Launch Slideshow

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Bridge from Tragedy

Bridge from Tragedy

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    The St. Anthony Falls Interstate 35W bridge is 76 feet wider than its predecessor and can accommodate the addition of light rail. Redundancy is designed into every element of the structure. Photo: Joe Nasvik

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    Twin City residents were invited to meet with project mangers like Minnesota DOT's Jon Chiglo (on the left with safety vest) every Saturday to learn about the reconstruction progress. Attendance ranged from four to 450 people; ultimately, 5,000 members of the public took advantage of the “sidewalk talks.” Photo: Joe Nasvik

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    Award-winning mathematicsMinnesota DOT's formula for determining the technical score of “best value” design-build proposals.

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    Flatiron/Manson cast the back-span sections of the bridge in-place and used precast segmental construction for the main spans, making it possible to work on all portions of the deck at the same time. They used the I-35W roadway south of the river as the site for the segmental precasting beds. Photo courtesy of Figg Engineering Group

Bidders had one month to research, design, and present proposals. In addition to being the only proposal using concrete rather than steel, the winning design:

  • Protects the bridge from collapse in the event of a 500-year storm event with 40-foot rock sockets drilled into bedrock. Even if 20 feet of limestone were to erode, the bridge will remain standing.
  • Removes piers from the river channel, minimizing the threat of scour at an adjacent bridge.
  • Fixes the design flaws of the approach ramps.
  • Routes stormwater runoff to treatment facilities at either end of the bridge rather than into the river.
  • Uses electronic sensors to monitor the bridge's performance.
  • Empowered the public to select pier shape, railing types, bridge color, and lighting through events like a full-day design charrette.

Despite the speed and scope of the project, Mn/DOT didn't alter specifications to facilitate project completion.

For example, it required the use of granite aggregates throughout. Under normal conditions, concrete mixes with demanding specifications are developed by creating trial mixes and testing them over time. But the deadline didn't allow for such luxuries.

To ensure consolidation around the reinforcement of the 8-foot-diameter drill shafts, the department permitted the use of self-consolidating concrete with a spread of 28 inches. An enhancement to DOT specifications, the department required test placements to ensure the concrete consolidated around the steel reinforcement properly. The requirement for strength was 5,000 psi within 56 days.

It was also the first time the department used #20 rebar; the Grade 75 straight bars are in the drilled shafts and help reduce steel congestion.

SAFETY THE ‘SMART' WAY

One of Flatiron/Manson's best-value offerings was an electronic “structural health” monitoring system of 323 sensors embedded in the superstructure and the substructure to provide insight into bridge conditions during and after construction.

“The firms providing the monitors said structural health systems are installed on older bridges,” says Figg Engineering Design Manager Alan Phipps. “So we're breaking new ground here.”

During construction, sensors monitored the temperature of mass concrete placements. Sensors embedded in the drilled shafts and piers provided real-time information about stresses and movements resulting from the loads imposed by the cantilevered segments before they were joined.

The Federal Highway Administration, in conjunction with the University of South Florida, placed the sensors. The University of Minnesota is collecting and analyzing data, working closely with Mn/DOT to track bridge conditions. Monitoring equipment includes:

Strain gauges. 75 in the substructure, located mainly in one of the piers and drill shafts. Of both “vibrating wire” and electrical resistance types, they provide insight into how substructure and superstructure forces interact. During construction, they transmitted data via cell phone. Also included are 146 strain gauges cast into the top and bottom of some box girder webs to measure strain in longitudinal, transverse, and vertical directions.