Determination of Optimal Load-Resistance Factors for Gravitational Loads-Governed Limit State of Korean Bridge Design Code

Abstract

This paper presents an optimization scheme to determine the optimal load-resistance factors for a gravitational loads-governed limit state in the Korean Highway Bridge Design Code. The random variables in the limit state function are normalized to the total nominal load effect, and the optimization is formulated using the normalized variables. The objective function is defined as an L2-norm of errors between the design strength obtained by the load-resistance factors and the target strength evaluated by the inverse reliability analysis for the target reliability index. Two different types of optimization, reference and sequential optimizations, are proposed. The former is to determine absolute optimal load-resistance factors, while the latter is proposed to obtain the factors suitable for actual design codes. The reference optimization results in load-resistance factors that yield a more uniform reliability index than the sequential optimization. The reliability levels provided by the proposed load-resistance factors are presented in detail and compared with those obtained by the factors in the Korean Highway Bridge Design Code. It is shown that the factors in the code lead to a much higher reliability index than the target while the optimal load-resistance factors yield a reliability index within ± 5% error bounds to the target reliability index.