Each of these bridges were built in a cookie cutter manner, with the basic Pennsylvania sytle through truss segment being the main building block. These bridges had between 3 and 5 of these truss segments. These state project bridges operated successfully until the 1950s. The major dam projects meant that all five of these bridges were to be flooded out as the Oahe, Big Bend, Fort Randal, and Gavins Point Dams were closed off and started to impound water.
In the case of the US-212 Bridge at Forest City, adding the depth of the water and the clearance for navigation resulted in a pier height of up to 190 feet tall. While the Wheeler Bridge was successfully reused, the required number of short trusses needed at Forest City would have resulted in a large number of very expensive piers needing to be built. It was determined that reusing the bridge was not practical. Rather, a cantalivered truss would be needed to span longer distances. The resulting bridge has 7 main channel spans. There are 3 sets of large trusses covering one span and a fourth of each adjacent span. A smaller Pennsylvania style through truss was used to connect these adjacent pairs of large trusses, plus one of these smaller through trusses was used at each end of the truss structure.
The US-212 Bridge was built with the help of the US Army Corps of Engineers. It opened in 1958. The Oahe Dam was closed off in 1961, and the resivour was full for the first time in 1962. The resulting structure is a very big metal monster. It is one of the largest inland bridges in the US. It was built relatively inexpensively, which is surprising when you compare it to the large bridges on the coasts. But those bridges are mostly freeway and multi-lane structures and many contain mass transit lines, whereas the Forest City Bridge is a much lighter two lane structure.
