The reduction of soil strength and stiffness caused by soil liquefaction is an important issue in designing pile or shallow foundations on liquefiable soils. In practice, strength and stiffness reduction is considered via a reduction factor ($D_E$) approach in seismic design. Different sets of values of $D_E$ are available according to the factor of safety against liquefaction ($F_L$) of the liquefiable soil; these ($F_L$) values are usually calculated using simplified liquefaction analysis procedures. Each set of $D_E$ is calibrated and constructed for specific simplified liquefaction analysis procedures or soil conditions. Thus, engineers have difficulty in selecting $D_E$ when using a simplified liquefaction analysis procedure without a corresponding set of $D_E$. In this study, values of $D_E$ for simplified liquefaction analysis procedures commonly used in Taiwan are obtained using a single element test via FLAC program and UBCSAND model. In addition, a novel application of $D_E$ is introduced to improve the usability of the simplified numerical procedure used to predict the liquefaction-induced settlement of a shallow foundation. Simulation results show that the simplified numerical procedure with $D_E$ approach yields a convincing and conservative result.