A composite rigid frame bridge replaces a certain portion of the concrete middle span of a bridge with a section of the steel girder. While the steel span improves the bending moment distribution of the rigid frame structure, it increases the stress level of certain cross-sections of the girder. There is little research reporting the effects of the addition of the steel span on the layout and structural design. This research studies the influence of the steel span on the structural behavior of the rigid frame bridge and conducts the structural optimization regarding the steel span ratio, curve order of the girder’s bottom line, and height-to-span ratio using the bending strain energy as the objective function. Finally, this study develops the computer program for structural analysis of composite rigid frame bridge structure using MATLAB, which can be used for advanced structural optimization.