Geode gives engineers a 17-surface puzzle
The development of plans for a large geode was clearly the most challenging assignment for McNamara Alumni Center's structural engineers, Minneapolis-based Meyer, Borgman and Johnson (MBJ). The unusual mass has 17 individual surfaces, all sloping at various angles, according to MBJ project engineer Jerod Hoffman.
The engineers used AutoCAD to translate the design for the geode's exterior cladding, separated by a 16-in. interstitial space, into a structural system that would support it. Each of the geode's 17 surfaces had its own computer-assisted drawing that referenced the same base model. This 3D-wire frame model was then imported into a structural-analysis computer program, which eliminated the traditional step of manually inputting the dimensional coordinates for each beam.
The thickness of the granite cladding varies from 1 1/4 inches to as much as 2 1/2 inches, depending on the plane of the surface to which it is attached. The thickest stone covers areas subject to the greatest wind and snow loads. About 90 percent of the stone has a thickness of either 1 1/4 inches or 1 3/4 inches.
Load data at the connections for each steel beam was extracted from the structural design program, organized in spreadsheet format and included as part of the structural documents. This data provided as much as six times the amount of information normally given to the steel fabricator, Hoffman says.
To minimize the flexure of the walls, which would transmit stress to the glazing and the stone joints, the framing was designed to be twice as stiff as normal.