The safety of construction in the long-term performance is the main goal of structural designers. An example is seen in reinforced concrete (RC) bridge deck slabs that have been subjected to a large number of load repetitions. Therefore, many numerical and experimental studies have been conducted to predict the fatigue life of these slabs. Most of these studies focused on the modeling of fatigue behaviors of RC slabs reinforced with deformed bars. Until 1965, plain bars were used for reinforcing RC bridge slabs in a part of Japan. The fatigue damage of those slabs is more significantly observed than that of slabs reinforced with deformed bars. Moreover, old slabs were economically designed by small thickness without considering the fatigue resistance. Until 1965, plain bars were used for reinforcing RC bridge slabs in a part of Japan. The fatigue damage of those slabs is more significantly observed than that of slabs reinforced with deformed bars. Therefore, it is important to predict the fatigue behaviors of these slabs to decide a suitable repairing method. For the main goal of this research, a numerical method based on the bridging stress degradation concept is presented in this study to simulate the fatigue behaviors of RC slabs reinforced with plain bars under a moving load. Moreover, this study provides a numerical investigation to understand the fatigue failure and the improvement mechanism of fiber reinforced polymer (FRP) strengthening. Finite element method (FEM) is used to solve a slab model of smeared crack elements. The cracked elements are modified according to the bridging stress degradation concept. This concept can be …