In this study, a numerical method for predicting solitary wave forces on a typical coastal bridge deck with girders is utilised in order to obtain an alternative way to assess solitary wave forces on coastal bridge decks with sufficient accuracy. Firstly, a wave model based on the solitary wave theory representing the incident wave of tsunamis is applied through a computational fluid dynamics computer program, where the shear stress transport k-ω model is adopted as the turbulent closure for the Reynolds Averaged Navier–Stokes model equations. Then, the numerical wave profiles and the predicted wave forces are compared with the analytical solutions and the reported laboratory measurements, respectively. These verifications assure the results in the following parametric study reliable. Finally, comparisons between the numerical results and those acquired through the empirical methods are conducted in order to examine the appropriateness of these empirical procedures regarding this specific case. Furthermore, an expanded formula is proposed with the definitions of the key parameters being discussed thereafter. This method can be expanded to cases where different deck cross sections rather than the typical one used in the present study are considered and to scenarios where different wave parameters are involved. It is hoped that the expanded formula could provide straightforward but advisable results for practicing engineers.