Editors Note: On Saturday morning April 26, 2008, 50 people from the Twin Cities area met the project managers from Mn/DOT and Flatiron/Manson to hear about reconstruction progress concerning the I-35W bridge. It was 30? F, snowing, and the wind was coming down the river channel at about 20 mph. As one aging resident put it, "We are Minnesotans. We're used to this" As many as 450 people show up for these weekly Saturday construction tours and as few as four. For the public and everyone involved in the construction, the rebuilding of the bridge is a very emotional issue.

On Aug. 1, 2007, news around the world carried the story of the collapse of the I-35W structural steel bridge crossing over the Mississippi River in Minneapolis, resulting in the deaths of 13 people and 145 injuries. It shocked the nation and caused awareness of the sad, deteriorating state of our nation's infrastructure. The rebuilding of the bridge became an immediate priority because the freeway is a vital cross-town link for the Twin City area causing expenses to citizens totaling more than $440,000 per day in additional user impacts. Shortly after the collapse, the Minnesota Department of Transportation (Mn/DOT) invited construction companies to submit "Best Value Design/Build" proposals for the reconstruction. Each of the four bidders, aligned with an engineering firm, presented their proposals. A panel of experts convened by Mn/DOT evaluated each proposal on the basis of price, time to construct, design, quality, safety, and the technical content of the proposal. On Oct. 8, Mn/DOT awarded the contract to the joint venture of Flatiron Constructors, Longmont, Colo., and Manson Construction, Seattle--with FIGG Engineering Group (FIGG), Tallahassee, doing the design work and being the engineer of record. Construction began on Oct. 15 and on Nov. 1, a test shaft was drilled. On Oct. 24, FIGG held a full day charrette to engage the public in the design process.

The winning proposal was the only one submitted for a structural concrete bridge. Flatiron's project manager, Peter Sanderson says they decided to use structural concrete because they would stand a better chance of completing the bridge on time and have better control of their very tight schedule.

The best value design/build construction approach

Terry Ward, Mn/DOT's deputy project manager, says there were three bid sections for this project: price, time, and "Best Value" In the Best Value section, the important areas included safety, quality, public relations (including public involvement in the design process), aesthetics, and enhancements. The concept of Best Value Design/Build is at the heart of the I-35W bridge project and Jon Chiglo, the project manager for Mn/DOT, says it's their seventh Best Value project. He likes this process for the following reasons:

  • Safety becomes a high priority.
  • There is a high emphasis on quality.
  • The design approach is taken into account in the bid process.
  • Design/build requires enhanced communication to be successful--more so than less aggressive projects.
  • The contractor has more control over the schedule.
  • Mn/DOT has the opportunity to evaluate the experience of the team and their approach as part of the selection process.
  • To achieve success, an enhanced partnering process is required.

Chiglo says Mn/DOT gives away some of their control on Best Value Design/Build projects, giving considerable latitude and flexibility to the bidder. In the case of the I-35W bridge, they provided the contractor with geometric layout, environmental requirements, drainage requirements, a deadline for completion of Dec. 24, 2008, and invited innovation. They also defined six sub-standard roadway geometric design elements to the original I-35W freeway approaches to the bridge in the original "Request for Proposals," inviting (but not requiring) bidders to eliminate them as part of the project. Mn/DOT also requires public involvement in decisions concerning the design of their bridges so contractors must provide details in the bids about how the public will be involved.

For the contractor, deciding to bid a Best Value Design/Build project imposes additional responsibilities. Each bidder was required to provide their own design for the bridge, so they approached bridge design/engineering firms to work with them. Because they are responsible for the design, they aren't entitled to change orders based on quantities or design conflicts. Ward says the upfront costs of Best Value contracts are often a little higher but over the life of a contract there is minimal cost growth due to change orders and the final numbers turn out to be close to other forms of bidding. Also schedule benefits can be quite significant.

The four entities who submitted bids had one month to do research, complete designs, and present their proposals to the review panel, which received three proposals for structural steel bridges and one for structural concrete.

