• Wind and Structures
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• 제1권1호
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• pp.43-57
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• 1998

A three-degree-of-freedom base hinged assembly (BHA) for aeroelastic model tests of tall building was developed. The integral parts of a BHA, which consists of two perpendicular plane frames and a flexural pivot, enable this modeling technique to independently simulate building translational and torsional degree-of-freedom. A program of wind tunnel aeroelastic model tests of the CAARC standard tall building was conducted with emphasis on the effect of (a) torsional motion, (b) cross-wind/torsional frequency ratio and (c) the presence of an eccentricity between center of mass and center of stiffness on wind-induced response characteristics. The experimental results highlight the significant effect of coupled translational-torsional motion and the effect of eccentricity between center of mass and center of stiffness on the resultant rms acceleration responses in both along-wind and cross-wind directions especially at operating reduced wind velocities close to a critical value of 10. In addition, it was sound that the vortex shedding process remains the main excitation mechanism in cross-wind direction even in case of tall buildings with coupled translational-torsional motion and with eccentricity.

• Structural Engineering and Mechanics
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• 제83권1호
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• pp.45-51
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• 2022

The need to account for accidental torsion in seismic design is no longer debatable, however, the seismic codes' requirement for accidental eccentricity has recently faced criticism. In order to get as close to real conditions as possible, this study investigated the impact of accidental torsion in symmetric RC multistory buildings caused by one of its many sources, the torsional earthquake component, and compared the results to those obtained by using the accidental eccentricity recommended by the codes (shifting the center of mass). To cover a wide range of frequencies and site conditions, two types of torsion seismic components were used: a recorded torsion accelerogram and five others generated using translation accelerograms. The main parameters that govern seismic responses, such as the number of stories (to account for the influence of all modes of vibration) and the frequency ratio (Ω) variation, were studied in terms of inter-story drift and displacement responses, as well as torsional moment. The results show that the eccentricity ratio of 5% required by most codes for accidental torsion should be reexamined and that it is prudent for computer analysis to use the static moment approach to implement the accidental eccentricity while waiting for new seismic code recommendations on the subject.

• 한국지진공학회논문집
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• 제7권2호
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• pp.21-27
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• 2003

일반적으로 유체-구조물 상호작용을 고려한 유체속 구조물들의 지진 및 진동해석에는 주어진 시스템에 대한 유체부가질량을 추정하여 구조물관 연계하는 단순해석 방법을 주로 사용한다. 실제로 유체속 구조물의 응답특성은 유체부가질량 뿐만 아니라 유체점성으로 인한 감쇠영향을 받으며 이들은 모두 연계항을 갖는 복잡한 행렬 형태로 나타난다. 본 연구에서는 비점성 및 점성 유체에 대한 Navier-Stokes 지배방정식의 선형화를 통한 유한요소 정식화를 유도하였다. 이를 이용하여 유한요소 해석 프로그램을 작성하고 6각형 단면특성을 갖는 액체금속로 노심에 대하여 덕트집합체 사이의 유체간격과 레이놀즈수 변화에 따른 유체부가질량과 유체감쇠에 대한 유한 요소 해석을 수행한 결과, 유체간격이 줄어들수록 유체부가질량은 유체점성의 영향을 크게 받고 유체감쇠는 점성으로 인하여 레이놀즈수의 영향을 크게 받는 것으로 나타났다. 또한 편심을 갖는 동축원통에 대한 유한요소 해석결과, 편심이 증가할수록 유체부가질량은 크게 증가하지만 유체감쇠는 편심이 작은 경우 거의 변화가 없으며 어느 일정 수준이상으로 편심이 커질 경우에는 크게 영향을 받는 것으로 나타났다.

