• Journal of Korean Association for Spatial Structures
• /
• v.8 no.5
• /
• pp.67-74
• /
• 2008

Multi-layered stone masonry monuments, such as stone pagoda can be modeled as a multi-degrees of freedom system. The dynamic behavior of these structures are mainly influenced by contour condition of contacting surface of stones. In this case the mass of the system can be easily estimated, mean while the estimation of stiffness at junction is not simple. In this paper a method for estimating the spring constant at the contacting surface of stone in proposed. This paper describes a method of computational modelling technique for structural analysis of stone pagodas using measurement of natural frequency and eigenvalue analysis. For this purpose Five story stone pagoda in Cchongnimsa site was selected as a model.

• Smart Structures and Systems
• /
• v.22 no.1
• /
• pp.41-55
• /
• 2018

Damping ratio and frequency have influence on dynamic serviceability or instability such as vortex-induced vibration and displacement amplification due to earthquake and critical flutter velocity, and it is thus important to make determination of damping ratio and frequency accurate. As bridges are getting longer, small scale model test considering similitude law must be conducted to evaluate damping ratio and frequency. Analysis conditions modified by similitude law are applied to experimental test considering different scale ratios. Generally, Nyquist frequency condition based on natural frequency modified by similitude law has been used to determine sampling rate for different scale ratios, and total time length has been determined by users arbitrarily or by considering similitude law with respect to time for different scale ratios. However, Nyquist frequency condition is not suitable for multimode system with noisy signals. In addition, there is no specified criteria for determination of total time length. Those analysis conditions severely affect accuracy of damping ratio. The focus of this study is made on the determination of minimum analysis conditions for different scale ratios. Influence of signal to noise ratio is studied according to the level of noise level. Free initial value problem is proposed to resolve the condition that is difficult to know original initial value for free vibration. Ambient and free vibration tests were used to analyze the dynamic properties of a system using data collected from tests with a two degree-of-freedom section model and performed on full bridge 3D models of cable stayed bridges. The free decay is estimated with the stochastic subspace identification method that uses displacement data to measure damping ratios under noisy conditions, and the iterative least squares method that adopts low pass filtering and fourth order central differencing. Reasonable results were yielded in numerical and experimental tests.

• Explosives and Blasting
• /
• v.39 no.4
• /
• pp.1-11
• /
• 2021

In this study, the vibration data were obtained to analyze the vibration generated during dismantling of slab using the mechanical dismantling method. The obtained vibration data were classified according to the attachment device and then the waveform and dominant frequency analysis were performed. And the correlation was analyzed by the different methods of measuring the distance between the work section and the measurement point. As a result of the waveform analysis for each attachment device, there was little change in the phase of the vibration waveform and only the change in amplitude, which is the magnitude of the vibration velocity. And as a result of frequency analysis, the frequency band was lower when using a crusher method than a braker method and frequency band were close of the natural frequency of the structure to be dismantled. As a result of the correlation analysis, the separation distance was estimated a higher correlation when evaluated as the path through which the vibration propagates along the structure frame than the straight distance between the measurement point and the working section.

• Food Science of Animal Resources
• /
• v.37 no.4
• /
• pp.579-592
• /
• 2017

This study assessed the quantitative microbial risk of non-enterohemorrhagic Escherichia coli (EHEC). For hazard identification, hazards of non-EHEC E. coli in natural and processed cheeses were identified by research papers. Regarding exposure assessment, non-EHEC E. coli cell counts in cheese were enumerated, and the developed predictive models were used to describe the fates of non-EHEC E. coli strains in cheese during distribution and storage. In addition, data on the amounts and frequency of cheese consumption were collected from the research report of the Ministry of Food and Drug Safety. For hazard characterization, a doseresponse model for non-EHEC E. coli was used. Using the collected data, simulation models were constructed, using software @RISK to calculate the risk of illness per person per day. Non-EHEC E. coli cells in natural- (n=90) and processed-cheese samples (n=308) from factories and markets were not detected. Thus, we estimated the initial levels of contamination by Uniform distribution ${\times}$ Beta distribution, and the levels were -2.35 and -2.73 Log CFU/g for natural and processed cheese, respectively. The proposed predictive models described properly the fates of non-EHEC E. coli during distribution and storage of cheese. For hazard characterization, we used the Beta-Poisson model (${\alpha}=2.21{\times}10^{-1}$, $N_{50}=6.85{\times}10^7$). The results of risk characterization for non-EHEC E. coli in natural and processed cheese were $1.36{\times}10^{-7}$ and $2.12{\times}10^{-10}$ (the mean probability of illness per person per day), respectively. These results indicate that the risk of non-EHEC E. coli foodborne illness can be considered low in present conditions.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.12 no.1
• /
• pp.95-105
• /
• 1988

The dynamic characteristics of a tennis racket were identified by experimental modal analysis and the sweet spot due to these was calculated. A tennis racket was selected as a model reference and modal parameters of the racket were estimated from the transfer function measured by experiment. The transfer function was reconstructed by the modified modal parameters on the assumption that the racket was locally modified, for example, a change in the material of the frame and the attachment of a damping material to the grip. The change of the dynamic characteristics, especially natural frequency and magnitude of the second and the third vibrational modes, were evaluated from the reconstructed transfer function. The change of the sweet spot due to the modified modal parameters was estimated and visualized through computer graphic simulation by the criterion of the sweet spot measured before structural modification. The modal parameters for improvement of the dynamic characteristics.

