Documents disponibles écrits par cet auteur

Expanded reliability-based design approach for drilled shafts / Yu Wang in Journal of geotechnical and geoenvironmental engineering, Vol. 137 N° 2 (Fevrier 2011) 

Public ISBD [article]  in Journal of geotechnical and geoenvironmental engineering > Vol. 137 N° 2 (Fevrier 2011) . - pp. 140-149 Titre : Expanded reliability-based design approach for drilled shafts Type de document : texte imprimé Auteurs : Yu Wang, Auteur ; Siu-Kui Au, Auteur ; Fred H. Kulhawy, Auteur Année de publication : 2011 Article en page(s) : pp. 140-149 Note générale : Géotechnique Langues : Anglais (eng) Mots-clés : Uncertainty  Foundation  Pile  Limit  states  Simulations Index. décimale : 624.1 Infrastructures.Ouvrages en terre. Fondations. Tunnels Résumé : This paper develops a more general reliability-based design approach for drilled shafts that formulates the design process as an expanded reliability problem in which Monte Carlo simulations (MCS) are used in the design. Basic design parameters, such as the shaft diameter (B) and depth (D), are formulated as discrete uniform random variables. Then the design process becomes one in which failure probabilities are developed for various combinations of B and D [i.e., conditional probability p(Failure∣B,D)] and are compared with a target probability of failure pT. Equations are derived for this expanded reliability-based design (RBDE) approach, and criteria are established for the minimum number of MCS samples to ensure a desired level of accuracy. Its usefulness is illustrated using a drilled shaft design example. This RBDE approach has the following advantages: (1) it gives results that agree well with current RBD designs, but it improves the resolutions of the designs; (2) it offers design engineers insight into how the expected design performance level changes as B and D change; (3) it gives design engineers the ability to adjust pT, without additional calculation effort, to accommodate specific needs of a particular project; and (4) it is transparent and “visible” to design engineers who are given the flexibility to include uncertainties deemed appropriate. Finally, the effects of uncertainties in the at-rest horizontal soil stress coefficient (K0) and allowable displacement (ya) are illustrated using this approach. DEWEY : 624.1 ISSN : 1090-0241 En ligne : http://ascelibrary.org/gto/resource/1/jggefk/v137/i2/p140_s1?isAuthorized=no [article] Expanded reliability-based design approach for drilled shafts [texte imprimé] / Yu Wang, Auteur ; Siu-Kui Au, Auteur ; Fred H. Kulhawy, Auteur . - 2011 . - pp. 140-149. Géotechnique Langues : Anglais (eng) in Journal of geotechnical and geoenvironmental engineering > Vol. 137 N° 2 (Fevrier 2011) . - pp. 140-149 Mots-clés : Uncertainty  Foundation  Pile  Limit  states  Simulations Index. décimale : 624.1 Infrastructures.Ouvrages en terre. Fondations. Tunnels Résumé : This paper develops a more general reliability-based design approach for drilled shafts that formulates the design process as an expanded reliability problem in which Monte Carlo simulations (MCS) are used in the design. Basic design parameters, such as the shaft diameter (B) and depth (D), are formulated as discrete uniform random variables. Then the design process becomes one in which failure probabilities are developed for various combinations of B and D [i.e., conditional probability p(Failure∣B,D)] and are compared with a target probability of failure pT. Equations are derived for this expanded reliability-based design (RBDE) approach, and criteria are established for the minimum number of MCS samples to ensure a desired level of accuracy. Its usefulness is illustrated using a drilled shaft design example. This RBDE approach has the following advantages: (1) it gives results that agree well with current RBD designs, but it improves the resolutions of the designs; (2) it offers design engineers insight into how the expected design performance level changes as B and D change; (3) it gives design engineers the ability to adjust pT, without additional calculation effort, to accommodate specific needs of a particular project; and (4) it is transparent and “visible” to design engineers who are given the flexibility to include uncertainties deemed appropriate. Finally, the effects of uncertainties in the at-rest horizontal soil stress coefficient (K0) and allowable displacement (ya) are illustrated using this approach. DEWEY : 624.1 ISSN : 1090-0241 En ligne : http://ascelibrary.org/gto/resource/1/jggefk/v137/i2/p140_s1?isAuthorized=no

