Browsing by Author "Su, W. C."

Now showing 1 - 11 of 11

An Application of Gram-Schmidt Process on Structural Modal Parameters Identification
(2017-01-01)
Su, W. C.
;
Huang, C. S.
;
土木工程學系
;
Department of Civil Engineering
Structural modal parameters identification based on the measured responses of structure is a major topic for structural health monitoring. This work presents the use of a Gram-Schmidt process to improve. the accuracy of the identified modal parameters. The wavelet transforms is applied to the measured acceleration responses of a structural system, and to reconstruct the autoregressive with exogenous input (ARX) model in wavelet domain. Next, the identified modal parameters would be calculated from the coefficients matrix. Finally, correcting the identified mode shapes via Gram-Schmidt process. The accuracy of this procedure is numerically confirmed; the effects of the wavelet parameters and noise on the ability to accurately estimate the dynamic characteristics are also investigated.
Application of Cauchy wavelet transformation to identify time-variant modal parameters of structures
(2016-12-01)
Huang, C. S.
;
Liu, C. Y.
;
Su, W. C.
;
交大名義發表
;
National Chiao Tung University
This work proposes a procedure for accurately identifying instantaneous modal parameters of a linear time-varying system using a time-varying autoregressive with exogenous input (TVARX) model with the continuous Cauchy wavelet transform (CCWT). An appropriate TVARX model is established using the velocity and displacement responses of the system under consideration. The time-varying coefficients of the TVARX are expanded as piecewise polynomial functions. CCWTs with various scale parameters are then applied to the TVARX model to evaluate the instantaneous modal parameters of different modes. The CCWTs of the velocity and displacement responses are analytically obtained from the CCWT of the measured acceleration responses. The effectiveness and accuracy of the proposed procedure are validated by numerical simulations of single and multiple degrees of freedom systems that have periodically varying and sharply varying stiffness and damping coefficients. The effects of noise, the Cauchy wavelet function and the order of the polynomial on the evaluation of the modal parameters are explored in processing the numerically simulated acceleration responses of systems with a single degree of freedom subjected to base excitation. Finally, the proposed procedure is adopted to determine the modal parameters of a five-story symmetric steel frame from its measured acceleration responses in a shaking table test. The measured strains reveal the yielding of columns in the first story. The variations of the identified instantaneous natural frequencies and modal damping ratios with time are consistent with the physical phenomena that are observed from the measured strains and base excitation acceleration. (C) 2016 Elsevier Ltd. All rights reserved.
Ground state of the dipolar Bose-Einstein condensate
(2006-12-01)
Jiang, T. F.
;
Su, W. C.
;
土木工程學系
;
物理研究所
;
Department of Civil Engineering
We present a variational method to study the ground state of the newly realized Cr-52 dipolar Bose-Einstein condensate. Besides the usual contact potential term in the mean-field equation, there is an additional long-range and anisotropic dipole-dipole interaction potential. We develop an efficient Newton-Raphson's scheme to solve the condensate state. The solution shows a double-peak feature in the loosely confined dimension. Compared to the existing single-peak calculations, the double-peak solution has lower energy and reflects the distinct property of the dipole-dipole interaction. Our method is easy and efficient to use for future investigations of the dipolar systems.
Identification of Instantaneous Modal Parameter of Time-Varying Systems via a Wavelet-Based Approach and Its Application
(2014-04-01)
Su, W. C.
;
Liu, C. Y.
;
Huang, C. S.
;
交大名義發表
;
National Chiao Tung University
This work presents an efficient approach using time-varying autoregressive with exogenous input (TVARX) model and a substructure technique to identify the instantaneous modal parameters of a linear time-varying structure and its substructures. The identified instantaneous natural frequencies can be used to identify earthquake damage to a building, including the specific floors that are damaged. An appropriate TVARX model of the dynamic responses of a structure or substructure is established using a basis function expansion and regression approach combined with continuous wavelet transform. The effectiveness of the proposed approach is validated using numerically simulated earthquake responses of a five-storey shear building with time-varying stiffness and damping coefficients. In terms of accuracy in determining the instantaneous modal parameters of a structure from noisy responses, the proposed approach is superior to typical basis function expansion and regression approach. The proposed method is further applied to process the dynamic responses of an eight-storey steel frame in shaking table tests to identify its instantaneous modal parameters and to locate the storeys whose columns yielded under a strong base excitation.
Identification of modal parameters of a time invariant linear system by continuous wavelet transformation
(2007-05-01)
Huang, C. S.
;
Su, W. C.
