• Geomechanics and Engineering
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• v.20 no.4
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• pp.323-331
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• 2020

Pile foundation is widely used for railway bridges in loess throughout northwestern China. Modeling of the loess-pile interaction is an essential part for seismic analysis of bridge with pile foundation at seismically active regions. A quasi-static test is carried out to investigate the hysteretic behaviors of pile foundation in collapsible loess. The failure characteristics of the bridge pile-loess system under the cyclic lateral loading are summarized. From the test results, the energy dissipation, stiffness degradation and ductility of the pile foundation in loess are analyzed. Therefore, a bilinear model with stiffness degradation is recommended for the nonlinearity of the bridge pier-pile-loess system. It can be found that the stiffness of the bridge pier-pile-loess system decreases quickly in the initial stage, and then becomes more slowly with the increase of the displacement ductility. The equivalent viscous damping ratio is defined as the ratio of the dissipated energy in one cycle of hysteresis curves and increases with the lateral displacement.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.03a
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• pp.587-598
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• 2008

In the trackbed design using elastic multi-layer model, the stress-dependent resilient modulus is an important input parameter, which reflects substructure performance under repeated traffic loading. The resilient moduli of crushed stone and weathered granite soil were developed using nonlinear dynamic stiffness, which can be measured by in-situ and laboratory seismic tests. The prediction models of resilient modulus varying with the deviatoric or bulk stress were proposed (Park et al., 2008). To investigate the performance of the prediction models proposed herein, the elastic response of the test trackbed near PyeongTaek, Korea was evaluated using a 3-D nonlinear elastic computer program (GEOTRACK) and compared with measured elastic vertical displacement during the passages of freight and passenger trains. The material types of the test sub-ballasts are crushed stone and weathered granite soil, respectively. The calculated vertical displacements within the sub-ballasts are within the order of 1mm, and agree well with measured values with the reasonable margin. The prediction models are thus concluded to work properly in the preliminary investigation. The prediction models proposed for resilient modulus were verified by the comparison of the calculated vertical displacements with measured ones during train passages.

• International Journal of Concrete Structures and Materials
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• v.9 no.4
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• pp.401-413
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• 2015

In this study the progressive collapse resisting capacity of a RC beam-column subassemblage with and without strengthening was investigated. Total of five specimens were tested; two unreinforced specimens, the one designed as gravity load-resisting system and the other as seismic load-resisting system, and three specimens reinforced with: (i) bonded strand, (ii) unbonded strand, and (iii) side steel plates with stud bolts. The two-span subassemblages were designed as part of an eight-story RC building. Monotonically increasing load was applied at the middle column of the specimens and the force-displacement relationships were plotted. It was observed that the gravity load-resisting specimen failed by fractures of re-bars in the beams. In the other specimens no failure was observed until the maximum displacement capacity of the actuator was reached. Highest strength was observed in the structure with unbonded strand. The test result of the specimen with side steel plates in beam-column joints showed that the force-displacement curve increased without fracture of re-bars. Based on the test results it was concluded that the progressive collapse resisting capacity of a RC frame could be significantly enhanced using unbonded strands or side plates with stud bolts.

• Steel and Composite Structures
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• v.16 no.3
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• pp.253-267
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• 2014

Based on the test on a 1/2.5-scaled model of a two-bay and three-story inner frame composed of reinforced concrete beams and lattice steel reinforced concrete (SRC) irregular section columns under low cyclic reversed loading, the failure process and the features of the frame were observed. The subsequence of plastic hinges of the structure, the load-displacement hysteresis loops and the skeleton curve, load bearing capacity, inter-story drift ratio, ductility, energy dissipation and stiffness degradation were analyzed. The results show that the lattice SRC inner frame is a typical strong column-weak beam structure. The hysteresis loops are spindle-shaped, and the stiffness degradation is insignificant. The elastic-plastic inter-story deformation capacity is high. Compared with the reinforced concrete frame with irregular section columns, the ductility and energy dissipation of the structure are better. The conclusions can be referred to for seismic design of this new kind of structure.

• Proceedings of the KSR Conference
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• 2008.06a
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• pp.1712-1723
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• 2008

In the trackbed design using an elastic multi-layer model, the stress-dependent resilient modulus is the key input parameter, which reflects substructure performance under repeated traffic loading. The prediction models of resilient modulus of crushed stone and weathered granite soil were developed from nonlinear dynamic stiffness, which can be combined by in-situ and laboratory seismic measurements. The models accommodate the variation with the deviatoric and/or bulk stresses. To investigate the performance of the prediction models proposed, the elastic response of the test trackbed near PyeongTaek, Korea was evaluated using a 3-D nonlinear elastic computer program (GEOTRACK) and compared with measured elastic vertical displacement caused by the passages of freight and passenger trains. The material types of the test sub-ballasts are crushed stone and weathered granite soil, respectively. The calculated vertical displacements within the sub-ballasts are within the order of 1mm, and agree well with measured values with the reasonable margin. The prediction models are thus concluded to work properly in the preliminary investigation.

