Check out Scenic Express website; they offer a plate girder in plastic that is bendable to 24" diameter to serve as the sides of a faux plate girder bridge; i.e., make the curve you want with a plywood base then add details, including the Scenic Express sides. I don't know if that would work for you.
If you want to consider a straight Truss bridge, Plastruct offers a 30" kit for $49.95 msrp in O scale; part no. 3002.
Good luck with your project.
