the same. But in other parts of the range the BEM is either inferior or inapplicable. The material tensile strength values identified by BEM have a much larger error than those obtained from the SEL after calibration by the cohesive crack model. From the theoretical viewpoint, several hypotheses of BEM are shown to be unrealistic. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/dbt/dbt.jsp?KEY=JENMDT&Volume=136&Issue=1

What did and did not cause collapse of world tade center twin towers in New York? / Zdenek P. Bazant in Journal of engineering mechanics, Vol. 134 N°10 (Octobre 2008) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 134 N°10 (Octobre 2008) . - pp. 892–906. Titre : What did and did not cause collapse of world tade center twin towers in New York? Type de document : texte imprimé Auteurs : Zdenek P. Bazant, Auteur ; Jia-Liang Le, Auteur ; Frank R. Greening, Auteur Année de publication : 2008 Article en page(s) : pp. 892–906. Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Collapse  Buildings  high-rise  Towers  New  York  New Résumé : Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade Center Towers. However, it remains to be determined whether the recent allegations of controlled demolition have any scientific merit. The present analysis proves that they do not. The video record available for the first few seconds of collapse is shown to agree with the motion history calculated from the differential equation of progressive collapse but, despite uncertain values of some parameters, it is totally out of range of the free fall hypothesis, on which these allegations rest. It is shown that the observed size range (0.01–0.1mm) of the dust particles of pulverized concrete is consistent with the theory of comminution caused by impact, and that less than 10% of the total gravitational energy, converted to kinetic energy, sufficed to produce this dust (whereas, more than 150t of TNT per tower would have to be installed, into many small holes drilled into concrete, to produce the same pulverization). The air ejected from the building by gravitational collapse must have attained, near the ground, the speed of almost 500 miles per hour (or 223m/s , or 803km/h ) on average, and fluctuations must have reached the speed of sound. This explains the loud booms and wide spreading of pulverized concrete and other fragments, and shows that the lower margin of the dust cloud could not have coincided with the crushing front. The resisting upward forces due to pulverization and to ejection of air, dust, and solid fragments, neglected in previous studies, are indeed found to be negligible during the first few seconds of collapse but not insignificant near the end of crush-down. The calculated crush-down duration is found to match a logical interpretation of seismic record, while the free fall duration grossly disagrees with this record. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%290733-9399%282008%29134%3A10%288 [...] [article] What did and did not cause collapse of world tade center twin towers in New York? [texte imprimé] / Zdenek P. Bazant, Auteur ; Jia-Liang Le, Auteur ; Frank R. Greening, Auteur . - 2008 . - pp. 892–906. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 134 N°10 (Octobre 2008) . - pp. 892–906. Mots-clés : Collapse  Buildings  high-rise  Towers  New  York  New Résumé : Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade Center Towers. However, it remains to be determined whether the recent allegations of controlled demolition have any scientific merit. The present analysis proves that they do not. The video record available for the first few seconds of collapse is shown to agree with the motion history calculated from the differential equation of progressive collapse but, despite uncertain values of some parameters, it is totally out of range of the free fall hypothesis, on which these allegations rest. It is shown that the observed size range (0.01–0.1mm) of the dust particles of pulverized concrete is consistent with the theory of comminution caused by impact, and that less than 10% of the total gravitational energy, converted to kinetic energy, sufficed to produce this dust (whereas, more than 150t of TNT per tower would have to be installed, into many small holes drilled into concrete, to produce the same pulverization). The air ejected from the building by gravitational collapse must have attained, near the ground, the speed of almost 500 miles per hour (or 223m/s , or 803km/h ) on average, and fluctuations must have reached the speed of sound. This explains the loud booms and wide spreading of pulverized concrete and other fragments, and shows that the lower margin of the dust cloud could not have coincided with the crushing front. The resisting upward forces due to pulverization and to ejection of air, dust, and solid fragments, neglected in previous studies, are indeed found to be negligible during the first few seconds of collapse but not insignificant near the end of crush-down. The calculated crush-down duration is found to match a logical interpretation of seismic record, while the free fall duration grossly disagrees with this record. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%290733-9399%282008%29134%3A10%288 [...]

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