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A cable-stay bridge comes to Minnesota at last


Midtown Greenway Bridge
Photo courtesy of Hennepin County Public Affairs
The Midtown Greenway bridge needed a cable-stay design because no piers could be built on Hiawatha Avenue.

 

Last week’s opening of the Midtown Greenway bike and pedestrian bridge over Hiawatha Avenue marks several firsts. The curving bridge over the busy highway completed the 5.5-mile greenway’s path from the Minneapolis lakes to the Mississippi River, a longtime dream of the greenway’s planners. And the graceful 2,200-foot-long structure became Minnesota’s first cable-stay bridge.

What’s the big deal? In a cable-stay bridge the deck is held up from above rather than below so the engineering becomes the aesthetic. And a spectacular look it is too, with nine pairs of steel cables flaring out from the 115-foot-high mast on the west flank of the bridge. URS of Minneapolis developed the $5.1 million design for Hennepin County.

It made me think about some other cable-stay designs that never came to be.

In 1993 New York artist Jamie Carpenter designed a V-mast cable-stay bridge for the Wabasha Street crossing over the Mississippi. It was touted as a St. Paul icon, the future Golden Gate of the Midwest. But its price tag of $41 million proved too steep so a much more modest $20 million concrete box-girder bridge was built.

Visions of grand design dashed
After the I-35W bridge collapsed in August, images of cable-stay bridges again appeared, though mostly in people’s heads. Many design-savvy folks, including Tom Fisher, dean of the University of Minnesota’s College of Design, envisioned a bridge design with an awe-inspiring structure above the deck, rather than the stripped-down concrete structure by Figg Bridge Design that the Minnesota Department of Transportation selected.

Minneapolis real estate consultant Peter Kitchak even solicited the interest of Spanish architect and engineer Santiago Calatrava, world-renowned for his soaring cable-stay structures. With only a few weeks before the selection deadline, Calatrava’s involvement seemed like a long shot, and MnDOT’s I-35W bridge project manager Jon Chiglo made it clear that Calatrava could be considered only if he joined one of the four teams already shortlisted. Surprisingly enough, he did. Calatrava submitted an alternative design for the Walsh/American Bridge team about a week before the selection deadline.

“We rejected it because it was a non-redundant design,” Chiglo explained the day the design by the team of Flatiron/Manson/Figg was unveiled. While the other rejected designs were made public, Calatrava’s was not. Since it was not reviewed as one of the submittals, the state retains no rights to it.

A good fit for Midtown Greenway
Chances are, without extenuating circumstances, the Midtown Greenway Bridge wouldn’t have been cable-stay either. (The structural technique does cost 2.5 to three times more, noted URS senior engineer Greg Brown.) But in this case it fit the job. No piers could be built on Hiawatha Avenue, so it was necessary to have a 220-foot clear span.

The bridge had to be threaded above the Hiawatha light rail line and under high-power transmission lines, so the thin deck of a cable-stay design was better. Community residents liked the transparency compared to the boxy Hiawatha Avenue Bridge over Lake Street. And the design allowed most of the construction to be done above the bridge rather than below—a definite advantage when a seven-lane highway and a light rail line are below.

The completion of the bridge and its beauty testify to the strength of the biking community, which turned out in full force (and cold-weather gear) for last Thursday’s opening, and to the commitment of Hennepin County Commissioner Peter McLaughlin.

“When we first envisioned this crossing for the greenway, we figured it would be a button to push and stop the traffic,” said cyclist and community activist George Puzak, whom McLaughlin called the “godfather of the Greenway.”

Instead, it’s a piece of infrastructure to love.

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