Determination of Importance Factors in Seismic Design for an RC Bridge Pier Considering the Damage Control Under Mainshock–Aftershock Sequences

ABSTRACT The importance factor is defined in codes for earthquake‐resistant design based on the functionality of bridges, and does not account for damage accumulation due to aftershocks. This study presents a novel approach by introducing the Bridge Aftershock Adjustment Importance Factor ( AIF ), which serves as a modification to current seismic design codes, specifically addressing the effects associated with aftershocks. A spectrum of AIF values can be established to accommodate various design scenarios. Using the ductility seismic design of a reinforced concrete (RC) bridge pier as an example, this study examines the relationship between the AIF , the scaling intensity of the aftershock SF 1 , the ductility capacity µ mon , and the equivalent height of the bridge pier H b . The Modified Equivalent Linearization Method (MELM) is employed to estimate the maximum deformation of an equivalent bridge pier system under a specified seismic sequence, which consist of one mainshock followed by three aftershocks, under near‐field (NF) or far‐field (FF) ground motions. Results indicate that the AIF effectively adjusts the seismic design level of bridges to meet design targets. For an RC bridge pier in Taiwan, with an allowable damage index set at 0.4, the AIF ranges from 1.5 to 2.3 for NF ground motions and from 1.1 to 2.0 for FF ground motions. Furthermore, this study introduces an equation that incorporates the AIF , SF 1 , µ mon , and H b , enabling engineers to adjust the base shear in seismic design for bridges accordingly. The proposed method offers valuable guidance for adjusting the seismic design level of bridges based on aftershock effects.