• Tribology and Lubricants
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• v.40 no.3
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• pp.103-110
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• 2024

Gas foil thrust bearings (GFTBs) are oil-free self-acting hydrodynamic bearings that support axial loads with a low friction during airborne operation. They need solid lubricants to reduce dry-friction between the runner and top foil and minimize local wears on their surfaces during start-up and shutdown processes. In this study, we evaluate the lift-off speeds and load capacity performance of a GFTB with Polytetrafluoroethylene (PTFE) surface coating by measuring drag torques during a series of experimental tests at increasing ambient temperatures of 25, 75 and 110℃. An electric heat gun provides hot air to the test GFTB operating in the closed booth to increase the ambient temperature. Test results show that the increasing ambient temperature delays the lift-off speed and decreases the load capacity of the test GFTB. An early developed prediction tool well predicts the measured drag torques at 60 krpm. After all tests, post inspections of the surface coating of the top foil are conducted. Scanning electron microscope (SEM) images imply that abrasive wear and oxidation wear are dominant during the tests at 25℃ and 110℃, respectively. A quantitative energy dispersive spectroscopy (EDS) microanalysis reveals that the weight percentages of carbon, oxygen, and nitrogen decrease, while that of fluorine increases significantly during the highest-temperature tests. The study demonstrates that the increasing ambient temperature noticeably deteriorates the static performances and degrades the surface coating of the test GFTB.

• Computational Structural Engineering
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• v.10 no.1
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• pp.37-45
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• 1997

대규모의 쉘 구조물 설계에서는 고정하중이 지배적이므로 구조물을 구성하는 부재단면은 거의 고정하중에 의해 결정되는 경우가 많다. 이미 보고된 정적하중에 대한 쉘의 좌굴에 관한 연구결과들이 말해주고 있듯이 구조물의 안전율이 어느정도 확보되어 있는가를 평가하는 일이 우선적으로 중요하다. 그러나 고정하중에 대한 안전율을 확보하여 설계된 쉘이 지진동을 받았을 경우 어떻게 거동할까, 어느정도의 지진동에 견딜 수 있는지를 조사하여 그 구조물의 내진성능을 파악하는 일은 설계상 반드시 필요한 일이라 생각된다. 지진응답에 관한 많은 연구들이 주로 수평지진동을 받는 구조물을 대상으로 하고 있다. 그러나 지진동은 원래가 3차원적인 것이고 따라서 직하형의 지진동을 받을 가능성도 있다. 특히 장span구조물은 상하지진동의 영향이 클 것으로 생각된다. 본 고에서는 상하지진동을 받는 단순지지된 편평쉘의 응답해석례를 소개하고자 한다.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.11
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• pp.725-730
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• 2017

The strength ratio of the main structures of buildings gradually increasing, due to the advances made in analysis and cost saving techniques. In this study, to examine the stability of industry buildings using sandwich panels as roof assemblies, we examine the changes in the moment strength ratio of the main structures caused by increasing the roof load. This study adopts the PEB structure and three H-steel structure as the structural analysis models. In the case where the additional load exceeds about 11% of the roof design load, the strength ratio exceeds 1 for the main structure. In the case where the additional load exceeds about 36%(of the roof design load), the working moment exceeds the plastic moments, which leads to major damage to the structure. This study compares 1) the maximum load according to the purlin spaces, 2) the maximum load by KS, and 3) the maximum load calculated from the test results of the manufacturer.The maximum bearing load of the panels determined by all three methods exceeds the structure failure threshold load of the main structure. This study provides evidence that an unexpected increase in the roof load might cause the whole structure to collapse, due to the failure of the main structural members, before the failure of the roof assemblies. Therefore, information on the structural performance of the sandwich panels is required for the structural design, and the sandwich panels should be considered to be an integral part of the overall structural design.

• Journal of the Korean Geotechnical Society
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• v.35 no.9
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• pp.29-36
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• 2019

Axial compressive failure loads ($P_n$) of diameter 500 mm and diameter 600 mm A type PHC pile were calculated as 7.7 MN and 10.6 MN, respectively. In the static pile load tests, the maximum axial compressive loads of the above 2 kinds of A type pile were measured as 6.9 MN and 8.8 MN respectively, therefore these measured maximum loads were at the level of 90% and 83% of $P_n$ respectively. Long-term allowable axial compressive loads ($P_a$) of the above 2 kinds of A type pile were 1.7 MN and 2.3 MN respectively. From the bi-directional pile load test data on the prebored PHC piles, it was confirmed that the allowable axial compressive bearing resistance was estimated as 131% of the long-term allowable compressive load of the PHC pile and showed higher than the allowable bearing capacity calculated by the current design method. Therefore, it has been verified that the PHC pile can be used up to the maximum long-term allowable compressive load, and it is suggested that the ultimate pile capacity formula used in the current design for prebored PHC piles should be improved to accommodate the actual capacity.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.6
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• pp.897-908
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• 2019

