How precast concrete helped to complete the project three months ahead of schedule.
By Sue McCraven
Above the Mississippi River, during steamy rush-hour traffic, baseball fans waited in bumper-to-bumper traffic on their way to a Minnesota Twins game along with other drivers as bridge repair barricades funneled everyone into one lane. Drivers and road crews alike had no warning when the roadway fell out from beneath them at 6:05 p.m., sending dozens of people into the water.
The collapse of the 40-year-old I-35W Highway Bridge in Minneapolis was one of the largest and most important accidents investigated in the National Transportation Safety Board’s history. Thirteen people died and 145 were injured on the weekday evening of Aug. 1, 2007, when the mainspan of the nearly 2,000-foot-long steel-truss arch bridge failed catastrophically in just four seconds, plummeting more than 100 feet into the water below. NTSB’s forensic investigation of the collapsed truss of the steel deck identifi ed fractured and inadequately designed gusset plates as the cause of the fatal accident.
In the aftermath of this Minnesota tragedy, an inspiring man-made structure of impressive strength, beauty and light stretches over the river. A remarkable accomplishment, the bridge was completed three months ahead of schedule with a new precast concrete segmental mainspan and cast-in-place concrete end spans securing the 504-foot reach over the Mississippi River. In an 11-month design/build project, a joint venture between Flatiron Construction Corp. and Manson Construction Co. (FMJV), builders worked day and night in close collaboration with the Minnesota Department of Transportation (Mn/DOT), the Federal Highway Administration (FHWA), the local community and designers from Figg Bridge Engineers Inc.
Rebuilding over the Mississippi
As owner of the I-35W highway bridge and a state public agency, Mn/DOT considered a number of bids for rebuilding the structure. Under the owner’s selection criteria, however, the low-cost bidder was not necessarily the winning bid. From the request for proposals through to opening the new bridge for traffic, this project was very different from a typical public transportation venture. “Almost everyone in Minneapolis, including the workers who built the bridge, knew someone who was personally affected by the collapse,” explains Todd Mittelstadt, director of operations for Stoneworks Architectural Precast Inc., the company that precast the sinuous gateway features. “The tragedy colored everything related to the project, but in a positive way.”
Mn/DOT is up front about its firm intent to exceed expectations on the project by making quality the basis of the selection process. What made this enterprise so different is that the completed bridge needed to do more than safely transport vehicles across the Mississippi River; this bridge had to restore public confidence, respect the past and demonstrate the structural strength and durability to deliver 100 years of service life.
Precast makes the difference for mainspan
Precast segmental bridge girders were the first choice of the design/build team for the 504-foot mainspan. “Given the very short window we had to complete this bridge, it was determined that casting the side spans in place on falsework while simultaneously precasting the mainspan was the most viable option,” states David Tullis, construction engineer with FMJV. Rapid erection of precast segmental girders was not the only competitive edge for precast in overtaking alternative systems, including structural steel designs from local bidders. “Higher quality can be a big advantage due to repetition and a closed environment. If you can do precasting in line with other activities, it can minimize the potential for superstructure erection to be on the critical path as it did
Running project operations at full throttle for precast segment superstructure production and at both bridge approaches simultaneously required careful planning and logistics. Orchestrating cast-in-place construction at each side of the river at one time meant that the necessary materials and equipment for duplicate operations had to be pre-ordered and at the ready. FMJV self-performed all the precast and erection work for the superstructure. To save more time and transport costs, an on-site casting yard was set up on the unused approach roadway for the old I-35 steel bridge. Precast bridge segments were formed and cured in constant-temperature warming houses using heated aggregates to combat bitter-cold conditions of a typical Minnesota winter. Long-line production ensured perfect end-to-end box girder match ups. Using eight casting beds allowed the fabrication of at least one and up to three segments per day at peak production.
Mainspan complete in 47 days
On May 25, 2008, the first precast box girder was placed. The last precast segment was erected on July 10, 2008. Such extraordinarily fast construction was not just due to a rapid on-site precast segment production; this testament to maximum efficiency lay with committed workers. “We worked every day, seven days a week, for 11 months,” stressed Bob Edwards, FMJV assistant project manager. “We started early October of 2007 and finished September 18, 2008. This meant we worked through one winter and one summer.”
