• Journal of the Computational Structural Engineering Institute of Korea
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• v.15 no.2
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• pp.209-218
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• 2002

The Vector Random Decrement technique has been developed as an efficient method for transforming ambient responses into free decays of linear structures. It is shown that the VRD functions nay contain as much information about the modes as the really measured free decay responses. In this paper, the theory of the VRD technique is extended by applying the concept of the mode shape ratio into the Ibrahim Time Domain modal parameter identification algorithm. The VRD function is not shifted in the correction procedures for constant time shifts of the proposed VRD technique. Thus, a number of points equal to the largest of the time shifts used in the vector triggering condition are not deleted. In the VRD functions, any influence of the input to the system is averaged out. The proposed technique is compared with the traditional VRD technique by assessment of the modal parameters. The applicability of the VRD technique has been justified through a simulation study and a study of the response of a laboratory beam model subject to ambient loads.

• Journal of the Society of Naval Architects of Korea
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• v.50 no.4
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• pp.217-223
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• 2013

This paper presents the wet damping estimation of the segmented hull model using the random decrement technique together with the continuous wavelet transform. The tested 16 sea states are grouped together based on the speed of the ship in order to figure out the possible influence of the ship speed on the damping ratio. The measured time histories of vertical bending moment for each tested sea state were processed with random decrement technique to derive the free decay signal, from which the damping ratios are estimated. Also, the autocorrelation functions of the filtered signal were calculated and comparison was made with the free decay signal obtained from the random decrement technique. Then the wet damping ratios for each sea state group, as well as precise wet natural frequencies, are estimated by using continuous wavelet transform. It turned out that the wet natural frequencies derived from the measured signal did not show any significant discrepancy compared with those obtained by wet hammering test, whereas the damping ratio did. It was considered that the discrepancy of the damping ratio between in calm and moving water may be attributed to the viscous effects caused by dramatically different flow pattern and relative velocity between the vibrating structure and surrounding fluid particles.

• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.6
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• pp.979-994
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• 2015

The mode shapes of a segmented hull model towed in a model basin were predicted using both the Proper Orthogonal Decomposition (POD) and cross random decrement technique. The proper orthogonal decomposition, which is also known as Karhunen-Loeve decomposition, is an emerging technology as a useful signal processing technique in structural dynamics. The technique is based on the fact that the eigenvectors of a spatial coherence matrix become the mode shapes of the system under free and randomly excited forced vibration conditions. Taking advantage of the simplicity of POD, efforts have been made to reveal the mode shapes of vibrating flexible hull under random wave excitation. First, the segmented hull model of a 400 K ore carrier with 3 flexible connections was towed in a model basin under different sea states and the time histories of the vertical bending moment at three different locations were measured. The measured response time histories were processed using the proper orthogonal decomposition, eventually to obtain both the first and second vertical vibration modes of the flexible hull. A comparison of the obtained mode shapes with those obtained using the cross random decrement technique showed excellent correspondence between the two results.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.15 no.3
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• pp.409-420
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• 2002

This paper proposed a technique of structural re-analysis for the evaluation of dynamic responses of bridge structure under moving loads using experimental modal results. For successful structural re-analysis, it is required to have accurate estimation techniques of the modal characteristics of bridge structures. The natural frequencies and mode shapes were identified by direct fourier analysis techniques and damping ratios by the random decrement method, respectively. An interpolation method was also proposed for the extension of mode shape measured on limited DOFs. Second, the structural reanalysis was performed using moving mass model and identified modal parameters. The results from the reanalysis show that the proposed technique is very reasonable to evaluate the actual behavior of bridge structures under moving loads.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.14 no.1
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• pp.77-85
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• 2001

본 연구에서는 차량하중에 의한 상시진동기록을 이용한 교량의 손상추정기법을 연구하였다. 즉, 차량진행 중 측정된 신호로부터 구조물의 모드특성을 구하고, 이를 이용하여 손상위치 및 손상정도를 추정하는 알고리즘을 제안하였다. 제안기법의 검증을 위하여 차량하중을 재하할 수 있는 모형교량을 제작하여 손상실험을 수행하였다. 차량진행 중 교량의 수직가속도를 계측하였으며, 측정된 가속도시계열로부터 random decrement(RD) 기법을 사용하여 자유진동신호를 구한 후, 이로부터 구조물의 모드특성을 추정하였다. 추정된 모드특성을 기초로 신경망기법을 적용하여 손상위치 및 손상정도를 추정하였으며, 추정된 결과는 실제 손상과 비교적 잘 일치하였다.

• Structural Engineering and Mechanics
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• v.12 no.4
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• pp.449-457
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• 2001

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.

• Wind and Structures
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• v.21 no.2
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• pp.159-182
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• 2015

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.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2009.04a
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• pp.205-208
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• 2009

POD(proper orthogonal decomposition)는 가해지는 하중(입력)의 계측없이 출력(응답)만으로 구조물의 동적특성을 파악할 수 있는 기법이다. 하지만 실제의 경우 측정데이터에 노이즈가 포함되어 있으면 분해가 완전하게 일어나지 않아 동적특성(특히 감쇠비)을 완벽히 파악하기 힘들다. 본 연구에서는 이러한 문제점을 보완하기 위해서 POD기법으로 추출된 각 모드의 자유진동파형에 RD(random decrement)법을 적용하여 노이즈에 의한 영향을 제거하는 방법을 제안하였다. 본 논문에서는 먼저 수치모델을 사용하여 계측노이즈가 있을 경우 제안된 방법을 사용하면 노이즈의 영향을 감소시킬 수 있음을 검증한 후 실험실 규모의 구조물모형에서 얻은 자유진동계측치에 제안된 기법을 적용하여 시스템식별을 수행하여 동특성을 파악하였다.

• Smart Structures and Systems
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• v.14 no.3
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• pp.445-468
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• 2014

The objective in this research is to determine the feasibility of using changes on the dynamic properties of a reinforced concrete (RC) structure to identify different levels of seismic induced damage. Damping ratio and natural frequency changes in a RC bridge column are analyzed using different signal processing techniques like Hilbert Transforms, Random Decrement and Wavelet Transforms. The data used in the analysis was recorded during a full-scale RC bridge column shake table test. The structure was subjected to ten earthquake excitations that induced different levels of inelastic demand on the column. In addition, low-intensity white noises were applied to the column in-between earthquakes. The results obtained show that the use of the damping ratio and natural frequency of vibration as damage indicators is arguable.

• Wind and Structures
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• v.8 no.5
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• pp.309-324
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• 2005

Aerodynamic damping often plays an important role in estimations of wind induced dynamic responses of super high-rise buildings. Across- and along-wind aerodynamic damping ratios of a square super high-rise building with a height of 300 m are identified with the Random Decrement technique (RDT) from random vibration responses of the SDOF aeroelastic model in simulated wind fields. Parametric studies on effects of reduced wind velocity, terrain type and structural damping ratio on the aerodynamic damping ratios are further performed. Finally formulas of across- and along-wind aerodynamic damping ratios of the square super high-rise building are derived with curve fitting technique and accuracy of the formulas is verified.