• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1998년도 봄 학술발표회 논문집
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• pp.291-300
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• 1998

This paper demonstrates how ambient vibration measurements at a limited number of locations can be effectively utilized to estimate parameters of a finite element model of a large-scale structural system involving a large number of elements. System identification using ambient vibration measurements presents a challenge requiring the use of special identification techniques, which ran deal with very small magnitudes of ambient vibration contaminated by noise without the knowledge of input farces. In the present study, the modal parameters such as natural frequencies, damping ratios, and mode shapes of the structural system were estimated by means of appropriate system identification techniques including the random decrement method. Moreover, estimation of parameters such as the stiffness matrix of the finite element model from the system response measured by a limited number of sensors is another challenge. In this study, the system stiffness matrix was estimated by using the quadratic optimization involving the computed and measured modal strain energy of the system, with the aid of a sensitivity relationship between each element stiffness and the modal parameters established by the second order inverse modal perturbation theory. The finite element models thus identified represent the actual structural system very well, as their calculated dynamic characteristics satisfactorily matched the observed ones from the ambient vibration test performed on a large-scale structural system subjected primarily to ambient wind excitations. The dynamic models identified by this study will be used for design of an active mass damper system to be installed on this structure fer suppressing its wind vibration.

• Structural Engineering and Mechanics
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• 제53권2호
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• pp.325-336
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• 2015

This paper presents a study based on the damage due to the fatigue life of thrust ball bearings using vibratory analysis. The main contribution of this work lies in establishing a relation between modal damping and the rolling contact fatigue damage of the thrust ball bearing. Time domain signals and frequency spectra are extracted from both static and dynamic experiments. The first part of this research consists in measuring the damping of damaged thrust ball bearings using impact hammer characterization tests. In a second part, indented components representing spalled bearings are studied to determine the evolution of damping values in real-time vibration spectra using the random decrement method. Dynamic results, in good agreement with static tests, show that damping varies depending on the component's damage state. Therefore, the method detailed in this work will offer a possible technique to estimate the thrust ball bearing fatigue damage variation in presence of spalling.

• Wind and Structures
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• 제22권2호
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• pp.185-209
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• 2016

Across-wind aerodynamic damping ratios are identified from the wind-induced acceleration responses of 15 aeroelastic models of rectangular super-high-rise buildings in various simulated wind conditions by using the random decrement technique. The influences of amplitude-dependent structural damping ratio and natural frequency on the estimation of the aerodynamic damping ratio are discussed and the identifying method for aerodynamic damping is improved at first. Based on these works, effects of turbulence intensity $I_u$, aspect ratio H/B, and side ratio B/D on the across-wind aerodynamic damping ratio are investigated. The results indicate that turbulence intensity and side ratio are the most important factors that affect across-wind aerodynamic damping ratio, whereas aspect ratio indirectly affects the aerodynamic damping ratio by changing the response amplitude. Furthermore, empirical aerodynamic damping functions are proposed to estimate aerodynamic damping ratios at low and high reduced speeds for rectangular super-high-rise buildings with an aspect ratio in the range of 5 to 10, a side ratio of 1/3 to 3, and turbulence intensity varying from 1.7% to 25%.

• Structural Engineering and Mechanics
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• 제55권5호
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• pp.965-977
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• 2015

Determination of dynamic parameters of a structure such as predominant frequency and damping ratio is one of the most important subjects in dynamics of structures. Different methods are used to determine predominant frequency. These methods are different in the cost, implement accessibility, accuracy, speed, applicability in different conditions, simplicity of calculations and required data accessibility. Calculation of damping ratio by using common experimental procedures is very difficult and costly, then it is assumed as a constant value in most calculations. Microtremor measurements and using spectral ratio method to determine the predominant frequency and damping ratio of structure is of interest in recent years. In this paper, as a case study, the effects of retrofitting on structural dynamic parameters of two four-story buildings by using microtremor measurements and also finite element analysis, is investigated. The results of this study show that microtremor measurements can be utilized to assess the improvement of dynamic behavior of the retrofitted structure and the effectiveness of the method of retrofitting.

• Structural Engineering and Mechanics
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• 제87권5호
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• pp.451-460
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• 2023

Recently, the use of smartphones for structural health monitoring in civil engineering has drawn increasing attention due to their rapid development and popularization. In this study, the structural responses and dynamic characteristics of a 421-m-tall skyscraper during the landfall of Typhoon Muifa are monitored using an iPhone 13. The measured building acceleration responses are first corrected by the resampling technique since the sampling rate of smartphone-based measurement is unstable. Then, based on the corrected building acceleration, the wind-induced responses (i.e., along-wind and across-wind responses) are investigated and the serviceability performance of the skyscraper is assessed. Next, the amplitude-dependency and time-varying structural dynamic characteristics of the monitored supertall building during Typhoon Muifa are investigated by employing the random decrement technique and Bayesian spectral density approach. Moreover, the estimated results during Muifa are further compared with those of previous studies on the monitored building to discuss its long-term time-varying structural dynamic characteristics. The paper aims to demonstrate the applicability and effectiveness of smartphones for structural health monitoring of high-rise buildings.

