Launch Slideshow

Image

Bridge from Tragedy

Bridge from Tragedy

  • Image

    http://www.pwmag.com/Images/tmp685%2Etmp_tcm111-1337801.jpg?width=300

    true

    Image

    300

    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

  • Image

    http://www.pwmag.com/Images/tmp686%2Etmp_tcm111-1337805.jpg?width=250

    true

    Image

    250

    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

  • Image

    http://www.pwmag.com/Images/tmp688%2Etmp_tcm111-1337811.jpg?width=300

    true

    Image

    300

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

  • Image

    http://www.pwmag.com/Images/tmp687%2Etmp_tcm111-1337808.jpg?width=300

    true

    Image

    300

    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

Temperature sensors. 48 located in box girders near the middle of each span to correlate temperature differences.

Accelerometers. 26 located midspan in each box girder to track traffic loads. The University of Minnesota placed 14 additional accelerometers in one box girder to get more data about the forces within a segment.

Fiber-optic cable sensors. A type of strain gauge, they're mounted in the main span of the same box girder as the accelerometers. The cables are 12 to 15 feet long and provide data about bridge curvature and deformation.

Corrosion-potential sensors. Four attached to sacrificial steel bars near the surface of the deck, which has a 2-inch wear surface. Monitoring the depth of chloride penetration will help Mn/DOT determine when to resurface.

Linear potentiometers. Used to measure the distance between two points; in this case, at the bearings over the piers and at the expansion joints. The bridge is designed to move at those locations in response to temperature variations, concrete creep, and shrinkage. Data will be used to compare expected to actual movement.

PROTECTING PUBLIC INVESTMENT

In addition to structural sensors, the bridge features “intelligent” traffic-monitoring systems. Video cameras monitor traffic on the approaches and on the bridge. Sensors embedded in the deck provide data about traffic volume and speed.

An automated anti-icing system is built into the structure. Sensors will monitor ambient weather conditions and predict when conditions favor the formation of black ice. A computer then calls for de-icing chemicals to be pumped onto the deck through small metal disks embedded in the deck 25 feet apart.

Security monitors will sound alarms when there is unauthorized access to the bridge interior.

Looking back, Chiglo is particularly pleased that cost growth was 1.5% compared to 7% for design-bid-build and 2% to 3% for design-build. During one construction tour, the first question a constituent asked was, “How long will it take this one to fall down?”

Chiglo says the answer was easy: It won't.

— Nasvik is senior editor of CONCRETE CONSTRUCTION, a sister magazine of PUBLIC WORKS.

The St. Anthony Falls Interstate 35W bridge

Owner: Minnesota DOT

General contractor: A joint venture of Flatiron Construction Corp., Longmont, Colo.; and Manson Construction Co., Seattle

Design and engineering: Figg Engineering Group, Tallahassee, Fla.

Length: 1,223 feet

Width: 189 feet (vs. 113 feet on for previous bridge), 10 lanes

Web Extra

For more information about the new I-35W bridge, visit the “article links” page under “resources” at www.pwmag.com.