;
土木工程學系
;
Department of Civil Engineering
The applications of wavelet transforms have received significant attentions in many fields. This work proposes a procedure for identifying modal parameters of a linear system using the continuous wavelet transform. The merits of the proposed procedure over the exiting schemes of applying the wavelet transform to system identification for a linear system are in use of the time invariance property and filtering ability of the transform to enhance the efficiency of identifying the modal parameters of a structure from its earthquake responses or free vibration responses. The effectiveness and accuracy of the proposed procedure are validated via numerical simulations. The effects of noise and wavelet function on identifying the modal parameters of the structure are also explored in processing the numerically simulated acceleration responses of a six-story shear building subjected to base excitation. The dynamic characteristics of close modes are accurately determined. Finally, the proposed procedure is adopted to obtain the modal parameters of a three-story non-symmetric steel frame from its measured acceleration responses in a shaking table test. A total of nine modes are identified, including modes with high frequencies and very small amplitude. (c) 2006 Elsevier Ltd. All rights reserved.
Identification of Time-Variant Modal Parameters Using Time-Varying Autoregressive with Exogenous Input and Low-Order Polynomial Function
(2009)
Huang, C. S.
;
Hung, S. L.
;
Su, W. C.
;
Wu, C. L.
;
土木工程學系
This work presents an approach that accurately identifies instantaneous modal parameters of a structure using time-varying autoregressive with exogenous input (TVARX) model. By developing the equivalent relations between the equation of motion of a time-varying structural system and the TVARX model, this work proves that instantaneous modal parameters of a time-varying system can be directly estimated from the TVARX model coefficients established from displacement responses. A moving least-squares technique incorporating polynomial basis functions is adopted to approximate the coefficient functions of the TVARX model. The coefficient functions of the TVARX model are represented by polynomials having time-dependent coefficients, instead of constant coefficients as in traditional basis function expansion approaches, so that only low orders of polynomial basis functions are needed. Numerical studies are carried out to investigate the effects of parameters in the proposed approach on accurately determining instantaneous modal parameters. Numerical analyses also demonstrate that the proposed approach is superior to some published techniques (i.e., recursive technique with a forgetting factor, traditional basis function expansion approach, and weighted basis function expansion approach) in accurately estimating instantaneous modal parameters of a structure. Finally, the proposed approach is applied to process measured data for a frame specimen subjected to a series of base excitations in shaking table tests. The specimen was damaged during testing. The identified instantaneous modal parameters are consistent with observed physical phenomena.
Identifying the Modal Parameters of a Structure from Ambient Vibration Data via the Stationary Wavelet Packet
(2014-11-01)
Su, W. C.
;
Huang, C. S.
;
Chen, C. H.
;
Liu, C. Y.
;
Huang, H. C.
Ambient vibration tests are conducted widely to estimate the modal parameters of a structure. The work proposes an efficient wavelet-based approach to determine the modal parameters of a structure from its ambient vibration responses. The proposed approach integrates the time series autoregressive (AR) model with the stationary wavelet packet transform. In addition to providing a richer decomposition and allowing for an improved time-frequency localization of signals over that of the discrete wavelet transform, the stationary wavelet packet transform also has significantly higher computational efficiency than the wavelet packet transform in terms of decomposing time-shifted signals because the former has a time-invariance property. The correlation matrices needed in determining the coefficient matrices in an AR model are established in subspaces expanded by stationary wavelet packets. The formulation for estimating the correlation matrices is shown for the first time. Because different subspaces contain signals with different frequency subbands, the fine filtering property enhances the ability of the proposed approach to identify not only the modes with strong modal interference, but also many modes from the responses of very few measured degrees of freedom. The proposed approach is validated by processing the numerically simulated responses of a seven-floor shear building, which has closely spaced modes, with considering the effects of noise and incomplete measurements. Furthermore, the present approach is employed to process the velocity responses of an eight-storey steel frame subjected to white noise input in a shaking table test and ambient vibration responses of a cable-stayed bridge.
Identifying the modal parameters of structure from Seismic Response in time-frequency domain
(2015-01-01)
Su, W. C.
;
Huang, C. S.
;
Liu, C. Y.
;
土木工程學系
;
Department of Civil Engineering
This work presents the use of a short time Fourier transform to determine the natural frequencies, damping ratios, and mode shapes of a structure from its free vibration or earthquake response data. The short time Fourier transform is applied to the measured acceleration responses of a structural system, and to reconstruct the autoregressive with exogenous input (ARX) model in time-frequency domain. The accuracy of this procedure is numerically confirmed; the effects of the length of frequency band and noise on the ability to accurately estimate the dynamic characteristics are also investigated. The feasibility of the present procedure to elucidate real structures is demonstrated through processing the measured responses of steel frames in shaking table tests.
Locating damaged storeys in a shear building based on its sub-structural natural frequencies
(2012-06-01)
Su, W. C.
;
Huang, C. S.
;
Hung, S. L.
;
Chen, U.
;
Lin, W. J.