• Wind and Structures
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• 제25권1호
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• pp.1-24
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• 2017

This paper presents a 1:25 multi-freedom aero-elastic model for a high lighting pole at the Zhoushan stadium. To validate the similarity characteristics of the model, a free vibration test was performed before the formal test. Beat phenomenon was found and eliminated by synthesis of vibration in the X and Y directions, and the damping ratio of the model was identified by the free decay method. The dynamic characteristics of the model were examined and compared with the real structure; the similarity results were favorable. From the test results, the major along-wind dynamic response was the first vibration component. The along-wind wind vibration coefficient was calculated by the China code and Eurocode. When the peak factor equaled 3.5, the coefficient calculated by the China code was close to the experimental result while Eurocode had a slight overestimation of the coefficient. The wind vibration coefficient during typhoon flow was analyzed, and a magnification factor was suggested in typhoon-prone areas. By analyzing the power spectrum of the dynamic cross-wind base shear force, it was found that a second-order vortex-excited resonance existed. The cross-wind response in the test was smaller than Eurocode estimation. The aerodynamic damping ratio was calculated by random decrement technique and the results showed that aerodynamic damping ratios were mostly positive at the design wind speed, which means that the wind-induced galloping phenomenon is predicted not to occur at design wind speeds.

• 한국정보통신학회논문지
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• 제17권11호
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• pp.2553-2562
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• 2013

광대역 무선 네트워크 환경에서 TCP의 혼잡 제어 알고리즘은 미디어 스트리밍 서비스가 요구하는 대역폭 및 지연 한계를 보장하기 어렵다. 본 논문에서는 고품질의 미디어 스트리밍 서비스를 제공하기 위한 혼잡 제어 기법인 COIN TCP (COncave INcrease TCP)를 제안하였다. COIN TCP는 라우터 큐의 데이터 양에 반비례하게 조절된 혼잡 윈도우의 증가율을 통해 혼잡 윈도우 크기를 오목하게 증가시킴으로써, 가용 대역폭의 빠른 점유와 Heavy Congestion의 방지가 가능하다. 또한 랜덤 손실을 고려한 패킷 손실률에 따라 혼잡 윈도우의 감소율을 조절하여 링크 활용도를 향상시켰다. 제안 기법의 실험 결과를 통해 광대역 무선 네트워크에서의 처리량 향상을 확인하였다.

• 한국노년학
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• 제36권4호
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• pp.1141-1155
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• 2016

본 연구는 식후저혈압 노인에서 식후에 녹차 또는 커피 한 잔을 섭취하는 것이 식후의 혈압하강에 어떤 영향을 미치는지 파악하기 위해 시도되었다. 실험은 식후저혈압이 있는 노인 30명을 대상으로 피험자내 반복측정 설계에 따라 참여노인들의 서면동의를 받은 후에 이루어졌다. 평소와 같은 일반 식사만 하는 것을 대조중재로 하였고, 동일한 일반식사 후에 120mL의 녹차 한 잔(카페인 15.8 mg)을 마시는 것을 실험중재1로, 120mL 커피를 한 잔(카페인 60mg)을 마시는 것을 실험중재2로 하였다. 각 노인이 세 가지의 중재에 참여하는 순서는 무작위로 배정하였다. 혈압과 맥박 수는 휴대용 자동혈압계(ABPM)를 사용하여 식전 30분부터 식후 120분까지 15분 간격으로 측정하였다. 수집된 자료는 IBM SPSS 19.0 program을 이용하여 분석하였다. 연구 결과, 참여노인의 사전 수축기혈압과 이완기혈압은 세 중재 간에 차이가 없었다. 식후 60분에 측정한 커피군의 수축기혈압은 $4.1{\pm}18.6mmHg$하강하여 대조군이 $15.1{\pm}17.9mmHg$ 하강한 것에 비해 유의하게 적었고(p=.032), 녹차군은 $13.4{\pm}22.4mmHg$ 하강하여 대조군과 차이가 없었다. 마찬가지로 식후 60분의 이완기혈압은 커피섭취군이 $0.8{\pm}13.4mmHg$ 하강하여 대조군이 $12.8{\pm}13.0mmHg$ 하강한 것보다 적었고, 이는 통계적으로 유의하였다(p=.033). 맥박 수는 세 군 모두 식전과 유의한 변화가 없었다. 따라서 식후저혈압 노인에 있어서 식후에 커피를 한 잔 마시는 것은 혈압하강을 예방하는 효과가 있다고 할 수 있다.