• Wind and Structures
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• 제21권1호
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• pp.25-40
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• 2015

Oscillating motions of flexible cylinders are associated to some extent with the aerodynamic response of plants. Tandem motions of reeds with flexible stems in a colony are experimentally investigated using an array of flexible cylinders made of polydimethylsiloxane (PDMS). Consecutive images of flexible cylinders subjected to oncoming wind are recorded with a high-speed camera. To quantify oscillating motions, the average bending angle and displacement of flexible cylinders are evaluated using point-tracking method and spectral analysis. The tandem motions of flexible cylinders are closely related to the flow characteristics around the cylinders. Thus, the dynamic motions of a tandem arrangement of flexible cylinders are investigated with varying numbers of cylinders arranged in-line, numbers of cylinders in a group (behaving like a single body), and Reynolds numbers (Re). When the number of cylinders in a group increases, the damping effect caused by the support of downstream cylinders is pronounced. These results would be provide useful information on the tandem-arranged design of complex structures and energy harvesting devices.

• Wind and Structures
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• 제2권4호
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• pp.267-284
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• 1999

Multiple tuned mass dampers are proposed to suppress the vertical and torsional buffeting and to increase the aerodynamic stability of long-span bridges. Each damper has vertical and torsional frequencies, which are tuned to the corresponding frequencies of the structural modes to suppress the resonant effects. These proposed dampers maintain the advantage of traditional multiple mass dampers, but have the added capability of simultaneously controlling vertical and torsional buffeting responses. The aerodynamic coupling is incorporated into the formulations, allowing this model to effectively increase the critical speed of a bridge for either single-degree-of-freedom flutter or coupled flutter. The reduction of dynamic response and the increase of the critical speed through the attachment of the proposed dampers to the bridge are also discussed. Through a parametric analysis, the characteristics of the multiple tuned mass dampers are studied and the design parameters - including mass, damping, frequency bandwidth, and total number of dampers - are proposed. The results indicate that the proposed dampers effectively suppress the vertical and the torsional buffeting and increase the structural stability. Moreover, these tuned mass dampers, designed within the recommended parameters, are not only more effective but also more robust than a single TMD against wind-induced vibration.

• Wind and Structures
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• 제9권2호
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• pp.109-124
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• 2006

This paper describes the characteristics of the fluctuating lift forces when a circular cylinder vibrates in the cross-flow direction. The response characteristics on elastically supported the circular cylinder was first examined by a free-vibration test. Next, flow-induced vibrations obtained by the free-vibration test were reproduced by a forced-vibration test, and then the characteristics of the fluctuating lift forces, the work done by the fluctuating lift, the behavior of the rolling-up of the separated shear layers were investigated on the basis of the visualized flow patterns. The main findings were that (i) the fluctuating lift forces become considerably large than those of a stationary circular cylinder, (ii) negative pressure generates on the surface of the circular cylinder when the rolling-up of separated shear layer begins, (iii) the phase between the fluctuating lift force and the cylinder displacement changes abruptly as the reduced velocity $U_r$ increases, and (iv) whether the generating cross-flow vibration becomes divergent or convergent can be described based on the work done by the fluctuating lift force. Furthermore, it was found that the generation of cross-flow vibration can be perfectly suppressed when the small tripping rods are installed on the surface of the circular cylinder.

• Structural Engineering and Mechanics
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• 제86권6호
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• pp.705-714
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• 2023

In order to establish a dynamic model test method of bridge pylons subjected to ocean waves, the similarity method of hydroelastic model test for bridge pylons were analyzed systematically, and a model design and production method was proposed. Using this method, a dynamic test model of a bridge pylon was made, and then a free vibration test on the model structure and a dynamic response test of the model structure under wave actions were conducted in a wave flume. The results of the free vibration test show that the primary natural frequencies of the structure by the model test are close to the design frequencies of the prototype structure, indicating that the dynamic characteristics of the bridge pylon are well simulated by the model structure. The results of the dynamic response test show that wave induced base shear forces and motion responses on the model structure are consistent with the numerical results of the prototype structure. The model test results confirm that the proposed model test design method is feasible and applicable. It has application and reference significances for model testing studies of such marine bridge structures.

