A mere 46 days after starting erection of the precast units for the main span, contractors on the new St Anthony Falls bridge in Minnesota installed the last of the 120 segments. An average of almost three units per day - quite an achievement by any standards.

But this new bridge cannot really be measured by the usual standards. Its very existence was born out of the tragedy of the I35W bridge collapse, and its design and construction in just over a year since the collapse took place already mark it out as something special. But this is not the only aspect of the job that is remarkable - it also sets new standards in working partnerships and community outreach that could offer important pointers for future construction projects.

When Bd&e visited the site in late May, erection of the first precast segments for the main span had just begun, and public awareness was high. The 'sidewalk superintendant' event that takes place every weekend had suddenly boomed in popularity, requiring the input of several of the project's key members of staff as well as portable loudspeakers and bits of prestressing cables and ducts acting as props (see below).

Such interest is entirely understandable, given its visibility, and the background to the project, which is being built to replace the I35W bridge that collapsed on 1 August 2007 with the loss of 13 lives. The Minnesota Department of Transportation knew that construction of a new bridge had to be carried out as quickly as possible, not only to replace a vital transportation link, but also to allow the city to move on from the tragedy and focus on the future. But as Mindot project manager John Chiglo explains, despite the urgency of the work, the DOT believed that the normal indicators of price and speed should not be the only standards by which the bids for the design and build package were assessed. Four teams submitted bids for the project, and these were evaluated for a 'best value' selection. The winning bid, by contractors Flatiron/Manson with designer Figg Bridge Engineers, was well ahead of all the other bidders, says Chiglo, even though it was not the lowest price.

He likens the bidding process to an 'open book' exam, since the DOT communicated very clearly to the bidders, the criteria against which the bids were going to be measured, and the weighting attached to each criterion. Bidders had to state how they would achieve each of nine specific goals highlighted by the DOT, including safety, quality, schedule and so on. One of the challenges of the design was to try and eliminate some of the six design exceptions that had been present in the original corridor, things such as stopping sight distances which did not meet current codes. The proposal by Figg was the only one of the bids that could eliminate all six of these, says Chiglo. Not only that, they went the furthest of all the bidders in trying to accommodate the state's requirement for public involvement, both during the design and the construction process.

Of course in order to attract well-thought-out, practicable bids the DOT must provide a clear, comprehensive brief, and Chiglo believes that this was central to the success of the process. As well as setting out how the bids were going to be assessed, Mindot also stated which structural types were allowable in the bid, gave minimum clearance requirements over the waterway, and made it clear that no piers were allowed in the water. The required capacity of the road was stated, and some geometrics and profile suggestions made, but there was a great deal of flexibility for the designers.

This enabled Figg Bridge Engineers to put forward a modern design for a prestressed concrete structure that would fit with the other bridges on the river and 'tell the story of its time in technology', as Linda Figg puts it. "Minneapolis has a great history," she explains, "but it also has some very modern architecture." The city features buildings designed by modern architects such as Frank Gehry, and European names Jean Nouvel and Herzog & de Meuron, which added to its wide streets and abundance of cyclists gives the city a very cosmopolitan feeling.

The need to straddle the river dictated the span of the bridge, and the fact that concrete is available locally steered the designers towards a concept of high strength, high performance concrete. "We wanted to develop a lot of redundancy for the bridge, which is why we went for four concrete box girders on four piers," explains Figg, "and as an added value component to our design, we offered the possibility of an additional pedestrian bridge suspended below the main span." As it is, the winning design has a viewing area incorporated at the base of the piers; the new bridge will have a lot more openness underneath that the previous structure, and Figg expects it will offer the general public a pleasant spot for enjoying the river and the new bridge.

Once the contract had been awarded, in early October, a day-long charrette was held with almost 90 people from the community, to choose from a range of options for the structure, put forward by Figg. The shape of the piers, final colour of the structure, design of the traffic parapet and the aesthetic lighting for the bridge, and the treatment of the abutment walls were all up for discussion with the final choice being down to the attendees. In addition to these elements of the structure, the team held a brainstorming session to discuss what people would like to have on the bridge to represent the 'gateway' to the crossing for drivers. And in order to ensure that the wishes of the community were carried right through to construction, a 'visual quality advisory team' was appointed for the duration of the project, consisting of 20 representatives of certain sections of the local community who gave feedback on these elements as the work progressed.

Team-working was very much central to achieving the incredible rate of construction progress that has been seen in Minneapolis, and naturally this starts right at the top of the tree, as Chiglo is aware. The use of a design-build contract helped in the process, but although Mindot has used this type of procurement before, this was the first time that it had been used with such an aggressive schedule, and the first time that it had been used on a major river crossing. "In these kind of circumstances it is very important to ensure that you have a class A team," he acknowledges, "so I have multiple project managers working with me on this project. We increased our staffing levels so that we could reduce the time taken to review and approve the paperwork, and ensured that we have people available who are qualified to make decisions and turn around the work very quickly."

The new crossing is actually two separate bridges, separated by a gap of just 2.6m, each of which will carry five lanes of traffic. It is also designed to be capable of carrying a light rail transit system in the future, if required. Each bridge has four spans; a main span of 154m, back spans of 101m and 74m, and a shorter back span on the north side of just 45m. A total of eight, 21m-high piers support the main span, while four additional piers support the shorter back span. The deck of each bridge is 28m wide, and this superstructure is formed of twin box girders which are connected along the centre.

