Abstract
A force reduction factor (R factor) is widely used to estimate the inelastic strength demand from the elastic strength of a bridge. In many seismic codes, a constant value is used for the R factor, although it has long been pointed out that R factor depends on the various factors. This paper presents how natural period of the system, the damping ratio and the ground condition affect the R factor, and a new model, which is formulated by a simple function with two parameters, is proposed to estimate the R factor. The parameters were obtained from the computation of the R factors for elasto-plastic SDOF systems for 70 ground motions. From these results, it was found that the proposed model predicts the mean values of R factors.
