• Tunnel and Underground Space
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• v.22 no.3
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• pp.163-172
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• 2012

As more diesel engines have been employed in underground limestone mines with large cross section, underground space environment is worsened by diesel exhausts and heat flow. This paper aims for the ultimate goal to optimize the work place environment through assuring the optimal required ventilation rate based on the analysis of the airflow, diesel exhaust gas concentrations and the effects of mechanization and deepening working face on temperature and humidity. Due to the insufficient capacity of the main exhaust fan and poor airway management, stagnant airflows were observed at various locations, while the flow direction was reversed instantly with passing diesel equipment and the flow reversal was also made by the seasonal variation of the outside surface weather. During the loading operation, CO concentration measurements were found to be frequently higher than the threshold limit of 50 ppm, and most of the $NO_2$ measurements during drilling and loading operations shows even more serious levels surpassing the permissible limit of 3 ppm. The actual ventilation quantity was considerably less than the required quantity estimated by the mine health and safety law, and this shortage problem was less serious in colder winter showing more effectiveness of the natural ventilation.

• Journal of the Korea Concrete Institute
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• v.28 no.5
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• pp.545-553
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• 2016

This paper presents experimental static test results of the precast concrete panels developed for short-span concrete bridge deck form. Different from LB-DECK, concrete rib attached to the bottom surface of concrete panel, and Top-bar is not used at the top surface of concrete panel. Number of concrete ribs and cross-section details of concrete rib are determined from the analytical results of parametric study considering the span length and the thickness of concrete bridge decks. Shear rebars are installed at the top surface of concrete panel for composite action between precast concrete panel and cast-in-place concrete. In order to evaluate the safety and the serviceability of the developed short-span concrete panel subjected to design load, static load test is conducted. Three test panels with span length of 1.6m are fabricated, and during the load test displacements, strains and cracks of test panels are measured and final failure modes are investigated. Serviceability of the test panels is evaluated based on the results of displacements, cracking load, and crack width at the design load level. Safety is also evaluated based on the comparison of the ultimate strength and the factored design load of test panels. Based on the test results, it is confirmed the short-span precast concrete panel satisfies the serviceability and safety regulated in design codes. In addition, the range of span length of concrete bridge decks for the short-span concrete panel is discussed.

• Journal of the Korean GEO-environmental Society
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• v.7 no.6
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• pp.101-111
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• 2006

In case of drilled shafts installed by drilling through soft overburden onto a strong rock, the piles can be regarded as end-bearing elements and their working load is determined by the safe working stress on the pile shaft at the point of minimum cross-section or by code of practice requirements. Drilled shafts drilled down for some depth into weak or weathered rocks and terminated within these rocks act partly as friction and partly as end-bearing piles. The base resistance component can contribute significantly to the ultimate capacity of the pile. However, the shaft resistance is typically mobilized at considerably smaller pile movements than that of the base. In addition, the base response can be adversely affected by any debris that is left in the bottom of the socket. The reliability of base response therefore depends on the use of a construction and inspection technique which leaves the socket free of debris. This may be difficult and costly to achieve, particularly in deep sockets, which are often drilled under water or drilling slurry. As a consequence of these factors, shaft resistance generally dominates pile performance at working loads. The efforts to improve the prediction of drilled shaft pile performance are therefore primarily concerned with the complex mechanisms of shaft resistance development. In this study, the numerical analyses are carried out to investigate the behavior characteristics of side of rock socketed drilled shafts varying the loading condition at the pile head. The difference of behavior characteristics of side resistance is also evaluated with the effects of modelling of asperity.

• Journal of the Korea Concrete Institute
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• v.24 no.3
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• pp.225-232
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• 2012

This paper presents the test results on the shear performance of large-size reinforced concrete beams using recycled fine aggregate to evaluate its applicability to structural concrete. The performance of these beams is compared to that of similar beams casted with natural coarse and fine aggregates. All of the beam specimens without shear reinforcement had $400mm{\times}600mm$ rectangular cross section and a shear span ratio (a/d) of 5.0. Five concrete mixtures with different replacement levels of recycled fine aggregates (0, 30, 60, 70 and 100%) were used to obtain a nominal concrete compressive strength of 28MPa. The test results of load-deflection curve, shear deformation, diagonal cracking load, crack pattern, ultimate shear strength, and failure mode are examined and compared. In addition, code and empirical equations from KCI, JSCE, CSA, Zsutty, and MCFT were considered to evaluate the applicability of these equations for predicting shear strength of reinforced concrete beam with recycled fine aggregate. The results showed that the overall shear behavior of reinforced concrete beams incorporating less than 60% recycled fine aggregate was comparable with that of conventional concrete beam. The MCFT gave good prediction and other code equations were conservative in predicting the shear strength of the tested beams. The beam specimens with replacement of 70 and 100% of natural fine aggregates by recycled fine aggregates showed different failure mode than other tested beams.

• Journal of Korean Society of Steel Construction
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• v.20 no.1
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• pp.131-149
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• 2008

A transmission tower was designed according to general and special load cases based on KEPCO Design Specifications. The special load case such as unbalanced load a cording to some broken wires has not been considered significantly. Therefore, this paper presents investigations on the stability and safety of main post members subjected to unbalanced load and design wind load. In this study, all cases totally considered. From the finite element analyses using LUSAS program, the stresses on the tower subjected to unbalanced load and design wind load were very high in comparison to the allowable stresses of the steel post member that was used. Some of the post member had higher stresses than the yield stress of the steel member. This paper also shows an example to improve the capacity of the post members using increased cross-section members. Based on the analyses results, when investigating the safety of the transmission tower, one must consider thenew design philosophy including ultimate strength of the member and reliability of the special loading cases.

