LSI - Dead River Bridge

I sometimes see these on similar viaducts and individual girder spans. I’ve heard them called “fishbellied” before Here’s an example on a short 50’ span:

http://bridgehunter.com/wi/eau-claire/bh84767/

Not yet sure on the history on that one, but it looks like it came from a larger viaduct. All spans on this branch line were moved from other places. Not sure why it would be on such a short span, as railroads routinely built even larger girders that were uniform depth. More examples:

http://bridgehunter.com/il/carroll/bh66537/

http://bridgehunter.com/mn/carver/bh53742/

Unusual to see this design on a deck girder but it's not a normal bridge.

Given that every span is like that, I doubt it's due to reuse.

If the limiting factor is bending, adding depth to the section may be the most efficient means of adding strength (and at midspan, the limiting factor is bending). Think of how many trusses have a deeper section midspan.

Also interesting is the girders with a shape like that of a turntable.

That sure is an unusual design.

I was thinking that it's odd that they don't have an upright above the center of the arch, rather than the deeper girder for the longer span, but I suppose that would significantly increase the loads in the arch.

Well that's nifty!