Bidders were given until 24 December 2008 to have the bridge completed and open to the public, with associated penalties and bonuses for late and early completion. At the time of writing, it seemed likely that Flatiron/Manson was going to be banking a considerable bonus with completion predicted for some time in September. Just finishing on time earns the team a bonus of US$7.5 million, and for every ten full days ahead of the deadline, they earn a further US$2 million, up to a maximum total of US$27 million. By contrast, the penalty for each day late is US$200,000.

But this would be just reward for the meticulous planning and extensive resources that have been put into the project, not to mention the doubtless stressful moments that the team has had to endure.

As project manager Peter Sanderson recalls, when the contract was awarded, there was still debris from the old bridge at various locations on the site, and investigators were still carrying out their work. The basic layout of the bridge was not approved until the end of November and there have been times when construction has literally come to a halt as approval of design changes have been awaited. In one particular incident, Sanderson was waiting for a phone call announcing the outcome of the design charette so that he could order the formwork for the piers the same day.

The construction work has not been particularly difficult, he says, but it is the logistics of coordinating with the design process that has been challenging. But no other type of structure could have been built this quickly, he concedes.

To maximise the amount of construction that could be carried out at the same time, shaving as much as possible off the construction programme, the main span and the back spans have been built using two different construction processes. A precast yard was set up close to the bridge, and site staff worked through the winter in four temporary buildings on eight casting beds to produce the 120 segments required to build the main spans. Being under cover was essential in the harsh winter weather that is experienced in Minneapolis, and the tents were heated to maintain a constant temperature and provide a controlled environment for curing of the concrete. In fact although the temperatures dropped very low, there was very little snow last winter, which could have set the programme back considerably.

At the same time as the precast yard was being set up, the piers were being built in situ and preparations were being made for casting the back spans, also in situ. Foundation construction began in December and the following February, construction of the piers started. Superstructure construction of the back spans, which were built on extensive falsework supports, began in March and was just being completed as Bd&e visited the site in time for the erection of the first segments.

At this point, explains assistant project manager Bob Edwards, his staff from the Manson side of the joint venture stepped in with their extensive maritime experience. The precast units were brought down to the river bank on a low loader, where they were cleaned and made ready for the final lift which was carried out by a ringer crane on a barge. A system of heavy anchors in the river and on land were used to secure the barge during the lifting process, along with two ‘tug’ boats – actually more like ‘push’ boats in practice – and two spuds on the barge which could be lowered into the river bed for further security. The river flow at this point is high, making it a tricky lift.

The first units to be installed were the pier units, which weigh 200t each. The process attracted huge public attention despite the fact that the contractor had not publicised the first lift, for obvious reasons. About a thousand people lined the adjacent bridge to enjoy the spectacle, which passed off without a hitch, says Edwards, despite the fact that there was a tornado warning and high winds that day.

Communicating effectively

Every Saturday morning, an eager crowd gathers in front of Grandma’s Saloon close to the construction site of the new St Anthony Falls bridge in Minneapolis. They are waiting for the start of the weekly Sidewalk Superintendent talk, part of the joint venture's extensive community outreach programme.

Senior staff such as project manager Peter Sanderson, his assistant Bob Edwards and Mindot project manager John Chiglo regularly host the talks, along with other site engineers and public relations staff, as demand dictates. Bd&e took the tour along with a couple of hundred members of the public, split into half a dozen groups, which had to be shepherded through the viewing points along the length of the project with careful choreography. This level of attendance has become commonplace as the visibility of the project has grown, and with the lifting of the main span segments it seems the whole city wants to know what is going on.

The tour starts next to the casting yard, and climaxes at the midspan of the road bridge adjacent to the new crossing, where there are superb views of the river and the erection process that is taking place. The engineers explain what is going on, using props such as lengths of prestressing cable and duct to demonstrate, and answer questions from the public.

Sanderson has become something of a celebrity on the sidewalk tours, his dry wit making him popular with the public, and one suspects that he rather enjoys this opportunity to explain the mysteries of engineering to a lay crowd. He has his regular fans too - one retired highway engineer has been to the talks every weekend since they began.

Such outreach is important not only in terms of strengthening ties with the local community and ensuring that they feel involved in the process, but also in terms of educating the public about what civil engineers do and even inspiring children to consider engineering as a career. One young teenager considering a career in the profession was thrilled to get the chance to talk to Linda Figg, asking her which college he should apply to, and what the process of becoming a registered engineer involves.

Of course Figg has been central to many of the community outreach aspects of the project, not only organising the design charettes to select certain features of the new bridge, but also in organising an educational outreach programme for students at 29 local schools. This was not in the original bid, but evolved during the contract process, explains Figg. Nearly 1800 students and teachers attended the programme which was run over a six-week period earlier this year, to learn about the the design and construction of the new bridge. The final stage of the programme involved each student making a mosaic tile with concrete and recycled glass aggregate, which will be used to decorate the walls of an underpass on the new bridge approach.

Other aspects of the outreach programme include the regularly-updated website, weekly email newsletter, information boards at viewing points along the adjacent bridge,interactive kiosks around the city, a project hotline and a speakers' bureau.

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