Best way i can think of: find reference picture with side view of your bridge, or draw it on paper. Make texture out of it, best like 2048x2048 dimension. Apply this texture to one of box static meshes that are in unreal. Static mesh not CSG one!. Your picture should nicely fit to side of static mesh. Scale that static mesh to match size you want. Now you have reference picture ready.
Now create Addtive only brush for one section of bridge. Copy it and vertex edit to match curve on reference picture. then copy next segment, and so on. But above everything stick to grid, or you summon HOM daemon (i hope they killed it finally but nobody knows for sure).
This all would be much easier in 3d max:
make and uv unwarp one section, remove surfaces that will be inside bridge.
copy segments until you have half of bridge length, attach them all together, weld verts
and then just bend tapper and twist until you happy.