Mutch, A tied arch type frame might be the way to go. An inclined member starting at the floor and slanting in helps support the roof simmilar to the frames in Mr. Hume's photo link to the Mass. project. Typically a steel shoe accepts the frame at the floor and is connected to rebar that runs in the floor slab to the shoe on the opposite side. The inclined members do eat into the useable space, but hey, you will have a 40' span!
Chris