• 한국농공학회:학술대회논문집
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• 한국농공학회 1999년도 Proceedings of the 1999 Annual Conference The Korean Society of Agricutural Engineers
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• pp.225-230
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• 1999

The purpose of this paper is to investigate the natural frequencies and mode shape of columns immersed in fluid. The beam model is based on the classical Bernoulli-Euler beam theory which neglects the effects of rotatory inertial and shear deformation. The eccentricity and rotatory inerital of the tip mass are taken into account . The governing differential equations forr the free vibrations of immersed columns are solved numerically using the corresponding boundary conditoins. The lowest four natural frequencies and corresponding mode shapes are calculated over a range of non-dimensional system parameters : the ratio of fluid depth to span length, the mass ratio, the dimensionless mass moment of inertial, and the eccentricity.

• Earthquakes and Structures
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• 제8권2호
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• pp.423-442
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• 2015

The effects of accidental eccentricity on the seismic response of four-storey steel buildings laterally stabilized by buckling restrained braced frames are studied. The structures have a square, symmetrical footprint, without inherent eccentricity between the center of lateral resistance (CR) and the center of mass (CM). The position of the bracing bents in the buildings was varied to obtain three different levels of torsional sensitivity: low, intermediate and high. The structures were designed in accordance with the seismic design provisions of the 2010 National Building Code of Canada (NBCC). Three different analysis methods were used to account for accidental eccentricity in design: (1) Equivalent Static Procedure with static in-plane torsional moments assuming a mass eccentricity of 10% of the building dimension (ESP); (2) Response Spectrum Analysis with static torsional moments based on 10% of the building dimension (RSA-10); and (3) Response Spectrum Analysis with the CM being displaced by 5% of the building dimension (RSA-5). Time history analyses were performed under a set of eleven two-component historical records. The analyses showed that the ESP and RSA-10 methods can give appropriate results for all three levels of torsional sensitivity. When using the RSA-5 method, adequate performance was also achieved for the low and intermediate torsional sensitivity cases, but the method led to excessive displacements (5-10% storey drifts), near collapse state, for the highly torsionally sensitive structures. These results support the current provisions of NBCC 2010.

• Structural Engineering and Mechanics
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• 제9권6호
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• pp.615-636
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• 2000

Some elevated water tanks have failed due to torsional vibrations in past earthquakes. The overall axisymmetric structural geometry and mass distribution of such structures may leave only a small accidental eccentricity between centre of stiffness and centre of mass. Such a small accidental eccentricity is not expected to cause a torsional failure. This paper studies the possibility of amplified torsional behaviour of elevated water tanks due to such small accidental eccentricity in the elastic as well as inelastic range; using two simple idealized systems with two coupled lateral-torsional degrees of freedom. The systems are capable of retaining the characteristics of two extreme categories of water tanks namely, a) tanks on staging with less number of columns and panels and b) tanks on staging with large number of columns and panels. The study shows that the presence of a small eccentricity may lead to large displacement of the staging edge in the elastic range, if the torsional-to-lateral time period ratio $({\tau})$ of the elevated tanks lies within a critical range of 0.7< ${\tau}$ <1.25. Inelastic behaviour study reveals that such excessive displacement in some of the reinforced concrete staging elements may cause unsymmetric yielding. This may lead to progressive strength deterioration through successive yielding in same elements under cyclic loading during earthquakes. Such localized strength drop progressively develop large strength eccentricity resulting in large localized inelastic displacement and ductility demand, leading to failure. So, elevated water tanks should have ${\tau}$ outside the said critical range to avoid amplified torsional response. The tanks supported on staging with less number of columns and panels are found to have greater torsional vulnerability. Tanks located near faults seem to have torsional vulnerability for large ${\tau}$.

• 대한기계학회논문집A
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• 제36권4호
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• pp.429-435
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• 2012

압연공정 중에 작업롤은 스트립과의 지속적인 마찰에 의해 마모가 발생하여, 질량 불평형이 발생하게 된다. 이러한 작업롤의 질량 불평형을 고려한 압연기의 수학적 모델을 제안하였고, 이 모델은 실험에 의해 검증되었다. 작업롤은 편심을 가진 강체 로터로 가정되어, 질량 불평형의 채터 진동에 대한 영향을 분석하였다. 스핀들의 준평형 해석을 통해 계산된 조인트 힘은 압연기의 작업롤에 외력으로 작용시켰다. 압연기의 운동방정식을 직접적분법으로 풀어 과도 응답을 얻고 주파수 분석을 수행하였다. 편심은 수직 방향의 진동보다 수평 방향의 진동에 더 큰 영향을 끼쳤는데, 작업롤 반경의 1%의 편심거리도 수평 방향 채터 주파수의 진폭을 크게 상승시킬 수 있다는 것을 확인하였다.

