A modified random decrement technique for modal identification from nonstationary ambient response data only

Chang-Sheng Lin,Dar-Yun Chiang 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.6

Modal identification is considered from response data of structural system under nonstationary ambient vibration. In a previous paper,we showed that by assuming the ambient excitation to be nonstationary white noise in the form of a product model, the nonstationary response signals can be converted into free-vibration data via the correlation technique. In the present paper, if the ambient excitation can be modeled as a nonstationary white noise in the form of a product model, then the nonstationary cross random decrement signatures of structural response evaluated at any fixed time instant are shown theoretically to be proportional to the nonstationary cross-correlation functions. The practical problem of insufficient data samples available for evaluating nonstationary random decrement signatures can be approximately resolved by first extracting the amplitude-modulating function from the response and then transforming the nonstationary responses into stationary ones. Modal-parameter identification can then be performed using the Ibrahim time-domain technique, which is effective at identifying closely spaced modes. The theory proposed can be further extended by using the filtering concept to cover the case of nonstationary color excitations. Numerical simulations confirm the validity of the proposed method for identification of modal parameters from nonstationary ambient response data.

나선형 형상의 초고층건물의 공력감쇠의 특성

김원술,이진학,타무라유키오 대한토목학회 2017 대한토목학회논문집 Vol.37 No.1

Characteristics of aerodynamic damping ratios of a helical 180° model which shows better aerodynamic behavior in both along-wind and across-wind responses on a super tall building was investigated by an aeroelastic model test. The aerodynamic damping ratio was evaluated from the wind-induced responses of the model by using Random Decrement (RD) technique. Further, various triggering levels in evaluation of aerodynamic damping ratios using RD technique were also examined. As a result, it was found that when at least 2000 segments were used for evaluating aerodynamic damping ratio for ensemble averaging, the aerodynamic damping ratio can be obtained more consistently with lower irregular fluctuations. This is good agreement with those of previous studies. Another notable observation was that for square and helical 180° models, the aerodynamic damping ratios in along-wind direction showed similar linear trends with reduced wind speeds regarding of building shapes. On the other hand, for the helical 180° model, the aerodynamic damping ratio in across-wind direction showed quite different trends with those of the square model. In addition, the aerodynamic damping ratios of the helical 180° model showed very similar trends with respect to the change of wind direction, and showed gradually increasing trends having small fluctuations with reduced wind speeds. Another observation was that in definition of triggering levels in RD technique on aerodynamic amping ratios, it may be possible to adopt the triggering levels of “standard deviation” or “ times of the standard deviation” of the response time history if RD functions have a large number of triggering points. Further, these triggering levels may result in similar values and distributions with reduced wind speeds and either may be acceptable. 본 연구에서는 변위 및 가속도 응답의 저감 효과에 있어서, 유리한 형상인 180° 나선형(Helical 180°) 초고층건물을 대상으로 공력진동실험 수행하여 나선형 초고층건물의 공력감쇠율의 특성을 조사하였다. 공력감쇠율은 RD법(Random decrement technique)을 이용하여 평가하였다. 또한 RD법에서 부분 샘플의 개수와 초기 조건 값의 변화가 공력감쇠율에 어떤 영향을 미치는지 조사하였다. 실험 결과, 최소 2000개 이상의 부분 샘플을 이용하여 앙상블 평균을 적용하면 공력감쇠율의 불규칙한 변동의 폭을 줄일 수 있음을 검증했고, 기존 연구들과도 잘 부합되는 것을 알 수 있었다. 정방형 모형과 180° 나선형 모형의 공력감쇠율의 결과를 살펴보면, 풍방향 공력감쇠율은 건물의 형상이 다름에도 불구하고 무차원 풍속에 따른 공력감쇠율은 매우 유사한 경향을 보였다. 한편, 정방형 모형에 대한 풍직각방향의 공력감쇠율은 180° 나선형모형의 공력감쇠율의 특성과는 다른 양상을 보이는 것을 알 수 있었다. 특히 풍향 변화에 따른 180° 나선형 모형의 Y방향에 대한 공력감쇠율은 풍향의 변화와 상관없이, 전반적으로 0에 가까운 값을 갖는 경향이 나타났고, 무차원 풍속의 증가와 함께 변동의 폭은 작지만 점진적으로 증가하는 경향을 보였다. 초기 조건 값의 변화에 따른 공력감쇠율을 평가한 결과, 초기 조건 값을 “응답의 표준편차” 또는 RD 함수에 대한 최적화 “ ×응답의 표준편차”를 적용하여 평가한 공력감쇠율은 매우 유사한 결과 값과 분포를 보이는 것으로 나타났다.

