LOOMING HIGH above Epcot Center in Walt Disney World, Fla., the worlds largest spherical structure, Spaceship Earth, has offered visitors a dazzling ride through the history of communications since 1982. This year, it is also the center of the resorts millennium activities.
Dr. Frank Heger, ASTM Fellow, was chief structural engineer for the 160 ft. diameter (48.8 m) steel orb from its beginning design in 1979, to completion of construction three years later. His firm, Simpson Gumpertz & Heger, Arlington, Mass., and San Francisco, Calif., was architect-engineer for the project. Its probably 180 ft. to the top, and about 20 ft. off the ground, he explains. The challenge was how to support this full sphere.
I dont know of any other full spheres that have been built using the geodesic framing concept. Most of the time these structures are half-spheres or three-quarter spheres, however, Disney wanted to have people be able to walk under it. So our contribution was to come up with, first of all, the practical geodesic geometry and structure and, secondly, the system for supporting the sphere along with the interior ride and show structure.
Geodesic geometry divides a sphere into 20 equal triangles with 12 poles, and further subdivides the resulting triangles. The construction went very smoothly and very quickly, recalls Heger, a former assistant professor of structural design at Massachusetts Institute of Technology. Its the kind of a structure thats like an erector set. If you make all the members exactly the right length, you bolt them all together and you dont need temporary scaffolding. It goes up very efficiently.
A supersized Mickey Mouse glove, wand, starbursts, and 2000 were erected near the sphere on a giant half-arch as millennium regalia. According to Heger, without knowing their father/son connection, Disneys exhibit contractor chose his son, Robert Heger, principal, Exhibit Engineers, Inc., Holliston, Mass., to design the structure for these large icons.
Involved in a broad range of international engineering and design projects, Frank Heger is also an award-winning structural researcher who has developed standards for nearly 40 years with ASTM Committee C13 on Concrete Pipe. He joined the committee in 1963 during a search for better ways to design concrete pipe, the subject of his doctoral thesis while on the MIT faculty. This partnership continued for decades after Heger and other faculty members formed Simpson Gumpertz & Heger and conducted sizable research projects on the structural behavior of concrete pipe. Today, 13 engineers at SGH are active ASTM committee members, including James Myers, an ASTM director and the chairman of Committee C24 on Building Seals and Sealants.
Heger was chosen for the Disney World project because of his prior collaboration on a 250 ft. [76 m] diameter, three-quarter geodesic sphere for Expo 67s U.S. Pavilion in Montreal. Both of these projects had their separate structural challenges, he says. One of the challenges for the Disney sphere was the design of the cladding. There was a concern that when it rains, as it often does in Florida, the rain would pour down on the people walking underneath the sphere. The cladding was designed as a double skin. One skin is made out of light gauge steel triangular panels that fill in the openings of the geodesic frame. They are waterproofed with a rubberized waterproofing. Then two feet out from the skin, supported on struts projecting from the geodesic frame, is the outer or aesthetic skin which is made out of aluminum frames; filled with Alucobond sandwich panels. Along each edge are one-inch wide slots that permit exterior rain water to flow between the two skins and be directed away by a large aluminum gutter at the horizontal diameter, thereby greatly reducing the amount of rainwater that can pour down on the people below.
Three months ago, Heger lectured to civil engineering graduate students at MIT about the challenges of developing unusual structures like the U.S. Pavilion and Spaceship Earthprojects that he really enjoyed. You dont get too many chances in your lifetime to design structures as interesting as these, he concludes. //
|Frank Heger, chief structural
engineer for Spaceship Earth.