Briefing: building for the future

E&T looks at why it took a tragedy to build a better bridge for Minneapolis.

It is more than a year since the collapse of the Interstate 35W Bridge in Minneapolis, Minnesota on 1 August 2007. The disaster left 13 people dead and over 100 injured. Now the city is focusing on the imminent opening of a swanky $235m replacement, to be called the St Anthony Falls Bridge.

But even now the official cause or causes of the collapse are not known - at least publicly. That won't occur until mid-November, when the US National Transportation Safety Board (NTSB) releases its ruling.

Meanwhile, the construction team, a joint venture of Colorado-based Flatiron and Seattle's Manson, has worked tirelessly since last October on the new bridge, designed by Figg Bridge Engineers of Tallahassee, Florida.

Minnesota Department of Transportation (MNDOT) opted for a design-build procurement process rather than a conventional design-bid-build. In that way, designers and contractors come together early on, giving more scope for innovation and flexibility in design, materials and construction methods. Moreover, construction can start before the detail design is finished, shortening the time to completion.

Figg's design manager, Alan Phipps, says that all the technical drawings were developed in computer-assisted design systems where embedded items such as ducts and reinforcing were modelled. The resulting 3D drawings could be manipulated to view from any angle and to perform "virtual construction" electronically, says Phipps.

Design features also include a sensor and monitoring system, which was key in helping build the bridge and will be used for its long-term management.

Dan Dorgan, state bridge engineer for MNDOT, explains: "Imagine starting on both sides of the river and then attaching pieces so that they meet in the middle." Models were also employed to predict construction effects in the face of Minnesota's "tremendous range in temperature."

Construction has been progressing far ahead of schedule, with a $27m financial incentive for the contractors hanging in the balance. At the time of writing, the MNDOT website promises an opening date "between the middle of September and the middle of October, 2008." As recently as mid-June the same site gave "by 24 December 2008" as the deadline.

This accelerated pace has garnered widespread attention from as far away as San Francisco and even Japan, as engineers pour in to see the bridge erected so quickly - even in the dead of a Minnesota winter.

The loss of I-35W is costing about $400,000 per day in lost revenue, increased commuter expenses and burden on surrounding roads, says Flatiron's public information officer on the project, Amy Barrett. An estimated 140,000 vehicles a day have had to find alternative routes, she claims.

But as the bridge goes up at what Figg calls an "aggressive" pace, the seemingly sluggish schedule of releasing the NTSB report frustrates many. Minneapolis Mayor RT Rybak, who is proud of the way his community and rescue teams responded the day of the disaster, is less kind towards state officials. "I am extremely disappointed that a year later we still don't know why a major freeway structure in the centre of an American city collapsed in rush hour. That's shocking to me," he told E&T.

Rybak, a Democrat, went on to blame the Republican governor of the state, Tim Pawlenty, for impeding progress. Pawlenty spokesman Brian McClung responded by accusing the mayor of simply trying to further a political agenda by politicising the disaster, asserting that the governor had funnelled $3bn into major highway projects in the state just in his first term.

Politics aside, NTSB issued a preliminary report in January that noted a design flaw in the original plans, drawn in the late 1960s by the collapsed bridge's contractor, now-defunct Sverdrup and Parcel, which was bought by Jacobs Engineering. (Called for comment, St Louis-based Jacobs Engineering never responded to E&T.)

Yet 'preliminary' means exactly that. Theories and observations have been bandied about - everything from faulty gussets to extreme loads - but the bottom line is that, for many years, American bridges have not been inspected as regularly as they should be.

Eight years ago, the Hoan Bridge in Milwaukee buckled only a week after engineers determined that cracks in the span did not pose imminent danger, as cited by the Milwaukee Journal Sentinel in a December 2000 report. Other bridges were inadequate as well, including those along the New Madrid fault in the US Midwest, a place not commonly considered geologically vulnerable.

Inspections

The question is, was the I-35W bridge inspected as frequently and as thoroughly as it should have been? Its last inspection before the collapse, in May 2007, was a partial one of the main span truss on the west side. A complete inspection had not been done in 2007 because construction by Progressive Contractors Inc (PCI) was taking place on the bridge at the time, Dorgan pointed out. He said a thorough inspection would have "likely" been done in September of 2007, after the work by PCI was complete.

Dorgan says the state only mandates inspections once every two years. Further, both Dorgan and a Federal Highway Administration spokeswoman, Nancy Singer, explained that 'deficient' doesn't necessarily mean ready to collapse. A quick check to the FHA website shows that even today, a quarter of the country's 600,000 bridges are defined as deficient, but that is split between 'structurally deficient' (72,524) and 'functionally obsolete' (79,792), which covers factors like inadequate capacity.

A fracture-critical bridge is one that has at least one 'fracture-critical' member, meaning that if it fails the bridge could collapse. The fallen I-35W bridge was a truss bridge, and Dorgan says most trusses are fracture-critical.

A cursory examination of the last full report issued by a team of inspectors in June, 2006, notes, among other problems, several cracks that needed monitoring including fatigue cracks more than 38mm wide at girders #1C and #3. The inspector said these areas should be checked "next year" for "any possible lengthening of the holes and drilling of possible stress relief holes."

Undoubtedly, the NTSB will consider whether the Minnesota Department of Transportation acted expeditiously enough in response to that report, which even includes a recommendation to replace the bridge deck if the full bridge was not soon replaced.

Whatever happened in the past, everyone involved in the new project is adamant that it will not happen again.

Thus, Figg Bridge Engineers was chosen after an elaborate points-based bidding process  despite initial public outcry about going outside the Minneapolis community. Undoubtedly, Figg's portfolio of work on a range of stylish and sturdy designs finally won over the public as it had the judges.

Linda Figg, president and CEO, added that the public participated in the final design. "We provided two options for the pier shape and they [the public] selected a curved pier shape that would fit with the theme of arches, water and reflection." The public also chose the open railing and aesthetic lighting.

The new bridge, built to last 100 years, is planned to be sturdy and not fracture-critical, based on a segmental box-girder design. The segments were pre-cast in forms on the construction site, explains Dorgan, "and then they were transported by barge underneath the bridge, lifted in place, and post-tensioned on to the previous segment so it builds out."

Flatiron's Peter Sanderson, who has managed the project from the beginning, has worked for months with his team. Asked if he's had any surprises, the 37-year industry veteran admits: "There's a surprise every two hours. But we've overcome the problems, to date." 

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