• Steel and Composite Structures
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• v.16 no.1
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• pp.21-44
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• 2014

The aim of this study is to determine the dynamic characteristics of long reinforced concrete highway bridges with post-tension tendons using analytical and experimental methods. It is known that the deck length and height of bridges are affected the dynamic characteristics considerably. For this purpose, Berta Bridge constructed in deep valley, in Artvin, Turkey, is selected as an application. The Bridge has two piers with height of 109.245 m and 85.193 m, and the total length of deck is 340.0 m. Analytical and experimental studies are carried out on Berta Bridge which was built in accordance with the balanced cantilever method. Finite Element Method (FEM) and Operational Modal Analysis (OMA) which considers ambient vibration data were used in analytical and experimental studies, respectively. Finite element model of the bridge is created by using SAP2000 program to obtain analytical dynamic characteristics such as the natural frequencies and mode shapes. The ambient vibration tests are performed using Operational Modal Analysis under wind and human loads. Enhanced Frequency Domain Decomposition (EFDD) and Stochastic Subspace Identification (SSI) methods are used to obtain experimental dynamic characteristics like natural frequencies, mode shapes and damping ratios. At the end of the study, analytical and experimental dynamic characteristic are compared with each other and the finite element model of the bridge was updated considering the material properties and boundary conditions. It is emphasized that Operational Modal Analysis method based on the ambient vibrations can be used safely to determine the dynamic characteristics, to update the finite element models, and to monitor the structural health of long reinforced concrete highway bridges constructed with the balanced cantilever method.

• Journal of the Earthquake Engineering Society of Korea
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• v.7 no.4
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• pp.71-79
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• 2003

As a critical member of cable-stayed bridges, stay cables play on important role of supporting the entire structure. Traffic, wind or rain-wind induced vibrations of stay cables would be a major cause of degrading both safety and serviceability of the bridge. One of the effective alternatives to solve this problem is to employ the cable dampers. In order to design the cable damper optimally, it is necessary to exactly estimate the dynamic characteristics of the existing cables. To achieve more reliable dynamic properties of stay cables, precise excitations inducing forced vibration are needed. Therefore, in this study, a cable excitation system(exciter) controlled digitally was developed. And to evaluate the performance of the cable exciter developed, a solution of the differential equation of cable motion considering the exciter was derived, Using the cable exciter, sine sweeping and resonance tests on a cable model were carried out to obtain the dynamic characteristics effectively.

• Steel and Composite Structures
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• v.7 no.6
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• pp.487-502
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• 2007

In recent decades there has been a trend towards improved mechanical characteristics of materials used in footbridge construction. It has enabled engineers to design lighter, slender and more aesthetic structures. As a result of these construction trends, many footbridges have become more susceptible to vibrations when subjected to dynamic loads. In addition to this, some inherit modelling uncertainties related to a lack of information on the as-built structure, such as boundary conditions, material properties, and the effects of non-structural elements make difficult to evaluate modal properties of footbridges, analytically. For these purposes, modal testing of footbridges is used to rectify these problems after construction. This paper describes an arch type steel footbridge, its analytical modelling, modal testing and finite element model calibration. A modern steel footbridge which has arch type structural system and located on the Karadeniz coast road in Trabzon, Turkey is selected as an application. An analytical modal analysis is performed on the developed 3D finite element model of footbridge to provide the analytical frequencies and mode shapes. The field ambient vibration tests on the footbridge deck under natural excitation such as human walking and traffic loads are conducted. The output-only modal parameter identification is carried out by using the peak picking of the average normalized power spectral densities in the frequency domain and stochastic subspace identification in the time domain, and dynamic characteristics such as natural frequencies mode shapes and damping ratios are determined. The finite element model of footbridge is calibrated to minimize the differences between analytically and experimentally estimated modal properties by changing some uncertain modelling parameters such as material properties. At the end of the study, maximum differences in the natural frequencies are reduced from 22% to only %5 and good agreement is found between analytical and experimental dynamic characteristics such as natural frequencies, mode shapes by model calibration.

• The KSFM Journal of Fluid Machinery
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• v.20 no.1
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• pp.15-20
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• 2017

The small turbocharger for the automotive application is designed to operate up to 200,000 rpm to increase system efficiency. Because of high rotation speed of turbocharger, floating ring bearing are widely adopted due to its low friction loss and high rotordynamic stability. This paper presents a linear and nonlinear analysis model for a turbocharger rotor supported by a semi-floating ring bearing. The rotordynamic model for the turbocharger rotor was constructed based on the finite element method and fluid film forces were calculated based on the infinitely short bearing assumption. In linear analysis, we considered fluid film force as stiffness and damping element and in nonlinear analysis, the fluid film force was calculated by solving the time dependent Reynolds equation. We verified the developed theoretical model by comparing to modal test results of test rotors. The analysis results show that there are two unstable modes, which are conical and cylindrical modes. These unstable modes appear as sub-synchronous vibrations in nonlinear analysis. In nonlinear analysis, frequency jump phenomenon demonstrated when vibration mode is changed from conical mode to cylindrical one. This jump phenomenon was also demonstrated in the test. However, the natural frequency measured in the test differs from those obtained using nonlinear analysis.

