Minnesota’s new I-35W St. Anthony Falls Bridge
On Sunday, May 25, 2008, a crowd of community spectators estimated between 800 and 1,000 strong lined Minneapolis’s 10th Street Bridge to watch history being made. The first 200-ton precast concrete segment was lifted into position for the 504-foot main span superstructure of the new I-35W St. Anthony Falls Bridge. Without any public notice of the event, word had spread throughout this community that something remarkable was about to happen at the bridge site. The following day, Memorial Day, similar crowds were on hand, watching a bridge for the future of their city take shape, high above the Mississippi River. Minnesota Department of Transportation (Mn/DOT) was in the midst of delivering one of the highest visibility bridge projects in our nation’s history.
The first superstructure main span segment went into place less than eight months after Oct. 8, 2007, when Mn/DOT issued notice to proceed to the Flatiron/Manson Joint Venture, with FIGG (Figg Bridge Engineers, Inc.) as its engineer. The new bridge is a modern, concrete bridge designed to complement its surroundings. The design-build team, which incorporates Mn/DOT and the Federal Highway Administration (FHWA), knew that around-the-clock focus and fortitude were essential to the success of quickly replacing Minnesota’s major interstate crossing of the Mississippi River.
On Aug. 23, 2007, Mn/DOT issued its request for technical proposals to a shortlist of design-build teams. The selection of the winning team was based on providing the best value for the citizens of Minnesota. Measures based on quality (50 percent), aesthetics/visual quality (20 percent), enhancements (15 percent), and public outreach/involvement (15 percent) were graded by 27 individuals from six agencies to provide a fair, open, and uniform basis of evaluation. Mn/DOT’s formula for determining the best value was as follows:
After the technical proposals were thoroughly reviewed and ranked, Mn/DOT selected Flatiron/Manson Joint Venture, with FIGG as the engineer for the design of the project.
Contractually, the new bridge had to be delivered by Dec. 24, 2008—in less than 15 months. Access to the bridge site came on Oct. 15 with the Flatiron/Manson Joint Venture immediately mobilizing. On Oct. 24, a FIGG bridge design charette was held with 88 community participants, including business owners and residents, as well as representatives from cultural, arts, government, University of Minnesota, and other special interest groups. Fifty percent of participants had lived in the city for more than 20 years; all were committed to creating a bridge that would be in harmony with the beauty of the Mississippi River and St. Anthony Falls, while respecting the urban architecture of Minneapolis.
During the charette, participants voted in a consensus manner to select the pier shape, railing style, abutment wall treatments, aesthetic lighting, and finished color of the bridge. Discussions on opportunities to create gateway elements for the entrances to the bridge were included in the charette, with final decisions deferred to the project’s Visual Quality Advisory Team (VQAT). The VQAT consists of representatives from the city of Minneapolis, Mn/DOT, FHWA, the state historic preservation office, National Park Service, and the Friends of the Mississippi River. The VQAT met regularly for nearly eight months to review and discuss design details.
The final bridge design consists of two parallel structures, each with two concrete box girders. Ten lanes of traffic are accommodated in the design, with provisions for future mass transit. The 1,223 foot long bridge has four spans and reflects modern arch forms in the variable depth, parabolic curved superstructure and sculpted piers. The 504 foot main span superstructure ranges in depth from 25 feet at the main piers to 11 feet at mid-span. Each of the 70 foot tall concrete main piers was cast-in-place and is founded on multiple drilled shafts that are socketed into rock.
Figure 1: The bridge design comprises two parallel structures, each with two concrete box girders, accommodating 10 lanes of traffic with provisions for future mass transit.
High-strength, high-performance concrete is used throughout the bridge. Concrete mix designs were tailored for each project element by Cemstone, a Minnesota-based company that is providing the concrete for the new bridge. Self-consolidating concrete was used in the drilled shafts, while superstructure mixes were tailored to provide low permeability and high freeze/thaw resistance. The post-tensioned concrete bridge design features bi-directional prestressing of the deck for maximum sustainability. The service life of the bridge is expected to be more than 100 years.
A unique aspect of the design-build team’s proposal was to incorporate innovative sensor-based monitoring systems in the bridge to provide information that supports construction activities, records structural behavior throughout the structure life, and enhances bridge security. These sensors, combined with the Intelligent Transportation System (ITS) elements and sensors for the deck’s automated de-icing system, provide Mn/DOT with a "smart bridge" that will increase operational and maintenance efficiency. The future of bridge design and construction will benefit from the analysis of information gathered from approximately 250 sensors in the bridge.
