• Title/Summary/Keyword: temperature distri bution

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Stress Analysis for Differential Drying Shrinkage of Concrete (콘크리트의 부등건조수축으로 인한 응력의 해석)

  • 김진근;김효범
    • Magazine of the Korea Concrete Institute
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    • v.6 no.4
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    • pp.102-112
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    • 1994
  • The drying shrinkage of concrete has a close relation to the water movement. Since the diffusion process of water in concrete is strongly dependent on the temperature and the pore humidity, the process is highly nonlinear phenomena. This study consists of two parts. The first is the development of a finite element program which is capable of simulating the rnoisture distri- ,bution in concrete, and the other is the estimation of the differential drying shrinkage and stress considering creep by using the modified elastic modulus due to inner temperature change and maturity. It is shown that the analytical results of this study are in good agreement with experlimental data in the literatures, and results calculated by BP-KX model. The internal stress caused by moisture distribution which was resulted from the diffusion process, was calculated :quantitatively. The tensile stress which occured in the drying outer zone mostly exceeded the tensile strength of concrete, and necessarily would result in crack formation.

A Study on Strengthening of Steel Girder Bridge using Multi-Stepwise Thermal Prestressing Method (다단계 온도프리스트레싱을 이용한 강거더교의 보강에 관한 연구)

  • Kim, Sang Hyo;Kim, Jun Hwan;Ahn, Jin Hee
    • Journal of Korean Society of Steel Construction
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    • v.18 no.6
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    • pp.717-726
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    • 2006
  • Traditional external post-tensioning method using either steel bars or tendons is commonly used as a retrofitting method for steel composite bridges. However, the method has some disadvantages such as stress concentration at anchorages and inefficient load-carrying capability of live loads. Multi-stepwise prestressing method using thermal expanded coverplate is a newly proposed prestressing method, which was originally developed for prestressing steel structures. A new retrofitting method for steel girder bridges founded on a simple concept of thermal expansion and contraction of cover plate, the method is a hybrid of and combines the advantages of external post-tensioning and thermal prestressing. In this paper, basic concepts of the method are presented and an illustrative experiment is introduced. From actual experimental data, the thermal prestressing effect was substantiated and the FEM approach for its analysis was verified. The retrofitting effects ofa single-span bridge were analyzed and the feasibility of the developed method was examined.

Preparation and Characteristics of $Y_2O_3-CeO_2-ZrO_2$Structural Ceramics ; I. Synthesis and Sinterability of Powder ($Y_2O_3-CeO_2-ZrO_2$ 구조세라믹스의 제조 및 특성 : I 분말의 합성 및 소결성)

  • 오혁상;이윤복;김영우;오기동;박흥채
    • Journal of the Korean Ceramic Society
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    • v.33 no.9
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    • pp.1057-1063
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    • 1996
  • Y2O3-CeO2-ZrO2 powders were prepared from water-soluble salts using a coprecipitation method. The forming process of oxide and the characteristics of the calcined powders treated in different drying conditions were investigated. The oxidation was occurred at the temperature of around 40$0^{\circ}C$ and the main crystallization of ZrO2 around $600^{\circ}C$. On calcination at $600^{\circ}C$ heating lamp-dried powders consisted of agglomerates of globular morphology with average agglomerate size of 2.27${\mu}{\textrm}{m}$ and specific surface area of 68.3m2/g and spray dried powders contained dense spheric particles with average agglomerate size of 1.35${\mu}{\textrm}{m}$ and specific surface area of 11.0m2/g which exhibited low agglomeration tendency. Removal of the water by a freeze-drying technique produced calcined powders containing flake-like secondary particle structures with wide agglomerate size distri-bution of 0.1-60${\mu}{\textrm}{m}$ and specific surface area of 24.5${\mu}{\textrm}{m}$. The 20 MPa-pressed density (36.8-41.4% T,D) of calcined powders did not nealy depend on drying methods whilst compaction ratio of calcined powders derived from freeze-drying was the highest ( 6.24) among three drying methods. On continuous heating up to 150$0^{\circ}C$ the sinterability of calcined powders derived from heating lamp-drying was superior to those derived from spray-and freeze-drying. The final sintered density of calcined powders was the highest (96% T,D at 150$0^{\circ}C$) in case of heating lamp-drying.

