Determination of the LoadResistance Factors for ReliabilityBased Codes by Optimization Changhoon Bae
ABSTRACT In current years, the limit state design is the general trend for bridge design specifications which suggests loadresistance factors for structural members of a bridge with dominant ultimate limit states to equally satisfy the target reliability index. To verify that existing loadresistance factors equally satisfy the target reliability index, the reliability assessment is executed for AASHTO LRFD and KHBDCLSD by calculating the required strength which can be generated from loadresistance factors. Prior to the reliability assessment, the normalization is introduced in this study, which nominal loads and nominal resistance are expressed as the ratio to the total nominal load. The generalized reliability assessment is possible by applying the normalization without the length and the cross sectional details of a bridge related to absolute values of nominal loads. According to the reliability assessment results, both specifications do not guarantee equally the target reliability level. To resolve this problem, it is necessary to establish calibration formulation which can generate loadresistance factors to satisfy evenly the target reliability index. By applying this calibration process, revised loadresistance factors are proposed for both specifications. In this study, the exact values of loadresistance factors are calculated by applying the inverse reliability analysis given statistical properties of variables. II From the results, ultimate limit state regions determined newly. In addition, optimization process within each ultimate limit state region is carried out for proposed loadresistance factors. Loadresistance factors are decided relatively by optimization following the variance of resistance and those of loads. According to the reliability assessment with proposed loadresistance factors, both specifications satisfy the target reliability level equally. This study is thoroughly based on statistical properties of loadresistance variables, which means there is a need of the strict research about statistical properties of variables considered in the bridge design preveniently. Key Word Limit state design, Minimum required strength, Inverse reliability analysis, Target reliability index, LoadResistance factor, Optimization