Fast bayesian FFT method for ambient modal identification with separated modes / Siu-Kui Au in Journal of engineering mechanics, Vol. 137 N° 3 (Mars 2011) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 137 N° 3 (Mars 2011) . - pp.214-226 Titre : Fast bayesian FFT method for ambient modal identification with separated modes Type de document : texte imprimé Auteurs : Siu-Kui Au, Auteur Année de publication : 2011 Article en page(s) : pp.214-226 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Bayesian  analysis  Field  tests  Modal  analysis  Spectral  analysis. Résumé : Previously a Bayesian theory for modal identification using the fast Fourier transform (FFT) of ambient data was formulated. That method provides a rigorous way for obtaining modal properties as well as their uncertainties by operating in the frequency domain. This allows a natural partition of information according to frequencies so that well-separated modes can be identified independently. Determining the posterior most probable modal parameters and their covariance matrix, however, requires solving a numerical optimization problem. The dimension of this problem grows with the number of measured channels; and its objective function involves the inverse of an ill-conditioned matrix, which makes the approach impractical for realistic applications. This paper analyzes the mathematical structure of the problem and develops efficient methods for computations, focusing on well-separated modes. A method is developed that allows fast computation of the posterior most probable values and covariance matrix. The analysis reveals a scientific definition of signal-to-noise ratio that governs the behavior of the solution in a characteristic manner. Asymptotic behavior of the modal identification problem is investigated for high signal-to-noise ratios. The proposed method is applied to modal identification of two field buildings. Using the proposed algorithm, Bayesian modal identification can now be performed in a few seconds even for a moderate to large number of measurement channels. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v137/i3/p214_s1?isAuthorized=no [article] Fast bayesian FFT method for ambient modal identification with separated modes [texte imprimé] / Siu-Kui Au, Auteur . - 2011 . - pp.214-226. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 137 N° 3 (Mars 2011) . - pp.214-226 Mots-clés : Bayesian  analysis  Field  tests  Modal  analysis  Spectral  analysis. Résumé : Previously a Bayesian theory for modal identification using the fast Fourier transform (FFT) of ambient data was formulated. That method provides a rigorous way for obtaining modal properties as well as their uncertainties by operating in the frequency domain. This allows a natural partition of information according to frequencies so that well-separated modes can be identified independently. Determining the posterior most probable modal parameters and their covariance matrix, however, requires solving a numerical optimization problem. The dimension of this problem grows with the number of measured channels; and its objective function involves the inverse of an ill-conditioned matrix, which makes the approach impractical for realistic applications. This paper analyzes the mathematical structure of the problem and develops efficient methods for computations, focusing on well-separated modes. A method is developed that allows fast computation of the posterior most probable values and covariance matrix. The analysis reveals a scientific definition of signal-to-noise ratio that governs the behavior of the solution in a characteristic manner. Asymptotic behavior of the modal identification problem is investigated for high signal-to-noise ratios. The proposed method is applied to modal identification of two field buildings. Using the proposed algorithm, Bayesian modal identification can now be performed in a few seconds even for a moderate to large number of measurement channels. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v137/i3/p214_s1?isAuthorized=no

Full-scale validation of dynamic wind load on a super-tall building under strong wind / Siu-Kui Au in Journal of structural engineering, Vol. 138 N° 9 (Septembre 2012) 

Public ISBD [article]  in Journal of structural engineering > Vol. 138 N° 9 (Septembre 2012) . - pp. 1161–1172. Titre : Full-scale validation of dynamic wind load on a super-tall building under strong wind Type de document : texte imprimé Auteurs : Siu-Kui Au, Auteur ; Ping To, Auteur Année de publication : 2012 Article en page(s) : pp. 1161–1172. Note générale : Génie civil Langues : Anglais (eng) Mots-clés : Bayesian  Measurement  Modal  identification  Structural  health  monitoring  Wind  tunnel  test Résumé : This paper presents work on validating the dynamic wind load predicted by a wind tunnel model with the values identified from full-scale field vibration data of a super-tall building in Hong Kong. Dynamic wind load in this work refers to the power spectral density (PSD) of the modal force near the natural frequency of a particular mode of the building, in the context of random vibration with stationary loading and response. This quantity reflects the turbulence characteristics integrated over the building body and is of importance in wind engineering design. This study presents a method to predict the modal PSD of wind load of the first two translational modes of a tall building. The method combines wind tunnel test information and mean wind data available in the free field; the latter taken to be at the Waglan Island station maintained by the Hong Kong Observatory. On the full-scale side, although the modal PSD is not directly observable, it is identified from field vibration data measured at the building top during strong wind events. A Bayesian modal identification method is used for determining the most probable value and the uncertainty of the modal parameters including the natural frequency, damping ratio, and the modal PSD. A number of recent strong wind events are analyzed. Reasonable agreement is observed between the wind tunnel model predictions and the values identified from field data. Factors that can potentially lead to discrepancy are discussed. As the same random vibration modeling consistent with conventional wind engineering design has been used in the wind tunnel prediction model and field identification model, the results are directly transferable among these three disciplines. ISSN : 07333-9445 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0000539 [article] Full-scale validation of dynamic wind load on a super-tall building under strong wind [texte imprimé] / Siu-Kui Au, Auteur ; Ping To, Auteur . - 2012 . - pp. 1161–1172. Génie civil Langues : Anglais (eng) in Journal of structural engineering > Vol. 138 N° 9 (Septembre 2012) . - pp. 1161–1172. Mots-clés : Bayesian  Measurement  Modal  identification  Structural  health  monitoring  Wind  tunnel  test Résumé : This paper presents work on validating the dynamic wind load predicted by a wind tunnel model with the values identified from full-scale field vibration data of a super-tall building in Hong Kong. Dynamic wind load in this work refers to the power spectral density (PSD) of the modal force near the natural frequency of a particular mode of the building, in the context of random vibration with stationary loading and response. This quantity reflects the turbulence characteristics integrated over the building body and is of importance in wind engineering design. This study presents a method to predict the modal PSD of wind load of the first two translational modes of a tall building. The method combines wind tunnel test information and mean wind data available in the free field; the latter taken to be at the Waglan Island station maintained by the Hong Kong Observatory. On the full-scale side, although the modal PSD is not directly observable, it is identified from field vibration data measured at the building top during strong wind events. A Bayesian modal identification method is used for determining the most probable value and the uncertainty of the modal parameters including the natural frequency, damping ratio, and the modal PSD. A number of recent strong wind events are analyzed. Reasonable agreement is observed between the wind tunnel model predictions and the values identified from field data. Factors that can potentially lead to discrepancy are discussed. As the same random vibration modeling consistent with conventional wind engineering design has been used in the wind tunnel prediction model and field identification model, the results are directly transferable among these three disciplines. ISSN : 07333-9445 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29ST.1943-541X.0000539

