• Nuclear Engineering and Technology
• /
• v.55 no.4
• /
• pp.1382-1399
• /
• 2023

Pressure relief valve (PRV) is one of the important control valves used in nuclear power plants, and its sealing performance is crucial to ensure the safety and function of the entire pressure system. For the sealing performance improving purpose, an explicit function that accounts for all design parameters and can accurately describe the relationship between the multi-design parameters and the seal performance is essential, which is also the challenge of the valve seal design and/or optimization work. On this basis, a surrogate model-based design optimization is carried out in this paper. To obtain the basic data required by the surrogate model, both the Finite Element Model (FEM) and the Computational Fluid Dynamics (CFD) based numerical models were successively established, and thereby both the contact stresses of valve static sealing and dynamic impact (between valve disk and nozzle) could be predicted. With these basic data, the polynomial chaos expansion (PCE) surrogate model which can not only be used for inputs-outputs relationship construction, but also produce the sensitivity of different design parameters were developed. Based on the PCE surrogate model, a new design scheme was obtained after optimization, in which the valve sealing stress is increased by 24.42% while keeping the maximum impact stress lower than 90% of the material allowable stress. The result confirms the ability and feasibility of the method proposed in this paper, and should also be suitable for performance design optimizations of control valves with similar structures.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.3 no.3
• /
• pp.71-86
• /
• 1983

Current R.C. retaining wall design is bared on WSD, but the reliability based design method is more rational than the WSD. For this reason, this study proposes a reliability based design criteria for the cantilever retaining wall, which is most common type of retaining wall, and also proposes the theoretical bases of nominal safety factors of stability analysis by introducing the reliability theory. The limit state equations of stability analysis and design of each part of cantilever retaining wall are derived and the uncertainty measuring algorithms of each equation are also derived by MFOSM using Coulomb's coefficient of the active earth pressure and Hansen's bearing capacity formula. The levels of uncertainties corresponding to these algorithms are proposed appropriate values considering our actuality. The target reliability indices (overturning: ${\beta}_0$=4.0, sliding: ${\beta}_0$=3.5, bearing capacity: [${\beta}_0$=3.0, design for flexure: [${\beta}_0$=3.0, design for shear: ${\beta}_0$=3.2) are selected as optimal values considering our practice based on the calibration with the current R.C. retaining wall design safety provisions. Load and resistance factors are measured by using the proposed uncertainties and the selected target reliability indices. Furthermore, a set of nominal safety factors, allowable stresses, and allowable shear stresses are proposed for the current WSD design provisions. It may be asserted that the proposed LRFD reliability based design criteria for the R.C. retaining wall may have to be incorporated into the current R.C. design codes as a design provision corresponding to the USD provisions of the current R.C. design code.

• Journal of the Korean GEO-environmental Society
• /
• v.19 no.4
• /
• pp.5-16
• /
• 2018

In the current study, the engineering behaviour of prebored and precast steel pipe piles was examined from a series of full-scale field measurements by conducting static pile load tests, dynamic pile load tests (EOID and restrike tests) and Class-A and Class-C1 type numerical analysis. The study includes the pile load - settlement relations, allowable pile capacity and shear stress transfer mechanism. Compared to the allowable pile capacity obtained from the static pile load tests, the dynamic pile load tests and the numerical simulation showed surprisingly large variations. Overall among these the restrike tests displayed the best results, however the reliability of the predictions from the numerical analysis was lower than those estimated from the dynamic pile load tests. The allowable pile capacity obtained from the EOID tests and the restrike tests indicated 20.0%-181.0% (avg: 69.3%) and 48.2%-181.1% (avg: 92.1%) of the corresponding measured values from the static pile loading tests, respectively. Furthermore, the computed results from the Class-A type analysis showed the largest scatters (37.1%-210.5%, avg: 121.2%). In the EOID tests, a majority of the external load were carried by the end bearing pile capacity, however, similar skin friction and end bearing capacity in magnitude were mobilised in the restrike tests. The measured end bearing pile capacity from the restrike tests were smaller than was measured from the EOID tests. The present study has revealed that if the impact energy is not sufficient in a restrike test, the end bearing pile capacity most likely will be underestimated. The shear stresses computed from the numerical analysis deviated substantially from the measured pile force distributions. It can be concluded that the engineering behaviour of the pile is heavily affected if a slime layer exists near the pile tip, and that the smaller the stiffness of the slime and the thicker the slime, the greater the settlement of the pile.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.1 no.1
• /
• pp.33-42
• /
• 1981