Experimental study of aerodynamic damping of a twisted supertall building

Kim, Wonsul,Yoshida, Akihito,Tamura, Yukio,Yi, Jin-Hak Elsevier 2018 Journal of wind engineering and industrial aerodyn Vol.176 No.-

<P><B>Abstract</B></P> <P>The aerodynamic damping ratios and aeroelastic instability of a 180° helical supertall building with good aerodynamic behavior in both the along-wind and across-wind responses were investigated using an aeroelastic model test (rocking vibration model test). The aerodynamic damping ratios for the 180° helical and square models were evaluated using the random decrement technique. Furthermore, the effect of the triggering level and number of time segments on the evaluation of the aerodynamic damping ratio was also investigated. It was found that both the mean and fluctuating displacement responses of the 180° helical model in the along- and across-wind directions showed better aerodynamic behaviors than those of the square model. Aeroelastic instability of the 180° helical model was not observed at any of the reduced velocities considered in the study. In the along-wind direction, the aerodynamic damping ratios of the 180° helical model exhibit similar trends to those of the square model. However, in the across-wind direction, the aerodynamic damping ratios of the 180° helical model show significantly different trends than those of the square model. Furthermore, there was no effect of the variation in the wind direction on the aerodynamic damping ratio of the 180° helical model. We found that in the evaluation of the aerodynamic damping ratio, it is possible to adopt the triggering level at the value of the standard deviation or √2 times of the standard deviation of the displacement responses if the random decrement signature has a high number of triggering points.</P> <P><B>Highlights</B></P> <P>Characteristics of aerodynamic damping ratio and aeroelastic instability of a 180° helical supertall building were studied through wind tunnel experiments using the rocking vibration model test.<UL> <LI> For the 180° helical model, both the mean and fluctuating displacement responses in the along-wind and across-wind directions show better aerodynamic behavior than those of the square model. </LI> <LI> Aeroelastic instability for the 180° helical model is not observed for any of the reduced velocities. </LI> <LI> In the across-wind direction, the aerodynamic damping ratios of the 180° helical model at different wind directions show a similar distribution regardless of the change in the wind direction, i.e., the effect of the aerodynamic damping force on the 180° helical model in the across-wind direction can be neglected. </LI> <LI> It might be possible to obtain a reliable aerodynamic damping ratio if the RD signature with the number of time segments over 2000 is used in the evaluation of the aerodynamic damping ratio through the rocking vibration model test. </LI> <LI> These results may be help practical designer in preliminary design of supertall buildings. </LI> </UL> </P>

Field measurement of damping in industrial chimneys and towers

Cho, K.P.,Tamura, Y.,Itoh, T.,Narikawa, M.,Uchikawa, Y.,Nishimura, I.,Ohshima, Y. Techno-Press 2001 Structural Engineering and Mechanics, An Int'l Jou Vol.12 No.4

In the design of industrial chimneys and towers, structural engineers must assume a level of the inherent damping in the structures. In order to better estimate the dynamic response of those structures, actual damping was measured from wind-induced vibration signals of chimneys and towers and its characteristics with respect to the response levels, the structural systems, and the wind direction were discussed. Damping ratio and natural frequency for three chimneys and two towers were calculated using random decrement technique.

Operational modal analysis on the hydroelastic response of a segmented container carrier model under oblique waves

Kim, Yooil,Park, Sung-Gun,Kim, Byoung-Hoon,Ahn, In-Gyu Elsevier 2016 Ocean engineering Vol.127 No.-

<P><B>Abstract</B></P> <P>Both vertical and torsional mode shapes of a segmented container carrier model towed in a model basin are estimated using the operational modal analysis. The operational modal analysis targets to identify the modal properties of a structure using measured data collected during the operation without any information about the excitation. The proper orthogonal decomposition, which is also known as Karhunen–Loeve decomposition, was used for the extraction of mode shapes, and the random decrement technique was applied for the derivation of free decay signal. The segmented hull model of 10,000 TEU container carrier with the scale of 1/60 was towed in a model basin under head and oblique waves and the strain time histories at the designated locations along the continuous backbone were measured. The sectional load, which was estimated based upon the measured strain, was processed using the proper orthogonal decomposition to obtain both vertical and torsional vibration modes. Finally, the free decay signal of each modes was extracted out of the processed data using the random decrement technique and wet damping ratios as well as damped natural frequencies were estimated.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Proper orthogonal decomposition was applied to the segmented hull model data to extract mode shapes and modal magnitude. </LI> <LI> Random decrement technique was applied to the derived modal magnitude and free decay signal was extracted. </LI> </UL> </P>

