• Journal of the Korea Society for Simulation
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• v.7 no.2
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• pp.33-43
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• 1998

To evaluate the server performance and forecast capacity requirements, we carry out simulation of Multimedia-on-demand(MOD) server. In multimedia service environment, especially for on-demand service, one of the key problems is capacity planning, which requires ensuring that adequate computer resources will be available to meet the future workload demands in a cost-effective manner. In this paper, we design and implement a simulation model for MOD server with failures of components (e.g., processors, disks and networks). By acquisition of utilization and queue length parameters, we can estimate desirable capacity of server components with various arrival rates of customers and failure rates of components. For a given failure probability, we also compute packet delay probability and reliability of the server. It is possible to derive some important design information of the MOD server by using the above parameters.

• Earthquakes and Structures
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• v.20 no.4
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• pp.389-403
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• 2021

Skewed bridges, being irregular structures with complicated dynamic behavior, are more susceptible to earthquake damage. Reliable seismic-resistant design of skewed bridges can be achieved by accurate determination of nonlinear seismic demands. However, the effect of geometric characteristics on the response modification factor (R-factor) is not accounted for in bridge design practices. This study attempts to investigate the effects of changes in the number of spans, skew angle and bearing stiffness on R-factor values and to assess the seismic fragility of skewed bridges. Results indicated that changes in the skew angle had no significant effect on R-factor values which were in consonance with code-prescribed R values. Also, unlike the increase in the number of spans that resulted in a decrease in the R-factor, the increase in bearing stiffness led to higher R-factor values. Findings of the fragility analysis implied that although the increase in the number of spans, as well as the increase in the skew angle, led to a higher failure probability, greater values of bearing stiffness reduced the collapse probability. For practicing design engineers, it is recommended that maximum demands on substructure elements to be calculated when the excitation angle is applied along the principal axes of skewed bridges.

• Journal of the Korean Society of Safety
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• v.32 no.4
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• pp.7-15
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• 2017

International standards such as IEC-61508 and IEC-61511 require Safety Integrity Levels (SILs) for Safety Instrumented Functions (SIFs) in process industries. SIL verification is one of the methods for process safety description. Results of the SIL verification in some cases indicated that several Safety Instrumented Functions (SIFs) do not satisfy the required SIL. This results in some problems in terms of cost and risks to the industries. This study has been performed to improve the reliability of a safety instrumented function (SIF) installed in hydrodesulfurization reactor heater using Partial Stroke Testing (PST). Emergency shutdown system was chosen as an SIF in this study. SIL verification has been performed for cases chosen through the layer of protection analysis method. The probability of failure on demands (PFDs) for SIFs in fault tree analysis was $4.82{\times}10^{-3}$. As a result, the SIFs were unsuitable for the needed RRF, although they were capable of satisfying their target SIL 2. So, different PST intervals from 1 to 4 years were applied to the SIFs. It was found that the PFD of SIFs was $2.13{\times}10^{-3}$ and the RRF was 469 at the PST interval of one year, and this satisfies the RRF requirements in this case. It was also found that shorter interval of PST caused higher reliability of the SIF.

• Smart Structures and Systems
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• v.17 no.1
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• pp.149-165
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• 2016

Floods have been known to be one of the main causes of bridge collapse. Contrary to earthquakes, flood events tend to occur repeatedly and more frequently in rainfall areas; flood-induced damage and collapse account for a significant portion of disasters in many countries. Nevertheless, in contrast to extensive research on the seismic fragility analysis for civil infrastructure, relatively little attention has been devoted to the flood-related fragility. The present study proposes a novel methodology for deriving flood fragility curves for bridges. Fragility curves are generally derived by means of structural reliability analysis, and structural failure modes are defined as excessive demands of the displacement ductility of a bridge under increased water pressure resulting from debris accumulation and structural deterioration, which are known to be the primary causes of bridge failures during flood events. Since these bridge failure modes need to be analyzed through sophisticated structural analysis, flood fragility curve derivation that would require repeated finite element analyses may take a long time. To calculate the probability of flood-induced failure of bridges efficiently, in the proposed framework, the first order reliability method (FORM) is employed for reducing the required number of finite element analyses. In addition, two software packages specialized for reliability analysis and finite element analysis, FERUM (Finite Element Reliability Using MATLAB) and ABAQUS, are coupled so that they can exchange their inputs and outputs during structural reliability analysis, and a Python-based interface for FERUM and ABAQUS is newly developed to effectively coordinate the fragility analysis. The proposed framework of flood fragility analysis is applied to an actual reinforced concrete bridge in South Korea to demonstrate the detailed procedure of the approach.

• Journal of the Earthquake Engineering Society of Korea
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• v.10 no.5 s.51
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• pp.11-23
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• 2006

This paper evaluates the seismic response of multi-span prestressed concrete girder bridges typically found in the New Madrid Seismic Zone region of the central United States. Using detailed nonlinear analytical models and synthetic ground motion records for Memphis, TN, nonlinear response history analyses are performed for two levels of ground motion: 10% probability of exceedance (PE) in 50 years, and 2% probability of exceedance (PE) in 50 years. The results show that the bridge performance is very good fur the 10% PE in 50 years ground motion level. However, the performance for the 2% PE in 50 years ground motion is not so good because it results in highly inelastic behavior of the bridge. Impact between decks results in large ductility demands on the columns, and failure of the bearings that support the girders. It is found that making the superstructure continuous, which is commonly performed for reducing dead load moments and maintenance requirements, results in significant improvement in the seismic response of prestressed concrete girder bridges.