Building a safe system
The glass plaza skylight may look like some of the other walkable glass projects around the country, but the resemblance ends there. The challenges in engineering were greater due to its wide span, size, and location—an external application exposed to the environment—as well as its dual function to act as both skylight and pedestrian way. Along with a robustness to handle live loads from above and slip resistance for walkability, the glass and frame system had to meet stringent seismic requirements for San Francisco as outlined by the American Society of Civil Engineers (ASCE) and the California Building Code (CBC).
Those familiar with the weather conditions in California’s Bay Area know weather resistance was also a vital component of the skylight. In addition to the non-skid surface, the unit had to be weatherproof and watertight. Finally, it had to be relatively easy and efficient to replace in case of damage, without sacrificing safety.
The eventual walkable glass floor the team designed comprises 290 1.3 x 1.3-m (4 x 4-ft) glass panels, each weighing about 453.5 kg (1000 lbs) within a unitized framing system suspended 36 m (118 ft) in the air over the station’s Grand Hall.
Each factory-sealed assembly unit consists of 13 layers of various glass types, measuring 165 mm (6 1/2 in.) overall thickness, in three sections.
The uppermost lite is the walking surface and acts as a sacrificial layer in case of breakage. It features solar reflective properties, translucent light diffusing laminating interlayer, and an anti-slip pattern silk print of 1.6 mm (163 mils) dots with 30 percent print pattern coverage.
The sacrificial glass layer on top was one of the first in the country to be tested for weather cycling. If the sacrificial layer experiences wear and tear or becomes damaged, the pane can be replaced onsite without removal of the lower fire-rated portion.
The second section ensures the loadbearing properties and comprises three layers of heat-strengthened, translucent laminated glass.
Following a black stainless steel spacer is a multichamber, fire-resistive glass that reacts to fire to block flames, smoke, and heat transfer for two hours. The bottom-most layer is laminated for overhead safety, and is clear.
Each panel sits in a load bypass frame system, which transfers the load from the structural glass without adding to the fire-rated glass layers. The sub-frame includes a backup V-channel internal gutter system in case of water infiltration. Seismic and thermal expansion joints down the middle of the structure and along the perimeter accommodate movement.
A specialty crating system was developed to allow a minimum of handling while permitting the framing and glass to be unitized and shipped direct to the site for installation.
Upon arrival to the jobsite, installers hoisted each crate to its designated spot, where the panel inside could be easily uncrated, strapped, and craned into place. Each panel is unitized for direct installation into the fire-rated structural steel floor system. The entire assembly is attached to the building’s primary concrete curbs.