• Proceedings of the Korean Geotechical Society Conference
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• 1995.10a
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• pp.15.2-22
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• 1995

Evaluating stiffness of near-surface materials has been one of the critically important tasks in many civil engineering works. It is the main goal of geotechnical characterization. The so-called deflection-response method evaluates the stiffness by measuring stress-strain behavior of the materials caused by static or dynamic load. This method, however, evaluates the overall stiffness and the stiffness variation with depth cannot be obtained. Furthermore, evaluation of a large-area geotechnical site by this method can be time-consuming, expensive, and damaging to many surface points of the site. Wave-propagation method, on the other hand, measures seismic velocities at different depths and stiffness profile (stiffness change with depth) can be obtained from the measured velocity data. The stiffness profile is often expressed by shear-wave (S-wave) velocity change with depth because S-wave velocity is proportional to the shear modulus. that is a direct indicator of stiffiiess. The crosshole and downhole method measures the seismic velocity by placing sources and receivers (geophones) at different depths in a borehole. Requirement of borehole installation makes this method also time-consuming, expensive, and damaging to the sites. Spectral-Analysis-of-Surface-Waves (SASW) method places both source and receivers at the surface, and records horizontally-propagating surface waves. Based upon the theory of surfacewave dispersion, the seismic velocities at different depths are calculated by analyzing the recorded surface-wave data. This method can be nondestructive to the sites. However, because only two receivers are used, the method requires multiple measurements with different field setups and, therefore, the method often becomes time-consuming and labor-intensive. Furthermore. the inclusion of noise wavefields cannot be handled properly, and this may cause the results by this method inaccurate. When multi-channel recording method is employed during the measurement of surface-waves, there are several benefits. First, usually single measurement is enough because multiple number (twelve or more) of receivers are used. Second, noise inclusion can be detected by coherency checking on the multi-channel data and handled properly so that it does not decrease the accuracy of the result. Third, various kinds of multi-channel processing techniques can be applied to f1lter unwanted noise wavefields and also to analyze the surface-wavefields more accurately and efficiently. In this way, the accuracy of the result by the method can be significantly improved. Fourth, the entire system of source, receivers, and recording-processing device can be tied into one unit, and the unit can be pulled by a small vehicle, making the survey speed very fast. In all these senses, multi-channel recording of surface waves is best suited for a routine method for geotechnical characterization in most of civil engineering works.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.2
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• pp.100-107
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• 2018

The material and geometrical nonlinear finite elment analysis of UHPFRC 50M composite box girder was carried out. Constitute law in tension and compressive region of UHPFRC and HPC were modeled based on specimen test. The accuracy of nonlinear FEM analysis was verified by the experimental result of UHPFRC 50M composite girder. The UHPFRC 50M segmental composite box girder which has 1.5% steel fiber of volume fraction, 135MPa compressive strength and 18MPa tensile strength was tested. The post-tensioned UHPFRC composite girder consisted of three segment UHPFRC U-girder and High Strength Concrete reinforced slab. The parts of UHPFRC girder were modeled by 8nodes hexahedron elements and reinforcement bars and tendons were built by 2nodes linear elements by Midas FEA software. The constitutive laws of concrete materials were selected Multi-linear model both of tension and compression function under total strain crack model, which was included in classifying of smeared crack model. The nonlinearity of reinforcement elements and tendon was simulated by Von Mises criteria. The nonlinear static analysis was applied by incremental-iteration method with convergence criteria of Newton-Raphson. The validation of numerical analysis was verified by comparison with experimental result and numerical analysis result of load-deflection response, neutral axis coordinate change, and cracking pattern of girder. The load-deflection response was fitted very well with comparison to the experimental result. The finite element analysis is seen to satisfactorily predict flexural behavioral responses of post-tensioned, reinforced UHPFRC composite box girder.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.8
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• pp.5318-5326
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• 2014

