A 1928 era bridge crossed the Mississippi at Alton, just north of St. Louis, but the narrow twisty 2 lane bridge was inadequate. It also had a number of short spans that required piers in the middle of the shipping lanes, which became an even bigger problem when the nearby lock and dam was expanded.
The Clark Superbridge is an impressively large cable stay bridge, featuring two towers that are each 250 feet tall, and extend up to 150 feet below the water level. These stats have lead folks to refer to this structure as the Superbridge. In fact, PBS made a TV mini-series about the building of this bridge called Super Bridge.
The Clark Superbridge is unique in two respects. First, it has cables that connect to both sides of the deck, but are gathered at one point on top of the bridge. The cables form two planes that intersect. On most cable stay bridges with two planes of cables, the towers have two different points where the cables are mounted, and the cable planes remain parallel. Put another way, most cable stay bridges with cables on both sides of the deck have towers that from large 'H' figures, whereas the cables form an inverted 'V' on the Clark Superbridge. Second, most cable stay bridges have the cables mounted to the towers. On the Clark Superbridge, the top of the tower is a saddle, and the cables go up and over rather than joining to the tower. While this is an innovative design, the saddle design has proven to have a serious problem, and is not likely to be attempted again in the near future.
