• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.5 s.98
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• pp.604-611
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• 2005

The relationship between floor impact sound and vibration has been studied by field measurements, and the vibration modal characteristics have been analyzed. Vibration levels impacted by a standard heavy-weight impact source have been predicted according to the main design parameters using finite element method. Experimental results show that the dominant frequencies of the heavy impact sounds range below 100 Hz and that they are coincident with natural frequencies of the concrete slab. In addition, simple 2-dimensional finite element models are proposed to substitute 2 types of 3-dimensional models of complicated floor structural slabs those by The analytical result shows that the natural frequencies from first to fifth mode well correspond to those by experiments with an error of less than $12\%$, and acceleration peak value iscoincident with an error of less than $2\%$. Using the finite element model. vibration levels areestimated according to the design Parameters, slab thickness, compressive strength, and as a result, the thickness is revealed as effective to increase natural frequencies by $20\~30\%$ and to reduce the vibration level by 3$\~$4 dB per 30 mm of extra thickness.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.12 s.105
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• pp.1347-1354
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• 2005

The free vibration characteristics of Al cantilever square plates with a brass inclusion were analyzed experimentally and numerically The experimentally obtained natural frequencies and mode shapes were compared with the FEM analysis results. The impulse exciting method was used for experiment and ANSYS software package was used for FEM analysis. The natural frequencies obtained iron experiment and numerical analysis matched within $0\%$. It was found that the natural frequencies of the Al cantilever square plates with a brass inclusion decrease as the size of inclusion increases. For the third mode shape, comparing the nodal line of the Al plate and the Al plate with a inclusion, the mode shape showed the reversed quadratic curve. The natural frequencies of inclusion plate were decreased as the location of inclusion moves from the clamped edge to the tree edge.

• International Journal of High-Rise Buildings
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• v.2 no.3
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• pp.171-178
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• 2013

The emergence of a growing number of tall buildings, often with unusual shapes and innovative structural systems, has led to the realization of the need for and the importance of field measurements. The new China Central Television (CCTV) Tower in Beijing is one of tall buildings with a highly unusual shape and a complex structural system, requiring field measurements to identify its dynamic characteristics for the subsequent dynamic analysis of the tower under wind excitation, seismic-induced ground motion and traffic-induced ground motion. The structural system and the finite element model of the CCTV Tower are first introduced in this paper. The computed natural frequencies and mode shapes are then presented as a reference for the field measurement. After introducing the arrangement of the ambient vibration measurement, the field measured natural frequencies and damping ratios of the CCTV Tower are presented and the measured natural frequencies are finally compared with the computed ones. It was found that the structural damping ratios of the CCTV Tower are small and the computed natural frequencies are smaller than the measured ones by about 12~17%.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.12
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• pp.1948-1952
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• 2012

The paper presents modal testing and analysis in order to obtain the dynamic characteristics of a middle spacer grids of a nuclear fuel rod. A spacer grid is one of the important structural elements supporting nuclear fuel rods. Such a fuel rod can be oscillated by its thermal expansion, neutron irradiation and etc. due to cooling water flow under the operation of a nuclear power plant. When the fuel rod vibrates, fretting wear due to repeated friction motion between the fuel rods and spacer grids can be occurred, and so the fuel rod is damaged. In this paper, through modal analysis and testing, natural frequencies and modes of a middle spacer grid were calculated, and the following conclusions were obtained. Firstly the numerical first-seven natural frequencies for spacer grids of a fuel rod having complicated structures have a small difference within 3.8% with experimental natural frequencies, and so the suitability of simulation results was verified. Secondly, experimental mode shapes for a middle spacer grid of a nuclear fuel rod were verified by obtaining lower non-diagonal terms through MAC(Modal Assurance Criteria), and were confirmed by the simulation modes.

