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Cable Stay Bridge Marks New Era in Minneapolis

Fri June 25, 2010 - Midwest Edition
Dick Rohland

A split second blast followed by bursts of light sparked the end of a transportation era last June for residents and commuters of the north side of Minneapolis.

At the same time, it marked the beginning of a new era when demolition crews dropped the steel trusses of the 104-year-old Lowry Avenue Bridge into the Mississippi River to clear the way for a new and much different looking bridge to serve the city’s north side for the next 100 years.

A neighborhood icon owned by Hennepin County, the Lowry Avenue Bridge was one of three bridges that connect the north side of Minneapolis to its downtown business and entertainment district along Lowry Avenue, a heavily used major arterial in the city.

Even though regular bridge users and neighborhood residents will eventually live with the daily frustrations of a three-and-a-half year detour, they will witness another, and quite different, bridge rise from the river and one that is certain to be an icon for future generations of commuters.

After a year of consulting, planning and open community and legislative participation, an $80 million arch, cable stay design was chosen to replace the original steel truss structure.

Hennepin County awarded the contract to Lunda Construction from Black River Falls, Wis., with a local office in Rosemount, Minn. After Bolander and Sons from St. Paul removed the old structure a year ago, Lunda crews began mobilizing last November to begin work on the new bridge.

In stark contrast to the angularity of the original steel truss bridge and most notable visual aspect of the new bridge will be the graceful, parabolic curves of the bridge’s twin arches. They will sweep into each other at their 90 ft. (27 m) peak and be stabilized by a set of arch transverse braces near the top. Two other braces will each be anchored into the arches halfway down from each side of the peak.

Fabricated by PDM Steel, nine arch segments with a cross section of 4 ft. (1.2 m) by 5 ft. (1.5 m) will form each arch and span 362 ft. (110 m) across the center span of the bridge.

Complementing the curves of the arches and providing the support for the 450 ft. (137 m) long bridge deck between the two river piers are 36 sets of multi-strand stay cables hanging vertically and anchoring the deck to the arches above.

Along with the visual aesthetics of the twin arches, the bridge will be lit by LED lighting.

Functionally, the bridge will carry two, 13 ft. (4 m) lanes; two, 15 ft. (4.5 m) shoulders and a 12 ft. (3.6 m) sidewalk on each side of the bridge on the approach decks leading up to the center river span where each sidewalk will widen to over 19 ft. (5.7 m).

A pair of overlooks will bow out from each side of the river piers and deicing equipment will be imbedded into the bridge deck; remotely monitored and controlled from the county’s public works headquarters.

Other quantities include 2,600,000 lbs. (1,180,000 kg) of reinforcement steel and 17,000 cu. yd. (13,000 cu m) of concrete.

The construction of the bridge also will be a unique opportunity for county staff and bridge workers.

“I’m just thrilled to be here,” said Ned Miller, a 17-year county veteran and chief inspector for the county on the construction site. “Every day, I leave work with a sense of accomplishment. I would not want to be anywhere else. This is probably the biggest project I’ll ever see in my career.”

It also is a first for Dale Even, project engineer for Lunda Construction, who succinctly wrapped up his thoughts on the construction by saying “it’s a challenge and an opportunity to learn new things.”

Opened in 1905, the first Lowry Avenue Bridge was 889 ft. (271 m) long and 57 ft. (17.3 m) wide. It was notable because it carried traffic on a steel grid deck that drained directly through the mesh. It was not until 2003 that crews replaced the steel grid deck with a concrete one.

Despite heavy duty modifications to the original bridge, including replacing five of its spans and the steel deck with a concrete one over the years, the original piers remained strong and stable. They brought the bridge, which originally carried horse and buggy traffic, into the 20th century with the rapid advance of the automobile.

However, it was the bridge’s third pier that cast its final fate. During a 2004 repainting project, inspectors discovered an 11 in. (28 cm) displacement of the bearings, according to county documents. Hennepin County contracted with Wiss, Janey, Elstner & Associates (WJE) to investigate and report on the cause and extent of the damage.

According to county documents, the consultants found evidence that the pier’s stability slowly eroded for years, perhaps as many as 50, through creep deflections caused by sustained lateral pressure at its foundation. Held in check over time by the bearing assemblies, the continued forces eventually overpowered their peak strength which allowed for the unrestrained and rapid movement of the pier.

Because WJE structural engineers could not predict the magnitude of future displacements, Hennepin County closed the bridge in April 2008 after it had safely carried millions of vehicles across the river for over a century.

A half dozen cranes now tower above the river banks including two American 100 ton (102 t) and 50 ton (51 t) lattice boom crawlers, two Terex 210 ton (213 t) and 165 ton (168 t) lattice boom crawlers, a Manitowoc 2250, 300 ton (305 t) lattice boom crawler and a Link Belt RTC 8040 rough terrain crane.

Lunda also is using APE pile driving and vibratory hammer equipment for the piles and sheeting.

Bridge workers will need plenty of heavy duty equipment just to erect a skeleton of temporary support towers and falsework to build the new bridge.

According to plan sketches supplied by Even, workers will build 20 support piers sitting on hundreds of temporary piles to support the 211 ft. (64 m) post tensioned box girder end spans coming out from each of the two river piers.

Once the end spans are in, a pair of mammoth pre-cast edge box girders will be hoisted into place for the center span. Each girder is 150 ft (96 m) long, weighing 1,000,000 lbs. (453,592 kg), and measuring 16 ft. (5 m) in width and 8 ft. (2.4 m) in height.

“We will hoist these girders with a pair of hoisting beams anchored to the structure we have already built. We’ll then cast closures at each end of the girders,” said Even.

From that point, crews will place a temporary deck platform above the end spans and across the girders for the center span. The temporary deck will then support temporary towers and falsework to build the arch.

Construction on the new bridge began last February in the dead of a cold, Minnesota winter. When crews rolled in with their cranes to a staging area 2,000 ft. (610 m) upstream from the construction site, they faced an ice-covered river. Not a problem for the Lunda crews.

“There was about 15 in. of ice in spots out on the river,” said Even. “We had to use a backhoe on a barge to break up the ice and float our cranes down here. We basically had to open a channel through the ice.”

Despite the fact that Lunda crews had to come up with a makeshift ice breaker just to get their cranes down to the bridge site, construction has otherwise been routine and on schedule, Even remarked.

After just five months of construction a lot has been accomplished even though much of it is hidden to the eye. Crews drove hundreds of 150 ft. (46 m) piles for the support of the three piers, Even said.

“The cofferdams have been installed for piers 5 and 6 and the foundations are in for pier 4 that sits on the west bank of the river,” said Even. “The two river piers are very unique because they are battered [angled] in two different directions.”

This is necessary, he explained, because they are angled upward to bear the forces of the twin arches and angled inward because the arches lean into each other at their peaks.

Bridge crews will work non-stop through this construction season and the next including the winters and expect to complete the new bridge in the fall 2011.

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