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Determination of Load-Resistance Factors and Material Factors for Gravitational Loads-Governed Limit State of Korean Highway Bridge Design Code Sang Won Song
ABSTRACT Most of the current bridge design codes including Korean Highway Bridge Design Code(Limit State Design) are based on load-resistance factor design which is founded on reliability theory. In load-resistance factor design, which is founded on reliability theory, the safety level is expressed as a reliability index, and the target reliability index should be secured for all the members constituting a structure. However, the reliability indices secured by the load-resistance factors of current Korean Highway Bridge Design Code(Limit State Design) is much higher than the target reliability index. Therefore, in this paper, a load-resistance factor determination method based on optimization is proposed. For the applicability of load-resistance factors to various structural types and load effects, the limit state function is standardized by the total nominal load effect and the concept of load ratios is introduced. The load-resistance factors, which secure the target reliability index uniformly, are calculated by using the objective function that minimizes the error between the required nominal strength calculated by the load-resistance factor and the target strength that accurately secures the target reliability index. Korean Highway Bridge Design Code(Limit State Design) applies the material factor to the design of reinforced concrete members. It is confirmed that the reliability indices secured by the load-material factors of the Korean Highway Bridge Design Code(Limit State Design) for reinforced concrete members are significantly higher than the target reliability index. Therefore, a method to determine the material factors based on optimization is proposed. For the flexural and compressive members, the objective function minimizing the errors between the equivalent resistance factor calculated by the material factors and the target resistance factor is constructed and the optimum material factors are calculated. It is confirmed that the calculated load-material factors secure the target reliability index uniformly. It is understood that the calibrated load-resistance and material factors should always be used together. In addition, the concept of calculating the material factor of the Eurocode is understood and some problems are pointed out in the process of the calculation. Solving these problems, the material factors for the Korean Highway Bridge Design Code(Limit State Design) are calculated by the method of the Eurocode and the fundamental problems of the method are pointed out. Finally, load-resistance and material factors suitable for Korean Highway Bridge Design Code(Limit State Design) are proposed by the optimization-based method described above. It is confirmed that the reliability indices secured by the proposed load-resistance and material factors are approximated to the target reliability index uniformly. Key Word Load-Resistance factors, Material factors, Optimization, Reliability index, Korean Highway Bridge Design Code, Eurocode, Code-calibration
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