This architectural information on the arch might help you. According to the article, "Each leg is an equilateral triangle with sides 54 ft. long at ground level, tapering to 17 ft. at the top." Keeping that in mind, you could simply draw an equilateral triangle in the top view then duplicate, translate, and rotate. Just keep duplicating translating and rotating triangles to form one side of the arch. You'll also want to scale the triangles as you get to the apex of the arch. Next, xoft the all the triangles. Finally, mirror the loft and attach both sides. Should work in theory.