Wednesday, September 13, 2017

Road Bridges over St. Croix River near Stillwater, MN

Satellite
(Old Bridge Hunter, no New Bridge Hunter, Old 3D Satellite, New Satellite (marked, not built), Old John Weeks, New John Weeks) The only thing that is currently on the satellite image for the new bridge is a couple of barges for building the piers for the towers.

The truss bridge was built in 1931 and rehabilitated in 1973. The replacement one-mile-long, bluff-to-bluff bridge started in 2013 was scheduled to be done in Fall 2016, but it was almost a year late (Aug 2, 2017). The new bridge does uses post-tensioning with almost 2,000 miles of strands segment strands. [video] (All of the videos are below in the order that they should be watched.)

John Weeks posted some bridge pictures with extensive comments on the photos.
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The old Stillwater Lift Bridge over the Saint Croix River. This bridge was just recently closed to highway traffic and it will reopen soon as a dedicated pedestrian and bicycle bridge. It will be part of a trail loop that goes over both the new and old bridge that is expected become a regional tourist attraction.
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The new Saint Croix Crossing bridge. A cell phone photo doesn't do it justice. This is the first and only bridge of its type in North America, an extradosed structure. It uses 5 sets of short towers to avoid spoiling the view from the river, whereas a more conventional cable stayed bridge would have had a pair of 500 foot towers that would have been totally out of character along a National Wild and Scenic River. — at The New Saint Croix River Crossing.

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Approaching the new Saint Croix Crossing bridge on the ramp leading up to the pedestrian / bicycle deck.

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A view of the bridge deck as seen from one of the bump-out observation decks. What makes this bridge type unique is the angle of the suspension cables. Having the cables go directly from the towers to the bridge deck is becoming an increasingly popular design (where there is no catenary cable strung between the towers), a type known as a cable stayed bridge. However, on this bridge, the cables are at a shallow angle where more of their force is pulling towards the bridge tower than is lifting the bridge deck. The bridge stays up not because the cables are holding up the deck, but because the cables are pulling the deck segments towards the towers making them a really strong beam. This is known as an extradosed design. I don't know where the word comes from, but it is the first of its type in North America.

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[I include this photo as a bonus.]
MNDOT from stcroixcrossing/gallery

(source of videos: bottom of stcroixcrossing)
If this part of the river had a rapids, then multiple, short towers would be relatively cheap because the bedrock would be close to the surface. But this video indicates the towers are rather expensive because the pier caissons go down as far as 140 feet.
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