odd bridge

[Sumpter250]

well went to the FT. Laramie historical site and found that it is a Bowstring Truss not a Arch type and there are many still standing today several still in use as a road bridge most as foot traffic or closed to all traffic . the top pices are for wind braces and some like the FT. Laramie one are elevated for clearance the longest one still standing from what i can find is 147 ft span.

I looked up "Bowstring Truss", and the difference between it and an arch bridge, is in the way the forces are resolved. An Arch distributes the force down, and out, at the ends of the arch. A Bowstring Truss distributes the forces straight down, and in tension along the "string". The Bowstring Truss doesn't require the massive abutments that an arch needs.The bow "string", however, is under no small amount of tension, which would limit the size, and the weight that could be carried.
I just might have to use several of these to provide a walkway out to the bridge tender's shack for the rolling lift bridge on the new modules. The total span would be 159'-6" ( three 53' bowstrings ), and would sit about 8' above mean high water ( it would parallel a through girder rail bridge that is also 8' above mean high water ). It would be something you don't see modeled very often, and I think, would look quite interesting, while not blocking the passage of small craft.