by Michael Chusid

Although precast concrete is an outstanding architectural material, its heavy weight can limit where and how it's used. A recently completed project demonstrates how a relatively new type of lighter-weight hybrid wall system combining cold-formed metal studs and precast concrete can expand opportunities to apply precast technologies.

The new Munger Research Center in San Marino, California adds 90,000 square feet (8,400 square meters) of space to the Huntington Library, home to an important collection of rare books, an extensive botanical garden, and a museum of fine art. The new building adds laboratories and work rooms for the conservation of historic documents, rare-book storage, administrative offices, and a reading room for scholars.

Design objectives were not only to safely house the library's collections, within a limited budget and tight schedule, but to blend with the neoclassical architectural style of the Huntington's original buildings.

Library director David Zeidberg worked with design-build contractor, Earl Corporation. Early in the project, they identified precast concrete as an appropriate cladding for the building's steel structure. It could be finished to match the plaster and natural stone of the existing buildings; precasting panels off site could accelerate construction; and its durability would make it suitable for the 100-year service life projected for the facility.  

Conventional precast concrete, however, would have been so heavy that it would have required additional and costly seismic bracing for the building's steel structure. Moreover, the need for a nearly airtight envelope — to maintain a protective interior environment for fragile documents — called for a minimum number of joints where air infiltration could occur and, therefore, a maximum panel size.

Wall Composition

Enter Metal Stud Crete. The company's technology makes it possible to prefabricate thin-shell concrete panels that are only two and a half inches (64 millimeters) thick, supported by light-gauge, cold-formed steel framing. Shear-transfer strips join the concrete and the metal framing to create a panel with composite strength.

Bert England, lead designer for the project and senior vice president of Earl Corporation, explains the panels' construction: The shear-transfer strip, he says, "is fabricated from galvanized steel sheet. The strips are screwed onto studs, and their Y-shaped flanges are embedded into the concrete to produce an economical and reliable composite panel."

Using the thin, lightweight panels, England continues, "enabled us to get the aesthetic and functional benefits of precast concrete without the normal limitations of the material. The panels were engineered to move independently from the structural steel frame, to resist cracking due to building movement, yet provide the long-lasting quality and appeal of concrete."

The panels' weight and strength made it practical to transport and erect panels up to 16 feet tall by 40 feet long (4.8 by 12.2 meters), much larger than most other wall panel systems. "It was very aggressive to make precast panels this large," says Bob Konoske, vice president and general manager of precast subcontractor Coreslab Structures, Inc. He explains that precast panels typically do not exceed 8 by 20 feet (2.4 by 6.1 meters).

"If these panels were a more conventional 4-1/2-inch- (114-millimeter-) thick precast concrete," Konoske says, "they would have been much heavier. Practically, we could not have made conventional panels this big; the panels would have had to be smaller, and more joints would have been exposed."

It is estimated that using the thin-shell composite precast panels reduced the length of joints on the Research Center by about 40 percent. Fewer joints, coupled with closed-cell foam insulation spray-applied to the interior of the panels, helped achieve a moisture barrier and thermal break, minimizing air intrusion and maintaining the required environmental conditions.

The large panels had to be shipped on a slanted easel at a 35-degree angle so they would stay under highway height and width limitations. Initial concerns that such large panels would be fragile were allayed after this test of their durability: surviving the 80-mile (130-kilometer) trip from Coreslab's plant to the project site without a single crack. There was also no cracking during installation, which was accomplished with a mobile crane.

Prefabrication Process

To create the panels, Coreslab used large flat casting tables with smooth fiberglass surfaces and side rails around the perimeter. The cold-formed steel framing was prefabricated into the required panel sizes, and the Metal Stud Crete shear transfer strips were screwed to the faces of the studs.

The framing was then set into the forms and secured in place above the casting table so that concrete could be cast to the required thickness. In some panels, it was necessary to pour the concrete first and then set the frames onto the concrete.

While the precast concrete is very thin, the designers wanted to recess the entrances and windows thirty inches (760 millimeters) to make the walls look thick and massive and to create dramatic shadows, as in the original building. Fabricating the deep returns required ingenuity to preserve the high-quality finish of the panels, and Coreslab chose to form the recesses in a two-step process.

First, they poured the concrete for the panel returns in a downcast position. The panel returns were then rotated into a vertical position and set into place in the forms so the panel faces could also be downcast. As a result of tight quality control, no pour lines or joints are visible at the transition between the two surfaces. Altogether, 325 precast components were cast and assembled to create a total of 146 building panels.

The architectural precast concrete contains integrally colored concrete and light colored aggregate. With a light sandblasted finish, the panels look like fine honed limestone.

The very large panels enabled almost all joints between panels to be to be concealed by architectural elements; vertical joints occur at changes in wall plane and horizontal joints are behind belt courses and cornice moldings. The result is an almost monolithic appearance, as if the entire building had been sculpted from a single massif of limestone.

Paul Clark, Jr., vice president of Metal Stud Crete, says this technology has been used to produce over two million square feet (186,000 square meters) of precast concrete panels, ranging from one-story load-bearing tilt-up walls to curtain-wall cladding for highrise buildings.

"This is the first time," he adds, "that the Metal Stud Crete system has been used to create panels with such deep returns. It demonstrates the design flexibility of thin-shell precast concrete." The system, he notes, is approved by the International Code Council Evaluation Service Report ER-5446.

Konoske credits the Metal Stud Crete system with allowing the period look to be achieved. And, given the project's technical requirements, this system was the only viable choice. "Metal studs and precast concrete is a nice marriage," Konoske says, citing strength, thin profile, and appearance.

The Research Center was completed on time and within budget. England says exterior walls accounted for just $1,500,000 of the project's $20,000,000 construction cost. Prefabrication began while the steel structure was being installed, and erection proceeded immediately after. The panels were installed in less than two months.

The technicians working in the labs and the scholars pouring over precious documents in the Huntington Library recognize the many benefits of the new structure. And thousands of visitors walk past the addition every day without recognizing that it is a new structure, so well does it harmonize with the traditional architecture of the institution's other buildings.

Courtesy: Architecture Week