by Stephanie Miller
From the Willis Tower Ledge overlooking Chicago and the Grand Canyon Skywalk to the revolving observation deck of the Seattle Space Needle, building owners and architects have long leveraged the power of glass in high places to elicit awe from visitors.
Each of these marvels relies on multiple layers of structural glass, and a lot of engineering.
Yet, there remained milestones to reach. When it opened in August, the Salesforce Transit Center in downtown San Francisco not only became the city’s latest hub of activity, it unveiled what is believed to be the country’s largest fire-rated glass floor.
Designed by Pelli Clarke Pelli Architects, the transit center is a marvel on its own, standing out for both its size—stretching five blocks along Mission Street—as well as its unique floating, undulating exterior façade.
The 139,355-m2 (1.5-million-sf) building features four above-ground and two below-grade levels. The center will accommodate 11 different transit systems via a bus plaza and an underground station serving Caltrain and, eventually, California’s anticipated high-speed rail link. The bus plaza is located on the third level and offers easy access, directly off the Bay Bridge. At street level, a Grand Hall with an atrium features retail shops.
The roof level has a 2.1-ha (5.4-acre) park complete with amphitheater, fountains, play area, and restaurant, stretching 427 m (1400 ft). The park will include vegetation from around the world and 469 trees, including a South African garden, a Mediterranean basin, and a redwood forest.
Skylights dot several places along the transit center’s roof, with the most prominent being a long glass walkway ending in a glass dome. On the inside, the plaza and dome create a vast light column, channeling light through the outer bus platforms and helping to form the Grand Hall on the street level. Steel columns from the dome funnel down to yet another walkable glass floor on the retail level; this circular floor carries the daylight further down to the first subterranean level.
“This is a benefit for a lot of transportation centers that are dark and dreary,” explains Darin Cook, senior associate principal at Pelli Clarke Pelli. “We were trying to get a light-filled, exuberant space.”
This desire kept the skylight system in the plans after it was nearly axed due to cost. While one approach could have been to make the entire glass roof a popup or have a greenhouse sitting on top, Cook says the walkable surface allowed for true integration into the park.
|Fire-resistant glass skylight brings new life to NYU building|
|Seen from above, the walkable glass floor at New York University’s Stern School of Business provides one hour of fire resistance and allows plenty of natural light, while still providing privacy.
Though the Salesforce Transit Center in San Francisco is expected to have the largest fire-rated glass walkway in the United States, it is not the first. Another example is the Stern School of Business at New York University (NYU), where officials sought to bring much-needed light into two levels of dark, below-ground classrooms in the school’s 1960s-era main building.
To capture daylighting, architects at Perkins+Will took advantage of the space’s position underneath a vast outdoor plaza, designing a skylight system into the plaza itself that is part window, part walkway. The solution not only funnels natural light into the below-ground corridors, lounge areas, and classrooms, but also creates a unique aesthetic for pedestrians above.
Due to the nature of the skylight and its position at the entrance of the building, the city classified it as a roof assembly and therefore required a one-hour tested assembly. So the architects set out to create a durable, insulated, water-resistant skylight system that was structurally designed per building code for up to 45 kg per 0.09 m (100 lb per 1 sf) live load, was made of safety glass, provided one-hour fire resistance and load capacity, and allowed plenty of natural light while still providing privacy.
The fire-resistant skylight system combines walkable glass and steel frames laminated to 60-minute insulated, fire-resistive glass. Combining the technologies provides for a structural loadbearing top glass layer with a fire-rated bottom unit.
The steel support structure includes a layer of intumescent paint and expansion joints to meet fire-resistance ratings. The steel structure was fastened to the building, then the glass was structurally glazed to the frame.
The top layer of glass also has a raised pattern to provide durable slip resistance in wet and dry conditions.
“The skylight surpassed everyone’s expectations,” said Matt Cornett, the project’s architect. “When one enters the space, the impact and presence of the skylights is immediately apparent. The once dark corridors are now overflowing with natural light—an unmistakable light that could only come from outside.”