Super-sized

Much-needed new crossing for Highway 63 at Fort McMurray pushes the envelope in bridge construction

With completion of a new five-lane bridge across the Athabasca River, Fort McMurray is now home to the biggest bridge in Alberta.


The $127-million bridge, where Highway 63 crosses the river at the northwestern corner of the city, is 33 metres wide and 472 metres long with a deck area of 15,576 square metres-the province's biggest deck. It includes a 4.2-metre-wide sidewalk and major utility lines.

The bridge has been designed and built to accommodate a large daily flow of traffic of 45,000-50,000 vehicles and the massive trucks and trailers ferrying huge pieces of equipment from the Edmonton area northwards to the oilsands mining sites and other industrial construction projects in the region. The new bridge can carry up to a 6.4-metre-wide, 1.1-million-kilogram (1,100-metric-tonne) overload, "which is 12.5 times our usual designs," according to Alberta Transportation.

The seven-span steel bridge runs parallel to two existing bridges, one built in the 1970s and the other a steel lattice truss bridge that cannot handle very high loads.

"The [older] 1950s bridge, with steel lattice over top, carried two lanes of northbound traffic, but it could not move large pieces of equipment," says Trent Bancarz, a spokesman for Alberta Transportation. "So, with a big loads, a northbound load had to run north on the southbound bridge. With traffic detours, that works, but it's not ideal. There are a lot of oversize loads moving northbound, and we need to have the infrastructure to accommodate that."

That would appear to be about to happen. The new Athabasca Bridge is part of more than $600 million spent by the province on transportation projects for Highway 63 since 2008. With the two older bridges remaining in operation and the planned addition of new interchanges nearby, the traffic delays and detours often involved in crossing the Athabasca at Fort McMurray should soon be a thing of the past.

The region's climate and the need to avoid damaging the river's ecosystems, including the fish that live there, imposed some constraints. The seven-span I-girder bridge has six concrete piers, with the first three constructed on the northwestern side of the river in 2008 and the next three in 2009.


CONSTRUCTION CONSTRAINTS

Construction of the berms could not begin before July and had to be complete before the end of September. In 2008, construction of the first berm began in mid-August and took less than a month.

"The berms had to be complete by then because of environmental regulation. There are fish biology issues," says Ken Tanner, Flatiron Construction Corp.'s project manager, in charge of all the builder's projects in the region. "They had to be removed by March, because of the river ice breakup in April. This was done to avoid the risk of the berms being washed into the river. Also, from March to July, ice flows pose a risk to safe construction."

With a berm in place, construction began on the first three piers. The foundation for piers can vary widely-from 300-foot steel beams that Flatiron used for a project in San Francisco Bay to, well, what was used for the Athabasca Bridge. Local terrain conditions, of course, are a major consideration. The bed of the Athabasca is hard limestone. The foundation for each pier consists of eight vertical shafts of reinforced concrete that descend 17 metres below the riverbed. The shafts are of substantial girth, with a diameter of 1.8 metres each.

Two options were considered for the bridge superstructure: pre-cast NU girders and steel I girders. NU girders were ruled out in part because the maximum span, 76 metres, exceeded the maximum span of 2.8-metre-deep NU girders, which is 63 metres.

"The only possible design option to make the NU girder system feasible would have been in conjunction with special variable-depth, pier-section, haunched-girder sections, with girder depth varying between 2.8 metres and 3.5 metres, with post tensioning," says Malika Ali, a structural engineer and bridge specialist at CH2M HILL. "This alternative was found to be very expensive."

A steel superstructure was chosen, with three-metre-deep I girders spaced 3.3 metres apart for the straight section of the bridge, the lion's share of the structure. Girders for the final flared span on the east side are 3.9 metres apart.

Extra stiffeners were added that run the length of the girders. It was relatively inexpensive to add the stiffeners at the fabrication stage rather than as a subsequent add-on after the bridge was complete. The increased bridge strength is substantial, however.

"The bridge was originally designed for an overload gross vehicle weight of 1,080 metric tonnes, but with the added stiffeners, it can now take 1,300 tonnes," Ali says.

Bridges of this kind are typically crane-erected, but not here. Instead, an approach known as the "incremental launching method" was used. It's a method that has been used since the 1940s, says Ali, but typically for smaller crossings.

"It has been successfully applied for the erection of torsionally stable, concrete-box girder structures throughout the world," Ali notes. "The technique has also been used for launching small steel bridges in North America and Europe. However, this method has rarely been used to launch a really large group of girders simultaneously. It is one of the widest launches ever done in North America."

With the total weight of the structural steel used for the bridge exceeding six million kilograms, it was also one of the heaviest steel bridge launches on the continent.


PREPARE FOR LAUNCH

Incremental launching method involves assembling sections of the bridge superstructure on one side of the crossing. The sections are then pushed horizontally, or launched, into their final position using jacks. The launching is done in a series of increments so that additional sections can be added to the rear of the superstructure unit prior to subsequent launches.

"An inclined launch nose attached to the leading segment of girders facilitated touchdown at the piers," Ali says. "At the end, you're pushing the whole weight of the girder system."

Using the incremental launching method instead of crane erection made it possible to install the superstructure on the 20-metre-high piers without the need for more berms. "As well as shortening the construction season, it helped minimize river disturbance," Ali says. "Otherwise, a berm for the girders would have had to be constructed."

Incremental launching method is also used for deep valleys, deepwater crossings, steep slopes or poor soil conditions and in other such locales whose characteristics make equipment access difficult.


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