• Journal of the Korean Recycled Construction Resources Institute
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• v.9 no.2
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• pp.208-215
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• 2021

In this study, a customiz ed bridge system was developed for North Korea application. For the application of North Korea, the customized bridge system design, fabrication, and construction performance evaluation were performed using ultra-high performance concrete a compressive strength 120MPa or more and a direct tensile strength 7MPa or more. The comparison of the North Korean truck luggage load(30, 40, 55) and the Korean standard KL-510 load showed that cross-section increased as the load increased. Furthermore, a bridge with a span length of 30m was fabricated with ultra-high performance concrete for the construction performance evaluation. The evaluation of the load condition analysis was performed by a flexural test. The results showed that a bridge with a span length of 30m secured about 167% of sectional performance under initial cracking load conditions and about 134% of load bearing capacity under ultimate load conditions. As a result of economic analysis, the customized bridge system using ultra-high-performance concrete was less than about 11% of the upper construction cost compared to the steel composite girder bridge. Therefore, these results suggest that the price competitiveness can be secured when applying the ultra-high-performance concrete long-span bridge developed through this study.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.34 no.6
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• pp.1731-1741
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• 2014

Recently, many overpasses, highway, and advanced transit systems have been constructed to distribute the traffic congestion, thus small size of curved bridges with small curvature such as ramp structures have been increasing. Many of early curved bridges had been constructed by using straight beams with curved slabs, but curved steel beams have replaced them due to the cost, aesthetic and the advantage in building the section form and manipulating the curvature of beams, thereby large portion of curved bridges were applied with steel box girders. However, steel box girder bridges needs comparatively high initial costs and continuous maintenance such as repainting, which is the one of the reason for increasing the cost. Moreover, I-type steel plate girder which is being studied by many researchers recently, seem to have problems in stability due to the low torsional stiffness, resulting from the section characteristics with thin plate used for web and open section forms. Therefore, in recent studies, researchers have proposed curved precast PSC girders with low cost and could secured safety which could replace the curved steel girder type bridges. Hence, this study developed a Smart Mold system to manufacture efficient curved precast PSC girders. And by using this mold system a 40 m 2-girder bridge was constructed for a static flexural test, to evaluate the safety and performance under ultimate load. At the manufacturing stage, each single girder showed problems in the stability due to the torsional moment, but after the girders were connected by cross beams and decks, the bridge successfully distributed the stress, thereby the stability was confirmed. The static loading test results show that the initial crack was observed at 1,400 kN when the design load was 450 kN, and the load at the allowable deflection by code was 1,800 kN, which shows that the safety and usability of the curved precast PSC bridge manufactured by Smart Mold system is secured.

• Journal of Dental Rehabilitation and Applied Science
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• v.18 no.3
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• pp.145-155
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• 2002

The results of the present feasibility study are summarized as follows, 1. The three unit bridge of knitted material and UD fibre reinforcement has both the rigidity and the strength against a vertical occlusal load of 75N. 2. Stress concentration at the junctional area between the bridge and the abutments, i.e. between the pontic and the knitted caps was observed. In the case of the bridge with reinforcement straps, it was partly shown that the concentration problem could be improved by simply increasing the fillet size at the area. Further refining in the surface of the junctional area will be needed to ensure a further improvement in the stress distribution. This will require some trade off in the level of the stress and the available space. A parametric study will help to decide the appropriate size of the fillet. 3. Design refinement is a must to improve the stress distribution and realize the most favourable shape in terms of fabrication. The current straight bar with a constant cross section area can be redesigned to a tapered shape. The curve from the dental arch should also be placed on the pontic design. In accordance with design refinement, the resistance of the bridge frame to other load cases should be evaluated. 4. Although not included in the present feasibility study, it is estimated that bridges of the anterior teeth can be made strong enough with the knitted material without further reinforcement using unidirectional materials. In this regard, a feasibility study on design concepts and stress analysis for 3, 4, 5 unit bridge is suggested. 5. Two types of bridge were analysed in terms of fatigue. The safe life design concept, i.e. fatigue design concept, looks reasonable for the bridge where if cracks should form and propagate there is virtually nothing a dentist to do. The bridge must be designed so that no crack will be initiated during the life span. In the case of crowns, however, if constructed with composite resin with knitted materials, it might be possible to repair them, which in general is impossible for crowns of PFM or of metal. Therefore for composite resin crowns, a damage tolerance design concept can be applied and reasonably higher operational stresses can be allowed. In this case, of course, a periodic inspection program should be established in parallel. 6. Parts of future works in terms of structural viewpoint which need to be addressed are summarized as the following: 1) To develop processing technology to accommodate design concepts; 2) More realistic modelling of the bridge and analysis-geometry and loading condition. Thickness variation in the knitted material, taper in the pontic, design for anterior tooth bridge, the effect of combined loads, etc, will need to be included; 3) To develop appropriate design concepts and design goals for the fibre composite FPD aiming at taking the best advantage of knitted materials, including the damage tolerance design concept; 4) To develop testing method and perform test such as static ultimate load test, fatigue test, repair test, etc, as necessary.