Nothing exemplifies humankind’s ability to tame the natural environment like a bridge, but the St. Croix Crossing between Minnesota and Wisconsin is awe-inspiring for another reason. It’s about a mile long, but affects seven miles of roads in two states and carries traffic over a federal protected waterway. To satisfy the requirements of 28 stakeholders, engineers proposed the nation’s first extradosed bridge: a combination concrete box girder (to shorten piers so they don’t detract from wooded bluffs on each side of the St. Croix River) and cable-stayed design (to reduce number of piers). The result is a gorgeous, graceful structure with piers that remind me of the eye of a needle, a shape that lets columns move 6.5 inches in either direction (thus accommodating hot upper-Midwest summers and brutally cold winters).
Minnesota DOT
Pier columns have an opening like the eye of a needle to mimic reeds along the river. The concrete "blades" are 4 feet thick and approximately 20 feet wide, and allow the columns to move 6.5 inches in direction to acommodate summer and winter temperature fluctuations.
Opportunities for this kind of innovation don’t happen every day, so they provide an excellent educational tool. You can earn professional development hours (PDHs) this fall in Cleveland at Public Works’ first Infrastructure Imperative conference. The St. Croix Crossing is one of three public projects we’ve chosen to see how teams streamlined environmental permitting among multiple jurisdictions and deployed new construction technologies to deliver the kind of project that impresses taxpayers. (Click here for more information.)
I’ll share tidbits about this and the other projects (transit-oriented development in Denver and a third we’re still debating) until the conference begins the evening of Nov. 13. In the meantime, be careful out there: The heat and humidity make for a dangerous work environment without proper hydration.
