Encana’s Bow building; the making of an icon

A stunning new emblem of homegrown industrial might is transforming the Calgary skyline

June 07, 2010

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Barnes’ team varied, including upwards of 20 or 25 architects. Together they came up with the optimum floor shape on the footprint given, along with a great deal of natural light, social space, and a sky garden design.

“And so through all these specific things, we ended up developing the curved floor plate which allowed for their project teams to all sit together,” says Barnes. Not only did the plan offer that accommodation – 85 to 90 per cent of the offices will be near the perimeter and have natural daylight.

Although internal considerations were the primary engine of the design, there are a number of factors unique to Calgary that contributed. “The climate is obviously one,” says Barnes. “The cold requires a great deal of heating, so we have the large southeast facing atrium which is a sustainable feature of the project that collects sun throughout the year. We draw energy from the sun and use that heat to offset the mechanical heating and ventilation required. It saves a great deal in energy costs.”

And he didn’t forget public space. There’s also the proximity of the building to Sixth Avenue, he says, the large open area “for all the employees and local downtown population to enjoy, including your Stampede. Then there are the sky gardens that provide social and formal meeting spaces which frame a view out towards the Rocky Mountains.”

Those pedestrians generally don’t like the wind tunnel and vortex generator effects caused by tightly packed highrises in downtown Calgary. Barnes says once the internal space design was looked after, they did extensive research on this external aspect of large buildings – their mountain-like capacity to generate a microclimate of turbulent weather.

“We did a great deal of wind tunnel testing, both physically and in the computer model. The building is actually aerodynamically shaped to point towards prevailing winds and in so doing can channel the wind freely around the building, which also in turn reduces impact on the structure.” That also allowed the architects to make a lighter steel structure. At the ground level, any wind vortexes coming off the building are directed in a much calmer trajectory at the ground plane, “which also helped inform the structural design,” adds Barnes.

There are two structural systems at work. On the front atrium on the south side, there’s a steel “diagrid” of triangular structures with diagonal support braces. On the main curving face on the back, there’s a brace frame. Both sides are steel construction.

“By using the triangular diagrid structure, we were able to create a near column-free space within the building that allowed for a very flexible floor plate. It maximizes views for employees and minimizes the amount of construction materials needed. It’s very efficient,” says Barnes. Not to mention that it makes a very striking addition to the city’s downtown skyline.

The look is very new but the use of steel girder construction and diagrid design isn’t – the 46-floor Hearst Tower on New York City’s West 57th Street, completed in 2006, uses it, as does the 108-storey Sears Tower (now called the Willis Tower) finished in 1973 in Chicago. And a variation of the theme was used on the world’s new tallest building, the 108-storey, 828-meter Burj Dubai (lately renamed the Burj Khalifa).

“We did the Hearst Tower, which is slightly different but a similar approach,” says Barnes. “It is again a steel triangulated structure. You see the steel braces of structures elsewhere and they are all variations on a theme.” Nevertheless, he says the layout is still rare enough to make the buildings look like unique civic or business art objects to their communities.

And when construction time arrived for the Calgary project, it started with a rare feature to establish a solid foundation in a city that lacks the natural bedrock of Manhattan. At the very bottom of the six-storey deep excavation, “A raft slab the same shape as the floor plans was poured,” says Bill Christensen, senior project manager with Matthews Southwest Developments, which is running the construction phase. “It’s about 10 feet thick of concrete and reinforcing steel. I believe it was the third largest concrete pour in the world.”

From there, steel columns six storeys high were raised up to ground level. “The ground floor deck and structure was put in place along with the concrete. That allowed us to start construction above grade while we were still below grade pouring the six floors of parkade slabs,” says Christensen. This technique made possible speedier construction overall, by eliminating the traditional wait until all of the subterranean work is complete before climbing skyward.

Three tower cranes were installed for the above grade work. The building is more or less divided into three vertical sections, one at the centre and one on each end. “The centre section goes up first,” explains Christensen. “We stand all the columns there – they stand three storeys high – and then we put in the beams and girders in between. Then the metal deck, with reinforcing steel and concrete, goes in.” From there, the construction sequence continues up the building to top out at floor 58, with “the centre portion of the building always three storeys ahead of the ends,” he says. “For the most part it’s repetitive; there are a few changes at the sky garden levels. Right now [in mid-April] we are standing columns up to floor 45.” Following completion of each vertical frame comes installation of the curtain wall.

Much of the 39,000 tonnes of structural steel going into the Bow are standard girders. But the diagrid on its front demands special attention. “You’ll notice it’s fairly elaborate,” says Christensen. “For these, there’s a considerable amount of fabrication in a shop to make those. It’s a joint venture between Supreme Steel out of Edmonton and Walters of Hamilton.”

As epitomized by the famed Mohawk “men of steel” ironworkers – also known as “fearless wonders” and “skywalkers” – who balanced on girders at the highest levels and gained their reputation on the Empire State Building and the Chrysler Building in the 1920s and 1930s, the specialists assembling the Calgary structure need to be highly skilled. “There’s an incredible amount of welding that has to take place on that diagrid,” marvels Christensen. “Each one of those nodes would take two welders a month to complete. And we’ve got 50 of those nodes on the job. We have about 60 or 70 welders right now.”

The 750 workers on the job as of this spring – a force Christensen says may grow to 1,200 or more – hope to complete the entire building by the first quarter of 2012, when Encana and its sister firm Cenovus Energy Inc. will move in and take exactly half of the space each. Until then, the construction workers are at it virtually 24 hours a day.

As each of those 24 hours passes, the vision gets more stunning. “I marvel at it every day,” says Christensen. “It’s kind of scary when I walk down the sidewalk and people aren’t paying attention to the traffic, they’re staring at the building.”

Completing this new Calgary icon will be something else to ogle – an as yet unnamed sculpture to grace the plaza at the front of the building. Barcelona sculptor Jaume Plensa, whose public pieces appear from Nice to Jerusalem to Toronto’s Lester B. Pearson Airport, has been commissioned to do a sculpture. It’ll be the final touch on what promises to be an icon for decades to come.

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