• Journal of the Computational Structural Engineering Institute of Korea
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• v.12 no.1
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• pp.95-102
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• 1999

본 연구에서는 중복되지 않는 고유치를 갖는 비비례 감쇠계의 고유치와 고유벡터의 민감도를 계산하는 새로운 방법을 제시하였다. 제안 방법에서는 (n+1)차의 대칭 행렬로 이루어진 대수방정식을 해석함으로써 n개의 자유도를 갖는 감쇠계의 고유치와 고유벡터의 설계변수에 대한 미분을 구한다. 제안 방법은 매우 간단하면서도 수치적 안정성이 보장되고 정확한 해를 주는 방법이다. 제안 방법의 검증을 위해 7자유도를 갖는 차량모델의 민감도해석을 예제에서 다루고 있다. 예제에서의 설계변수는 콘테이너의 질량으로 하였다.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.12 no.1
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• pp.103-109
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• 1999

본 연구에서는 중복 고유치를 갖는 비비례 감쇠 진동계의 고유치와 고유벡터의 민감도를 계산하는 새로운 방법을 제시하였다. 제안 방법은 매우 간단하면서도 수치적 안정성이 보장되고 정확한 해를 주는 방법이다. 제안 방법에서는 (n+m)차의 대칭 행렬로 이루어진 대수방정식을 해석함으로써 n개의 자유도를 갖는 감쇠계에 있어서 m차의 중복도를 갖는 고유치와 고유벡터의 설계변수에 대한 미분을 구한다. 제안 방법의 검증을 위해 5자유도를 갖는 단순구조물의 민감도해석을 예제에서 다루고 있다. 예제에서의 설계변수는 모델의 부분강성으로 하였다.

• Computers and Concrete
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• v.10 no.3
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• pp.277-294
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• 2012

The aim of the study is to determine the modal parameters of a prototype damaged arch dam by operational modal analysis (OMA) method for some damage scenarios. For this purpose, a prototype arch dam-reservoir-foundation model is constructed under laboratory conditions. Ambient vibration tests on the arch dam model are performed to identify the modal parameters such as natural frequency, mode shape and damping ratio. The tests are conducted for four test-case scenarios: an undamaged dam with empty reservoir, two different damaged dams with empty reservoirs, and a damaged dam with full reservoir. Loading simulating random impact effects is applied on the dam to crack. Cracks and fractures occurred at the middle of the upper part of the dams and distributed through the abutments. Sensitivity accelerometers are placed on the dams' crests to collect signals for measurements. Operational modal analysis software processes the signals collected from the ambient vibration tests, and enhanced frequency domain decomposition and stochastic subspace identification techniques are used to estimate modal parameters of the dams. The modal parameters are obtained to establish a basis for comparison of the results of two techniques for each damage case. Results show that approximately 35-40% difference exists between the natural frequencies obtained from Case 1 and Case 4. The natural frequencies of the dam considerably decrease with increasing cracks. However, observation shows that the filled reservoir slightly affected modal parameters of the dam after severe cracking. The mode shapes obtained are symmetrical and anti-symmetrical. Apparently, mode shapes in Case 1 represent the probable responses of arch dams more accurately. Also, damping ratio show an increase when cracking increases.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.22 no.5
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• pp.480-488
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• 2012

The aircrafts with high aspect ratio wings made by a composite material have been developed, which enable high energy efficiency and long-term flight by reducing air resistance and structural weight. However, they have difficulties in securing the aeroelastic stability such as the flutter because of their long and flexible wings. The flutter is unstable self-excited-vibration caused by interaction between the structural dynamics and the aerodynamics. It should be verified analytically prior to first flight test that the flutter does not happen in the range of flight mission. Normally, the finite element model is used for the flutter analysis. So it is important to construct the finite element model representing dynamic characteristics similar to those of a real aircraft. Accordingly, in this research, to acquire dynamic characteristics experimentally the modal test of the aircraft with high aspect ratio composite wings was conducted. And then the modal parameters from the finite element analysis(FEA) were compared with those from the modal test. To make analysis results closer to test results, the finite element model was updated by means of the sensitivity analysis on variables and the optimization. Finally, it was proved that the updated finite element model is reliable as compared with the results of the modal test.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.18 no.8
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• pp.854-861
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• 2007

In this paper, a compact L-shaped shorted pin patch to be used at local oscillator port in radar detector is proposed. Shorted pin patch operates as short-open cavity while conventional patch operates as open-open cavity. Shorted pins are L-shaped to obtain quarter wave length and three-quarter wave length resonance for the fundamental(X-band) and $2^{nd}(3^{rd})$(K- and Ka-band) mode resonance frequency, respectively. Thus, the proposed patch can be compact. It is also possible to operate at triple band(X-, K-, Ka-band) with small return loss in radar detector, It is expected that shorted pin patch could improve radar detector sensitivity.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.05a
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• pp.980-985
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• 2002

Several tests are performed to evaluate road booming noise. Baseline test delivers the information of road noise characteristics. Coupling effect between structure and acoustics is obtained from the mode shapes and the natural frequencies by the modal test. Equivalent stiffness at joint areas between chassis and car-body system can be determined by the input point inertance test. Noise sensitivity of body mounting point of a chassis part can be obtained from the noise transfer function test with input point inertance test. Operational deflection shape makes us analyze the actual vibration modes of the chassis system under actual loading and find noise sources very easily. finally, the transfer function analysis is used to identify noise paths through the chassis system. However, all of the tests above mentioned must be performed to evaluate road booming noise. The objectives and the procedures of the tests are described in this paper. Also, the guideline for efficient road noise evaluation test can be found.

