GRANITE FALLS -- It has withstood major floods and more than 70 years of the river's relentless flow with no more than the smallest of tilts to show for all it has endured.
But the tilts -- measured as 9/32 of an inch per foot to the north on one pier and as little as 3/32 of an inch per foot to the north and 5/16 inch per foot to the west on the other pier -- may be enough to bring down the Granite Falls pedestrian bridge that many consider the community's icon.
"It is an icon of the strength of the community,'' said Terri Dinesen, a member of the Granite Falls Historical Society. She said the bridge's ability to withstand floodwaters in 1997 and in 2000 served as a rallying point for the beleaguered community.
The local historical group is encouraging the Granite Falls City Council to look for ways to repair and preserve rather than replace the bridge, which was originally constructed in 1935.
City officials are mulling their options. City Manager Bill Lavin said the issue has come up as the city seeks transportation aid monies to repair the bridge so that it could also serve as a link in the planned Minnesota River Valley recreational trail.
ADVERTISEMENT
The Minnesota Department of Transportation has expressed concerns about investing in the bridge when its structural integrity can be called into question, he said.
While there is no apparent danger posed by the small tilts that have been measured in the bridge's two piers, they raise the question of what could happen in the future, according to Tim Moe, an engineer with Widseth, Smith, Nolting and Associates of Alexandria. "The question is what will happen in the next 70 years,'' he said.
The U.S. Army Corps of Engineers also expressed concerns about the bridge's structural integrity during the 1997 flood: "As debris caught on the upstream face of the bridge, the bridge visibly twisted from the load, (and) the tower tilted. There was a failed tension cable and a torn steel support member of the downstream end of the bridge,'' the Corps reported.
Repairs were subsequently made to the bridge. It remains a much traveled pedestrian link connecting the east and west sides of the community.
Despite the tremendous forces the bridge was up against in both the 1997 and 2000 floods, it appears neither is responsible for the tilts measured in the piers. The tilts were first measured in the two piers in 1995, and measurements made after the 1997 and 2000 floods showed no changes whatsoever. One of the piers is sunk into bedrock and the other is in pilings, according to Moe.
It is impossible to know what caused the tilts first measured in 1995, but the most likely explanation is a frazil ice event that occurred in December 1985, according to the engineer. The unusual ice -- crystals that form in a flowing river -- poured over the Granite Falls dam and jammed up in the downtown river channel until it lifted the east end of the bridge by a number of feet, he said.
Another possibility for the problems is the bridge's alignment across the river. It is slightly skewed to the flow, said Moe. He said the bridge was built parallel to a wooden pedestrian bridge it replaced and does not face the river's flow directly as bridges should.
The 1935 bridge is among only a handful of suspension bridges in Minnesota still seeing service. Its original plans and the steel cables that hold its 285-foot span across the Minnesota River in downtown Granite Falls were developed by the John A. Roebling and Sons Company. John Roebling (1806-1869) is known as the builder of the Brooklyn Bridge and inventor of the twisted steel cable that holds the loads of suspension bridges.
ADVERTISEMENT
Those facts along with the bridge's age are attributes that may merit a historic designation for the bridge, according to Dinesen. Until now, no such designation has been sought.
The city did not apply for state funding to repair the bridge under a special historic preservation category, according to Lavin, the city manager. That is now an option the council may consider, he said.
Lavin said the city's initial state aid application was based on the bridge's functional use for the planned trail. Along with the structural concerns, MnDOT expressed concerns about the bridge being too narrow to serve as a bicycle trail link, said Lavin.
The bridge deck offers a clear width of 7.64 feet, according to Moe. A width of 10 feet is the minimum usually required for bicycle trails, and a 12-foot width is preferred, he said. The towers on the bridge also place lower-than-desired height restrictions for trail use, he added.
But more than anything else, the economics of the situation may determine the fate of the bridge. An engineering analysis indicates it would cost $419,380 to rehabilitate the bridge. "It's a lot of money to put into a structure that has questions about its structural integrity,'' said Moe.
It would cost an estimated $547,450 to replace the bridge with a stronger truss bridge.
Lavin said MnDOT state aid requirements do not support repairing structures if that cost exceeds 60 percent of the cost for a new structure.
Dinesen said that the estimated costs for rehabilitating the bridge may be too high. She noted that one of the larger expenses in the estimate is for sandblasting and capturing the lead paint that now covers the bridge. It may be possible instead to cover the lead-based paint at far less cost, she said.
ADVERTISEMENT
No matter the cost, she said it is hard for her to imagine that residents of the community would be willing to see the bridge removed. "Everyone has feelings about it,'' she said. "Nobody wants to see that bridge come down.''
The city has time to consider its options, and there are many. Moe said one option for the city is to replace the bridge but incorporate its towers and other features in a new bridge. The towers could become the "portals'' to enter the bridge at either end, he said.
The city's initial aid application sought funding for 2009 construction at the earliest. Lavin said it is probably more realistic to anticipate either replacement or rehabilitation of the bridge to occur in 2011 or 2012, if a transportation aid application is approved in the next year or two.
