The I-35W bridge, carrying traffic over the Mississippi River in downtown Minneapolis, opened to traffic in 1967. Over the last 40 years, both local traffic and the local economy grew increasingly dependent on it and it is estimated that in the year 2007, the bridge carried around 140,000 vehicles, including 5,700 commercial vehicles.
When the steel truss bridge collapsed on 1 August last year, not only did it result in the tragic loss of 13 lives, it also left a gaping hole in the city’s road network. Rebuilding needed to start as soon as possible.
Preparing a brief for a bridge and subsequent tendering process is normally a drawn
-out process. However, Minnesota Department of Transport (Mn/DOT) needed to move quickly. It used existing templates to produce the request for proposal, or design brief, by 23 August 2007.
"We already had a template established and specifications in place that we’d used in other major projects, which establishes the minimum expectations," says Mn/DOT project manager, Jon Chiglo. "We modified it to the uniqueness of the project to establish the framework."
Technical proposals for the £234M bridge had to be submitted by 14 September followed by proposed budgets four days later. The winner was announced on 19 September.
"It was a competitive process between four teams with three key factors: time, cost and technical score," says Chiglo. "The time and cost remained sealed until it was opened in front of the public. [A joint venture of contractors] Flatiron-Manson won as it had the highest score."
The winning consortium is made up of the Flatiron-Manson joint venture and designer FIGG Bridge Engineers. It won on the high technical proposal score, as it had managed to improve on the alignment and lines of vision of the original roadway.The consortium also thought about how it could allow for input from the public in its design, which is a US state and federal requirement.
"In just seven weeks, we had established the criteria in the request for proposals, although the public and legislature were concerned that we were moving too fast," says Chiglo.
"We required the contractor to allow flexibility to allow public input and develop ownership. FIGG has a system called the Charrette Design Process, where it designed two bridges in parallel. It gave options, including pier types, colour, railings, lighting and retaining walls."
The final design – including pier shape, colour and other aesthetic issues – was decided on in just one day. Members of the public were invited to a consultation, walked through the different options, and then voted.
However, when it came to starting on site, there was a slight delay. "One of the main problems that delayed us starting work for three weeks was that the old bridge was still in place where we wanted to do our site investigations," explains Flatiron project manager Peter Sanderson.
Locating the new piers was difficult, as there were a number of hidden underground structures that needed to be avoided. "We took between 8 October and 30 November just to find an area to place the piers," says Sanderson.
"One of the new piers, for example, is at the same location as one from the old bridge. We had to thread the foundations among the old drilled shafts. There are also big stormwater tunnels running under the bridge piers on either side. On one side, the foundations for the piers span over the tunnel, and on the other side the piers for the bridge are staggered."
Each pier sits on eight piles that are around 2m to 2.5m in diameter and socketed into the bedrock 30m down. The concrete footing on top of the piles is formed with standard wood formwork, but steel shutters were used to achieve the curved pier shapes.
"The piers were gangformed [formed in large steel moulds] and all done at once, so there was no reuse," says Sanderson. "We had as much steel formwork as we had concrete surface."
The abutments of the new bridge are being built on the path of the old one. Gabion
walls support the side of the earth abutments. The approach spans are cast in reinforced concrete, while the main span comprises precast concrete segments.
The replacement crossing is actually two separate bridge structures – one for northbound traffic and one for southbound. The gap between the two allows easier access for inspection.
Building for the future
The new bridge will be 24m wider than the original bridge, allowing for an extra lane and wider shoulders that could accommodate a bus lane or light rail transit.
"It’s designed for 100 years, so we need to make it accommodating and flexible for the future," says Mn/DOT project manager Jon Chiglo.
The bridge also incorporates sensors that will provide engineers with information about its structural behaviour. For example, vibrating wire strain gauges will measure how the bridge is resisting loads and the temperature of the concrete. Other sensors will measure bridge movements, deflections, and chloride penetration.
"I am convinced that the design submitted is the only one that could be done in the time available," says Sanderson.
"We looked at steel girders, we looked at going all the way across with falsework, lifting
the main span, casting it in place with travellers. It wasn’t until 30 November when we had approval for the bridge layout and also for where we could place the piers. We couldn’t
have designed steel, had it fabricated and delivered in the time we had."
The main spans are each formed from 120 precast concrete box girder sections, made on a site downriver and towed upstream by barge. A 600t barge-mounted ringer crane then winched the pieces into position over a 46-day period.
The sections were erected incrementally from the piers on either bank, cantilevering until
a 2m gap was left. Teeth had been cast into the sides of the precast segments to form a
shear key between them. As each section was erected, cables were threaded through ducts embedded in the units, while hydraulic jacks pulled on the tendons and compressed the segments together.
For the closure pour, frames were inserted in the 2m gap, pushing apart the two sections by 50mm. The concrete for the northbound span was poured on 15 July and left to cure overnight, when the thermal effects were at their lowest. Pressure on the jacks was released the following morning, introducing compression into the concrete.
The programme is currently three months ahead of schedule. "The greatest challenge is getting it done and getting subcontractors to take you seriously," says Sanderson. "They say, 'you never said you wanted it that fast' and then you show them the programme they were sent and they just don’t believe you."