This paper presents the results of flexural test of concrete beams reinforced with GFRP and conventional steel reinforcement for comparison. The beams were tested under a static load to examine the effects of the reinforcement ratio and compressive strength of concrete on cracking, deflection, ultimate capacity, and modes of failure. The test results showed that the ultimate capacity of the GFRP-reinforced beams increased with increasing reinforcement ratio and concrete strength, showing a 41.3~51.6% increase compared to steel reinforced beams. The deflections at maximum loads of the GFRP reinforced beams were 4.1~6.3 times higher that of steel reinforced beams. The measured deflections of GFRP reinforced beams decreased approximately 31% compared to the theoretical predictions because the theoretical flexural stiffness was underestimated at the maximum loads. For the GFRP-reinforced beams, the ACI code 440 design method resulted in conservative flexural strength estimates.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.1
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• pp.34-46
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• 2020

Computational assessment of gas-dynamic characteristics is explored for a three-dimensional counter-flow thrust vector control system in a rectangular supersonic nozzle. This convergent-divergent nozzle is designed by Method of Characteristics and its design Mach number is specially set as 2.5. Performance variations of the counter-flow vector system are illustrated by varying the gap height of the secondary flow duct. Key parameters are quantitatively analyzed, such as static pressure distribution along the centerline of the upper suction collar, deflection angle, secondary mass flow ratio, and resultant thrust coefficient. Additionally, the streamline on the symmetry plane, three-dimensional iso-Mach number surface contour, and three-dimensional turbulent kinetic energy contour are presented to reveal overall flow-field characteristics in detail.

• Structural Engineering and Mechanics
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• v.71 no.2
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• pp.185-196
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• 2019

In this investigation, study of the static and dynamic behaviors of functionally graded beams (FGB) is presented using a hyperbolic shear deformation theory (HySDT). The simply supported FG-beam is resting on the elastic foundation (Winkler-Pasternak types). The properties of the FG-beam vary according to exponential (E-FGB) and power-law (P-FGB) distributions. The governing equations are determined via Hamilton's principle and solved by using Navier's method. To show the accuracy of this model (HySDT), the current results are compared with those available in the literature. Also, various numerical results are discussed to show the influence of the variation of the volume fraction of the materials, the power index, the slenderness ratio and the effect of Winkler spring constant on the fundamental frequency, center deflection, normal and shear stress of FG-beam.

• Computers and Concrete
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• v.30 no.3
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• pp.175-183
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• 2022

To study the working mechanism and size effect of an innovative dovetail UHPC joint originated from the 5th Nanjing Yangtze River Bridge, a large-scale testing subject to negative bending moment was conducted and compared with the previous scaled specimens. The static responses, i.e., the crack pattern, failure mode, ductility and stiffness degradation were analyzed. It was found that the scaled specimens presented similar working stages and working mechanism with the large-scale ones. However, the post-cracking ductility and relative stiffness degradation all decrease with the enlarged length/scale, apart from the relative stiffness after flexural cracking. The slab stiffness at the flexural cracking stage is 90% of the initial stiffness while only 24% of the initial stiffness reserved in the ultimate stage. Finite element model (FEM) was established and compared with the experiments to verify its effectiveness in exploring the working mechanism of the innovative joint. Based on this effective method, a series of FEMs were established to further study the influence of material strength, pre-stressing level and ratio of reinforcement on its deflection-load relationship. It is found that the ratio of reinforcement can significantly improve its load-carrying capacity among the three major-influenced factors.