• Journal of the Korean Society of Safety
• /
• v.7 no.2
• /
• pp.71-79
• /
• 1992

This paper presents a least-square algorithms of lattice structures and their use for adaptive prediction of time series generated from the dynamic system. As the view point of adaptive prediction, a new method of Identification of dynamic characteristics by means of estimating the parameters of linear auto regressive model is proposed. The fast convergence of adaptive lattice algorithms is seen to be due to the orthogonalization and decoupling properties of the lattice. The superiority of the least-square lattice is verified by computer simulation, then predictor coefficients are computed from the linear sequential time data. For the application to the dynamic characteristic analysis of unknown system, the transfer function of ideal system represented in frquency domain and the estimated one obtained by predicted coefficients are compared. Using the proposed method, the damping ratio and the natural frequency of a dynamic structure subjected to random excitations can be estimated. It is expected that this method will be widely applicable to other technical dynamic problem in which estimation of damping ratio and fundamental vibration modes are required.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.12 no.5
• /
• pp.1016-1025
• /
• 1988

A force cancelling observed to control the vibration of a single degree of freedom elastic system subjected to an arbitrary, unmeasurable disturbance is considered in this paper. The main idea of a force cancelling observer is how an estimate of the excitation can be derived and used to generate a control force which reduces the vibration. This control is shown to be robust with respect to the parameters describing the behavior of the system. Experimental and numerical results are presented which show the efficacy of the observer when the system is excited by periodic, random, and impulsive torques.

• Journal of Ocean Engineering and Technology
• /
• v.38 no.1
• /
• pp.1-9
• /
• 2024

The vertical center of gravity (VCG) has a significant impact on the roll motion response of a surface ship, particularly oil tankers based on the oil level in the tanker after discharging oil at several stations or positional changes, such as changes in the superstructure and deck structure. This study examined the motion response of the Korea very large crude carrier 2 (KVLCC2) at various VCGs, especially roll motion when the VCG changed. The potential theory in the Ansys AQWA program was used as a numerical simulation method to calculate the motion response. On the other hand, the calculations obtained through potential theory overestimated the roll amplitudes during resonance and lacked precision. Therefore, roll damping is a necessary parameter that accounts for the viscosity effect by performing an experimental roll decay. The roll decay test estimated the roll damping coefficients for various VCGs using Froude's method. The motion response of the ship in regular waves was evaluated for various VCGs using the estimated roll-damping coefficients. In addition, the reliability of the numerical simulation in motion response was verified with those of the experiment method reported elsewhere. The simulation results showed that the responses of the surge, sway, heave, pitch, and yaw motion were not affected by changing the VCG, but the natural frequency and magnitude of the peak value of the roll motion response varied with the VCG.

• Journal of Korean Society of Steel Construction
• /
• v.18 no.4
• /
• pp.503-514
• /
• 2006

FBG sensors are capable of measuring the strain of structures easily and more durably than electric resistance gauges. Thus, many researches are dedicated to the application for the response monitoring or non-destructive evaluation of structures using FBG sensors. Additionally, the measured strains at the top and bottom of a cross-section can be transformed into the curvature of the section, which can be used to calculate its vertical displacement. Hence, this study aims to measure the dynamic strain signals of a steel section simply supported beam and to estimate the dynamic displacement from the strain signals, after which the estimated displacement is com pared with the measured displacement. The dynamic characteristics (natural frequency, damping ratio and mode shape) of the beam are predicted from both the estimated and measured displacement signals, and from the strain time history of the FBG sensors. The predicted properties are compared with those of an analytical model of the beam. The estimated displacement. However, the predicted dynamic properties from both the estimated displacements and the measured strains are well-correlated with those from the measured displacement. It is therefore appreciated that the estimation of the dynamic properties of FBG sensor signals is reasonable. Especially, the strain signal of the FBG sensor was amplified at a higher-frequency region in comparison with the displacement estimation with higher-mode properties.

• Journal of the Society of Naval Architects of Korea
• /
• v.55 no.1
• /
• pp.37-44
• /
• 2018

To understand the dynamic characteristics of the vessel with hydroelastic response, it is very important to estimate the dynamic modal parameters such as mode shapes, natural frequency, and damping ratio. These dynamic modal parameters of full scale ship are a priori unknowns, hence to be estimated directly based upon the full scale measurement data. In this paper, dynamic modal parameters were extracted by signal processing of acceleration and strain data measured from a large container ship whose loading capacity is 9400TEU. The mode shapes of the vibrating hull were identified using the proper orthogonal decomposition and the vibration response of hull was decomposed into its modal magnitudes. Natural frequencies of specific modes were derived via Fourier transform of these modal magnitude. Also, the free decay signal of the vibrating hull was obtained through the random decrement technique and the damping ratio was estimated with accuracy.