Construction has progressed quickly, with multiple operations occurring simultaneously as Flatiron/Manson Joint Venture invested in multiple forms and equipment to compress the schedule. Crews have worked seven days a week and around the clock, despite bitter cold in January and severe weather this springtime. Measures have been taken to keep working, regardless of the weather, including moveable enclosures that were constructed to provide heated spaces for the long-line casting beds established on the southern end of the bridge site. The enclosures allowed concrete to be placed, despite winter days when the temperature stayed below 0° F. Eight long-line beds were constructed, each bed approximately 250 feet long. Each of the beds produced 15 segments to form one half of each of the four box girders in the 504-foot main span across the Mississippi River.
Concurrent with precasting activities, foundations were completed and the superstructure backspans over land were cast-in-place on falsework. All of the 120 main span precast concrete segments were produced by June 6; just days after the final concrete had been placed in the superstructure back spans. Precast segments are staged along the southern bank of the river, prepped and ready for installation in the main span cantilevers. The casting bed and roadway has been demolished and the area is being reconstructed to provide for more lanes of traffic and improve the overall bridge and roadway geometry.
On June 19, 2008, Mn/DOT’s commissioner, Tom Sorel, said, "The I-35W bridge project will be complete sometime between mid-September and mid-October, more than two months ahead of schedule; the project is already more than 70-percent complete. Restoring this vital river crossing earlier than planned will allow 140,000 motorists to return to their normal route." He added, "The early re-opening may also save as much as $40 million in travel costs to the public." A Mn/DOT study concluded that road-user costs resulting from the unavailability of the river crossing are approximately $400,000 per day.
Reaching out to educate and inform
The Flatiron/Manson Joint Venture, along with FIGG, has demonstrated a commitment to completing the project successfully, while reaching out to share project development and status with the public through many delivery channels. Weekly updates are made to the robust project website (http://projects.dot.state.mn.us/35wbridge/index.html). The site includes two web cams, construction images, profiles of those building the bridge, and more. Outreach includes community open houses; electronic kiosks displaying renderings and project information, and the popular Saturday Sidewalk Superintendent Tours.
Every Saturday morning at 11 a.m., the public is invited to participate in a tour led by one the project’s leaders. Despite bitter winter weather, the community has come out to hear about the bridge progress. On Saturday, June 14, with 35 precast concrete superstructure segments in place over the Mississippi River, more than 300 people attended the Sidewalk Superintendents Talk to ask questions, learn about bridge design and construction, and watch the bridge get closer to completion.
Another notable element of the public outreach was an educational program for school children that FIGG envisioned during the project’s development. During April and May 2008, nearly 2,000 students, teachers, and children of workers involved in the project participated in "Casting the Future." This educational program began as an outgrowth of the aesthetic feature design for the 2nd Street Bridge, a smaller bridge on the north end of the project that will carry I-35W over a local city street and provide pedestrian access. Students experienced hands-on learning by making concrete mosaic tiles that utilize recycled glass as an exposed aggregate. These tiles will be placed in the wall areas of the new 2nd Street Bridge.
Students, typically fifth graders, participated in a day-long field trip that included learning about the I-35W St. Anthony Falls Bridge design and construction; the roles of engineers, contractors, and construction trade workers; the basics of concrete; and the importance of using sustainable materials. Following a safety briefing, the students put on gloves, safety glasses, and aprons to protect their hands, eyes, and clothes. Representatives from Cemstone (concrete provider for the bridge) demonstrated the materials used to make concrete and then each student formed concrete in a 16-inch x 6-inch x 1-inch mold. Tiles formed by the previous day participants were then cleaned and signed by the students, reinforcing their contributions to the new bridge. Students also participated in quizzes, learning about the construction trades, how concrete cylinders are tested for strength, and enjoyed lunch before going to the bridge construction site for a tour from the 10th Avenue Bridge. Students each received a Sidewalk Superintendent’s pin, along with a miniature recycled glass tile and a "build a bridge kit" based on the I-35W St. Anthony Falls Bridge. Teachers received classroom Bridge Basics Kits, as developed by the National Building Museum with support from FIGG, that were further customized to the I-35W St. Anthony Falls Bridge.
Jon Chiglo is the I-35W project manager for Mn/DOT. Linda Figg is president/CEO of FIGG.