The 65 ft tall interactive sculpture was created from wood from the 165 ft long schooner Wawona, which sailed from 1897 to 1947. It is connected to the museum at its top and bottom. © John Grade
The new sculpture at the Museum of History and Industry in Seattle required a great deal of structural analysis to ensure it can withstand seismic forces—and curious visitors.
January 29, 2013—It’s plain to see that turning the remains of a sailing schooner more than a century old into a 65 ft tall interactive sculpture for the Museum of History and Industry (MOHAI) in Seattle, required a great deal of artistic vision. But what might not be as obvious is the amount of structural analysis required to ensure the sculpture maintains its structural integrity under seismic forces and human-induced loads.
Dedicated to preserving and sharing the history of Seattle and the Puget Sound region, the MOHAI commissioned Seattle-based artist John Grade to create a sculpture for the museum’s new home: the renovated Naval Reserve Armory along the shore of Lake Union. Shortly after winning the commission, Grade was entrusted with the remains of the dismantled schooner Wawona.
Built by influential shipbuilder Hans Ditlev Bendixsen, the 165 ft long schooner Wawona sailed from 1897 to 1947, carrying lumber and, later, fish through Puget Sound. After it was decommissioned, the ship was on display until 2009, when it was disassembled. A great deal of the vessel’s wood had rotted significantly by that time, but much of the planking was in good condition despite having been under water all those years. Leaching from the ship’s wrought iron and temporary copper cladding and staining from the oils of the fish it once carried brought out a variety of colors within the wood, making it perfect for the sculpture, Grade said in written responses to questions posed by Civil Engineering online.
Named for the vessel from which the wood was derived, the 10,000 lb Wawona sculpture extends from the floor of the museum to its ceiling. Grade said the vertical orientation of the sculpture references the old-growth trees that once covered the landscape near the museum. Its tapered shape—roughly 10 ft in diameter at its base and approximately 4 ft in diameter at its top— recalls a ship’s hull, he said. To further connect it to the natural world, the sculpture penetrates the museum’s floor and ceiling, enabling visitors who walk within the hollow form to see the lake below and the sky above, Grade explained.
Creating such a large sculpture required a great deal of engineering expertise. Grade worked with the Seattle office of Arup, an international engineering firm headquartered in London, to turn his vision into a reality. “Arup provided structural engineering services, starting at concepts ... on through artwork permitting, construction documents, and construction administration,” said Hans-Erik Blomgren, P.E., P.Eng., M.ASCE, the project manager for Arup, in written responses to questions from Civil Engineering online. “Interactive artwork of this scale, weight, and permanence requires it to be treated as an element and component of the building from a code standpoint.”
Artist John Grade wanted visitors to be able to walk within the
sculpture, to be able to view the lake below and the sky above, and
to be able to touch and push on the sculpture without damaging it.
© John Grade
But before they began getting into the details of the structural design, Arup’s engineers suggested using digital technology to fabricate the components of the sculpture. As a result, Grade engaged the University of Washington College of the Built Environments to individually cut the sculpture’s more than 200 wooden pieces using computer numerical control (CNC) fabrication. The CNC technology was also used to fashion the steel hangers and rods that hold the sculpture’s wooden components together horizontally and vertically. The digital fabrication plans were developed parallel with the structural design.
Arup had several performance objectives for the project, including ensuring that the sculpture could endure human-induced movement, resist metal fatigue under cyclic motion, and withstand seismic forces. The team also had to ensure that the wood was kiln dried to the correct moisture content and that the sculpture would hang vertically through its center of mass over the correct location on the museum’s floor, Blomgren said.
To meet these objectives, Arup and the team from the college used Rhino 3D and a parametric plug-in tool known as Grasshopper (both produced by Seattle-based Robert McNeel & Associates) to model each of the sculpture’s steel and wooden components for fabrication and structural analysis. They then exported a wireframe model from the Rhino files into another program that enabled them to analyze the sculpture under its own weight, seismic forces, and human-induced loads, Blomgren said.
For the seismic forces, the team treated the sculpture as an element and component as defined in ASCE Standard 7-05 (Minimum Design Loads for Buildings and Other Structures, ASCE, Reston, Virginia, 2006.) “We then applied an equivalent static force in orthogonal directions,” Blomgren said. On the basis of the analysis, the team added rubber damper restraints at the base of the sculpture to provide lateral support under seismic loads and make the sculpture flexible between the floor and the ceiling, Blomgren said.
Arup engineers used computer numerical control techniques to
fabricate the more than 200 wooden pieces that form the sculpture
as well as the steel hangers and rods that hold those wooden
components together. © Arup
The human-induced forces were studied as a 200 lb force at a height of roughly 4 ft from the sculpture’s base. “We wanted to be sure the wooden planks supporting this concentrated load were sufficient,” Blomgren explained. As a result of the analysis, the team included a steel rod that extends from the underside of the sculpture to approximately 10 in. below the floor. This rod is allowed to articulate but a viscoelastic polymer disk provides resistance to quickly dampen human-induced oscillation, Blomgren said. “It was important to me that people directly, physically interact with the sculpture,” Grade explained. “I wanted it to have enough flex that a small child could push the base of the sculpture into movement but that movement would stop quickly and not endanger other people simultaneously interacting with the sculpture.”
Following 20 months of design, the Wawona sculpture was unveiled when the MOHAI opened at its new location on December 29, 2012. “The MOHAI is a fantastic and rich place full of cherished Seattle artifacts,” Blomgren said. “The Wawona sculpture, however, stands apart as something distinct because it is a new creation that employs state-of-the-art design but is richly and brilliantly tied to the Pacific Northwest’s history of shipbuilding, fishing, and old-growth forests.”