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Structural Analysis and Magnctic Propcrics of Amorphous $Fe_{78}Si_{9}B_{13}$ Alloy (비정질 $Fe_{78}Si_{9}B_{13}$ 합금의 구조와 자성 연구)

  • 이희복;송인명;유성초;임우영
    • Journal of the Korean Magnetics Society
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    • v.3 no.3
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    • pp.179-184
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    • 1993
  • The X-ray diffraction pattern of amorphous $Fe_{78}Si_{9}B_{13}$ alloy was analyzed to obtain the radial distribution function (RDF) where the first peak was in the form of Gaussian function. The calculated coordination number of the form of Gaussian functiono The calculated coordination number of the sample is 13.5, the mean distance betweeon near-neighbor atoms $r_{0}$ is $2.595{\AA}$ and a Gaussian parametet ${\delta}r$ indicating near-neighbor atomic distri-bution is $0.27{\AA}$. The temperature dependence of saturated magnetization at low temperature could be explained by spin wave excitations theory yielding the spin wave stiffness constant as $117.8\;meV\;{\AA}^2$. Also, we tried to fit the observed temperature dependence of saturated magnetization with the Handrich's equation of the modified molecular field theory for the amorphous ferromagnet. Nice fittings are obtained when we used the parameters ${\Delta}=0.32$(S=1/2) and ${\Delta}=0.23$(S=1), respectively. Finally, the calculated spin wave stiffness constant using the parameters and the structural data are $149\;meV\;{\AA}^2$ for S=1/2 and $138\;meV\;{\AA}^2$ for S=1, respectively. The mean exchange coupling integral between near-neighbor atoms was estimated to be 17.9 meV for S=1/2 and 6.7 meV for S=1.

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A study on the improvement of distribution system by overseas agricultural investment (해외농업투자에 따른 유통체계 개선방안에 관한 연구)

