by Michael Chusid
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.
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
Library director David Zeidberg worked
with design-build contractor,
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.
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.
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.
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
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.
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
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
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.