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Concrete Ribbons Beautify Courtyard in San Francisco

Concrete ribbons, benches made of twisted concrete
The benches have a different finish on each face.

When the U.S. General Services Administration decided to renovate the federal office building at 50 United Nations Plaza in downtown San Francisco for its own use, officials set out to make the building a model of environmental sustainability.

The American Recovery and Reinvestment Act of 2009 funded the project — the 77-year-old building’s first renovation — and international firm HKS Architects undertook a renovation that included environmental remediation, installation of environmentally friendly building systems, restoration of historically significant interiors, and restoration of the central courtyard.

When the building was constructed in 1936, architect Arthur Brown designed a courtyard with classical fountains, which were never built. For the courtyard renovation, sculptor Cliff Garten conceived a design of plaited ribbons of concrete pavement accented by modern cubical granite fountains. Sections of the ribbons appear to rise from the pavement and twist to form benches.

Garten wanted the precast benches to complement the color of the granite fountains, and the pavement to match the benches.

To cast the benches, Garten called on Quick Crete, a precast-concrete manufacturer based in Norco, California. “We work with Cliff quite a bit,” says Rick Crook, Quick Crete’s president.

Garten created drawings of the benches in Rhinoceros, a 3-D modeling program. Quick Crete used the Rhino files to CNC-machine a model of the bench. Then technicians laid fiberglass around the master to create a mold for the concrete. Quick Crete’s mold-making process is proprietary. “We use CNC pieces,” Crook says. “It’s a little bit of technology and a little bit of old-fashioned work with your hands.”

Concrete benches, courtyard, United Nations Plaze

Each bench was made in two or three sections. A traditional precast joint is 1/4 inch, Crook says, but at Garten’s request, Quick Crete made the tolerances tighter than that to make the joints inconspicuous. After Quick Crete staff fitted the sections together at the plant and Garten approved their appearance, they transported the sections to San Francisco and shimmied them together.

Each exposed face that twists around the benches has a different finish. One is polished and the other two have different sandblasted finishes. “It looks like you’ve got a lot of motion there,” Crook says.

As for the pavement ribbons, general contractor Hathaway Dinwiddie poured those using a mix formulated to match the color of the benches. The ribbons of pavement border pervious walkways made with decomposed granite. Plantings of birch trees and shade-tolerant, drought-tolerant plants complete the courtyard.

The 50 United Nations Plaza project is expected to earn a LEED Platinum rating, and with its sustainable plantings and recycled concrete, the courtyard is expected to earn a LEED Gold rating. But the courtyard is a treasure for other reasons as well. Don Douglass, fine artist specialist for the GSA Pacific Rim Region’s Art in Architecture program, calls it “a wonderful place for employees to relax momentarily during their workday.”

Concrete benches, courtyard, United Nations Plaze
San Francisco general contractor Hathaway Dinwiddie poured ribbons of pavement that define pervious walkways made with decomposed granite.

Project at a Glance

Client: U.S. General Services Administration

Decorative concrete contractor: Quick Crete, Norco, California | www.quickcrete.com

General contractor: Hathaway Dinwiddie, San Francisco | www.hdcco.com

Design and project management: HKS Architects | www.hksinc.com

Scope of project: Precast concrete benches integrated with pavement in an undulating design

Most challenging aspect: The precast benches had to match the pavement; sections had to be fitted together with tight tolerances.

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