• Journal of the Korean GEO-environmental Society
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• v.24 no.2
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• pp.31-40
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• 2023

We installed temporary strong motion seismometers at the ground surface, 1 m, 2 m, and 9 m at an existing seismic station that houses permanent seismometers installed at 20 m and 100 m, to investigate the influence of installation depth on the recorded ambient and anthropogenic noise level and the characteristics of earthquake signals. Analysis of the ambient noise shows that anthropogenic noise dominates where vibration period T < 1 s at the studied site, whereas wind speed appears to be strongly correlated with the noise level at T > 1 s. Frequency-wavenumber analysis of 2D seismometer array suggests that ambient noise in short periods are predominantly body waves, rather than surface waves. The level of ambient noise was low at 9 m and 20 m, but strong amplification of noise level at T < 0.1 s was observed at the shallow seismometers. Both the active-source test result and the recorded earthquake data demonstrated that the signal level is decreased with the increase of depth. Our result also shows that recorded motions at the ground and 1 m are strongly amplified at 20 Hz (T = 0.05 s), likely due to the resonance of the 3 m thick soil layer. This study demonstrates that analysis of ambient and active-source vibration may help find optimal installation depth of strong motion seismometers. We expect that further research considering various noise environments and geological conditions will be helpful in establishing a guideline for optimal installation of strong motion seismometers.

• Geophysics and Geophysical Exploration
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• v.27 no.1
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• pp.23-36
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• 2024

In this study, we analyze the structure of the Korea-Japan Joint Development Zone (JDZ) using gravity, magnetic, and seismic data. Gravity and magnetic data analysis confirmed that the Jeju Basin exhibits low anomalies compared to adjacent areas. We applied the total gradient to the Bouguer anomaly to identify basin boundaries, and computed the analytic signal from the total magnetic anomaly data to enhance the edges of the magnetic anomalies. The Taiwan-Sinzi Belt, exhibits high magnetic anomalies and crosses the center of the JDZ in the northeast-southwest direction; we presume that intrusive rocks are sporadic in the JDZ. The 3D inversion results of the gravity and magnetic data show a strong correlation between magnetic susceptibility and density (i.e. a low-density zone in the Jeju Basin and the Ho Basin, and a high magnetic susceptibility distribution in the Taiwan-Sinzi Belt). Comparison of the density and seismic profiles of the Jeju Basin shows that high densities are associated with sill, horst, and basement highs, whereas low densities are associated with basement low and grabens. These results suggest that interpretations based on seismic, gravity and magnetic data can effectively reveal the subsurface structure of the JDZ.

• 한국지구물리탐사학회:학술대회논문집
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• 2004.08a
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• pp.25-48
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• 2004

Structures show the phehomena of deformation and lowering of function with time-lapse by artificial environments and changes of geotechnical conditions or accumulation of initial deformation elements. This study aims the structural assessment of cultural property, Chum-Sung-Dae, located in Kyeongju city, Korea. It was built about 1,300 years ago, and has undergone deformation and ground-subsidence with time-lapse. Non-destructive evaluation techniques were applied to the Chum-Sung-Dae, to protect it from survey Because of this reason, 3D precise laser scanning surveying system was applied to measure the exact size of Chum-Sung-Dae, displacement and declining angles. Geophysical exploration also was applied to study the subsurface distribution of geotechnical parameters or physical properties. Natural frequencies were measured from real and model of Chum-Sung-Dae to study the dynamic characteristics of vibration and/or earthquake load and stiffness of structures.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.12a
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• pp.43-51
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• 2007