Current standard code for R.C. design consists of two conventional design parts, so called WSD and USD, which are based on ACI 318-63 and 318-71 code provisions. The safety factors of our WSD and USD design criteria which are taken primarily from ACI 318-63 code are considered to be not appropriate compared to out country's design and construction practices. Furthermore, even the ACI safety factors are not determined from probabilistic study but merely from experiences and practices. This study investigates the safety level of R.C. flexural members designed by the current WSD safety provisions based on Second Moment Reliability theory, and proposes a rational but efficient way of determining the nominal safety factors and the associated flexural allowable stresses of steel bars and concretes in order to provide a consistent level of target reliability. Cornell's Mean First-Order Second Moment Method formulae by a log normal transformation of resistance and load output variables are adopted as the reliability analysis method for this study. The compressive allowable stress formulae are derived by a unique approach in which the balanced steel ratios of the resulting design are chosen to be the corresponding under-reinforced sections designed by strength design method with an optimum reinforcing ratio. The target reliability index for the safety provisions are considered to be ${\beta}=4$ that is well suited for our level of construction and design practices. From a series of numerical applications to investigate the safety and reliability of R.C. flexural members designed by current WSD code, it has been found that the design based on WSD provision results in uneconomical design because of unusual and inconsistent reliability. A rational set of reliability based safety factors and allowable stress of steel bars and concrete for flexural members is proposed by providing the appropriate target reliability ${\beta}=4$.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.27 no.6
• /
• pp.152-161
• /
• 2023

In general, large-capacity hydrogen storage vessels, typically in the form of vertical cylindrical vessels, are constructed using steel materials. These vessels are anchored to foundation slabs that are specially designed to suit the environmental conditions. This anchoring method involves pre-installed anchors on top of the concrete foundation slab. However, it's important to note that such a design can result in concentrated stresses at the anchoring points when external forces, such as seismic events, are at play. This may lead to potential structural damage due to anchor and concrete damage. For this reason, in this study, it selected an vertical hydrogen storage vessel based on site observations and created a 3D finite element model. Artificial seismic motions made following the procedures specified in ICC-ES AC 156, as well as domestic recorded earthquakes with a magnitude greater than 5.0, were applied to analyze the structural behavior and performance of the target structures. Conducting experiments on a structure built to actual scale would be ideal, but due to practical constraints, it proved challenging to execute. Therefore, it opted for an analytical approach to assess the safety of the target structure. Regarding the structural response characteristics, the acceleration induced by seismic motion was observed to amplify by approximately ten times compared to the input seismic motions. Additionally, there was a tendency for a decrease in amplification as the response acceleration was transmitted to the point where the centre of gravity is located. For the vulnerable components, specifically the sub-system (support columns and anchorages), the stress levels were found to satisfy the allowable stress criteria. However, the concrete's tensile strength exhibited only about a 5% margin of safety compared to the allowable stress. This indicates the need for mitigation strategies in addressing these concerns. Based on the research findings presented in this paper, it is anticipated that predictable load information for the design of storage vessels required for future shaking table tests will be provided.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.9 no.4
• /
• pp.119-126
• /
• 2005

The objective of this paper is to evaluate the flexural tensile strength of unreinforced concrete masonry wall to ensure the structural safety in out-of-plane behaviors under the wind or earthquake loads. Flexural tensile strength of unreinforced concrete masonry wall has been obtained from the full scale tests of total 327 specimens and the statistical analysis are performed for each of the cases. The flexural tensile strength derived from experiments is classified as 13 groups according to masorny units, mortar ingredients, and the direction of tensile stresses and the mean tensile strength and the variable coefficient are obtained for each case. The uniform and concentrated transverse loads have been applied over the face of the wall specimens. The ultimate mean flexural tensile strengths are distributed from 1,564 kPa to 363 kPa according to masonry units, mortar ingredients, and other factors. The allowable flexural tension stress criteria will be established based on the mean flexural tensile strengths in the future.