The Flatiron/Manson best value proposal

Having worked on other projects with FIGG, Flatiron/Manson involved them as their design/engineering partner. FIGG is known for their innovative work with segmental concrete bridge designs so it was clear that structural concrete would be the chosen material. The venture partnership believed that concrete construction would be faster and they would have better control of their schedule. Key elements of their proposal included:

  • Drilled shafts with 40-foot rock sockets into the bedrock. In the event of a 500-year storm, it's expected that 20 feet of stone could be eroded away, posing no threat to the bridge.
  • No piers would be located in the Mississippi River channel, eliminating a threat of increased scour at the adjacent 10th Ave. bridge and to barge traffic.
  • All elements of the bridge would provide a minimum 100-year service life.
  • Concrete was recommended for lowest maintenance.
  • The identified six sub-standard design elements of the I-35W ramps would all be resolved in the new construction.
  • Mn/DOT required the elimination of drainage from the bridge into the river. All runoff water from the bridge would be routed to treatment facilities located at either end of the bridge.
  • "Smart Bridge" technology would be included by embedding electronic sensors to monitor bridge performance. Some of this data would be used for research purposes, benefiting the engineering and design community.
  • The public would be involved during the design phase. FIGG would conduct a full-day public bridge design charrette with community participants selecting design elements including pier shape, railing types, bridge color, and lighting. Flatiron/Manson would conduct weekly public construction tours led by construction and Mn/DOT management to keep people informed on construction progress. Linda Figg, president of FIGG, would develop an educational program to educate and involve public school children in the casting of 1800 decorative concrete tiles for the project.

Beginning construction

Sanderson, says that Flatiron is the 70% partner: providing leadership, scheduling, and organization. They also self-performed many aspects of the construction, including concrete work. They contracted Cemstone, Mendota Heights, Minn. to supply concrete. Manson is the 30% partner with expertise working on water and with large cranes. Their responsibility would involve transporting the precast segments by barge to location and lifting them into position using a 600-ton "ringer" crane mounted on two barges located in the river. FIGG is a subcontractor to the venture partners.

Flatiron/Manson signed the contract with Mn/DOT on Oct. 8, with a completion date set for Dec. 24, 2008--15 months to complete a project considered vital to the flow of Twin City traffic. There were penalties for finishing late and rewards for early completion. They decided to finish the project before the contracted date by scheduling multiple operations simultaneously. This decision meant increasing their investment in forms, maximizing the size of the labor force, and working 24/7--the only day not worked being Christmas Day.

Building high quality, building fast

Fast track building starts with employing a large, productive labor force. At the height of construction, more than 600 people worked in shifts 24 hours a day, 7 days a week, concurrently working on several elements of the construction. As a result, more equipment and forms were needed. Ward says, for example, that contractors typically buy one set of bridge pier forms, constructing one pier at a time, but Flatiron/Manson bought forms to construct all substructure elements at the same time. Using concrete, the bridge could be built with the majority of labor from the local workforce. Also, contractors typically set up one or two casting beds onsite to produce precast segments, but here eight casting beds were set up.

To further decrease the time of construction, the venture decided to cast the back-span sections of the bridge in-place and use precast segmental construction for the main spans, making it possible to work on all portions of the deck at the same time. Most of this work proceeded through the winter months. They used the I-35W roadway south of the river as the site for the segmental precasting beds and the venture provided enough false-work and forms to work on all the back spans.

Ward says that one of the advantages of building the bridge with structural concrete was that it allowed some flexibility in the precise location of the 100-plus drilled shafts that were a maximum of 8 feet in diameter, enabling construction to begin before all the design work was completed.

Building fast requires more focus on quality and safety. These needs were addressed in part by scheduling work with ample time for completion and quality checking. Workers weren't asked to work faster; they were provided the time needed to complete tasks well before the deadline, allowing time for quality inspections and revisions. Flatiron/Manson hired more people to inspect for quality and safety issues and made an agreement with OSHA to provide additional personnel on the jobsite to ensure safe operations.