Cullinan Throws a Curve
The gridshell is not a new idea. In the 1940s, Frei Otto developed models using various techniques with suspended nets to establish and analyze the form of lightweight compression structures such as arches and shells.
Building the Gridshell
Local materials were used as much as possible, but volunteer curator Alan Old recalls that this was not always as easy as it sounds. "Getting the right type of green oak in the United Kingdom meant bringing it long distances by road," he says. "It was easier and quicker from Normandy, which is closer than Scotland or the West Country."
Shaped into a giant peanut shell form, the undulating structure is 164 feet (50 meters) long, 39 feet (12 meters) wide and 33 feet (10 meters) high. The gridshell is based on straight lines that are criss-crossed and twisted to form a rotating grid plane.
The shell was constructed, top-down, over a scaffolding platform, 25 feet (7.5 meters) above the workshop floor. Rather than fight gravity by pushing the grid mat into the air, the builders used gravity to shape the gridshell into its planned form.
The team of carpenters laid the grid of oak laths on top of the scaffolding cube. Each lath was 120 feet (36 meters) long (spliced together from six shorter lengths), only 2 by 1-3/8 inches (50 by 35 millimeters) in section, and spaced at 39-inch (one-meter) centers. The grid was connected at the intersections by specially designed node clamps.
The weather remained wet throughout the construction phase, which was fortunate, because it kept the wood's moisture content high, ensuring its flexibility. The carpenters performed a bending operation by lowering the scaffolding at predetermined points by a few centimeters each day. The engineers monitored the dimensions while the carpenters watched the oak laths. Whenever a piece broke, it was mended with solid blocking between laths at the point of failure.
The 20-foot (6-meter) lengths were then joined on site using traditional scarf joints to produce continuous laths. Of the 10,000 finger-joints in the structure, there were only about 145 breaks during shell forming.
After seven weeks, the edges of the grid touched the perimeter of the timber deck, which forms the ceiling of the basement store. The carpenters then applied additional laths horizontally and vertically to ensure the structure's rigidity.
Douglas fir joists were used to support the roof system made up of a Roofkrete covering that undulates with the shape of the gridshell. It comprises four layers of steel mesh stapled to the ply deck and covered with a trowel-applied cement layer.
Polycarbonate clerestories were used instead of glass because of the building's movement. The wood tends to expand and contract throughout the day, so glass was not considered flexible or safe enough.
Heavy timber frames were erected at the building's east and west ends. As well as forming the museum entrances, these large arches form and support the ends of the gridshell; they in turn are supported laterally by the gridshell.
The building is clad in locally grown 6.5-foot (2-meter) red cedar planks. They were originally a light brown but have now weathered to silver. Inside, an aluminum foil inner lining provides a layer of insulation and enhances illumination in the space.
The building's lower level, containing the archive storage, is dug into the chalk hillside. Its retaining walls are insulated on their outer faces, creating a temperature-stabilizing earth shelter. The thermal mass of the walls moderates the environment for the archive.
Key to the gridshell's construction were node clamps, or metal coupling plates, that were placed wherever wooden laths intersected on the grid. The clamps keep the long oak lath pieces in place while allowing them to rotate and distort, by bending and lifting, to form the three-dimensional structure.
A clamp consists of three plates, the middle one having pins to locate the grid geometry of the middle lath-layers. The outer plates hold the outer laths loosely in place, allowing sliding during the formation of the shell.
Old explains: "After the diagonal bracing was in place, two of the four bolts on the plates were tightened just enough to provide stiffness throughout the shell. Once the structure had reached its final shape, all four bolts of each coupling were tightened to compress the wood together."
Customized software was used to map the structure so that each joint in the lattice was positioned in a way that would allow the wood the right degree of curvature to form the shell.
Richard Harris of Buro Happold says: "The computer 'form finding' is based on the 'dynamic relaxation' technique. Dynamic relaxation is an interactive process of computer analysis that solves a set of nonlinear equations. The technique modifies an initial approximation to the desired shape by monitoring the kinetic energy of the model as it is made to oscillate."
This is the computer technique used in the modeling and analysis of tensile (fabric) structures. The established method was developed further by Mike Barnes, professor of civil engineering at the University of Bath to make allowance for the bending stiffness of the timber laths.
As you stand inside the Downland Gridshell, a number of uses come to mind — a barn, a domestic home and workshop, even a modern church. "You can see it as an engineer's building or a carpenter's building," says Johnson. "It's both — but it's also architecture."
Don Barker is a freelance writer and photographer in London, UK, who has lived and worked in Europe, Australia, Thailand, Sri Lanka, Hong Kong, and Singapore. He is a contributing editor to ArchitectureWeek.