• Structural Engineering and Mechanics
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• v.86 no.2
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• pp.167-180
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• 2023

This work applies a four-known quasi-3D shear deformation theory to investigate the bending behavior of a functionally graded plate resting on a viscoelastic foundation and subjected to hygro-thermo-mechanical loading. The theory utilizes a hyperbolic shape function to predict the transverse shear stress, and the transverse stretching effect of the plate is considered. The principle of virtual displacement is applied to obtain the governing differential equations, and the Navier method, which comprises an exponential term, is used to obtain the solution. Novel to the current study, the impact of the viscoelastic foundation model, which includes a time-dependent viscosity parameter in addition to Winkler's and Pasternak parameters, is carefully investigated. Numerical examples are presented to validate the theory. A parametric study is conducted to study the effect of the damping coefficient, the linear and nonlinear loadings, the power-law index, and the plate width-tothickness ratio on the plate bending response. The results show that the presence of the viscoelastic foundation causes an 18% decrease in the plate deflection and about a 10% increase in transverse shear stresses under both linear and nonlinear loading conditions. Additionally, nonlinear loading causes a one-and-a-half times increase in horizontal stresses and a nearly two-times increase in normal transverse stresses compared to linear loading. Based on the article's findings, it can be concluded that the viscosity effect plays a significant role in the bending response of plates in hygrothermal environments. Hence it shall be considered in the design.

• Journal of the Korea Concrete Institute
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• v.22 no.5
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• pp.609-616
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• 2010

FRP-concrete composite deck, an innovative system, is composed of concrete in the top and FRP panel in the bottom. Bottom FRP panel can reduce self weight and improve workability. This system requires strong connection between FRP and concrete. Therefore coarse sand coating was previously applied on FRP to improve the bonding. In this study, concrete wedge method is newly introduced to enhance both vertical bond and fatigue performance. Three FRP-concrete composite deck specimens with the concrete wedges were manufactured, and static and fatigue tests were carried out. The results showed that the new FRP-concrete composite deck satisfied deflection and crack width limits set by the design codes. And the fatigue test showed that the composite deck was capable of two million load cycles under 50% of its static strength. Based on the results, it can be concluded that that this new system has outstanding mechanical and durability performance, and therefore, satisfactorily be used in designing FRP-concrete composite deck.

• Journal of the Korean Society for Railway
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• v.13 no.2
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• pp.186-193
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• 2010

Track system and periodic live load are characteristics of railway bridges. In the design and construction of railway bridge, periodic live load increases the importance of dynamic behavior. And It is well known that behavior of railway bridge may be affected by track system in real bridge. Through experimental study, static and dynamic behaviors were investigated. Deflection and stress due to bending moment were measured, the location of neutral axis of each section, natural frequency, damping ratio were analyzed for each three track systems - girder only, installed ballast track system and installed concrete slab track system. According to measured values for the each type of track system, concrete track system increases the stiffness of bridge by 50%, and ballast system does by 7%, dynamic responses of structure change linearly with the magnitude of load and location of neutral axis of each sections varies with each track system. Damping ratio is almost equal without and with track. Therefore, the effects of track system on the integral behaviors of railway bridge can not be ignored in the design of bridge, especially in the case of concrete slab track system. So study of the quantitative analysis method for effects of track system must be performed.

• Journal of the Korea institute for structural maintenance and inspection
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• v.15 no.2
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• pp.51-60
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• 2011

In this study, structural safety capacity analysis of the overpass railway bridge between Konkuk Univ. and Guui station railroad has been performed. The overpass is expected to have suffered durability reduction by deterioration. The weight reduction of the overpass has been implemented to prevent further durability reduction and to improve performance capacity. To reduce the weight, 3 procedures of replacing concrete soundproofing wall to light-weight soundproofing wall, replacing gravel ballast to concrete ballast, and reducing the weight of trough have been performed. The analysis of static/dynamic behaviors and improved capacity of the light-weighted overpass bridge has been performed. The structural safety verification of the improved structure has been implemented by using rating factors of load carrying capacity of PSC I girder. The results have shown that the deflection has been reduced by 2.6mm and tensile strength has been improved by 1.07MPa, which indicate that the structural capacity has effectively been improved. Also, the natural frequency has improved by approximately 30% where vibration reduction and dynamic behavior improvement have been achieved. Moreover, in the rating factor evaluation based on analysis and test results, an improvement from 1.82 to 1.93 has been observed. Therefore, weight reduction method for the overpass is effective considering overall results.