Abstract: The emphasis of this paper is placed on the description of a series of calculating analyses of the extreme internal forces on the members of a prestressed concrete cantilever arched-truss bridge under live loads. The calculations are made by adopting the finite alement method by plane beam elements. We apply the dynamic programming to decide the use of the truck loading and carry out five different bending stiffness ratios and area ratios of upper chord's section to lower chord's section, thus the magnitude range of these ratios is found.