Engineering a Pei Cantilever - Dallas City Hall
Pei wanted the building to feel like an organic product of the region. He felt this would be achieved by using a buff-colored concrete that resembled the local earth tones.
"Color of the concrete was the issue," recalls Musho. "You can't put a block of concrete that big on the Earth without doing it in the right color. That buff color was the rose for the lady."
Another important issue, both aesthetically and structurally, was to minimize shrinkage cracking in the concrete. The design team toured the region observing large cast-in-place buildings, paying attention to cracking characteristics. Then, different aggregates and mix designs were tested, casting more than 50 sample panels, but none were entirely satisfactory.
Rosenlund had some previous experience with shrinkage-compensating concrete and believed it would provide the needed control of drying-shrinkage cracking and reduce loss of post-tensioning force due to drying shrinkage.
Local cement producer Texas Industries was a licensee of the patent for Type K cement used to make shrinkage-compensating concrete. They had already produced buff-colored portland cement, made by oxidizing the clinker in the kiln. For this project, they developed a buff-colored Type K cement. Amberg recalls that the Pei firm felt it was not "buff enough" and induced the producer to enhance the color by reprocessing the clinker with an extra burn in the kiln.
Consistency of color was the focus of a great deal of effort, because concrete color is affected by many variables including the batch-to-batch variations of cement and aggregate properties, placement temperature, water-to-cementitious-material (w/cm) ratio, and curing conditions. Texas Industries was also ready-mix supplier for the project, and made a commitment to keep the concrete consistent over the period of 18 months needed to finish the building.
"If the project were being done today," explains Louis Valenzuela, Type K product manager for CTS Cement Mfg, the current producer of Type K cement, "the buff color of the shrinkage-compensating concrete would be created by adding integral pigments to the mix at the batch plant, a technique that gives a wide range of available colors."
The formalism of design is echoed in the regulated way it was constructed. The formwork, for instance, was not simply specified: the orientation of every sheet of plywood was designed with an eye towards the final pattern the wall would present.
Pei's firm used repetition of structural elements as a way of making a demanding design more affordable. The coffered ceilings were formed on fiberglass domes that could be reused numerous times without deterioration. The walls were originally specified as board-formed, but this was changed to a phenolic resin-coated plywood that was extremely dense and smooth, and also extensively reusable. The 4-by-8-foot (1.2-by-2.4-meter) sheets were held in form guides to keep alignment uniform and joints tight. Musho comments that the surface texture produced by those forms was extremely smooth.
The temperature extremes in Dallas over the course of a year posed a challenge to concrete color-consistency. "Aggregates were stock-piled in the blistering Texas sun," recalls Amberg. "and you could fry a chicken on them."
There was also concern that built-up heat of hydration within the massive concrete walls would cause cracking. A placement-temperature of 85 degrees Fahrenheit (29.4 degrees Celsius) was specified. According to Bob May, who was then Texas Industries' Vice President of Concrete, extraordinary measures were required to meet that specification, including heating the aggregate in winter, and cooling the mix water and aggregate in summer using liquid nitrogen.
The Pei firm insisted on building a small mock-up before embarking on construction. This was a regular practice of the company, intended to allow all the workers to become familiar with the specific practices demanded by the project, and how they all functioned together. Pei would then request that the same crews perform the same tasks throughout construction.
The mock-up was a freestanding structure 14'-0" (4.3 meters) wide, 23'-4" (7.11 meters) long, and one story in height, comprising three typical sloped windows and nine ceiling coffers. It was complete with lighting fixtures, so that as many trades as possible could see how their work fit into the whole. It also gave the architect an opportunity to work out methods with the contractor. It cost $100,000, just 0.3% of the total cost of the building, which Amberg considers well worth the price.
Crack control was a priority. Close attention was paid when the basement levels of the bearing walls were placed. Since that area would not ultimately be seen by the public, it was considered a good test-location to observe the performance of the shrinkage-compensating concrete and work out any final adjustments in its mix.
Concrete was much more massive below grade level, however: it was solid across the 14-foot (4.3-meter) width of the wall-pairs. Small cracking was noted, and adjustments were made in mix design, placement and curing techniques. Valenzuela explains that proper curing is critical with shrinkage-compensating concrete: it must be kept wet for seven days after placement, the expansion-phase of the material.
There is evidence that the cracking in the basement was caused by heat build-up creating a level of stress that could not be compensated. Below ground, the walls are not separated by a gap; they are 14-foot- (4.3-meter-) thick masses of concrete, which were subject to high buildup of the heat generated by the chemical reaction of cement hydration. Thermocouples placed in a wall on a hot day showed a temperature differential between the concrete interior and exterior of 50 degrees Fahrenheit (10.0 degrees Celsius): 100 degrees Fahrenheit (37.8 degrees Celsius) on the wall surface, 150 degrees Fahrenheit (65.6 degrees Celsius) in the concrete interior.
