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The new I-35W bridge across the Mississippi.
I-35W St. Anthony Falls Bridge
Jon Chiglo, Minnesota Department of Transportation
On Thursday, August 2, 2007—less than 24 hours after the collapse of the I-35W bridge—a group of approximately 30 individuals from the City of Minneapolis, Federal Highway Administration, and the Minnesota Department of Transportation (Mn/DOT) met to discuss how to begin the rebuilding process. The collapse resulted in the death of 13 people and injured 145. It also severely impacted the public trust and confidence in the Department of Transportation's ability to provide a service dependent on safety to the citizens of Minnesota.
The discussion during the meeting primarily evolved around two delivery processes that Mn/DOT has already used successfully: the more traditional design-bid-build process and the design-build process. The pros and cons of each process and their anticipated times of delivery were discussed. Ultimately, the design-build process was selected in large part because of the speed of delivery, risk transfer, and the ability to allow more flexibility and innovation. Immediately after the decision was made to use this method, Mn/DOT established a team with past design-build experience to lead the charge and begin the process of rebuilding the I-35W bridge and to take the first steps in re-establishing public trust and confidence.
The I-35W project ultimately validated the use of design-build as the appropriate delivery process under these circumstances. The Flatiron-Manson Team, using FIGG Engineering as the lead designer, provided a design and construction approach with many innovations including those in structural design, concrete mix design, construction staging, and cold weather protection. This allowed the successful completion of a project that many in the bridge industry and the public doubted could ever be achieved.
The Mn/DOT design-build delivery process does not allow a “pure” design-build approach with complete flexibility. The design and construction criteria encompass an approach that enhances the ability of the contractor and the designer to take advantage of each team's strengths. Mn/DOT has learned through each of their seven previous design-build projects that allowing flexibility in design and construction enables the design-build teams to capitalize on innovative approaches to solving problems. Assigning risk to the party that can best manage that risk is also a key to success but it does not mean assigning all the risk to the design-build team. Mn/DOT accepts the risks in the areas that it can best manage.
The bridge consists of twin 1223-ft (373-m) long parallel structures that carry northbound and southbound traffic. Each structure consists of two variable depth box girders joined together through their top flanges to provide a 90-ft 4-in. (27.5-m) wide deck. Each box girder is supported on separate columns with pairs of columns sharing the same footing. The footings are supported on drilled shafts that are socketed into bedrock. Span lengths are 330, 504, 242, and 47 ft (101, 154, 74, and 14 m).
As part of this project, Mn/DOT encouraged innovation through establishing performance specifications associated with the concrete mixes. Each element of this project seemed to produce a different challenge to overcome when it came to the concrete. The substructure consisted of 7- and 8-ft (2.1- and 2.4-m) diameter drilled shafts, up to 95-ft (29-m) deep cast with self-consolidating concrete. Each pier footing was 13 ft (4.0 m) deep, 109 ft (33.2 m) long, and 34 ft (10.4 m) wide and contained over 1700 yd3 (1300 m3) of concrete. Those footings needed both mass concrete considerations and cold weather protection. The main span of the superstructure was built using the cantilever method with 120 precast segments match cast using the long-line method. The side spans were cast-in-place on falsework. The contract requirements associated with the full depth deck approach ensured long-term performance and durability through permeability and strength parameters.
100-year Service Life
The request for proposals required the contractor to submit a corrosion protection plan to assure a 100-year service life for the structural elements of the bridge. The plan was required to assess the effects of concrete permeability, corrosion thresholds, corrosion rates, impact of cracked concrete, and time to repair. Recommendations on the use of calcium nitrite, silica fume, sealers, membranes, reinforcement coatings, concrete cover, and other corrosion inhibitors were required.
The new I-35W bridge opened on September 18, 2008—only 11 months from notice-to-proceed. The performance of Flatiron-Manson, FIGG team working with Cemstone Concrete Products was exemplary in solving all of the concrete-related challenges during this project through innovation and a depth of experienced personnel second to none. The innovative approaches that were used to achieve this remarkable project will continue to be used to enhance future projects; thereby ensuring that the value of this project will be sustained.
A significant amount of information about this project is available at http://projects.dot.state.mn.us/35wbridge/index.html or contact the author at firstname.lastname@example.org. for further information.
HPC Bridge Views, Issue 52, Nov/Dec 2008