• Structural Engineering and Mechanics
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• v.32 no.6
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• pp.725-740
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• 2009

A new simple relation for the estimation of modal correlation coefficients is presented. It is obtained from the decomposition of covariances of modal responses into background and resonant contributions, as it is commonly done for the variances. Thanks to appropriate assumptions, the modal correlation coefficients are estimated as weighted sums of two limit values, corresponding to the background and resonant responses respectively. The weighting coefficients are expressed as functions of the background-to-resonant ratios, which makes the proposed formulation convenient and easily accessible. The simplicity of the mathematical formulation facilitates the physical interpretation. It is for example proved that modal correlation coefficients can be non negligable even in case of well separated natural frequencies, which is sometimes unclear in the litterature. The new relation is mainly efficient in case of large finite element models. It is applied and validated on a finite element buffeting analysis of the Viaduct of Millau, the highest bridge deck ever built so far.

• Proceedings of the KSR Conference
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• 2005.11a
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• pp.211-216
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• 2005

Resonance of railroad bridge can be broken out when natural frequency of the bridge coincides with exciting frequency of moving forces. In order to avoid aforementioned unpleasant response of the structure, exact determination of dynamic structural properties is important to understand dynamic behavior of the structure under moving train loads. In the present paper, a 25 meters long full scale IPC girder and 15m Precom girder models were fabricated as a test specimen and modal testing was carried out to evaluate modal parameters including natural frequencies and modal damping ratios. In the modal testing, a digitally controlled vibration exciter as well as an impact hammer is applied to obtain frequency response functions more exactly and the modal parameters are evaluated varying with structural status.

• Structural Engineering and Mechanics
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• v.33 no.3
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• pp.325-337
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• 2009

Structural damage detection, damage localization and severity estimation of jacket platforms, based on calculating modal strain energy is presented in this paper. In the structure, damage often causes a loss of stiffness in some elements, so modal parameters; mode shapes and natural frequencies, in the damaged structure are different from the undamaged state. Geometrical location of damage is detected by computing modal strain energy change ratio (MSECR) for each structural element, which elements with higher MSECR are suspected to be damaged. For each suspected damaged element, by computing cross-modal strain energy (CMSE), damage severity as the stiffness reduction factor -that represented the ratios between the element stiffness changes to the undamaged element stiffness- is estimated. Numerical studies are demonstrated for a three dimensional, single bay, four stories frame of the existing jacket platform, based on the synthetic data that generated from finite element model. It is observed that this method can be used for damage detection of this kind of structures.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.10a
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• pp.216-221
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• 1997

A strain modal testing method has been applied to a cantilever beam to investigate the characteristics of the method. By applying the method to an analytical and an experimental system, it was shown that accurate modal parameters can be estimated from the FRFs using a current modal parameter extraction algorithm. The modal parameters estimated by the method are more accurate than those by the conventional method which uses accelerometers when the tested system is of light weight. The strain response for a given excitation force and the force which causes the response can be predicted using the measured strain FRFS.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.275-278
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• 2005

The dynamic characteristics of rotor shafts for electric motors were investigated through the modal tests. The natural frequencies and modal dampings in each manufacturing stage of rotor core assembly were analyzed from the frequency response functions fer all 6 motors of a product model. The deviation of the each individual modal feature was found dependent on the mode shapes as well as the rotor assembly stage. The core stacking itself is known to widen the deviation of modal properties but fellowing processes of rotor bar insertion and swaging are confirmed to reduce the deviation. Finally the equivalent diameter of core part was estimated from the comparison of measured and calculated results to include the stiffness of core part.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.79-83
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• 2005

SEA is a useful tool to predict noise and vibration response in high frequency region but has a weak point not to be able to express modal behavior in low frequency region. For a structure with middle subsystem having relatively higher modal density than excited subsystem and receiving subsystem, we studied the possibility that the modal behavior of receiving subsystem can express by considering finite mobility of excited subsystem. For a simply three-coupled beams which is chosen for feasibility study, the response of receiving beam was investigated with varying the length & area moment of inertia of middle beam. In case that the middle beam has relatively higher modal density than exciting beam, the application to finite mobility of excited beam led to express modal behavior of receiving beam relatively well.

• Structural Engineering and Mechanics
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• v.22 no.1
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• pp.53-70
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• 2006

The paper summarizes the dynamic-based assessment of a reinforced concrete arch bridge, dating back to the 50's. The outlined approach is based on ambient vibration testing, output-only modal identification and updating of the uncertain structural parameters of a finite element model. The Peak Picking and the Enhanced Frequency Domain Decomposition techniques were used to extract the modal parameters from ambient vibration data and a very good agreement in both identified frequencies and mode shapes has been found between the two techniques. In the theoretical study, vibration modes were determined using a 3D Finite Element model of the bridge and the information obtained from the field tests combined with a classic system identification technique provided a linear elastic updated model, accurately fitting the modal parameters of the bridge in its present condition. Hence, the use of output-only modal identification techniques and updating procedures provided a model that could be used to evaluate the overall safety of the tested bridge under the service loads.

• Smart Structures and Systems
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• v.15 no.1
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• pp.81-97
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• 2015

Recently, a new output-only modal identification method based on time-frequency independent component analysis (ICA) has been developed by the authors and shown to be useful for even highly-damped structures. In many cases, it is of interest to identify the complex modes of structures with non-proportional damping. This study extends the time-frequency ICA based method to a complex ICA formulation for output-only modal identification of non-proportionally-damped structures. The connection is established between complex ICA model and the complex-valued modal expansion with sparse time-frequency representation, thereby blindly separating the measured structural responses into the complex mode matrix and complex-valued modal responses. Numerical simulation on a non-proportionally-damped system, laboratory experiment on a highly-damped three-story frame, and a real-world highly-damped base-isolated structure identification example demonstrate the capability of the time-frequency complex ICA method for identification of structures with complex modes in a straightforward and efficient manner.

• Current Optics and Photonics
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• v.5 no.2
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• pp.93-100
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• 2021

Three factors that degrade the accuracy of modal decomposition are extensively studied using simulated and measured beams. These include a beam size mismatch, beam center mismatch, and signal-to-noise ratio of the images. The beam size and beam center are scanned using simulated noisy beams, and the result of the modal decomposition is compared with that of real beams. Based on the suggested procedure, error functions of approximately 1-4 × 10-3 can be acquired for real beams. This study provides important information regarding the impact of the three factors on the practical modal decomposition and tolerances of a mismatch, helping estimate the achievable values of the error function in a real beam modal decomposition.

• Nuclear Engineering and Technology
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• v.51 no.4
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• pp.1117-1131
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• 2019

Tanks are used extensively in many engineering areas for spent fuel pool structures at nuclear power plants or for water storage tanks in bulk carriers. To ensure the structural integrity of such tanks when under dynamic loads, modal characteristics such as natural frequencies, participation factors and mode shapes should be known. Investigated in this study are the modal characteristics of a square tank by the finite element method. This approach can be used with subsequent dynamic analyses such as a response spectrum analysis or a harmonic analysis. Finite element models are prepared to determine the natural frequencies and mode shapes, which are easy to find the modal characteristics of a fluid-filled square tank. The effects of the fluid contained in the tank and the boundary conditions at top and bottom ends on the modal characteristics are assessed by several finite element analyses.