• The korean journal of orthodontics
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• v.36 no.3 s.116
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• pp.188-197
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• 2006

Fiber reinforced composite (FRC) is usually used as a connector joining a few teeth into one unit in orthodontics. However, fracture often occurs during the two to three years of the orthodontic treatment period due to repeated occlusal loading or water sorption in the oral environment. We simulated the repair by overlapping and attaching portions of two FRC strips in the middle and performed a three-point bending test to investigate the changes of the repair strength among the different FRC groups. The specimens were grouped according to the overlapping lengths of the two FRC strips, which were 1, 2, 3 and 4 mm (group E1, E2, E3 and E4, respectively) and the control group consisted of unrepaired, intact FRC strips. Each group consisted of 6 specimens and were cured with a light emitting diode curing unit. Group E4 showed the highest maximum loads of 2.67 N, then the control group (2.39 N), group E3 (2.35 N), E2 (2.10 N), and E1 (1.75 N) in decreasing order. Group E4 also showed the highest stiffness, which was 2.32 N/mm, however, the stiffness of group E3 (2.06N/mm) was higher than that of the control group (1.88 N/mm). According to the visual examination, the specimens tended to be bent rather than being fractured into two pieces with an increased length of overlapping portions. The above results suggest that a minimum overlapping length of 3 mm was necessary to obtain an adequate repair of a 10 mm length of FRC connector. In addition, the critical section adjacent to the joint area, where the thickness decreased abruptly, should be reinforced with flowable resin to minimize the bending tendency.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.38 no.2
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• pp.357-367
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• 2018

Recent earthquakes in Gyeongju and Pohang have raised interest in liquefaction in South Korea. Liquefaction, which is a phenomenon that excessive pore pressure is generated and the shear strength of soil is decreased by repeated loads such as earthquakes, causes severe problems such as ground subsidence and overturning of structures. Therefore, it is necessary to identify and prepare for the possibility of liquefaction in advance. In general, the possibility of liquefaction is quantitatively assessed using the Liquefaction Potential Index (LPI), but it takes a lot of time and effort for performing site response analysis which is essential for the liquefaction evaluation. In this study, a simple method to evaluate the liquefaction potential without executing the site response analysis in a downtown area with a lot of borehole data was proposed. In this simple method, the correlation between the average shear wave velocity of the target location ground and the LPI divided by thickness of liquefiable layer was established. And the applicable correlation equation for various rock outcrop accelerations were derived. Using the 104 boreholes information in Seoul, the correlation equation between LPI and the shear wave velocity (ground water level: 0m, 1m, 2m, 3m) is obtained and the possibility of liquefaction occurrence in Seoul and Gyeongju is evaluated. The applicability of the proposed simple method was verified by comparing the LPI values calculated from the correlation equation and the LPI values derived using the existing site response analysis. Finally, the distribution map of LPI calculated from the correlation was drawn using Kriging, a geostatistical technique.

• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.1
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• pp.73-80
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• 2022

In this study, details of NRC beam-column connections were developed in which beam and columns pre-assembled in factories using steel angles were bolted on site. The developed joint details are NRC-J type and NRC-JD type. NRC-J type is a method of tensile joining with TS bolts to the side and lower surfaces of the side plate of the NRC column and the end plate of the NRC beam. NRC-JD type has a rigid joint with high-strength bolts between the NRC beam and the side of the NRC column for shear, and with lap splices of reinforcing bar penetrating the joint and the beam main reinforcement for bending. For the seismic performance evaluation of the joint, three specimens were tested: an NRC-J specimen and NRC-JD specimen with NRC beam-column joint details, and an RC-J specimen with RC beam-column joint detail. As a result of the repeated lateral load test, the final failure mode of all specimens was the bending fracture of the beam at the beam-column interface. Compared to the RC-J specimen, the maximum strength of the specimen by the positive force was 10.1% and 29.6% higher in the NRC-J specimen and the NRC-JD specimen, respectively. Both NRC joint details were evaluated to secure ductility of 0.03 rad or more, the minimum total inter-story displacement angle required for the composite intermediate moment frame according to the KDS standard (KDS 41 31 00). At the slope by relative storey displacemet of 5.7%, the NRC-J specimen and the NRC-JD specimen had about 34.8% and 61.1% greater cumulative energy dissipation capacity than the RC specimen. The experimental strength of the NRC beam-column connection was evaluated to be 30% to 53% greater than the theoretical strength according to the KDS standard formula, and the standard formula evaluated the joint performance as a safety side.