Floor slabs of deep basement structures are subjected to large compressive forces due to soil and hydraulic lateral pressures. In this study, nonlinear finite-element analyses were carried out to improve the existing moment magnifier method so that it is applicable to the long-term behavior of the flat plates. For this purpose, a computer program that can address creep and shrinkage as well as geometrical and material nonlinearity was developed. Parametric studies were performed to investigate the characteristics of the long-term behavior and the load-carrying capacities of flat plates. In the proposed moment magnifier method, the creep factor was introduced to accommodate the effect of creep on the load-carrying capacity. A design example was presented to demonstrate the proposed design method.