Simply supported bridges consisting of five I-section concrete girders are analyzed using the finite element method. The main parameters of this study are: girder spacing (1.8 to 2.7 meters), span length (25 to 35 meters), skew angle (0° to 60°) and different arrangements of internal transverse diaphragms. In this analysis, 3-dimensional decks are modeled for considering interaction between the structural elements of the decks, and consequently the distribution factor of design moment for various arrangements of internal diaphragms are obtained. The results are compared and the advantages of each arrangement with respect to the others are derived. Results of reliable analysis based on the finite element method show that in right bridges, AASHTO distribution factors are conservative and in skew bridges, these factors are very conservative, such that, in skew bridges with internal diaphragms perpendicular to longitudinal girders, distribution factors of external and internal girders are 37% and 45% less than their equivalents in AASHTO, respectively. At the end, the simple relations are proposed to determine design moment distribution factors of both external and internal girders for the decks with internal diaphragms perpendicular to longitudinal girders.