• Smart Structures and Systems
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• v.12 no.3_4
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• pp.235-250
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• 2013

Optimal sensor placement (OSP) technique plays a key role in the structural health monitoring (SHM) of large-scale structures. According to the mathematical background and implicit assumptions made in the triaxial effective independence (EfI) method, this paper presents a novel multi-dimensional OSP method for the Canton Tower focusing on application demands. In contrast to existing methods, the presented method renders the corresponding target mode shape partitions as linearly independent as possible and, at the same time, maintains the stability of the modal matrix in the iteration process. The modal assurance criterion (MAC), determinant of the Fisher Information Matrix (FIM) and condition number of the FIM have been taken as the optimal criteria, respectively, to demonstrate the feasibility and effectiveness of the proposed method. Numerical investigations suggest that the proposed method outperforms the original EfI method in all instances as expected, which is looked forward to be even more pronounced should it be used for other multi-dimensional optimization problems.

• Structural Engineering and Mechanics
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• v.10 no.6
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• pp.517-532
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• 2000

A direct output feedback control scheme was recently proposed by the authors for single-story building structures resting on flexible soil body. In this paper, the control scheme is extended to mitigate the seismic responses of multi-story buildings. Soil-structure interaction is taken into account in two parts: input at the soil-structure interface/foundation and control algorithm. The former reflects the effect on ground motions and is monitored in real time with accelerometers at foundation. The latter includes the effect on the dynamic characteristics of structures, which is formulated by modifying the classical linear quadratic regulator based on the fundamental mode shape of the soil-structure system. Numerical result on the study of a $\frac{1}{4}$-scale three-story structure, supported by a viscoelastic half-space of soil mass, have demonstrated that the proposed algorithm is robust and very effective in suppressing the earthquake-induced vibration in building structures even supported on a flexible soil mass. Parametric studies are performed to understand how soil damping and flexibility affect the effectiveness of active tendon control. The selection of weighting matrix and effect of soil property uncertainty are investigated in detail for practical applications.

• Current Optics and Photonics
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• v.5 no.1
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• pp.66-71
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• 2021

The resonance characteristics of H-shaped metamaterials, whose widths were varied while keeping the height constant, were investigated in the terahertz (THz) frequency range. The H-shaped metamaterials were numerically analyzed in two modes in which the polarization of the incident THz electric field was either parallel or perpendicular to the width of the H-shaped structure. The resonant frequency of the metamaterial changed stably in each mode, even if only the width of the H shape was changed. The resonant frequency of the metamaterial operating in the two modes increases without significant difference regardless of the polarization of the incident electromagnetic wave as the width of the H-shaped metamaterial increases. The electric field distribution and the surface current density induced in the metamaterial in the two modes were numerically analyzed by varying the structure ratio of the metamaterial. The numerical analysis clearly revealed the cause of the change in the resonance characteristics as the width of the H-shaped metamaterial changed. The efficacy of the numerical analysis was verified experimentally using the THz-TDS (time-domain spectroscopy) system. The experimental results are consistent with the simulations, clearly demonstrating the meaningfulness of the numerical analysis of the metamaterial. The analyzed resonance properties of the H-shaped metamaterial in the THz frequency range can be applied for designing THz-tunable metamaterials and improving the sensitivity of THz sensors.

• Structural Engineering and Mechanics
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• v.83 no.3
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• pp.363-375
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• 2022

Flexural capacity prediction is a challenging problem for externally prestressed concrete beams (EPCBs) due to the unbonded phenomenon between the concrete beam and external tendons. Many prediction equations have been provided in previous research but typically ignored the differences in deformation mode between internal and external unbonded tendons. The availability of these equations for EPCBs is controversial due to the inconsistent deformation modes and ignored second-order effects. In this study, the deformation characteristics and collapse mechanism of EPCB are carefully considered, and the ultimate deflected shape curves are derived based on the simplified curvature distribution. With the compatible relation between external tendons and the concrete beam, the equations of tendon elongation and eccentricity loss at ultimate states are derived, and the geometric interpretation is clearly presented. Combined with the sectional equilibrium equations, a rational and simplified flexural capacity prediction method for EPCBs is proposed. The key parameter, plastic hinge length, is emphatically discussed and determined by the sensitivity analysis of 324 FE analysis results. With 94 collected laboratory-tested results, the effectiveness of the proposed method is confirmed, and comparisons with the previous formulas are made. The results show the better prediction accuracy of the proposed method for both stress increments and flexural capacity of EPCBs and the main reasons are discussed.