  • Sun, Il-Suck;Lee, Dong-Ok
    • Journal of Distribution Science
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    • v.8 no.3
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    • pp.17-26
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    • 2010
  • Recently concerns have been raised due to the unbalanced supply of crops: the price of crops has been unstable and at one point the price went up so high that the word Agflation(agriculture+ inflation) was coined. Korea, in particular, is a small-sized country and needs to secure the stable supply of crops by investing in the produce importation at a national level. Investment in foreign produce importation is becoming more important as a measure for sufficient supply of crops, limited supply of domestic crops, weakened farming conditions worldwide, as well as recent changes in the use of crops due to the development of bio-fuels, influence of carbon emission on crops, the price increase in crops, and influx of foreign hot money. However, there are many problems with investing in foreign produce importation: lack of support from the government; lack of farming information and technology; difficulty in securing the capital; no immediate pay-off from the investment and insufficient management. Although foreign produce is originally more price-competitive than domestic produce, it loses its competiveness in the process of importation (due to high tariffs) and poor distribution system, which makes it difficult to sell in Korea. Therefore, investment in foreign produce importation is being questioned for feasibility; to make it possible, foreign produce must maintain the price-competitiveness. Especially, harvest of agricultural products depends on natural and geographical conditions of each country and those products have indigenous properties, so distribution system according to import and export of agricultural products should be treated more carefully than that of other industries. Distribution costs are differentiated into each item and include cost of sorting and wrapping, cost of wrapping materials, cost of domestic transport, cost of international transport and cost of clearing customs for import and export. So transporting and storing agricultural products generates considerable costs compared with other products. Also, due to upgrade of dietary life, needs for stability, taste and visible quality toward food including agricultural products are being raised and wrong way of storage causes decomposition of food and loss of freshness, making the storage more difficult than that in room temperature, so storage and transport in distribution of agricultural products needs specialty. In addition, because lack of specialty in distribution and circulation such as storage and wrapping does not solve limit factors in distance, the distribution and circulation has been limited to a form of import and export within short-distant region. Therefore, need for distribution out-sourcing which can satisfy specialty in managing distribution and circulation and it is needed to establish more effective distribution system. However, existing distribution system of agricultural products is exposed to various problems including problems in distribution channel, making distribution and strategy for distribution and those problems are as follows. First, in case of investment in overseas agricultural industry, stable supply of the products is difficult because areas of production are dispersed widely and influenced by outer factors due to including overseas distribution channels. Also, at the aspect of quality, standardization of products is difficult, distribution system is quite complicated and unreasonable due to long distribution channels according to international trade and financial and institutional support is not enough. Especially, there are quite a lot of ineffective factors including multi level distribution process, dramatic gap between production cost and customer's cost, lack of physical distribution facilities and difficulties in storage and transport due to lack of wrapping containers. Besides, because import and export of agricultural products has been manages under the company's own distribution according to transaction contract between manufacturers and exporting company, efficiency is low due to excessive investment in fixed costs and lack of specialty in dealing with agricultural products causes fall of value of products, showing the limit to lose price-competitiveness. Especially, because lack of specialty in distribution and circulation such as storage and wrapping does not solve limit factors in distance, the distribution and circulation has been limited to a form of import and export within short-distant region. Therefore, need for distribution out-sourcing which can satisfy specialty in managing distribution and circulation and it is needed to establish more effective distribution system. Second, among tangible and intangible services which promote the efficiency of the whole distribution, a function building distribution environment which includes distribution information, system for standard and inspection, distribution finance, system for diversification of risks, education and training, distribution administration and tax system is wanted. In general, such a function building distribution environment is difficult to be changed and supplement innovatively because its effect compared with investment does not appear immediately despite of its necessity. Especially, in case of distribution of agricultural products, as a function of collecting and distributing is performed individually through various channels, the importance of distribution information and standardization is getting more focus due to the problem of repetition of work and lack of specialty. Also, efficient management of distribution is quite difficult due to lack of professionals in distribution, so support to professional education is needed. Third, though effort to keep self-sufficiency ratio of staple food, rice is regarded as important at the government level, level of dependency on overseas of others crops is high. Therefore, plan for stable securing food resources aside from staple food is also necessary. Especially, governmental organizations of agricultural products distribution in Korea are production-centered and have unreasonable structure whose function at the aspect of distribution and consumption is quite insufficient. And development of new distribution channels which can deal with changes in distribution environment and they do not achieve actual results of strategy for distribution due to non-positive strategy for price distribution. That is, it implies the possibility that base for supply will become vulnerable because it does not mediate appropriate interests on total distribution channels such as manufacturers, wholesale dealers and vendors by emphasizing consumer protection excessively in the distribution of agricultural products. Therefore, this study examined fundamental concept and actual situation for our investment to overseas agriculture, drew necessities, considerations, problems, etc. of overseas agricultural investment and suggested improvements at the level of distribution for price competitiveness of agricultural products cultivated in overseas under five aspects; government's indirect support, distribution's modernization and distribution information function's strengthening, government's political support for distribution facility, transportation route, load and unloading works' improvement, price competitiveness' securing, professional manpower's cultivation by education and training, etc. Here are some suggestions for foreign produce importation. First, the government should conduct a survey on the current distribution channels and analyze the situation to establish a measure for long-term development plans. By providing each agricultural area with a guideline for planning appropriate production of crops, the government can help farmers be ready for importation, and prevent them from producing same crops all at the same time. Government can sign an MOU with the foreign government and promote the importation so that the development of agricultural resources can be stable and steady. Second, the government can establish a strategy for an effective distribution system by providing farmers and agriculture-related workers with the distribution information such as price, production, demand, market structure and location, feature of each crop, and etc. In order for such distribution system to become feasible, the government needs to reconstruct the current distribution system, designate a public organization for providing distribution information and set the criteria for level of produce quality, trade units, and package units. Third, the government should provide financial support and a policy to seek an efficient distribution channel for foreign produce to be delivered fresh: the government should expand distribution facilities (for selecting, packaging, storing, and processing) and transportation vehicles while modernizing old facilities. There should be another policy to improve the efficiency of unloading, and to lower the cost of distribution. Fourth, it is necessary to enact a new law covering exceptional cases for importing produce in order to maintain the price competitiveness; currently the high tariffs is keeping the imported produce from being distributed domestically. However, the new adjustment should be made carefully within the WTO regulations since it can create a problem from giving preferential tariffs. The government can also simplify the distribution channels in order to reduce the cost in the distribution process. Fifth, the government should educate distributors to raise the efficiency and to modernize the distribution system. It is necessary to develop human resources by educating people regarding the foreign agricultural environment, the produce quality, management skills, and by introducing some successful cases in advanced countries.

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