Engineering a Pei Cantilever - Dallas City Hall
by Steven H. Miller
Innovative architecture often requires equally innovative engineering and technologies for successful realization. An outstanding example of design and engineering interdependence can be seen in the Dallas City Hall, a landmark building completed in 1977, designed with daring vision by one of the world's leading architectural teams, I.M. Pei & Partners.
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The building is a fascinating amalgam, equal parts raw truth and magic. The 560-foot- (170.7-meter-) long structure is a cast-in-place monolith of exposed concrete, unabashedly checkered with the imprints of the resin-coated plywood panels used to form it. The long north facade, however, leans forward at a 34-degree angle, each floor 9'-4" (2.84 meters) farther into space than the one beneath it, seeming to hover weightlessly despite the obvious massiveness of the structure. Its appearance is often described as "gravity defying."
The famous "inverted pyramid" is an early example of post-tensioning used for large-scale construction, and its innovative post-tensioning system was crucial to executing the building's unique shape. The building also employed another then-new technology — shrinkage-compensating concrete — to achieve the beauty and "truth" of its concrete.
"You guys are crazy," said Dallas Mayor Erik Jonsson to the architects, upon seeing an early model of the building. "People will think it's going to fall over."
Theodore Musho, AIA, Pei's lead designer on the project, still remembers the conversation well, because Jonsson's opinion was important. The mayor was closely involved with the project, the prime mover in the drive to build a new city hall and in the selection of the Pei firm to create it.
The project was conceived in the aftermath of the assassination of President John F. Kennedy. Dallas was reeling from the tragedy, grieving and simultaneously under attack from around the world as a "City of Hate."
When Jonsson took over as mayor, he made it a priority to reinvent the city's image, and created the "Goals for Dallas" program to accomplish this transformation. One of goals, Design of the City, was summarized by the statement, "We demand a city of beauty and functional fitness that embraces the quality of life for all its people." That was the impetus to build a new city hall to replace the small and outdated structure that had served Dallas since the early 20th century.
Musho relates that a study of the space requirements of city government concluded that a great deal of square footage was needed for the functions of the bureaucracy, but much less was required for the specific activities that interfaced with the public. "Dallas had an outreach mentality, though," recalled Musho in a recent interview. "People still came to City Hall to pay their water bill."
Musho and Pei recognized that the building had to welcome the public, so they wanted to concentrate the offices and counters where the public came to conduct its business at ground level. This suggested a small space at the bottom of the building, with increasing floor space higher up to house the offices that ran the government. As they began to play with sketches, the inverted pyramid profile took shape.
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This case study by Steven H. Miller was previously published in the August 2008 issue of the PTI Journal under the title "An Inclination for Innovation." It is presented here with permission of the publisher.