• Journal of the Earthquake Engineering Society of Korea
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• v.8 no.3
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• pp.1-12
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• 2004

Due to the 1989 Loma Prieta, 1995 Hyogoken Nambu earthquakes, etc, a number of bridge columns  were collapsed in flexure-shear failures as a consequence of the premature termination of the column longitudinal reinforcement. Nevertheless, previous researches for the performance of bridge columns were concentrated on the flexural failure mode. It is well understood that the seismic behaviour of RC bridge piers was dependent on the performance of the plastic hinge of RC bridge piers, the ductility of which was desirable to be computed on the basis of the curvature. Experimental investigation was made to evaluate the variation of the curvature of the plastic hinge  region for the seismic performance of earthquake-damaged RC columns in flexure-shear failure mode. Seven test specimens in the aspect ratio of 2.5 were made with test parameters: confinement ratios, lap splices, and retrofitting FRP materials. They were damaged under series of artificial earthquakes that could be compatible in Korean peninsula. Directly after the pseudo-dynamic test, damaged columns were retested under inelastic reversal cyclic loading under a constant axial load, $P=0.1f_{ck}A_g$. Residual seismic capacity of damaged specimens was evaluated by analzying the moment-curvature hysteresis and the curvature ductility. Test results show that the biggest curvature was developed around 15cm above the footing, which induced the column failure. It was observed that RC bridge specimens with lap-spliced longitudinal steels appeared to fail at low curvature ductility but significant improvement was made in the curvature ductility of RC specimens with FRP straps wrapped around the plastic hinge region. Based on the experimental variation of the curvature of RC specimens, new equivalent length of the plastic hinge region was proposed by considering the lateral confinement in this study. The analytical and experimental relationship between the displacement and the curvature ductility were compared based on this proposal, which gave excellent result.

• The Journal of Korean Academy of Prosthodontics
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• v.41 no.2
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• pp.232-242
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• 2003

Statement of problem : Fiber-reinforced posts have lower modulus of elasticity than titanium post or cast post-core. With this similar elasticity to that of dentin, fiber-reinforced posts have been known to have a tendency to reduce the risk of root fracture. However, there were few studies on the teeth restored with fiber-reinforced posts under the condition of reduced periodontal support. Purpose : The purpose of this study was to evaluate the fracture strength and failure mode of endodontically treated teeth restored with fiber-reinforced posts and titanium posts under the condition of reduced periodontal support. Material and method : Extracted human maxillary incisor roots were divided into 3 groups (group 1 carbon fiber post, group 2 : glass fiber post, and group 3 : titanium alloy post). After coronectomy and endodontic treatment, teeth were restored with each post systems and resin core according to the manufacturer's recommendation. Then, teeth with simulated periodontal ligament were embedded in the acrylic resin blocks at the level of 4 mm below the cemento-enamel junction. Each specimen was exposed to $10^5$ load cycles with average 30 N force in $36.5^{\circ}C$ water using a computer-controlled chewing simulator. Loads were applied at $45^{\circ}$ angle to the long axis of the teeth. After cyclic loading, teeth were subjected a compressive load until failure at a crosshead speed of 0.5 mm/min. Fracture strength (N) and failure mode were examined. The fracture strength was analyzed with one-way ANOVA and the Scheffe adjustment at the 95% significance level. Results and conclusion : The results were as follows. 1. There was no statistically significant difference in the mean fracture strength among the groups (P<.05). 2. Carbon fiber post and glass fiber post group showed less root fracture tendency than control group. 3. All specimens with root fractures showed fracture lines above the level of acrylic resin block, except for only one specimen in group 3.

• Journal of the Korea Concrete Institute
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• v.13 no.2
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• pp.107-113
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• 2001

This study was conducted preliminarily to develop the crack source location technique for plain concrete beam using acoustic emission(AE). Before the main experiment, the test of virtual An source location was achieved in plain concrete block. The sensor layout was mutually compared between triangular layout and rectangular layout. As the results of test, AE source location by triangular layout was evaluated more effective than that by rectangular layout. The specimen to apply he source location technique was man in total nine specimens (each three in 40 %, 50%, 60% of W/C ratio) which the experiment variable was the compressive strength level(W/C ratio). The bending loading method is selected by cyclic loadings to evaluate the degree of concrete damage. It is seen that Kaiser effect and Felicity effect exists through analysis of AE parameters in coming failure experiment. As a result of analyzing the felicity ratio(FR) values, it is shown that this values can be used for evaluating the degree of concerto damage. AE activity is started highly at the 70% of failure load without the compressive strength level. Thus considered by a index in constructing the system of the failure warning at application of the field structure. And the results compared the real cracking location with the source location has perceived by AE monitoring before it is appeared the primary crack by visual observation.

• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.4
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• pp.46-55
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• 2021

Concrete structures can cause various problems as the number of common years increases when exposed to external extreme climate conditions. Among these problems, freezing and thawing occur due to the action of extreme climate factors such as heavy rain and heavy snow, which have become the most problematic in recent years. In this study, we present a rapid freezing and thawing test method of concrete in the air, referring to KS F 2456, as Seoul exhibits very dry weather during the period of freezing and thawing. Concrete test specimens and RC beams were fabricated to perform rapid freezing and thawing of 0, 100, 200, and 300 cycles, and the performance evaluation confirmed the degradation of each subject in material and member units. The design strength of 24 MPa, which performs rapid freezing and thawing in the air up to 300 cycles, decreases by 5.24 MPa (21%), and as rapid freezing and thawing in the air increases the stress burden on reinforced concrete bending members, reducing the energy absorption (dissipation) ability of structures due to earthquakes.

• Journal of the Korean Geotechnical Society
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• v.35 no.7
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• pp.15-27
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• 2019

The purpose of this study is to investigate the mechanism about damages occurring at quay wall and backfill in Youngilman Port during Pohang earthquake (M5.4) on November 15, 2017. In the field investigation, the horizontal displacement of the caisson occurred between 5 cm and 15 cm, and the settlement at backfill occurred higher than 10 cm. 2D-effective Stress Analysis was performed to clarify the mechanism for the damage. The input earthquake motion used acceleration ($3.25m/s^2$) measured at bedrock of Pohang habor. Based on a numerical analysis, it was found that the effective stress decreased due to the increase of excess pore pressure in the backfill ground and the horizontal displacement of the caisson occurred by about 14 cm, and the settlement occurred by about 3 cm. In backfill, the settlements occurred between 6 cm and 9 cm. This is similar to field investigation results. Also, it was found that the backfill soil was close to the Mohr-Coulomb failure line due to the cyclic loading from the effective stress path and the stress-strain behavior. It may be related to decreasing of bearing capacity induced by the reduction of effective stress caused by the increase of the excess pore water pressure.

• Structural Engineering and Mechanics
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• v.83 no.3
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• pp.305-326
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• 2022

To realize the recycling utilization of waste concrete and alleviate the shortage of resources, 11 specimens of steel reinforced recycled concrete (SRRC) filled circular steel tube columns were designed and manufactured in this study, and the cyclic loading tests on the specimens of columns were also carried out respectively. The hysteretic curves, skeleton curves and performance indicators of columns were obtained and analysed in detail. Besides, the finite element model of columns was established through OpenSees software, which considered the adverse effect of recycled coarse aggregate (RA) replacement rates and the constraint effect of circular steel tube on internal RAC. The numerical calculation curves of columns are in good agreement with the experimental curves, which shows that the numerical model is relatively reasonable. On this basis, a series of nonlinear parameters analysis on the hysteretic behaviors of columns were also investigated. The results are as follows: When the replacement rates of RA increases from 0 to 100%, the peak loads of columns decreases by 7.78% and the ductility decreases slightly. With the increase of axial compression ratio, the bearing capacity of columns increases first and then decreases, but the ductility of columns decreases rapidly. Increasing the wall thickness of circular steel tube is very profitable to improve the bearing capacity and ductility of columns. When the section steel ratio increases from 5.54% to 9.99%, although the bearing capacity of columns is improved, it has no obvious contribution to improve the ductility of columns. With the decrease of shear span ratio, the bearing capacity of columns increases obviously, but the ductility decreases, and the failure mode of columns develops into brittle shear failure. Therefore, in the engineering design of columns, the situation of small shear span ratio (i.e., short columns) should be avoided as far as possible. Based on this, the calculation model on the skeleton curves of columns was established by the theoretical analysis and fitting method, so as to determine the main characteristic points in the model. The effectiveness of skeleton curve model is verified by comparing with the test skeleton curves.

• Composites Research
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• v.36 no.1
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• pp.16-24
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• 2023

This paper presents a multi-scale progressive fatigue damage model incorporating the model for interfacial debonding between fibers and matrix. The micromechanics model for the progressive interface debonding was adopted, which defined the four different interface phases: (1) perfectly bonded fibers; (2) mild imperfect interface; (3) severe imperfect interface; and (4) completely debonded fibers. As the number of cycles increases, the progressive transition from the perfectly bonded state to the completely debonded fiber state occurs. Eshelby's tensor for each imperfect state is calculated by the linear spring model for a damaged interface, and effective elastic properties are obtained using the multi-phase homogenization method. The fatigue damage evolution formulas for fiber, matrix and interface were proposed to demonstrate the fatigue behavior of CFRP laminates under cyclic loading. The material parameters for the fiber/matrix fatigue damage were characterized using the chaotic firefly algorithm. The model was implemented into the UMAT subroutine of ABAQUS, and successfully validated with flat-bar UD laminate specimens ([0]₈,[90]₈, [30]₁₆) of AS4/3501-6 graphite/epoxy composite.

