• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.743-754
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• 2020

This study provides an insight of the nonlinear behavior of the Offshore Wind Turbine (OWT) structure using the distributed plasticity approach. The fiber section beam-column element is applied to construct the finite element model. The accuracy of the proposed model is verified using linear analysis via the comparison of the dynamic characteristics. For collapse risk assessment of OWT, the nonlinear effects considering the earthquake Incident Angle (IA) have been evaluated first. Then, the Incremental Dynamic Analysis (IDA) has been executed using a set of 20 near-fault records. Lastly, fragility curves are developed to evaluate the vulnerability of structures for different limit states. Attained results justify the accuracy of the proposed approach for the structural response against the ground motions and other environmental loads. It indicates that effects of static wind and wave loads along with the earthquake loads should be considered during the risk assessment of the OWT structure.

• Journal of the Earthquake Engineering Society of Korea
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• v.8 no.5 s.39
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• pp.79-89
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• 2004

This paper presents the results of experimental studies on the equipment isolation effect in the nuclear containment. For this purpose, shaking table tests were performed. The purpose of this study is enhancement of seismic safety of equipment in the Nuclear Power Plant. The isolation system, known as Friction Pendulum System (FPS), combines the concepts of sliding bearings and pendulum motion was selected. Peak ground acceleration, bidirectional motion, effect of vertical motion and frequency contents of selected earthquake motions were considered. As a result, these are founded that the vertical motion of seismic wave affect to the base isolation and the isolation effect decreased in case of near fault earthquake motion.

• Journal of the Earthquake Engineering Society of Korea
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• v.8 no.5 s.39
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• pp.65-77
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• 2004

In this study, the base isolation systems for equipment in the NPP are presented and the responses of each isolation system are investigated. As for the base isolation systems, a natural rubber bearing (NRB) and a high damping rubber bearing (HDRB) are selected. As input motions, artificial time histories enveloping the US NRC RG 1.60 spectrum and the probability-based scenario earthquake spectra developed for the Korean nuclear power plant site as well as a typical near-fault earthquake record are used. Uniaxial, biaxial, and triaxial excitations are conducted with PGAs of 0.1, 0.2 and 0.25g. The reduction of the seismic forces transmitted to the equipment models are determined for different isolation systems and input motions.

• Journal of the Earthquake Engineering Society of Korea
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• v.9 no.4 s.44
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• pp.55-66
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• 2005

The effectiveness of fuzzy supervisory control technique for the control of seismic responses of smart base isolation system is investigated in this study. To this end, first generation base isolated building benchmark problem is employed for the numerical simulation. The benchmark structure under consideration is an eight-story base isolated building having irregular plan and is equipped with low-damping elastometric bearings and magnetorheological (MR) dampers for seismic protection. Lower level fuzzy logic controllers (FLC) for far-fault or near-fault earthquakes are developed in order to effectively control base isolated building using multi-objective genetic algorithm. Four objectives, i.e. reduction of peak structural acceleration, peak base drift, RMS structural acceleration and RMS base drift, are used in multi-objective optimization process. When earthquakes are applied to benchmark building, each of low level FLCs provides different command voltage and supervisory fuzzy controller combines two command voltages io one based on fuzzy inference system in real time. Results from the numerical simulations demonstrate that base drift as well as superstructure responses can be effectively reduced using the proposed supervisory fuzzy control technique.

• Journal of the Earthquake Engineering Society of Korea
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• v.9 no.1 s.41
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• pp.61-70
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• 2005

Shaking table tests are carried out on a single-degree-of-freedom mass that is equipped with a hybrid base isolation system. The isolator consists of a set of four specially-designed friction pendulum systems (FPS) and a magnetorheological (MR) damper. The structure and its hybrid isolation system are subjected to various intensities of near- and far-fault earthquakes on a large shake table. The proposed fuzzy controller uses feedback from displacement or acceleration transducers attached to the structure to modulate resistance of the semi-active damper to motion. Results from several types of passive and semi-active control strategies are summarized and compared. The study shows that a combination of FPS isolators and an adjustable MR damper can effectively provide robust control of vibration for a large full-scale structure undergoing a wide variety of seismic loads.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.33 no.5
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• pp.319-329
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• 2020

Among the bridges used in Korea, those that are more than 30 years old account for approximately 11% of the total bridges. Therefore, developing a seismic performance-evaluation method is necessary by considering the bridge age. Three composite steel-concrete box girder bridges with port, elastic-rubber, and lead-rubber bearings were selected, and a structural analysis model was developed using the OpenSEESs program. In this study, pier aging was reflected by the reduction in the area of the longitudinal and transverse rebars. Four conditions of 5%, 10%, 25%, and 50% in the degree of pier aging were used. As input earthquakes, 40 near-fault and far-field earthquakes were used, and the maximum displacement and maximum shear-force responses of the piers were obtained and compared. The result shows that as the aging degree increases, the pier strength decreases. Therefore, the pier displacement response increases. To analyze the effects of displacement response and shear resistance, displacement ratio D_ratio and shear-force ratio F_ratio were evaluated. The older the sample bridge is, the greater is the tendency of D_ratio to increase and the smaller is the tendency of F_ratio to decrease.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.18 no.4
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• pp.269-282
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• 2006