Vibration-based damage detection methods are simple and conventionally adopted to monitor the structural health of buildings. This work proposes a simple and efficient approach to locating the storeys whose properties (stiffness and mass) change in the life cycle of a shear building. The storeys that may be damaged are determined by comparing the natural frequencies of sub-structures in different stages in the life cycle of a building. An appropriate ARX (autoregressive with exogenous input) model of a sub-structure of a shear building is established from the sub-structural dynamic responses in terms of acceleration or velocity. The parameters in an ARX model of a sub-structure are identified through the continuous wavelet transform, and the natural frequency and damping ratio of the sub-structure are estimated directly through these identified parameters. The effectiveness of the proposed procedure is verified using the numerically simulated earthquake acceleration responses and ambient vibration responses of a six-storey shear building that is damaged at one or two storeys with different damage levels. The effect of noise on the accuracy of the proposed approach is also examined. The proposed scheme is demonstrated to be superior to COMAC (Coordinate Modal Assurance Criterion) and the frequency response function curvature method in identifying damaged storeys. The proposed method is also applied to process the dynamic responses of three five-storey steel frames, which are not shear buildings, in shaking table tests. The differences in the floor mass or storey stiffness among these frames are accurately reflected in the sub-structural natural frequencies that are obtained by the proposed approach. (C) 2012 Elsevier Ltd. All rights reserved.
Thermodynamic Characterization of Polymorphs in Bulk-Crystallized Syndiotactic Polystyrene via Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry
(2009-06-23)
Su, C. H.
;
Jeng, U.
;
Chen, S. H.
;
Cheng, C. -Y.
;
Lee, J. -J.
By means of in situ small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC), we examined evolutions of lamellar crystal thickness for alpha and beta crystals, respectively, in bulk-crystallized syndiotactic polystyrene (sPS) during the partial melting-reorganization process upon progressive heating up to 290 degrees C. For the SAXS data analysis, the Kratky-Porod approximation proves to be particularly helpful in extracting the crystal thickness when approaching final melting where crystalline lamellae (near equilibration with the melt) exist in low concentrations as dispersed entities instead of in arrays. On the basis of the crystal thicknesses at elevated temperatures under solid-melt equilibration, we constructed melting lines of the two separate forms in the Gibbs-Thomson phase plane. The extrapolated (to infinite lamellar thickness) equilibrium melting temperature T(m,alpha)* approximate to 294 degrees C of the a form is moderately lower than T(m,beta)* approximate to 306 degrees C of the beta form. The two melting lines intercept at a crossover temperature T(Q) approximate to 284 degrees C and crystal thickness l(Q) approximate to 9.6 nm, where the relative thermal stability of the two phases inverses. For crystals thicker than l(Q) (practically hard to reach for bulk crystallization under ambient pressure), the beta form is the stable phase; for crystals thinner than I(Q) (the commonly accessible case), the a form is circumstantially more stable. With crystallinity-corrected values of the heat of fusion Delta H(f,alpha) approximate to 82 MJ m(-3) and Delta H(f,beta) approximate to 146 MJ m(-3) obtained from a combination of DSC and WAXS results, we determined from the slope (= 2 sigma(e)/Delta H(f)) of the melting line that basal surface energy sigma(e,alpha) approximate to 8.2 mJ m(-2) and sigma(e,beta) approximate to 26.8 mJ m(-2) which are considerably lower than those expected for tight folds, indicative of nonadjacently re-entered or loosely looped folds. The combination of lower T(m)*, Delta H(f), Delta S(f), and sigma(e) values renders the a phase highly competitive in the rate of nucleation at low temperatures but much less so at high temperatures as compared to the beta phase. The higher sigma(e,beta) value is also consistent with the observation that the beta phase is more responsive to externally added heterogeneous nucleation agents.
Wavelet-based approach of time series model for modal identification of a bridge with incomplete input
(1970-01-01)
Huang, C. S.
;
Le, Q. T.
;
Su, W. C.
;
Chen, C. H.
;
交大名義發表
Identification of modal parameters of a bridge from its earthquake responses is crucial for performing damage assessment of the structure. However, all the input base excitations of the bridge may not be measured because of economic concerns and sensor malfunctions. Consequently, evaluating the modal parameters of a bridge under the consideration of incomplete input measurements is a challenging and important task. An approach that combines the continuous Cauchy wavelet transform with an autoregressive time-varying moving average with exogenous input (AR-TVMA-X) model is proposed in this study to identify the modal parameters of a multispan bridge under multiple support earthquake excitations with incomplete measurements. The efficiency and efficacy of the proposed approach are first validated using numerically simulated responses of a three-span continuous beam subjected to multiple support nonstationary excitations. A standard procedure of using the proposed approach to identify the modal parameters is established according to comprehensive studies on the effects of noise in the data, the number of supports whose excitations are used in the AR-TVMA-X model, and the orders of the AR-TVMA-X model on the accuracy of identifying the modal parameters. This procedure is further applied to process the earthquake responses of a two-span cable-stayed 510-m-long bridge to demonstrate the engineering applicability of the proposed approach.