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Daylighting is a defining element of San Francisco’s Salesforce Transit Center because it carries natural light from the rooftop park through the Grand Hall and toward train platforms below street level. The centerpiece is not a conventional roof window. It is a custom, walkable, fire-rated glass floor and skylight installation engineered to function as public paving, a weather barrier, and a light-transmitting building enclosure.
Completed in 2018 and designed by Pelli Clarke Pelli Architects, the transit center uses this “light column” to connect an above-street-level park, the Grand Hall, and platforms two stories underground. The project demonstrates how daylighting can become circulation infrastructure and civic architecture at the same time.
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The project in context
Salesforce Transit Center is a large, vertically layered transportation complex in San Francisco. A Transbay Joint Powers Authority fact sheet describes it as approximately one million square feet, about 1,400 feet long, with four levels above ground and two below. The Grand Hall is the primary public space, while the rooftop is a public park above the transit functions.
That arrangement created an architectural opportunity and a technical problem. A roof over the transit center could have made the hall below feel disconnected from the park above. Traditional electric lighting would provide visibility, but it would not create a visible relationship between the two public spaces. The design team instead made the roof itself transmit daylight, allowing people in the hall to see changing natural light and understand that another civic space is directly overhead.
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Building Enclosure’s September 21, 2018 article describes the feature as a key element of the center. Saint-Gobain Glass and Vetrotech later published project case studies with additional dimensions and product information. Those manufacturer accounts document the system’s design and reported specifications; they are not independent post-occupancy performance studies.
How the daylighting system works
Light begins in the rooftop park
Openings in the above-street-level park admit sunlight into a specially designed glass assembly. Instead of stopping at a ceiling, the light travels through a vertical sequence of glazed panels. The result is a light column that visually and physically links spaces with very different functions: an outdoor park, a busy arrival hall, and rail infrastructure below.
Saint-Gobain Glass describes the daylight path as extending more than 30 meters below the park. Building Enclosure describes the same relationship as more than 100 feet below the above-street-level park. These are approximate descriptions from different sources, and the metric and imperial figures are broadly equivalent.
The Grand Hall receives the main daylight effect
The walkable glass floor and skylight are located in the Grand Hall, the transit center’s principal public room. Sunlight passes through the horizontal glazing and produces a bright, legible focal point in a space that otherwise sits beneath a roof and below the park deck.
The largest light column continues the daylight source toward train platforms two stories below. Daylight is therefore doing more than illuminating one room: it helps establish a vertical orientation system. A traveler can read the building as a stack of connected public levels rather than as an underground hall isolated from the city.
What was built
Reported quantities vary slightly by source because one account counts panels while another describes sheets or individual glass pieces. The consistent picture is a large, bespoke installation rather than a small skylight.
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| Project detail | Reported value | Qualification |
|---|---|---|
| Glass installation area | About 1,400 square feet (130 m²) | Reported by Vetrotech for the CONTRAFLAM LITEFLAM-XT-120 project installation |
| Panels | 298 individual panels; another source says almost 300 | Building Enclosure gives 298; Saint-Gobain Glass uses “almost 300” |
| Glass components | More than 4,000 | Sources use “sheets” or “individual pieces,” so the count should be treated as approximate |
| Vertical daylight reach | More than 30 meters, or more than 100 feet, below the park | Approximate descriptions from Saint-Gobain Glass and Building Enclosure |
| Transit center completion | 2018 | Reported by the project and manufacturer case studies |
Vetrotech identifies the system as CONTRAFLAM LITEFLAM-XT-120. The name describes a fire-rated glazing product used in the project, but the complete floor-and-skylight assembly was developed for this particular building. It should not be interpreted as a standard, ready-to-install commercial skylight specification.
Why a walkable skylight is difficult to engineer
Two-hour fire protection
A transit center carries high occupant loads and must maintain protected routes during an emergency. The project called for a two-hour fire rating. That requirement applies to a horizontal glazed element that also forms part of a public walking surface, making the design more demanding than a typical vertical window.
Fire performance had to be considered together with framing, joints, supports, and the surrounding floor construction. A rating assigned to a piece of glass alone would not establish the performance of the installed floor; the complete tested assembly is what matters.
Pedestrian live loading
People walk over the glass, gather around it, and move rolling luggage across it. The glazing therefore had to resist pedestrian live loads and the concentrated forces that occur when feet, wheels, or dropped objects contact the surface. The assembly also had to remain stable under everyday movement rather than functioning only as a visual ceiling.
Public floors require a usable walking surface as well as strength. Slip resistance, edge details, visual comfort, and the ability to inspect damage are practical concerns for any project considering a transparent floor. The published project descriptions establish the loading requirement, but they do not provide a complete slip-resistance value, load test report, or maintenance schedule.
Seismic and blast resistance
San Francisco’s seismic environment made movement accommodation a central design constraint. The daylighting system was required to address seismic resistance, so supports, joints, and interfaces had to tolerate building movement without losing fire or weather performance.
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The project brief also included blast resistance. That requirement does not mean the glass is invulnerable to every explosive event; it means the design team had to account for specified blast conditions as part of the project’s protective design. The available case-study material does not state the tested pressure, threat scenario, or acceptance criteria.