• Journal of the Korean Geotechnical Society
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• v.19 no.2
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• pp.57-64
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• 2003

The liquefaction potential of saturated sands under seismic loading conditions has been carefully considered by many investigations. Typical of these investigations is the laboratory determination of cyclic strength of sands by means of cyclic triaxial tests. This study was conducted to investigate the effects of the method of sample preparation on the liquefaction characteristics of remolded samples of saturated uniform sands. Cyclic triaxial tests were performed on saturated uniform sand compacted to the same density by 3 different procedures of pluvial compaction through air, pluvial compaction through water and vibratory compaction. It was validated that the cyclic stress ratio of remolded saturated uniform sands by different compaction procedures at the same density was very different.

• Journal of Korean Association for Spatial Structures
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• v.21 no.4
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• pp.31-38
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• 2021

The precast-buckling restrained braces(PC-BRB) reinforced with engineering plastics that can compensate for the disadvantages in the manufacturing process of the existing buckling restrained brace. In this study, to examine the applicability of PC-BRB to actual structures, example structures similar to school facilities were selected and the reinforcement effect was analyzed analytically according to the damping design procedure of PC-BRB. Load-displacement curve through the incremental loading test appeared similar to the bilinear curve. Applying test result, Analytical model of PC-BRB model was constructed and applied to the example structure. As a result of the analysis, the PC-BRB showed stable hysteresis behavior without lowering the strength, and the inter story drift ratio and the shear force were reduced due to the damping effect. In addition, the reduction ratio of the shear force was similar to the reduction ratio assumed when designing the damping device.

• Steel and Composite Structures
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• v.42 no.6
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• pp.791-804
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• 2022

As a type of semi-rigid connection, the top and seat angle connections are popular in current structures owing to their good cyclic performance and simple erection. However, their stiffness and load bearing capacity are relatively insufficient. This study proposes two strengthening methods to further increase the stiffness and strength of bolted-angle joints while maintaining satisfactory energy dissipation capacity (EDC) and ductility. Cyclic loading tests were conducted on six joint specimens with different strengthened angle components. Based on the test results, the influence of the following important factors on the cyclic behavior of steel joint specimens was investigated: the position of the rib stiffeners (edge rib stiffeners and middle rib stiffener), steel strength grade of rib stiffeners (Q345 and Q690), and additional stiffeners or not. In addition, the finite element models of these specimens were built and validated through a comparison of experimental and numerical results. The stiffness and bearing capacity of the bolted-angle joints could be improved significantly by utilizing the novel strengthened joints proposed in this study. Moreover, this can be achieved with almost no increase in the amount of steel required, and the EDC of this joint could also satisfy the requirements of seismic codes from various countries.

• Steel and Composite Structures
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• v.42 no.5
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• pp.657-669
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• 2022

Structural materials can experience large plastic deformation under extreme cyclic loading that is caused by events like earthquakes. To evaluate the seismic safety of a structure, accurate numerical material models should be used. For a steel structure, the cyclic strain hardening behavior of structural steel should be correctly modeled. In this study, a combined hardening model, consisting of one isotropic hardening model and three nonlinear kinematic hardening models, was used. To determine the values of the combined hardening model parameters efficiently and accurately, the improved opposition-based particle swarm optimization (iOPSO) model was adopted. Low-cycle fatigue tests were conducted for three steel grades commonly used in Korea and their modeling parameters were determined using iOPSO, which was first developed in Korea. To avoid expensive and complex low cycle fatigue (LCF) tests for determining the combined hardening model parameter values for structural steel, empirical equations were proposed for each of the combined hardening model parameters based on the LCF test data of 21 steel grades collected from this study. In these equations, only the properties obtained from the monotonic tensile tests are required as input variables.

• Journal of the Earthquake Engineering Society of Korea
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• v.28 no.1
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• pp.67-75
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• 2024

In the case of the Pohang earthquake, which had a magnitude of 5.4 in 2017, geotechnical damages such as liquefaction and ground settlement occurred. The need for countermeasures has emerged, and experimental research in the Pohang area has continued. This study collected undisturbed samples from damaged fine-grained soil areas where ground settlement occurred in Pohang. Cyclic tri-axial tests for identifying the dynamic characteristics of soils were performed on the undisturbed samples, and the results were analyzed to determine the cause of ground settlement. As a result of the study, it was determined that in the case of fine-grained soils, ground settlement occurred because the seismic load as an external force was relatively more significant than the shear resistance of the very soft fine-grained soils, rather than due to an increase in excess pore water pressure.