We numerically studied tsunami propagation characteristics through Korean Straits based on nonlinear shallow water equation, a robust wave driver of the near field tsunamis. Tsunamis are presumed to be generated by the earthquake in Tsuhima-Koto fault line. The magnitude of earthquake is chosen to be 7.5 on Richter scale, which corresponds to most plausible one around Korean peninsula. It turns out that it takes only 60 minutes for leading waves to cross Korean straits, which supports recently raised concerns at warning system might be malfunctioned due to the lack of evacuation time. We also numerically obtained the probability of tsunami inundation of various levels, usually referred as tsunami hazard, along southern coastal area of Korean Peninsula based on simple seismological and Kajiura (1963)'s hydrodynamic model due to tsunami-generative earthquake in Tsuhima-Koto fault line. Using observed data at Akita and Fukaura during Okushiri tsunami in 1993, we verified probabilistic model of tsunami height proposed in this study. We believe this inundation probability of various levels to give valuable information for the amendment of current building code of coastal disaster prevention system to tame tsunami attack.

• Smart Structures and Systems
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• v.25 no.1
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• pp.65-80
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• 2020

In order to protect a structure over its full life cycle, a novel tuned mass damper (TMD), the so-called semi-active eddy current pendulum tuned mass damper (SAEC-PTMD), which can retune its frequency and damping ratio in real-time, is proposed in this study. The structural instantaneous frequency is identified through a Hilbert-Huang transformation (HHT), and the SAEC-PTMD pendulum is adjusted through an HHT-based control algorithm. The eddy current damping parameters are discussed, and the relationship between effective damping coefficients and air gaps is fitted through a polynomial function. The semi-active eddy current damping can be adjusted in real-time by adjusting the air gap based on the linear-quadratic-Gaussian (LQG)-based control algorithm. To verify the vibration control effect of the SAEC-PTMD, an idealized linear primary structure equipped with an SAEC-PTMD excited by harmonic excitations and near-fault pulse-like earthquake excitations is proposed as one of the two case studies. Under strong earthquakes, structures may go into the nonlinear state, while the Bouc-Wen model has a wild application in simulating the hysteretic characteristic. Therefore, in the other case study, a nonlinear primary structure based on the Bouc-Wen model is proposed. An optimal passive TMD is used for comparison and the detuning effect, which results from the cumulative damage to primary structures, is considered. The maximum and root-mean-square (RMS) values of structural acceleration and displacement time history response, structural acceleration, and displacement response spectra are used as evaluation indices. Power analyses for one earthquake excitation are presented as an example to further study the energy dissipation effect of an SAECPTMD. The results indicate that an SAEC-PTMD performs better than an optimized passive TMD, both before and after damage occurs to the primary structure.

• Journal of the Earthquake Engineering Society of Korea
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• v.23 no.6
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• pp.311-319
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• 2019

The number of aged bridges is increasing so that bridges over 30 years old account for about 11% of all bridges. Consequently, the development of a seismic performance evaluation method that considers the effects of ageing is essential for a seismic retrofitting process for improvement of the seismic safety of existing old bridges. Assessment of the damage situation of bridges after the recent earthquakes in Korea has been limited to the bearings, anchor, and concrete mortar on piers. The purpose of this study is to evaluate the seismic responses of PSC box girder bridges by considering the ageing effect of rubber bearings (RBs) and lead-rubber bearings (LRBs). The modification factor proposed by AASHTO is used to take into account the ageing effect in the bearings. PSC box girder bridges with RBs and LRBs were 3D modeled and analyzed with the OpenSEES program. In order to evaluate the ageing effect of RBs and LRBs, 40 near fault and 40 far field records were used as the input earthquakes. When considering the effect of ageing, the displacement responses and shear forces of bridge bearings (RBs and LRBs) were found to increase mostly under the analytical conditions. It was shown that the effect of ageing is greater in the case of RBs than in the case of LRBs.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.3-7
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• 2005

Focal mechanism of the real and imaginary faults in the western coast of Japan has been assumed by examining the previous studies on the seismicity, seismic gap, fault behaviors, seismic zoning, and faults. In the area of no seismic information, the focal mechanism has been assumed to have the maximum influence on the tsunami height in the eastern coast of Korea. The tsunami height in a particular point of the eastern coast of Korea can vary up to 7 orders with the variation of the strike of the fault in a particular source point of the western coast of Japan with constant magnitude. Tsunami scenario DB including tile arrival times and tsunami heights has been constructing by using the assumed focal mechanism of the western coast of Japan. Tsunami warning system will be reinforced by using this tsunami scenario DB near future.