Concrete is poured inside to build each.
And, presto, soon there will be four new piers underneath the Washington Avenue bridge.
The piers are the added support the span over the Mississippi River will need to support the Central Corridor light rail trains when the line starts operating in 2014.
Here’s how the National Academies’ Transportation Research Board describes the project:
The structure is a unique double-decker bridge carrying an upper-level pedestrian deck with an enclosed walkway and a lower-level vehicular deck with two roadway lanes in each direction. The CCLRT Project proposes to replace one roadway lane in each direction to accommodate the light rail transit.
Read about how the Central Corridor folks explain the process here.
And last but certainly not least, here’s a Web page with bridge history and a batch of cool photos.