• Earthquakes and Structures
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• 제25권6호
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• pp.455-467
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• 2023

This study focuses on demonstrating the effectiveness of vibration control of tuned rotary mass damper (TRMD) for reducing the bidirectional and torsional response of the irregular asymmetric structure with voided slabs under earthquake excitations. The TRMD arranged in plane of one-story eccentric structure is proposed as a distributed tuned rotary mass damper (DTRMD) system. Lagrange's equation is used to derive the equations of motion of the controlled system. The optimum position and number of TRMD are numerically investigated under harmonic excitation and the control effects of different distributions are discussed. Furthermore, a shaking table test is conducted under different excitation cases, including free vibration, forced vibration and seismic wave to investigate the absorption performance of the device. The numerical simulations of different distributions of the TRMDs show that the DTRMDs are more effective in reduction of the displacement response of the asymmetric structure under the same mass ratio, even when the degree of eccentricity becomes large. However, with small degree of eccentricity, the unreasonable asymmetrical arrangement may cause the increase of the peak value of the rotational angular displacement. Finally, the experimental investigations exhibit similar results of translational displacement of the structure. It is concluded that the vibration of the irregular asymmetric structure can be controlled more economically and effectively by reducing the mass ratio through reducing the quantity of TRMDs at the high stiffness end.

• 한국지진공학회논문집
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• 제22권3호
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• pp.139-147
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• 2018

In the present study, effective hydrodynamic pressure modeling methods for three-dimensional earthquake safety analysis of a dam intake tower structure are investigated. Time history analysis results using the Westergaard added mass and Chopra added mass methods are compared with the one by the CASI (Coupled Acoustic Structural Interaction) method, which is accepted as giving almost exact solutions, to evaluate the difference in displacement response, stress and dynamic eccentricity. The 3D time history analysis of a realistic intake tower, which has the standard geometry widely used in Korea, shows that the Chopra added mass method gives similar results in displacement and stress and less conservative results in dynamic eccentricity to CASI ones, while the Westergaard added mass yields much more conservative results in all measures. This study suggests to use the CASI method directly for three-dimensional earthquake safety analysis of a dam intake tower, if computationally possible.

• Earthquakes and Structures
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• 제6권5호
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• pp.473-494
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• 2014

Progressive collapse, which is referred to as the collapse of the entire building under local damages, is a common failure mode happened by earthquakes. The collapse process highly depends on the whole structural system. Since, asymmetry of the building plan leads to the local damage concentration; it may intensify the progressive collapse mechanism of asymmetric buildings. In this research the progressive collapse of regular and irregular 6-story RC ordinary moment resisting frame buildings are studied in the presence of the earthquake loads. Collapse process and collapse propagation are investigated using nonlinear time history analyses (NLTHA) in buildings with 5%, 15% and 25% mass asymmetry with respect to the number of collapsed hinges and story drifts criteria. Results show that increasing the value of mass eccentricity makes the asymmetric buildings become unstable earlier and in the early stages with lower number of the collapsed hinges. So, with increasing the mass eccentricity in building, instability and collapse of the entire building occurs earlier, with lower potential of the progressive collapse. It is also demonstrated that with increasing the mass asymmetry the decreasing trend of the number of collapsed beam and column hinges is approximately similar to the decreasing trend in the average story drifts of the mass centers and stiff edges. So, as an alternative to a much difficult-to-calculate local response parameter of the number of collapsed hinges, the story drift, as a global response parameter, measures the potential of progressive collapse more easily.