• Journal of the Society of Naval Architects of Korea
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• v.35 no.4
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• pp.27-37
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• 1998

The sloshing is very important in a safe transport of the liquid cargo by a ship. With the increasing number of supertanker and LNG carriers, this problem has become increasingly more important. In order to study the magnitude and characteristics of impact pressures due to sloshing, experiments ware performed with a rectangular tank and compared with numerical results. Structural responses of tank wall under impulsive pressures were measured. Structural vibrations induced by the sloshing load were analysed by including hydroelastic erects in terms of added mass and damping. To check the validity of the numerical model, the natural frequencies of plate in air and water were compared with measurements, and a good agreement was found. In the case that a plate vibrates under impulsive loads, the pressure on the flexible plate is larger than that on the rigid plate without hydroelastic effects, which was confirmed experimentally. The frequency of oscillatory pressure as well as accel%pion coincides with the natural frequency of plate in water.

• The Journal of the Acoustical Society of Korea
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• v.16 no.2
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• pp.10-15
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• 1997

The hydrophones is mounted in many applications on a vibrating surface and functions as an underwater acoustic signal receiver without sensing the vibrations from the mounting surface. However, their performance is usually degraded by the interference of exterior noises such as acoustic cavitation in water stream, host structural vibration in the hull, and propeller motions. This paper describes the design and evaluation of a self noise suppressing hydrophones which shows very poor sensitivity to the external noises, first, effects of the external noise on the its receiver performance is simulated with finite element method(FEM). Second, the geometrical variations are implemented on the original structure that include additional air pockets and acoustic walls which work as acoustic shied or scatter of the noises. The results show that the effect of the external noise is the most significant when it is applied near to the bottom of the side wall of the hydrophones. The transverse noise induced by the outside water flow is isolated most effectively when a thin compliant (damping) layer combined with two air pockets is inserted to the circumference of the nose. Noise level is reduced about fifty nine percent of that of the original structure.

• Journal of the Institute of Convergence Signal Processing
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• v.22 no.4
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• pp.199-205
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• 2021

The distributing board in directly installed on the ground or the bottom surface of the building, and when vibrations such as earthquakes or external shocks occur, the possibility of damage or malfunction of electric components such as internal power devices, wiring, and protection relays increases. Recently, the need for a seismic type distributing board is increasing, and research and development of a distributing board having a vibration damping function and product launch are being conducted. In this paper, an IoT-based data collection device system capable of measuring vibration and environmental data of distributing board was designed and implemented. When vibration occurred on the distributing board, data was stored and visualized in the MySQL DB through Node-RED for monitoring and data storage using the MQTT protocol for reliable messaging transmission. The test was conducted by attaching the IoT device of the distributing board, and data was collected in real-time and monitored through Node-RED.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.51 no.11
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• pp.601-611
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• 2002

As resent trends in structural construction have been to build taller and larger structures than any time in the past, they have had high flexibility and low damping that can cause large vibration response under severe environmental loading such as earthquakes, winds, and mechanical excitations. The damper with mass and sqring is one aproach to safeguarding the structure against excessive vibrations. In this paper, a large capacity hybrid-type linear motor damper(LMD) was designed and fabricated for the application to the vibration control of a large building structure model. It has been designed to be able to move the damper mass, 1,500 kg up to ${\pm}250mm$ strokes at the first mode natural frequency of the building structure model, ${\pm}0.51Hz$. Linear motor is consisted of the fixed coil and the movable NdFeB permanent magnets field part. The PM field part composed magnet modules and iron yoke, is the damper mass itself, 1500kg. LMD therefore has a simplified structure and requires a few elements in the driving system, being compared with a rotary motor damper and a hydraulic damper. However, the manufacture of large PM linear actuator is difficult because of the limit of PM size and the attraction and repulsion at the assembly of PM. Therefore, large damper system is manufactured and tested for dynamic characteristics and frequency response.

• Tribology and Lubricants
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• v.31 no.6
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• pp.302-307
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• 2015

The use of turbocharger in internal combustion engines has increased as it is a key components for improving system efficiency without increasing engine size. Because of increasing demand, many studies have evaluated rotordynamic performance so as to increase rotation speed. This paper presents a linear and nonlinear analysis model for a turbocharger rotor supported by a floating ring bearing. We constructed rotor model by using the finite element method and approximated bearings as being infinitely short. In the linear model, we considered fluid film force as stiffness and damping element. In nonlinear analysis, calculation of the fluid film force involved solving the time dependent Reynolds equation. We verified the developed model by comparing the results to those of previous research. The analysis results show that there are four unstable modes, which are rigid body modes combining ring and rotor motion. As the rotating speed increases, the logarithmic decrement shows that certain unstable modes goes into the stable area or the stable mode goes into the unstable area. These unstable modes appear as sub-synchronous vibrations in nonlinear analysis. In nonlinear analysis frequency jump phenomenon demonstrated in several experimental studies appears. The analysis results also showed that frequency jump phenomenon occurs when the vibration mode changes and the sequence of unstable mode matches the linear analysis result. However, the natural frequency predicted using linear analysis differs from those obtained using nonlinear analysis.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.36 no.5
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• pp.315-321
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• 2023

Long-span bridges are flexible structures with low natural frequencies and damping ratios, making them susceptible to vibrational serviceability problems. However, the current design guideline of South Korea assumes a uniform threshold of wind speed or vibrational amplitude to assess the occurrence of harmful vibrations, potentially overlooking the complex vibrational patterns observed in long-span bridges. In this study, we propose a pointwise vortex-induced vibration (VIV) detection method using a deep-learning-based signalsegmentation model. Departing from conventional supervised methods of data acquisition and manual labeling, we synthesize training data by generating sinusoidal waves with an envelope to accurately represent VIV. A Fourier synchrosqueezed transform is leveraged to extract time-frequency features, which serve as input data for training a bidirectional long short-term memory model. The effectiveness of the model trained on synthetic VIV data is demonstrated through a comparison with its counterpart trained on manually labeled real datasets from an actual cable-supported bridge.