Has anyone noticed that bridges running over major waterways have curved girders?
Is this common in your area?
https://goo.gl/maps/AARoPXkxYXs (https://goo.gl/maps/AARoPXkxYXs)
They exist over highways as well: https://goo.gl/maps/GmQ1R2dmPkr
I think it has to do with the length of the girder; not anything to do with what's underneath it.
Structurally accounts for higher moment at the piers/bents. This is one option of many of doing a bridge as it uses less steel than a constsnt-depth superstructure.