• Earthquakes and Structures
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• v.26 no.2
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• pp.97-115
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• 2024

In this paper, a database consisting of the dynamic shear modulus ratio and damping ratio test data of clay obtained from 406 groups of triaxial tests is constructed with the starting area of Xiong'an New Area as the research background. The aim is to study the nonlinear dynamic properties of clay in this area under cyclic loading. The study found that the effective confining pressure and plasticity index have certain influences on the dynamic shear modulus ratio and damping ratio of clay in this area. Through data analysis, it was found that there was a certain correlation between effective confining pressure and plasticity index and dynamic shear modulus ratio and damping ratio, with fitting degree values greater than 0.1263 for both. However, other physical indices such as the void ratio, natural density, water content and specific gravity have only a small effect on the dynamic shear modulus ratio and the damping ratio, with fitting degree values of less than 0.1 for all of them. This indicates that it is important to consider the influence of effective confining pressure and plasticity index when studying the nonlinear dynamic properties of clays in this area. Based on the above, prediction models for the dynamic shear modulus ratio and damping ratio in this area were constructed separately. The results showed that the model that considered the combined effect of effective confining pressure and plasticity index performed best. The predicted dynamic shear modulus ratio and damping ratio closely matched the actual curves, with approximately 88% of the data falling within ±1.3 times the measured dynamic shear modulus ratio and approximately 85.1% of the data falling within ±1.3 times the measured damping ratio. In contrast, the prediction models that considered only a single influence deviated from the actual values, particularly the model that considered only the plasticity index, which predicted the dynamic shear modulus ratio and the damping ratio within a small distribution range close to the average of the test values. When compared with existing prediction models, it was found that the predicted dynamic shear modulus ratio in this paper was slightly higher, which was due to the overall hardness of the clay in this area, leading to a slightly higher determination of the dynamic shear modulus ratio by the prediction model. Finally, for the dynamic shear modulus ratio and damping ratio of the engineering site in the starting area of Xiong'an New Area, we confirm that the prediction formulas established in this paper have high reliability and provide the applicable range of the prediction model.

• The Journal of Korean Academy of Prosthodontics
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• v.46 no.4
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• pp.396-408
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• 2008

Statement of problem: Within the elastic limit of the screw, the greater the preload, the tighter and more secure the screw joint. However, additional tensile forces can incur plastic deformation of the abutment screw when functional loads are superimposed on preload stresses, and they can elicit the loosening or fracture of the abutment screw. Therefore, it is necessary to find the optimum preload that will maximize fatigue life and simultaneously offer a reasonable degree of protection against loosening. Another critical factor in addition to the applied torque which can affect the amount of preload is the joint connection type between implant and abutment. Purpose: The purpose of this study was to evaluate the influence of tightening torque on the implant-abutment screw joint stability. Material and methods: Respectively, three different amount of tightening torque (20, 30, and 40 Ncm) were applied to implant systems with three different joint connections, one external butt joint and two internal cones. The initial removal torque value and the postload (cyclic loading up to 100,000 cycles) removal torque value of the abutment screw were measured with digital torque gauge. Then rate of the initial and the postload removal torque loss were calculated for the comparison of the effect of tightening torques and joint connection types between implant and abutment on the joint stability. Results and conclusion: 1. Increase in tightening torque value resulted in significant increase in initial and postload removal torque value in all implant systems (P < .05). 2. Initial removal torque loss rates in SS II system were not significantly different when three different tightening torque values were applied (P > .05), however GS II and US II systems exhibited significantly lower loss rates with 40 Ncm torque value than with 20 Ncm (P < .05). 3. In all implant systems, postload removal torque loss rates were lowest when the torque value of 30 Ncm was applied (P < .05). 4. Postload removal torque loss rates tended to increase in order of SS II, GS II and US II system. 5. There was no correlation between initial removal torque value and postload removal torque loss rate (P > .05).

• Journal of the Korean Geotechnical Society
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• v.21 no.4
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• pp.65-70
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• 2005

Many case histories of re-liquefaction phenomena seem to support the idea that sand deposits, if they once have been liquefied, could be reliquefied again by a subsequent earthquake even though the earthquake is smaller than the previous one. The magnitude of the strains induced in the initial liquefaction has a significant influence on the resistance of the sample to re-liquefaction. The deposits undergoing liquefaction experience large shear strain during liquefaction. And this previous strain changes the microstructure into highly anisotropic structure such as columnlike structure and connected voids. This type of anisotropy is so unstable that it can reduce re-liquefaction resistance. It is blown that the extent of anisotropic structural change depends on the gradation characteristics of ground. The purpose of this study is to estimate the correlation between the gradation characteristics of the sand and the ratio of re-liquefaction resistance to liquefaction resistance. In this study, 1-g shaking table tests were carried out on five different kinds of sands. During the tests the values of excess pore pressure at various depths and surface settlements were measured. Re-liquefaction resistances were not affected by the initial void ratio and the effective confining pressures, and the deposits of all test sands which had once been liquefied were reliquefied in the cyclic loading number below 1 to 1.5. The ratio of re-liquefaction resistance to liquefaction resistance linearly decreased as $D_{10}/C_u$ increased, and was constant as about 0.2 above the value of $D_{10}/C_u$, 0.15 mm.