On Aug. 1, 2007, a steel deck truss bridge carrying motorists in and out of Minneapolis collapsed into the Mississippi River during the evening rush hour. An investigation revealed that 16 gusset plates, which connected steel beams in the center span to the load-bearing columns, were too thin to function properly. As a result, they fractured and caused the bridge to collapse. Within hours, officials vowed to rebuild the bridge and to restore public confidence by incorporating a variety of advanced safety features. The result was the St. Anthony Falls Bridge, which opened in September 2008.
This box girder bridge truly is a marvel of modern engineering. It's made with high-performance concrete, which provides great strength and inhibits corrosion. More than 15 million pounds (6.8 million kilograms) of rebar -- all of it coated with epoxy to prevent the steel from getting weak and brittle -- reinforce the concrete [source: McCarthy]. At the same time, 323 sensors embedded in the bridge substrate constantly monitor the integrity of the structure, collecting data that can be analyzed to determine unusual stress points or other trouble spots. While it forms a solid foundation, the concrete is also cleaning the air. That's because it's made with cement containing TX Active, an agent that, in the presence of light, breaks down air pollutants, including carbon monoxide, nitrogen oxide and benzene.
High-tech materials such as TX Active are becoming commonplace in modern bridges. One such bridge is located to the east of Minneapolis, in Fond du Lac, Wis.