There were many challenges for the crews, working in -20 F (-29 C) in the winter to more than 90 F (32 C) in the summer, along with wind, rain and tornados.“ In fact, we had a tornado starting to form while placing the first two precast segments right in our location.” An example of what a Minnesota winter can deliver: one bone-numbing February day hurled wind chills of -65 F (-54 C) at anyone venturing outside a shelter at the bridge site.
“The erection went like clockwork,” continues Edwards. “The barge crews and topside erection crew worked together without any problems. Our average setting was four sections a day, and we set eight on one long day.” In the current U.S. economy, it is important to note that this bridge was built with local materials and local labor. With this kind of dedicated and focused work force, the entire project was completed three months ahead of schedule. To appreciate this feat, one need only consider that the new I-35W bridge was open to traffic just 11 months after the award of the contract to FMJV.
Biggest challenges for the design/build team
Quality control and quality assurance are never such heavy hitters as when the cost of hindsight is beyond mere numerical calculation. For owner Mn/DOT, there would be no second guessing on the proven quality of the new bridge. “From the production side, we dealt with four levels of QC/QA,” says Tullis, “which often meant inspections that lasted for hours. We overcame this by being consistent, doing quality work, and using our experience to gain the owner’s confidence.” Time was the narrow gauntlet that engineers endured from
start to finish to get drawings completed and approved. In many instances, the design/build team took “some calculated risks” in ordering materials based on yet-to-be-approved designs and details. The clock wouldn’t wait.
Mn/DOT out-specified AASHTO (American Association of State Highway & Transportation Officials) in some of its exacting requirements for this bridge. A specific example of the great care taken by the owner was in the requirement for the precast segmental box girders to have extra service strength beyond AASHTO specifications. The bridge deck was to provide a minimum of 250 psi in residual compressive service stress, longitudinally. This additional strength requirement meant that the I-35W precast girders contain extra longitudinal post-tensioning. Because the deck was cast integrally as part of the precast box girders, designers focused on meeting this strength criterion.
On the erection side of things, hoisting the 200-ton, 45-foot-wide and 25-foot-deep precast segments into place called for seasoned problem-solving skills. Edwards said, “Our biggest challenge was to find a way to support a large crane to erect the precast segments. The lock on the Mississippi River was only 56 feet wide. If we were to use a barge-mounted crane, we would need to have a barge at least 100 feet wide for stability. We looked at many different arrangements for cranes, from stiff-leg cranes on the pier tables to floating equipment. We went through many engineering plans to see what would work and the challenges in getting the equipment to the site and in place. We finally decided to go with a barge-mounted 4600 Series III Ringer Manitowoc mounted onto two barges. We designed a steel grillage to transfer the weight from the crane to the two barges and keep the list of the barge less than 1 degree to meet the manufacturer’s requirements.”
Precast box girders were made with high-strength, high-performance concrete with silica fume and fly ash added for durability and corrosion resistance. From a shoreline holding yard, 120 precast segments were shipped upriver on flat-bottom barges and lifted into place with the ringer crane. The post-tensioned and cantilevered precast girders were jacked apart (using 560 tons of force on each side of the superstructure) for placement of a 7-foot-deep, highly reinforced CIP closure girder to complete the cantilevered mainspan in the center. After the center section had gained sufficient strength, high-strength post-tensioning cables were tensioned through all girder sections and grouted in place. To minimize adverse environmental impact, bridge runoff drainage is routed through the interior of the girders. There is enough space within the girders for utility ducting and
With redundancy in design, high-performance concrete, and more than 300 “smart bridge” sensors (including real-time monitors for traffic patterns, joint movement, security, strain and chloride ingress), the new bridge achieves the highest levels of safety standards. On Sept. 18, 2008, the $234 million I-35W Bridge was open for traffic, three months earlier than the 15-month project schedule. Technically and structurally, this bridge is a forward-looking ideal of the very best that can be accomplished by hardworking and dedicated teams. But this particular bridge had to deliver more than safe transport over the Mississippi.
And it has succeeded: Locals claim that the most impressive view of this remarkable man-made crossing is from beneath; looking up from the swirling waters of the Mississippi is the best way to appreciate the brilliant design and reassuring strength of the structure.
Sue McCraven, NPCA Senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.