Engineering a Pei Cantilever - Dallas City Hall
Pei intended the inclined facade to welcome visitors and, on the grander scale, to create a visual dialogue with the growing downtown area to the north of the site. He persuaded the city to acquire an additional six acres (2.4 hectares) in front of the building — two full city blocks — as a plaza and buffer zone for his grand public structure.
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The cost of construction and the extra land proved a challenge for the city. It took six years to raise the money, during which time the project was scrapped and then revived by a new mayor, Wes Wise. Funding was eventually supplemented by income from the 1,325-car parking garage that was built beneath the plaza.
Pei's firm subscribed to a creed of "honesty of materials." Jonsson described Pei as a purist. "Pei... doesn't like to see a thin coating of marble on the outside that makes you think you're looking at an honest marble building. He wants it solid marble or something of a single material that's homogenous... and won't be a source of worry... for the first hundred years anyway." Jonsson recalled that Pei chose concrete because it was the only thing that the city could afford that met the design criteria.
When Jonsson reacted to the apparent top-heaviness of the building's shape, Pei and Musho created the three cylindrical pillars that appear the hold up the structure. These are, in actuality, stairwells that had originally been concealed within the design, but were brought forward to lend visual support; they do not bear the load of the building.
The cantilevered floors are supported by 14 large bearing walls, 18 inches (46 centimeters) thick, arranged in seven pairs. The pairs describe a width of 14 feet (4.3 meters), except in instances where they flank the staircase towers. The 11-foot (3.4-meter) wide areas enclosed by the pairs are used to house mechanical and electrical services. Between the pairs are 65'-4" (19.91-meter) spans of office space.
The north edges of the bearing walls slope outwards flush with the facade. The south edges terminate in sheer verticals at the interior atrium. The floor-to-floor height is 14'-0" (4.3 meters); 3'-8" (1.12 meters) of this height is occupied by a coffered concrete ceiling, raised floor deck, and integral air-duct system.
The task of engineering the daring design was given to Jack Rosenlund, P.E., of the Dallas firm of Terry-Rosenlund & Associates. Rosenlund brought in additional engineering assistance from Weisskopf and Pickford, New York.
The structural strategy was an innovative implementation of vertical and horizontal post-tensioning. The bearing walls are post-tensioned vertically through the portion directly over the wall's narrow footprint. Bonded post-tensioning was used, in which tendons are fed through steel ducts and then grouted in place after stressing. The ducts ran down one wall of each pair, into the foundation, looped across the bottom of the 14-foot (4.3-meter) gap and came up the other wall of the pair. Thus, each tendon applied compressive force to both walls of the pair.
Each floor had its own separate set of tendons. As the building rose floor by floor, tendons were fed through the ducts, anchored in one wall and stressed from the other.
The horizontal post-tensioning ran from the front of the sloped facade, where the tendons were anchored, back to the edge of the interior atrium. Openings for doorways penetrating the bearing walls were limited to 6'-8" (2.03 meters) in height, leaving more than 7 feet (2.1 meters) of solid wall height at each floor for post-tensioning.
The horizontal tendons carry the over-turning force of the cantilever back to the vertical tendons, which then transfer it down to the foundation. The foundation and basement levels are considerably wider than the apparent footprint of the structure, extending out beneath the inclined facade. The cantilevered roof is 200 feet (61 meters) wide, the ground floor is 126 feet (38.4 meters) wide, and the basement 230 feet (70.1 meters) wide. The footing beneath the second basement is also post-tensioned, with a force of 4 million pounds (1.8 million kilograms).
The interior atrium is 96 feet (29.3 meters) of unbroken height. Across the top, 14-foot- (4.3-meter-) high box-beams tie the sloping front structure to the more orthodox shape of the rear half of the building, providing balance and additional stability.
Throughout the building, post-tensioned concrete coffered ceilings span the 65'-4" (19.91-meter) distance between bearing walls. The ceilings are three feet deep and configured in 4'-8" (1.42-meter) squares. Above the ceilings are 8-inch- (20-centimeter-) high voids that function as integral HVAC ducts. Cast concrete curbs on the top side of the ceiling coffers support the floor above.
Since concrete was both the primary structural and finish material, close attention was paid to every aspect of its mix and placement. "We had learned more every time we did a concrete building," remembered Ted Amberg, AIA, in a recent interview. Amberg was an associate partner of the Pei firm and ran the company's Dallas office during the construction of the project.
"In Dallas City Hall, the use of concrete and care of concrete was stretched to its limit. To build formwork and place concrete that would let the structure speak for itself, let structure speak to the architecture, that was very satisfying."
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