One of the biggest architectural challenges involved the huge glass walls and roof that had a combined surface area of 33,000-square-feet and rise more than 120 feet in the air. The vertical aspect of the glass envelope on the northwest elevation needed to be as transparent as possible to provide passers-by with a seamless view of the cascading water. Because this elevation points out toward the harbor, occupants of watercraft also enjoy the view. A lightweight, unobtrusive roof structure underneath the skylight was designed to avoid detracting from the natural habitat within. To challenge the design team further, only a limited budget was available for the monumental glazed atrium. 

Propose and Solve
In resolving the design and construction challenge of the glazed atrium, CSP began by developing the architectural form of the space. A sloping glazed skylight sits atop two glazed walls and slopes in two directions at approximately 15 degrees. In plan, the skylight is square with dimensions of approximately 110 x 110 feet. The Northeast wall (B wall) measures 95 inches with a high point of 120 feet sloping down to 95 feet. The Northwest wall (A wall) faces the harbor and measures 120 feet wide with a high point of 98 feet tapering down to 68 feet.

Though transparency was a key design objective, it became clear early in the process that there were not enough funds available to achieve it on all glass surfaces. So the A wall and skylight were targeted as the key surfaces to accomplish a light and airy design effect. To add to its natural appearance, the A wall has an 11-foot return at the parapet and cantilevers 12 feet beyond its intersection with the B wall. The resulting glass plate seems to float, appearing independent of the structure. To enhance its transparency, CSP conceived a point-fixed glass wall supported by a lightweight framing system. To create a more subtle roof structure, while being sensitive to budget, the architect envisioned a lightweight truss used to support a more conventional glazing system.

For the A wall, thermal specifications required insulating glazing panels with a low-E coating. Because the natural appearance of the waterfall when viewed from the outside was paramount, low-iron or water-white glazing was recommended to limit the green cast. To reduce structure, large insulating lites 6-feet tall by 12-feet wide were selected. These were attached to the structure using a system that has articulating fastener heads that allow the glass to rotate out of plane while reducing stress at the hole. Because insulating, point-supported glass is particularly sensitive to stress at the support holes, the stress has to be limited to avoid seal failure and condensation into the unit. After a finite element structural analysis, it was determined that the glass would need to be fixed at six points per panel. 

To support the wall and tie in with the roof structure, four vertical-tension truss masts were engineered. The columns were pre-tensioned with cables in order to reduce the vertical pipe diameter to 12 inches. Horizontal, secondary, “mini” masts were used at each glass joint. It was possible to enhance transparency with a 3.5-inch pipe that was tensioned economically by painted rods. Further economy and transparency was provided by reducing the vertical hangers used to support each mini mast from five to two locations. 

For the skylight, insulating glazing was again used with a low-E coating. Additional savings resulted from substituting a two-sided captured system in lieu of the four-sided structurally glazed concept originally considered. A lightweight, yet economical, planar tension truss was designed by combining a wide flange top chord, tubular webs with a galvanized cable as the bottom chord. The roof trusses were stabilized in the other direction by adding additional pre-stressed cables perpendicular to the bottom chords. 

Architects' Guide to Glass & Metal
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