In order to investigate the velocity structure of the southern part of the Korean peninsula, exploded seismic signals were recorded for 120 s along a 294-km WNW-ESE line and 150 s along a 335-km NNW-SSE line in 2002 and 2004, respectively. Velocity tomograms were derived from inverting P-wave and S-wave first arrival times. The raypaths indicate several midcrust interfaces. The shallowest one is at the approximate depth of $2{\sim}3\;km$ with refraction velocities of approximately Vp=6.0 and Vs=3.5 km/s, respectively. The second one of $15{\sim}17\;km$ depth has refraction velocities of approximately Vp=7.1 and Vs=3.7 km/s, respectively. The deepest significant interface varies in depth from 30.8 km to 36.1 km. The critically refracting Vp of $7.8{\sim}8.1\;km/s$ and Vs of $4.2{\sim}4.6\;km/s$ along this interface which may correspond to the Moho discontinuity. The velocity tomograms show (1) existence of a low-velocity zone centered at $6{\sim}7\;km$ depth under the Okchon fold belt and the Yeongnam massif, (2) extension of the Yeongdon fault down to greater than 10 km, and (3) existence of high-velocity materials under the Gyeongsan basin less than 4.2 km thick.

• Steel and Composite Structures
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• v.36 no.4
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• pp.447-462
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• 2020

The paper aims to investigate the mechanical mechanism and seismic effect of stiffeners in blind bolt endplate connection to CFST column. A precise 3D finite element model with considering the cyclic properties of concrete and steel materials was established, and the efficiency was validated through monotonic and cyclic test data. The deforming pattern and the seismic performance of the unstiffened and stiffened blind bolt endplate connections were investigated. Then a parametric analysis was conducted to analyze the contribution of stiffeners and the joint working behaviors with endplate under cyclic load. The joint stiffness classifications were compared and a supplement stiffness classification method was proposed, and the energy dissipation ability of different class connections were compared and discussed. Results indicated that the main deformation pattern of unstiffened blind bolt endplate connections was the local bending of end plate. The vertical stiffeners can effectively alleviate the local bending deformation of end plate. And influence of stiffeners in thin endplate and thick endplate was different. Based on the stiffness of external diaphragm welded connection, a more detailed rigidity classification was proposed which included the pin, semi-rigid, quasi-rigid and rigid connection. Beam was the main energy dissipation source for rigid connection. For the semi-rigid and quasi-rigid connection, the extended endplate, stiffeners and steel beam would all participate in the energy dissipation.

• Wind and Structures
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• v.30 no.6
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• pp.663-678
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• 2020

Robust semi-active vibration control of wind turbines using tuned mass dampers (TMDs) is a promising technique. This study investigates a 1.5 megawatt wind turbine controlled by eight different types of tuned mass damper systems of equal mass: a passive TMD, a semi-active varying-spring TMD, a semi-active varying-damper TMD, a semi-active varying-damper-and-spring TMD, as well as these four damper systems paired with an additional smaller passive TMD near the mid-point of the tower. The mechanism and controllers for each of these TMD systems are explained, such as employing magnetorheological dampers for the varying-damper TMD cases. The turbine is modelled as a lumped-mass 3D finite element model. The uncontrolled and controlled turbines are subjected to loading and operational cases including service wind loads on operational turbines, seismic loading with service wind on operational turbines, and high-intensity storm wind loads on parked turbines. The displacement and acceleration responses of the tower at the first and second mode shape maxima were used as the performance indicators. Ultimately, it was found that while all the semi-active TMD systems outperformed the passive systems, it was the semi-active varying-damper-and-spring system that was found to be the most effective overall - capable of controlling vibrations about as effectively with only half the mass as a passive TMD. It was also shown that by reducing the mass of the TMD and adding a second smaller TMD below, the vibrations near the mid-point could be greatly reduced at the cost of slightly increased vibrations at the tower top.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.10a
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• pp.860-868
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• 2008

Seismic safety analysis of rockfill dams are consist of the stability analysis as an simplifed method and the dynamic analysis as an detailed method. When high risk dams such as Multi-purpose dams were often applied detailed method by dynamic analysis, dynamic properties of dam materials such as shear modulus are considered as most important factor. Dynamic material properties such as shear modulus had to be investigated by cyclic triaxial test et al. during design and construction stage but these were not conducted because of the condition of domestic seismic design technique. MASW and SASW methods had been applied as a non destructive method to investigate dynamic material properties of existing rockfill dam, has no problems in dam safety at present. These methods were usually performed under the assumptions that the subsurface can be described horizontally homogeneous and isotropic layers. Recent studies(Marwin, 1993, Kim, 2001) showed that surface waves generated through inclined structures have different characteristics from those through a horizontally homogeneous layered model. further Kim et al(2005) and Min and Kim(2006) showed that central core type rockfill dam overestimated the shear wave velocities as increasing the depth through the 3D numerical modelling dut to the effect of outer rockfill and geometrical reasons In this study the results of shear wave velocities of seven rockfill dams form comprehensive facility review, was carried out from 2003 to 2007, were collected and analysed to establish the shear wave velocity distribution characteristics in increasing confining stress in rockfill dams and surface wave velocity ranges in rockfill dam through MASW and the limitation in application are discussed to be utilized as an reference value for dynamic analysis.