Professor Milos Polivka, an expert on expansive concrete from the University of California, was consulted. He calculated the internal stress as 175 psi (1,210 kilopascals), significantly higher than could be counteracted by the shrinkage-compensating cement available at that time. Advances in shrinkage-compensating technology now make it practical to compensate for tensile forces of 175 psi or more. Computing the cumulative width of the cracks, Polivka found that they were half as much as could be expected with conventional concrete, and concluded that the shrinkage-compensating concrete was performing properly.
Polivka's conclusion was proved correct as construction progressed. In the above-ground levels, where the walls are not as thick, there was little or no cracking. The crack-control effect was judged satisfactory by Rosenlund, and even satisfied the notoriously exacting standards of the architect. Thirty-five years later, the above-ground levels of the building remain largely crack-free.
At the point where the bearing walls met grade level, there was a dense forest of steel reinforcement bar to help transfer load to the extending "foot" of the foundation. Concrete placement was especially tricky there, and even with careful vibration, consistent distribution was difficult. Eventually, superplasticizer was added to make the concrete flow more readily into the tangle of steel.
Forms were kept in place an unusually long time, according to Amberg, and were covered with wet blankets to cure the shrinkage-compensating concrete. The concrete was specified as 4,000 psi (27,600 kilopascals) compressive strength, with post-tensioning stress to be applied when the concrete achieved 75% of its target strength, or 3,000 psi (20,700 kilopascals). Shrinkage-compensating concrete is documented to achieve approximately 20 to 25% greater compressive strength than portland cement concrete of similar mix design, so 3,000 psi was reached in as little as four days in parts of the City Hall, and in no case longer than six days.
Construction took five years. The building was dedicated with great ceremony in 1977.
Test of Time
More than 30 years after construction was completed, the building stands as proof of the structural soundness of its design. Methodical control and consistent practices paid off with highly uniform color and texture of the structure's finish.
Contemporary architectural criticism of the building was generally positive, and inspired terms like "monument" to describe it. It had its intended effect on the city, launching a series of important structures in Dallas that brought a sense of pride and dignity.
Ed Rice, Chairman of CTS Cement Manufacturing Company, and David Crocker of Texas Industries contributed to the development of this article. The author would also like to acknowledge the generous assistance of John Slate, archivist of the City of Dallas; Stephen Fagin, oral historian of the Sixth Floor Museum at Dealey Plaza; Theodore Musho; Ted Amberg; Harry Barone; and Bob May.
Steven H. Miller, CDT, is an award-winning freelance journalist and photographer specializing in the construction industry. He is a consultant to Chusid Associates, an architectural materials and technology consulting firm, and is a regular contributor to the blog Building Product Marketing Online.
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.
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J. Erik Jonsson Oral History Interview 2 of 3, 8/17/1992, by Wes Wise with Bob Porter. Oral History Collection/The Sixth Floor Museum at Dealey Plaza, Dallas, Texas.
J. Erik Jonsson Oral History Interview 3 of 3, 11/10/1992, by Wes Wise with Bob Porter. Oral History Collection/The Sixth Floor Museum at Dealey Plaza, Dallas, Texas.
Papademetriou, Peter, "Angling for a Civic Monument," Progressive Architecture, May 1979.
Pastier, John, "Bold Symbol of a City's Image of Its Future," AIA Journal, Mid-May, 1978.
Polivka, Milos and Willson, Cedric, "Properties of Shrinkage-Compensating Concretes," from Klein Symposium on Expansive Cement Concretes, Publication SP-38, American Concrete Institute, Detroit MI, 1973.
Rosenlund, Jack E., "Use of Shrinkage-compensating Concrete in the Dallas Municipal Center," from Cedric Willson Symposium on Expansive Cement, Publication SP-65, American Concrete Institute, Detroit MI, 1980.
Wiseman, Carter, I.M. Pei: A Profile in American Architecture, revised edition, Harry N. Abrams, New York, 2001.
Design Architect: I.M. Pei & Partners
Associate Architect: Harper and Kemp, Dallas
Structural Engineer: Terry-Rosenlund and Company, Dallas
Mechanical and Electrical Engineer: Gaynor and Sirmen, Inc., Dallas
Urban Design and Master Planning: I.M. Pei & Partners
Landscape Architect: Myrick, Newman & Dahlberg, Dallas
Planning and Traffic: Ponte-Travers & Associates, Clifton, New Jersey
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