• Journal of Korean Society of Steel Construction
• /
• v.25 no.3
• /
• pp.307-320
• /
• 2013

This paper proposes a new hybrid support structure by the driven piles which removes disadvantages of the existing type of support structure for offshore wind turbines. The hybrid type of support structure is combined with concrete cone and steel shaft, and is supported not only by gravity type foundations but also by driven piles. For three dimensional analysis of the huge and thick concrete structure, a solid-shell element that is capable of exact modeling and node interpolations of stresses is developed. By applying wave theory of stream function and solid-shell element in XSEA simulation software for fixed offshore wind turbines, a quasi-static analysis and natural frequency analysis of proposed support structure are performed with the environmental condition on Southwest Coast in Korea. In the result, lateral displacement is not exceed allowable displacement and a superiority of dynamic behavior of new hybrid support structure is validated by natural frequency analysis. Consequently, the hybrid support structure presented in this study has a structural stability enough to be applied on real-site condition in Korea. The optimized structures based on the preliminary design concept resulted in an efficient structure, which reasonably reduces fabrication costs.

• The Journal of Engineering Geology
• /
• v.25 no.2
• /
• pp.237-250
• /
• 2015

Underground structures such as a tunnel have been considered as safer than structures on the ground during earthquake. However, severe damages of underground structures occurred at subway tunnel during 1995 Kobe Earthquake and such damages are gradually increased. In this study, a dynamic behavior of a cut and cover tunnel surrounded by weathered soils is investigated using Mohr-Coulomb Model. Parametric study was carried out for boundary conditions, tensile strength, and earthquake magnitudes. The results of numerical analyses in terms of ground deformations and stresses acting on the lining were quite dependent on the side boundary condition (free or fix conditions) and tensile strength of surrounding soils. The ground was deformed upward at the end of earthquake when the side boundary condition was fixed, whereas residual deformations were not predicted when it was free. When the tensile strength of a soil was set to the same as its cohesion, residual deformation was less than 1cm, regardless of side boundary conditions or input accelerations. In addition to that, stress conditions at the maximum deformation and end of earthquake were within an allowable range and considered as safe. Proper boundary conditions and material properties such as tensile strength are quite important because they may significantly impact on the results of dynamic analyses.

• Journal of Korea Water Resources Association
• /
• v.49 no.11
• /
• pp.931-939
• /
• 2016

Bangudae Petroglopys of the national treasure No. 285 located in elevation of 53 m to 57 m have been damaged by repetition of submergence and exposure due to the Sayeon-dam of EL.60 m constructed in down stream. In this study, as a preservation plan of the petroglyphs from the contact with water, the construction of eco-levee was suggested and its effect was investigated in the views of hydraulic engineering. It was designed to be located aside of 80 m from Bangudae Petroglyphs with the length of 440 m in streamwise direction, and it was need to construct a new channel maintaining the original hydraulic capacity and conveyance. Hydraulic characteristics such as water surface elevations and velocities near Bangudae Petroglyphs were measured after the eco-levee was installed in the hydraulic model with the scale of 1:50. It showed that there were not much changes of water surface elevations and velocities between sayeon-dam spillway EL. 60 m (Suggestion 1) and EL. 54 m (Suggestion 2). It was concluded the eco-levee could be made of natural materials like soil, pebble, gravel in terms of allowable velocity and shear stresses. The slope of water surface at Suggestion 2 was steeper, and velocities near Bangudae Petroglyphs were also faster than Suggestion 1. As the vorties occured at the left side in Suggestion 2, more detailed study is required.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.2 no.3
• /
• pp.87-99
• /
• 1982

This study proposes a reliability based design criteria for the R.C. superstructures of highway bridges. Uncertainties associated with the resistance of T or rectangular sections are investigated, and a set of appropriate uncertainties associated with the bridge dead and traffic live loads are proposed by reflecting our level of practice. Major 2nd moment reliability analysis and design theories including both Cornell's MFOSM(Mean First Order 2nd Moment) Methods and Lind-Hasofer's AFOSM(Advanced First Order 2nd Moment) Methods are summarized and compared, and it has been found that Ellingwood's algorithm and an approximate log-normal type reliability formula are well suited for the proposed reliability study. A target reliability index (${\beta}_0=3.5$) is selected as an optimal value considering our practice based on the calibration with the current R.C. bridge design safety provisions. A set of load and resistance factors is derived by the proposed uncertainties and the methods corresponding to the target reliability. Furthermore, a set of nominal safety factors and allowable stresses are proposed for the current W.S.D. design provisions. It may be asserted that the proposed L.R.F.D. reliability based design criteria for the R.C. highway bridges may have to be incorporated into the current R.C. bridge design codes as a design provision corresponding to the U.S.D. provisions of the current R.C. design code.