The objective of this study is to evaluate the field support performance of highperformance lattice girder (BK-Lattice Girder) by using numerical analysis. Three types (50, 70, 95-type) of existing and high performance lattice girders were applied to the cross section of highway 2, 3, and 4 lane tunnels to compare the supporting performance. The numerical analysis was the finite element method and the lattice girder was modeled in three dimensions with an elasto-plastic frame. The ground was modeled as a spring receiving only compression. The load was applied as a concentrated load on the central ceiling of the tunnel section. The yield strengths of the lattice girders were determined from the numerical results to compare the supporting performance of lattice girder. In case of 50-type, the yield strengths of high-performance lattice girders were increased by 6.7~10.0% compared with those of the existing lattice girders. In the case of 70-type, the high-performance lattice girders increased yield strengths by 12.1~14.9% than the existing lattice girder. In the case of 95-type, the high-performance lattice girders increased yield strengths by 13.3~20.0% than the existing lattice girder. As a result of numerical analysis, it was considered that the high-performance lattice girder supported better than the existing lattice girder when only the lattice girders were constructed.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.7
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• pp.9-16
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• 2017

To improve concrete-filled tube (CFT) columns, an octagonal concrete-filled tube (OCFT) column was developed. Because the OCFT column requires a small boring diameter, the OCFT column is suitable for Top-Down construction method. In this study, the compression performance of OCFT column to be used as Top-Down pile foundation was verified using centrifuge equipment. Under 12 g centrifugal acceleration, the bearing capacities of the pile foundations of OCFT and H-shaped sections were tested. When the pile foundations were embedded in soil of full depth, 45 % of the design strength, which was assumed to be the construction load, was supported by the OCFT and H-shaped sections in the elastic states. When the pile foundations were embedded in soil of half depth, the buckling of the pile foundations was not investigated. After the loading test, the rock at the bottom of pile foundation, which had a strength of 3.5 MPa, was not damaged due to 45 % of the design strength.

• Journal of Korean Association for Spatial Structures
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• v.7 no.5
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• pp.83-87
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• 2007

Elastic critical loads of sinusoidally tapered bars with simply supported ends are determined by finite element method. The parameters considered in the analysis are taper parameter (=a) and section property parameter (=m). The analysis result for the special case of porismatic bar (a=0) shows good agreement with the existing value. The changes of the critical load coefficients are expressed by an algebraic equation. The coefficients appearing in the equations are determined by regression technique. The critical loads coefficients estimated by the proposed equations reveal little errors when they are compared with those determined by finite element method.

• Fire Science and Engineering
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• v.25 no.6
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• pp.64-72
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• 2011

The concrete void slab structure with the existing mushroom slab, is the structure that maximizes the advantages, while minimizing the weakness with removing useless body force of the concrete part, located on the center of the slab cross-section, which does not need to support the structural weight. In this research, a fire test is performed to analyze how the blaze behave according to the thickness of slab cover, with the practical span length of concrete void slab for the slab length 7.5 m. With this heating test, we assumed the uniform-load-model considering fixed loads and live loads, and chose the standard fire test condition. We measured the temperature changes and the deflection character according to the depth from the heat exposure side, and assessed the resisting capability according to the standard KS F 2257-1. The result comes out with the EPS model can secure about 2 hour fire-resisting-capability with 50 mm of cover depth.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.5
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• pp.1063-1070
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• 1988

The influence of the inlet pressure on bearing performance of tilting pad bearings in laminar regime is examined. A simple flow model is presented to calculate the inlet pressure in inlet flow that occurs at a short distance ahead of the bearing inlet. The bearing performances are obtained, load capacity, friction torque and lubricated flow-rate, etc, numerically for the inlet pressure boundary conditions with and without pressure jump. The computed results of both cases show that bearing performance and the optimum pivot position changes remarkably according to the bearing operating conditions. The influence of the inlet pressure on bearing performance must be considered to analyze the bearing performance precisely.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.37 no.2
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• pp.357-367
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• 2017

Bending and axial forces simultaneously occur at the cross-section of a shotcrete lining reinforced with steel supports due to the tunnel geometry. The shotcrete has changing flexural stiffness depending on the axial forces and, as a result, severely nonlinear behavior. The mechanical properties of a shotcrete-steel composite also depend on the type of steel support. This study presents a fiber section element model considering the effect of axial force to evaluate the nonlinear behavior of a shotcrete-steel composite. Additionally, the model was used to analyze the effects of different types of steel supports on the load capacity. Furthermore, a modified hyperbolic model for ground reaction, including strain-softening, is proposed to account for the ground-lining interaction. The model was validated by comparing the numerical results with results from previous load test performed on arched shotcrete specimens. The changes in mechanical responses of the lining were also investigated. Results show a lining with doubly reinforcement rebar has similar load capacity as a lining with H-shaped supports. The use of more materials for the steel support enhances the residual resistance. For all types of steel reinforcement, the contribution of steel supports during peak load decreases as the ground becomes stiffer.