• 한국공간구조학회논문집
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• 제7권5호
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• pp.57-65
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• 2007

철골조 고층건물을 대상으로 풍응답계측을 실시하여, 감쇠율과 고유진동수의 진폭의존성을 검토하였다. 진폭의존성의 검토를 위하여 RD법을 사용하였다. RD법에 의한 진동특성을 하프파워법에 의한 진동특성과 비교하여 두 방법 사이의 상관관계를 분석하였다. 또한 일본, 캐나다기준에 의한 강풍시의 사용성평가를 통하여 두 가지 방법의 적용성을 검토하였다. RD법에 의하여 감쇠율의 진폭의존성을 확인하였으며, 고유진동수의 진폭의존성은 공학적인 의미에서 상대적으로 매우 작은 것을 확인하였다. 또한 계측건물은 사용성을 만족하였으며, 일본기준이 캐나다기준보다 더욱 엄격함을 확인하였다.

• 국제초고층학회논문집
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• 제4권1호
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• pp.9-25
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• 2015

Field measurements of various structures have been conducted for many purposes. Measurement data obtained by field measurement is very useful to determine vibration characteristics including dynamic characteristics such as the damping ratio, natural frequency, and mode shape of a structure. In addition, results of field measurements and modal identification can be used for modal updating of FEM analysis, for checking the efficiency of damping devices and so on. This paper shows some examples of field measurements and modal identification for structural health monitoring. As the first example, changes of dynamic characteristics of a 15-story office building in four construction stages from the foundation stage to completion are described. The dynamic characteristics of each construction stage were modeled as accurately as possible by FEM, and the stiffness of the main structural frame was evaluated and the FEM results were compared with measurements performed on non-load-bearing elements. Simple FEM modal updating was also applied. As the next example, full-scale measurements were also carried out on a high-rise chimney, and the efficiency of the tuned mass damper was investigated by using two kinds of modal identification techniques. Good correspondence was shown with vibration characteristics obtained by the 2DOF-RD technique and the Frequency Domain Decomposition method. As the last example, the wind-induced response using RTK-GPS and the feasibility of hybrid use of FEM analysis and RTK-GPS for confirming the integrity of structures during strong typhoons were shown. The member stresses obtained by hybrid use of FEM analysis and RTK-GPS were close to the member stresses measured by strain gauges.

• 대한토목학회논문집
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• 제28권5A호
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• pp.657-664
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• 2008

사장교 케이블은 구조적으로 휨강성과 감쇠력이 작아 풍우에 의해 쉽게 유해진동이 발생한다. 이러한 풍우진동을 저감시키기 위한 효과적인 방법으로 부가댐퍼를 장착하여 케이블의 감쇠력을 증가시키는 제어시스템이 널리 사용되어왔다. 그러나 사장교의 장대화로 인해 구조적으로나 미적으로 충분한 감쇠력을 제공할 수 있는 위치에 부가댐퍼를 장착하기 어렵게 되었다. 최근 본 저자는 사장교의 미관을 해치지 않으면서 기존의 제어시스템보다 효과적으로 케이블의 진동을 저감시킬 수 있는 새로운 개념의 사장교 케이블 진동 제어시스템을 제안하였다. 본 논문에서는 기존의 연구를 확장하여 새그(Sag)를 고려한 Movable anchorage 시스템을 제안하였으며, 수치해석을 통한 제어성능을 평가하였다. 수치해석의 결과 제안된 시스템은 새그를 고려하였을 때에도 고려하지 않은 경우와 마찬가지로 베어링장치의 강성이 작을수록 기존의 비감쇠 시스템이나 일반적인 수동제어시스템보다 훨씬 좋은 성능을 나타내었다. 제안된 시스템에 있어서 최적의 제어성능을 제공하기 위해서는 새그의 크기를 고려해서 최적의 감쇠계수를 결정해야 할 것으로 사료된다.

• 대한토목학회논문집
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• 제37권1호
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• pp.9-17
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• 2017

본 연구에서는 변위 및 가속도 응답의 저감 효과에 있어서, 유리한 형상인 $180^{\circ}$ 나선형(Helical $180^{\circ}$) 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 $180^{\circ}$ 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 $180^{\circ}$ 나선형모형의 공력감쇠율의 특성과는 다른 양상을 보이는 것을 알 수 있었다. 특히 풍향 변화에 따른 $180^{\circ}$ 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 "${\sqrt{2}}{\times}$응답의 표준 편차"를 적용하여 평가한 공력감쇠율은 매우 유사한 결과 값과 분포를 보이는 것으로 나타났다.