• Smart Structures and Systems
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• v.20 no.2
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• pp.247-261
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• 2017

In the field of dynamic measurement and structural damage identification, it is generally known that modal frequencies may be measured with higher accuracy than mode shapes. However, the number of natural frequencies within a measurable range is limited. Accessing additional forms of modal frequencies is thus desirable. The present study is concerned about the extraction of artificial boundary condition (ABC) frequencies from modal testing. The ABC frequencies correspond to the natural frequencies of the structure with a perturbed boundary condition, but they can be extracted from processing the frequency response functions (FRF) measured in a specific configuration from the structure in its existing state without the need of actually altering the physical support condition. This paper presents a comprehensive experimental investigation into the measurability of the ABC frequencies from physical experiments. It covers the testing procedure through modal testing, the data processing and data analysis requirements, and the FRF matrix operations leading to the extraction of the ABC frequencies. Specific sources of measurement errors and their effects on the accuracy of the extracted ABC frequencies are scrutinised. The extracted ABC frequencies are subsequently applied in the damage identification in beams by means of finite element model updating. Results demonstrate that it is possible to extract the first few ABC frequencies from the modal testing for a variety of artificial boundary conditions incorporating one or two virtual pin supports, and the inclusion of ABC frequencies enables the identification of structural damages without the need to involve the mode shape information.

• Smart Structures and Systems
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• v.26 no.3
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• pp.291-309
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• 2020

To study oceanic and meteorological problems related to climate change, Korea has been operating several ocean research stations (ORSs). In 2011, the Gageocho ORS was attacked by Typhoon Muifa, and its structural members and several observation devices were severely damaged. After this event, the Gageocho ORS was rehabilitated with 5 m height to account for 100-yr extreme wave height, and the vibration measurement system was equipped to monitor the structural vibrational characteristics including natural frequencies and modal damping ratios. In this study, a mass reallocation method is presented for structural model updating of the Gageocho ORS based on the experimentally identified natural frequencies. A preliminary finite element (FE) model was constructed based on design drawings, and several of the candidate baseline FE models were manually built, taking into account the different structural conditions such as corroded thickness. Among these candidate baseline FE models, the most reasonable baseline FE model was selected by comparing the differences between the identified and calculated natural frequencies; the most suitable baseline FE model was updated based on the identified modal properties, and by using the pattern search method, which is one of direct search optimization methods. The mass reallocation method is newly proposed as a means to determine the equivalent mass quantities along the height and in a floor. It was found that the natural frequencies calculated based on the updated FE model was very close to the identified natural frequencies. In conclusion, it is expected that these results, which were obtained by updating a baseline FE model, can be useful for establishing the reference database for jacket-type offshore structures, and assessing the structural integrity of the Gageocho ORS.

• Journal of the korean Society of Automotive Engineers
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• v.3 no.1
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• pp.38-45
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• 1981

In this paper, the natural frequencies of curved taper leaf springs using on vehicle suspension systems are studied. By applying the Castigliano's definition, Rayleigh's principle and Dunkerley's equation, new formulas defining the natural frequency of such shaped spring are derived. Numerical calculations are in very good agreement with experimental results on actual models. We found that the natural frequencies of curved taper spring are increased by 21-28% compared with the spring having same weight, span and curve but uniform section.

• Transactions of the Korean Society of Automotive Engineers
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• v.5 no.4
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• pp.152-159
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• 1997

This paper presents a vibration analysis method of crank shaft of six cylinder internal combustion engine. For simple analysis journal, pin and arm parts were assumed to have uniform section. Transfer Matrix Method was used, considering branched part and coordinate transformation part. Natural frequencies, modeshapes and transfer functions of crank shaft were investigated based upon the Euler beam theory: It was shown that the calculated natural frequencies, modeshapes agree well with the existing paper results.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.11a
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• pp.396.1-396
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• 2002

The AVT under traffic-induced vibrations was carried out on Namhae Suspension bridge in Korea. Mode shapes as well as natural frequencies up to the 15th mode were acquired exactly, and the effect of traffic mass and temperature on measured natural frequencies was investigated. The results from the AVT are compared with those from forced vibration test(FVT) and FE analysis. (omitted)

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2006.04a
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• pp.675-680
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• 2006

This paper deals with the flexural free vibrations of strip foundations. Based on dynamic equilibrium equations of a beam element resting on Winkler foundation, differential equations governing free vibration of strip foundation are derived, in which effects of rotatory inertia and shear deformation are included. For obtaining the natural frequencies, differential equations are solved by numerical methods. As the numerical results, relationships between natural frequencies and various strip parameters are obtained and presented in Tables and Figures.