Reliability analysis of single-degree-of-freedom elastoplastic systems. I / Siu-Kui Au in Journal of engineering mechanics, Vol. 133 N°10 (Octobre 2007) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 133 N°10 (Octobre 2007) . - pp. 1072–1080. Titre : Reliability analysis of single-degree-of-freedom elastoplastic systems. I : Critical excitations Type de document : texte imprimé Auteurs : Siu-Kui Au, Auteur ; Heung Fai Lam, Auteur ; Ching Tai Ng, Auteur Année de publication : 2007 Article en page(s) : pp. 1072–1080. Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Critical  load  Elastoplasticity  Monte  Carlo  method  Structural  reliability  Vibration  Noise  Excitation Résumé : This paper investigates the application of importance sampling method to estimating the first passage probability of single-degree-of-freedom elastoplastic systems subjected to white noise excitations. The importance sampling density is constructed using a conventional choice as a weighted sum of Gaussian distributions centered among design points. It is well known that the design points, or equivalently the critical excitations in the function space, are difficult to obtain for nonlinear hysteretic systems. An efficient method has been developed recently for finding the critical excitations, on which this paper is based. Characteristics of the critical excitation for elastoplastic systems are explored and the efficiency of the resulting importance sampling strategy is critically assessed. It is found that some efficiency is gained by importance sampling over direct Monte Carlo method but to a lesser extent compared to its linear-elastic counterparts. The cause of this drop in efficiency will be investigated. The study calls for revisiting a basic assumption of importance sampling densities constructed using design points, where they are expected to generate samples lying frequently in the failure region, but in reality their capability should not be taken for granted. A companion paper investigates the approximation of the critical excitation that allows its simple determination. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%290733-9399%282007%29133%3A10%281 [...] [article] Reliability analysis of single-degree-of-freedom elastoplastic systems. I : Critical excitations [texte imprimé] / Siu-Kui Au, Auteur ; Heung Fai Lam, Auteur ; Ching Tai Ng, Auteur . - 2007 . - pp. 1072–1080. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 133 N°10 (Octobre 2007) . - pp. 1072–1080. Mots-clés : Critical  load  Elastoplasticity  Monte  Carlo  method  Structural  reliability  Vibration  Noise  Excitation Résumé : This paper investigates the application of importance sampling method to estimating the first passage probability of single-degree-of-freedom elastoplastic systems subjected to white noise excitations. The importance sampling density is constructed using a conventional choice as a weighted sum of Gaussian distributions centered among design points. It is well known that the design points, or equivalently the critical excitations in the function space, are difficult to obtain for nonlinear hysteretic systems. An efficient method has been developed recently for finding the critical excitations, on which this paper is based. Characteristics of the critical excitation for elastoplastic systems are explored and the efficiency of the resulting importance sampling strategy is critically assessed. It is found that some efficiency is gained by importance sampling over direct Monte Carlo method but to a lesser extent compared to its linear-elastic counterparts. The cause of this drop in efficiency will be investigated. The study calls for revisiting a basic assumption of importance sampling densities constructed using design points, where they are expected to generate samples lying frequently in the failure region, but in reality their capability should not be taken for granted. A companion paper investigates the approximation of the critical excitation that allows its simple determination. ISSN : 0733-9399 En ligne : http://ascelibrary.org/doi/abs/10.1061/%28ASCE%290733-9399%282007%29133%3A10%281 [...]