초고층건물의 공력감쇠율

김용철(Kim, Yong-Chul) 대한건축학회 2014 大韓建築學會論文集 : 構造系 Vol.30 No.8

Aerodynamic damping ratios play an important role in the estimations of wind-induced responses of modern super-tall and slender buildings and structures. In the present study, along- and across-wind aerodynamic damping ratios of a square cross-sectional conventional and tapered super-tall buildings with height of 448m were investigated using the Random Decrement Technique (RDT) for the responses obtained from the aeroelastic wind tunnel tests. Wind tunnel tests were conducted under the urban flow condition. Effects of building shape, normalized velocity and wind direction on aerodynamic damping ratios were comprehensively discussed. It was found that the along-wind aeroelastic aerodynamic damping ratios show similar linear increasing trends on the normalized velocities at small wind directions regardless of building shapes. But the across-wind aerodynamic damping ratios show clear differences depending on building shapes, showing different peaks and different normalized velocities at which peaks of aerodynamic damping ratios occur and positive aerodynamic damping ratios become negative. Lastly the effect of trigging value in RDT on aerodynamic damping ratios was examined by changing it from one standard deviation to three times of standard deviation. By enlarging the trigging value, the differences in aerodynamic damping ratios increase, implying the optimal trigging values may exist between one standard deviation and √2 times of standard deviations in the current experimental conditions.

Amplitude dependency of damping of tall structures by the random decrement technique

An Xu,Zhuangning Xie,Ming Gu,Jiurong Wu 한국풍공학회 2015 Wind and Structures, An International Journal (WAS Vol.21 No.2

This study focuses on the amplitude dependency of damping of tall structures by the random decrement technique (RDT). Many researchers have adopted RDT to establish the amplitude dependency of damping ratios in super-tall buildings under strong wind loads. In this study, a series of simulated examples were analyzed to examine the reliability of this method. Results show that damping ratios increase as vibration amplitudes increase in several cases; however, the damping ratios in the simulated signals were preset as constants. This finding reveals that this method and the derived amplitude-dependent damping ratio characteristics are unreliable. Moreover, this method would obviously yield misleading results if the simulated signals contain Gaussian white noise. Full-scale measurements on a super-tall building were conducted during four typhoons, and the recorded data were analyzed to observe the amplitude dependency of damping ratio. Relatively wide scatter is observed in the resulting damping ratios, and the damping ratios do not appear to have an obvious nonlinear relationship with vibration amplitude. Numerical simulation and field measurement results indicate that the widely-used method for establishing the amplitude-dependent damping characteristics of super-tall buildings and the conclusions derived from it might be questionable at the least. More field-measured data must be collected under strong wind loads, and the damping characteristics of super-tall buildings should be investigated further.

Investigation of the Dynamic Behavior of a Super High-rise Structure using RTK-GNSS Technique

Chunbao Xiong,Yanbo Niu 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.2

Super high-rise structures have a significant deformation under ambient excitations such as earthquake and wind, which may lead to structural instability and even damage. To capture the dynamic characteristics of a super high-rise structure under construction (i.e., Tianjin 117 tower), Real Time Kinematic - Global Navigation Satellite Systems (RTK-GNSS) sensors are employed to derive the horizontal displacement of the structure. Considering the defects in measurement accuracy of RTK-GNSS sensors, a Type 1 Chebyshev high-pass digital filter is firstly employed and thus the output results are smoothed. Subsequently, based on the smoothed signals, the natural frequencies and the corresponding damping ratios are extracted via FFT (Fast Fourier Transform) and RDT-LDM (random decrement technique combined with logarithmic decrement method). It is found that the results from the field measurement coincide with the numerical simulation. Finally, the structural parameters are successfully obtained and illustrated graphically.

Amplitude dependency of damping of tall structures by the random decrement technique

Xu, An,Xie, Zhuangning,Gu, Ming,Wu, Jiurong Techno-Press 2015 Wind and Structures, An International Journal (WAS Vol.21 No.2

This study focuses on the amplitude dependency of damping of tall structures by the random decrement technique (RDT). Many researchers have adopted RDT to establish the amplitude dependency of damping ratios in super-tall buildings under strong wind loads. In this study, a series of simulated examples were analyzed to examine the reliability of this method. Results show that damping ratios increase as vibration amplitudes increase in several cases; however, the damping ratios in the simulated signals were preset as constants. This finding reveals that this method and the derived amplitude-dependent damping ratio characteristics are unreliable. Moreover, this method would obviously yield misleading results if the simulated signals contain Gaussian white noise. Full-scale measurements on a super-tall building were conducted during four typhoons, and the recorded data were analyzed to observe the amplitude dependency of damping ratio. Relatively wide scatter is observed in the resulting damping ratios, and the damping ratios do not appear to have an obvious nonlinear relationship with vibration amplitude. Numerical simulation and field measurement results indicate that the widely-used method for establishing the amplitude-dependent damping characteristics of super-tall buildings and the conclusions derived from it might be questionable at the least. More field-measured data must be collected under strong wind loads, and the damping characteristics of super-tall buildings should be investigated further.

랜덤 감쇠기법을 이용한 분할모형의 접수 감쇠계수 추정

김유일(Yooil Kim),박성건(Sung-Gun Park) 대한조선학회 2013 大韓造船學會 論文集 Vol.50 No.4