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Michael, Two quick things ... If these are modern highway bridges then they would be constructed of concrete slab spans. About an eighteen to twenty inch thick slab for the 20 foot bridge, and about 36 inches P/S slab for the forty foot bridge. The forty foot bridge could be a nine inch slab on 30 inch deep rolled beams at about eight or nine foot centers, but in most places I think a P/S slab would be cheaper. BTW, most modern highway bridges have concrete parapet on the sides. In HO scale, I think RIX makes the stuff in a plastic section of 20 or so scale feet. Second thing, highway bridges this short are supported on a rubbery compound called neoprene instead of expensive steel bearings. On a slab span, you wouldn't see the neoprene bearings. If you built the slab on beam bridge described above then you would have small black rectangular pads under each beam end, something like 8 x 12 inches and two inches thick. Just cut a piece of plastic and paint it black and you have a neoprene bearing. Normally you would see a sole plate 9 x 16 inches and one inch thick over a neoprene bearing this size. Another easy piece of plastic. Maybe I'll get my own model bridge forum someday. Later and Good Luck. - Ed
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