Two-Stage Structural Health Monitoring Approach for Phase I Benchmark Studies / Yuen, Ka-Veng in Journal of engineering mechanics, Vol. 130 N°1 (Janvier 2004) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 130 N°1 (Janvier 2004) . - 16-33 p. Titre : Two-Stage Structural Health Monitoring Approach for Phase I Benchmark Studies Titre original : Approche Structurale à Deux étages de Surveillance de Santé pour des Etudes de Repère de la Phase I Type de document : texte imprimé Auteurs : Yuen, Ka-Veng, Auteur ; Siu-Kui Au, Auteur ; J. L. Beck, Auteur Article en page(s) : 16-33 p. Note générale : Génie Mécanique Langues : Anglais (eng) Mots-clés : Damage  assessment  Bench  marks  Bayesian  analysis  Structural  safety  Evaluation  de  dommages  Mettre  hors  jeu  les  marques  Analyse  bayésienne  Sûreté  structurale Index. décimale : 621.34 Résumé : This paper presents a two-stage structural health monitoring methodology and applies it to the Phase I benchmark study sponsored by the IASC-ASCE Task Group on Structural Health Monitoring. In the first stage, modal parameters are identified using measured structural response from the undamaged system and then from the (possibly) damaged system. In the second stage, these data are used to update a parametrized structural model of the system using Bayesian system identification. The approach allows one to obtain not only estimates of the stiffness parameters but also the probability that damage in any substructure exceeds any specified threshold expressed in terms of a fractional stiffness loss. It successfully identifies the location and severity of damage in all cases of the benchmark problem. Cet article présente une méthodologie structurale à deux étages de surveillance de santé et s'applique l'à l'étude de repère de la phase I commanditée par l'IASC-ASCE Chargent le groupe sur la surveillance de santé structurale. Dans la première étape, des paramètres modaux sont identifiés en utilisant la réponse structurale mesurée du système intact et puis (probablement) du système endommagé. Dans la deuxième étape, ces données sont employées pour mettre à jour a parametrized le modèle structural du système en utilisant l'identification bayésienne de système. L'approche permet à on d'obtenir non seulement des évaluations des paramètres de rigidité mais également de la probabilité que les dommages en n'importe quelle sous-structure excèdent n'importe quel seuil indiqué exprimé en termes de perte partielle de rigidité. Elle identifie avec succès l'endroit et la sévérité des dommages dans tous les cas du problème de repère. En ligne : jimbeck@caltech.edu [article] Two-Stage Structural Health Monitoring Approach for Phase I Benchmark Studies = Approche Structurale à Deux étages de Surveillance de Santé pour des Etudes de Repère de la Phase I [texte imprimé] / Yuen, Ka-Veng, Auteur ; Siu-Kui Au, Auteur ; J. L. Beck, Auteur . - 16-33 p. Génie Mécanique Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 130 N°1 (Janvier 2004) . - 16-33 p. Mots-clés : Damage  assessment  Bench  marks  Bayesian  analysis  Structural  safety  Evaluation  de  dommages  Mettre  hors  jeu  les  marques  Analyse  bayésienne  Sûreté  structurale Index. décimale : 621.34 Résumé : This paper presents a two-stage structural health monitoring methodology and applies it to the Phase I benchmark study sponsored by the IASC-ASCE Task Group on Structural Health Monitoring. In the first stage, modal parameters are identified using measured structural response from the undamaged system and then from the (possibly) damaged system. In the second stage, these data are used to update a parametrized structural model of the system using Bayesian system identification. The approach allows one to obtain not only estimates of the stiffness parameters but also the probability that damage in any substructure exceeds any specified threshold expressed in terms of a fractional stiffness loss. It successfully identifies the location and severity of damage in all cases of the benchmark problem. Cet article présente une méthodologie structurale à deux étages de surveillance de santé et s'applique l'à l'étude de repère de la phase I commanditée par l'IASC-ASCE Chargent le groupe sur la surveillance de santé structurale. Dans la première étape, des paramètres modaux sont identifiés en utilisant la réponse structurale mesurée du système intact et puis (probablement) du système endommagé. Dans la deuxième étape, ces données sont employées pour mettre à jour a parametrized le modèle structural du système en utilisant l'identification bayésienne de système. L'approche permet à on d'obtenir non seulement des évaluations des paramètres de rigidité mais également de la probabilité que les dommages en n'importe quelle sous-structure excèdent n'importe quel seuil indiqué exprimé en termes de perte partielle de rigidité. Elle identifie avec succès l'endroit et la sévérité des dommages dans tous les cas du problème de repère. En ligne : jimbeck@caltech.edu