The free-standing twin-arch span will be a first in the state and the continent's longest such bridge.
Sleek twin arches, artfully lit at night, will grace the novel Mississippi River crossing now taking shape in Hastings. The arches will give Minnesota its first free-standing, above-deck arch bridge and will constitute the longest such span in North America, transportation officials said.
Last week, diesel hammers whammed test piles into the Mississippi bed to ensure that underlying bedrock will bear the 11,500-ton load expected on the single-span bridge.
The $120 million structure will ace out the George Washington Carver Bridge in Des Moines, Iowa, the current record-holder with a 278-foot arch, said Steve Kordosky, bridge project manager for the Minnesota Department of Transportation (MnDOT).
The longest single-span arch bridge in the Western Hemisphere opened this year in Nevada, but its arch is below the bridge deck. That $240 million span extends 1,060 feet across the Colorado River, just downstream of the Hoover Dam.
The Hastings arch segment will be built on five barges in the river next summer, Kordosky said. It will be floated into place, hopefully by year's end before the river freezes. He said the distinctive arched-rib style costs more than a traditional girder bridge but suits the city's nearby historic downtown and its history of iconic bridges. That includes the Spiral Bridge, built for horse-drawn traffic in 1895.
"We are excited in Hastings about the new bridge," said Mayor Paul Hicks. "We were looking for a bridge that stood out a little bit. We have had a history of bridges that were new and breathtaking."
The Spiral Bridge was demolished in 1952 when the current bridge, the state's first with a steel truss style, was built, Hicks said. The 58-year-old truss bridge is overdue for replacement, but improvements have been made to keep it safe until the new bridge is open, he said.
Like the Interstate 35W bridge that collapsed in 2007 in Minneapolis, the old Hastings bridge is fracture-critical, which means if one load-carrying part breaks, the whole bridge could fall.
In recent years, the city's history has been mired in rush-hour snarls from the two-lane bridge that can back up traffic on Hwy. 61 more than a dozen blocks into downtown. Hicks noted that the bridge is the only Mississippi River crossing between Red Wing and the Wakota Bridge, which connects South St. Paul and Newport.
The new four-lane bridge will ease traffic and shoppers into downtown, he said. He noted the bridge will split into separate spans for north and southbound lanes when it hits the south bank. That will allow daylight to filter between the spans onto a planned plaza. The bridge abutment wall will display stone mosaics being designed by artist David Craig, who did stone murals outside Target Field. His stone art is likely to include a Spiral Bridge portrayal, the mayor said.
The twin arch ribs will have cross sections that are four-sided trapezoid boxes -- picture a triangle with the top cut off -- that will be 9 feet wide at the deck and narrow to about 6 feet wide at the 94-foot apex, said MnDOT spokesman James Gillach. The trapezoid shape strengthens the arches to withstand crosswinds.
The completed bridge -- center arch section, north and south approaches -- will be 1,938 feet long. The deck will rest on 10 piers, each containing 11 to 21 pilings, Kordosky said. Soon testing devices will ignite explosives atop test pilings to simulate almost twice the 1,650-ton load each piling is designed to carry, bridge engineer Vincent Gastoni said.
Work began in September, and two-way traffic will remain open, with few exceptions, during the 33-month project. Access to local businesses, including two marinas on the north end, and residences should not be affected. Half of the new bridge is scheduled to be open to two-lane traffic in the summer of 2012, with all four lanes to open by June 2013.
Jim Adams • 952-707-9996