The unique architectural design of the roof of the
Crystal Bridges Museum of American Art in Bentonville, Ark., is as distinct as the collection it showcases. Supported by Wal-Mart heiress Alice Walton and the Walton Family Foundation, the museum displays her collection of four centuries of American art.
Designed by renowned architect Moshe Safdie, the museum is located in a ravine with a creek fed by Crystal Springs, from which the museum draws its name. Made up of seven buildings, the museum and cultural center features two suspended-cableand- wood buildings that span the ravine, creating two ponds. The project team explains that the structures serve as dams as well as bridges. “The design aims to protect the natural beauty of the site and create a powerful sense of place in harmony with its setting,” says Safdie Architects. “Our goal was to use materials primarily drawn from the region, including wood, fieldstone and limestone aggregate. Copper roofs were a natural complement to these materials.”
Safdie Architects worked closely with M.G. McGrath Inc., Maplewood, Minn., to design the roof system, from the Grace Ultra vapor barrier from Grace Construction Products, Cambridge, Mass., to the 68,000 square feet of 20-ounce copper roofing, which was fabricated on-site.
The building forms and materials needed to integrate the roofs as a primary visual element of the museum complex, since they can be seen from all directions. “The geometry of the roofing system- concave on the ‘land’ buildings and convex on the cable-supported
‘bridge’ buildings-reflects the relationship of the building forms to the site topography,” says Safdie Architects.
To create the rod-suspended roof structures, engineers used bridge technology and turned to Portsmouth, R.I.-based Ronstan Tensile Architecture. A total of 650 rods are located in the building’s interiors, crisscrossed between the glulam beams of the suspended roofing structure, providing the roof diaphragms for the hanging bridge structure.
The project team explains that the primary challenge came in defining, detailing and documenting the convex ‘bridge’ roofs for construction. “The complex geometries of these roofs-in which there are no repeating conditions-required precise 3-D modeling and close coordination between architects, engineers, fabricators and builders.”
“There are seven unique buildings all with different radii and all presenting a unique challenge,” explains Matt Rassmussen, project manager, M.G. McGrath. For example, the Great Hall has a roof with geometry resembling a turtle shell, requiring the craftsman to customize every panel.
To allow for the controlled use of natural light throughout the building interiors, the copper roofs and their glulam structural members are designed to accommodate the integration of skylights. Mequon, Wis.-based Super Sky Products supplied and installed the skylights, working closely with M.G. McGrath to successfully integrate the two systems.
“The skylights are designed to move and flex with the structure,” says Rassmussen. “The copper roofing had to effectively seal the skylight system but also allow it to move freely.”
According to Safdie Architects, copper was the material choice for the museum roofs from the beginning. “Because of the excellent air quality of the site environment, natural copper will probably never achieve the green patina of aged copper. However, the natural aging process of the oxidizing copper will reflect the constant evolution of its natural setting. Copper was also chosen for its ability to accommodate the varying geometries and dimensions of the roof, as well as its longevity-lasting for many decades with little maintenance.”
“Copper has been the best choice for long-term roofing solutions that greatly accentuate a building’s design and give it a look unlike any other,” Rasmussen adds. “As the copper ages it will draw the eye more and more to appreciate the design and flow of the museum.”
The project took 14 months for McGrath to manufacture and install the 68,000 square feet of 20-ounce copper roofing, which was completed in September 2011. The museum officially opened on Nov. 11, 2011.
Crystal Bridges Museum of American Art, Bentonville, Ark.
Completed: November 2011
Total Square Feet: 93,000 square feet
Owner’s representative: Hines, Houston
Architect: Moshe Safdie and Associates Inc., Somerville, Mass.,
www.msafdie.com
Construction manager/builder: Linbeck/Nabholz Joint Venture, Bentonville
Engineer: Buro Happold, New York City
Metal roofing panels/installer: M.G. McGrath Inc., Maplewood, Minn., www.mgmcgrath.com
Skylights: Super Sky Products, Mequon, Wis., www.supersky.com
Tensile rods: Ronstan Tensile Architecture, Portsmouth, R.I.,
www.ronstantensilearch.com
Vapor barrier: Grace Ultra, Grace Construction Products, Cambridge, Mass., www.na.graceconstruction.com