Assessment of Reliability Levels and Adjustment of Load-resistance Factors Using Optimization for Gravitational Loads-governed Limit States of the AASHTO LRFD Bridge Design Specifications

Hae Sung Lee, Changhoon Bae and Ji Hyeon Kim

ABSTRACT

 This paper presents the reliability levels provided by the load-resistance factors of the AASHTO LRFD Bridge Design Specifications for the Strength Limit State-I and -IV, and an optimization scheme for adjusting the load-resistance factors of the specifications. It is shown that the load-resistance factors of the specifications result in lower reliability indices than the target reliability index and widely varying reliability indices with the load composition for reinforced concrete, steel and pre-stressed concrete members. An optimization scheme is proposed to obtain a new set of the load-resistance factors that yield more uniform reliability levels and better approximation to the target reliability index in a wide range of load compositions. The objective function of the proposed optimization scheme is defined as the L2-norm of the error between the target and calculated reliability indices by load-resistance factors over a given interval of load composition. The results of the optimization show that the dead load factor for structural components should be increased by 0.1 for the Strength Limit State-I, and that the vehicular live load should be included in the Strength Limit State-IV. The use of exact figures up to the second decimal point is recommended for the resistance factors. A unified limit state for gravitational loads is proposed to replace the two limit states with a single limit state.

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