Waterproofing above occupied space
The upper surface sits in an outdoor environment, while the Grand Hall and transit functions below need protection from rain. Horizontal glazing creates numerous potential leak paths at panel edges, joints, frames, and transitions to adjacent paving. Waterproofing therefore had to be integrated with the structural and fire-rated design rather than added as a separate finish.
Dirt and condensation control
Outdoor dust and foot traffic can quickly make a transparent floor look opaque. Temperature differences between the park, the glass layers, and the conditioned interior can also produce condensation. Vetrotech reports that Vetrotech and Greenlite Glass Systems combined horizontal fire-rated glass, structural glass, spacers, and air gaps to address dirt and condensation risks.
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How the glass can be walked on
A walkable glass floor is safe only when the entire assembly has been engineered and tested for its intended use. In this project, the design brief combined four demands that often conflict: transparency, a two-hour fire rating, resistance to pedestrian loading, and protection from weather and environmental exposure.
- Layered construction: Multiple glass layers and structural components provide capacity and fire protection rather than relying on one thin pane.
- Framing and support: The supporting grid transfers pedestrian forces into the surrounding structure and must accommodate movement at joints.
- Air gaps and spacers: These help manage condensation and keep contaminants from turning the light path permanently cloudy.
- Tested assembly: Fire, structural, seismic, and blast requirements must be evaluated for the installed configuration, including interfaces and restraints.
- Operational controls: A public owner needs inspection, cleaning, damage-response, and replacement procedures even when the original design passes its tests.
The available project accounts do not publish a complete maintenance manual or an independent record of long-term performance. That absence is significant for anyone attempting to copy the idea: a visually similar glass floor would not automatically have the same safety or durability characteristics.
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Daylight as wayfinding and public-realm design
In a transit facility, daylight can perform several jobs at once. It improves visual orientation, gives passengers a reference to the outside world, and marks a major public destination. At Salesforce Transit Center, the roof park and Grand Hall are connected by a visible environmental cue rather than by signs alone.
The light column also gives the underground platforms a recognizable architectural address. Travelers moving down through the building can follow the brightest and most distinctive spatial element toward the rail levels. This is particularly useful in a complex with multiple entries, circulation routes, and changes in elevation.
Daylight changes throughout the day and with weather. That variability can make a large hall feel less like a sealed infrastructure room and more like part of the city. It also creates design responsibilities: glare, reflections, contrast, and nighttime lighting must be coordinated so that the feature remains legible without compromising comfort or safety.
What the project teaches design and construction teams
Start with the complete performance brief
The Salesforce installation was not designed by selecting a decorative skylight and then adding protection later. The brief combined fire, structure, seismic movement, blast resistance, waterproofing, pedestrian use, and environmental control from the beginning. A comparable project should write those requirements before selecting glass, framing, or a daylight target.
Coordinate architecture and enclosure engineering early
A transparent floor affects the roof, slab, waterproofing, fire compartments, drainage, lighting, access routes, and finishes around it. Treating the glazing as an isolated specialty package increases the risk of incompatible tolerances and unserviceable joints. Early coordination should include the structural engineer, fire-protection consultant, enclosure specialist, glazing fabricator, transit operator, and facilities team.
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Any daylighting system exposed to pedestrians and weather needs an access strategy. Teams should identify how the upper surface will be cleaned, how condensation or dirt will be detected between layers, how a damaged panel will be isolated, and whether replacement can occur without closing a major passenger route. The case study confirms that these risks were considered, but it does not publish the center’s current inspection intervals or replacement process.
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Separate documented facts from transferable assumptions
The reported area, panel count, product designation, and project requirements describe this installation. They do not establish a universal cost, energy saving, service life, or performance advantage over other daylighting approaches. A new project would need its own calculations, mock-ups, fire tests where required, structural analysis, condensation modeling, and maintenance plan.
Why this is a case study, not an off-the-shelf specification
Kevin Norcross, general manager of Vetrotech Saint-Gobain North America, said in the Saint-Gobain account: “When ground broke on this project, this technology didn’t exist.” The statement captures the project’s development challenge, but it is a vendor and project-team characterization rather than an independent technical assessment.
Darin Cook, senior associate principal with Pelli Clarke Pelli, described the design ambition this way: “Architectural design is, in some ways, about imagining the unimaginable and bringing it to life.” In this building, that ambition required translating a conceptual connection between park and platforms into a tested enclosure system that people could occupy every day.
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Evidence and unanswered performance questions
The project sources establish the 2018 completion, the Grand Hall location, the approximate size and component counts, the vertical daylight route, and the principal engineering requirements. They also identify the CONTRAFLAM LITEFLAM-XT-120 system and the collaboration between Vetrotech and Greenlite Glass Systems.
They do not provide an independent post-occupancy study, a published lifecycle-cost comparison, measured daylight levels at the platforms, energy savings, current inspection findings, or a complete long-term maintenance record. Those questions matter when evaluating whether a similar feature is appropriate for another transit center. Until such data are available, the Salesforce project is best understood as a documented example of bespoke architectural glazing solving a difficult public-building problem, not as proof that every transit facility should use a glass floor.
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