• Steel and Composite Structures
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• v.27 no.6
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• pp.727-745
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• 2018

The design criteria and the corresponding performance levels characterize the response of extended stiffened end-plate beam-to-column joints. In order to guarantee a ductile behavior, hierarchy criteria should be adopted to enforce the plastic deformations in the ductile components of the joint. However, the effectiveness of thesecriteria can be impaired if the actual resistance of the end-plate material largely differs from the design value due to the potential activation of brittle failure modes of the bolt rows (e.g., occurrence of failure mode 3 in the place of mode 1 per bolt row). Also the number and the position of bolt rows directly affect the joint response. The presence of a bolt row in the center of the connection does not improve the strength of the joint under both gravity, wind and seismic loading, but it can modify the damage pattern of ductile connections, reducing the gap opening between the end-plate and the column face. On the other hand, the presence of a central bolt row can influence the capacity of the joint to resist the catenary actions developing under a column loss scenario, thus improving the joint robustness. Aiming at investigating the influence of these features on both the cyclic behavior and the response under column loss, a wide range of finite element analyses (FEAs) were performed and the main results are described and discussed in this paper.

• Journal of the Earthquake Engineering Society of Korea
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• v.20 no.6
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• pp.379-389
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• 2016

This study intends to evaluate the conservativeness of the fixed-base analysis as compared to the soil-structure interaction (SSI) analysis for the seismically isolated model of a nuclear power plant in Korea. To that goal, the boundary reaction method (BRM), combining frequency-domain and time-domain analyses in a twofold process, is adopted for the SSI analysis considering the nonlinearity of the seismic base isolation. The program KIESSI-3D is used for computing the reaction forces in the frequency domain and the program MIDAS/Civil is applied for the nonlinear time-domain analysis. The BRM numerical model is verified by comparing the results of the frequency-domain analysis and time-domain analysis for the soil-structure system with an equivalent linear base isolation model. Moreover, the displacement response of the base isolation and the horizontal response at the top of the structure obtained by the nonlinear SSI analysis using BRM are compared with those obtained by the fixed-base analysis. The comparison reveals that the fixed-base analysis provides conservative peak deformation for the base isolation but is not particularly conservative in term of the floor response spectrum of the superstructure.

• Structural Engineering and Mechanics
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• v.39 no.1
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• pp.125-145
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• 2011

Pushover analysis has gained significant popularity as an analytical tool for realistic determination of the inelastic behaviour of RC structures. Though significant work has been done to evaluate the demands realistically, the evaluation of capacity and realistic failure modes has taken a back seat. In order to throw light on the inelastic behaviour and capacity evaluation for the RC framed structures, a 3D Reinforced concrete frame structure was tested under monotonically increasing lateral pushover loads, in a parabolic pattern, till failure. The structure consisted of three storeys and had 2 bays along the two orthogonal directions. The structure was gradually pushed in small increments of load and the corresponding displacements were monitored continuously, leading to a pushover curve for the structure as a result of the test along with other relevant information such as strains on reinforcement bars at critical locations, failure modes etc. The major failure modes were observed as flexural failure of beams and columns, torsional failure of transverse beams and joint shear failure. The analysis of the structure was by considering all these failure modes. In order to have a comparison, the analysis was performed as three different cases. In one case, only the flexural hinges were modelled for critical locations in beams and columns; in second the torsional hinges for transverse beams were included in the analysis and in the third case, joint shear hinges were also included in the analysis. It is shown that modelling and capturing all the failure modes is practically possible and such an analysis can provide the realistic insight into the behaviour of the structure.