San Francisco is booming with construction and one of its largest and most challenging projects is changing the face of a downtown neighbourhood. Situated south of Mission Street from Second to Beale Streets, the landmark Transbay Transit Center (TTC) project is replacing the Transbay Terminal with a modern and integrated transit hub built to accommodate both current and future needs. BART, AC Transit, Caltrain, Golden Gate Transit, Greyhound, Muni, SamTrans, WestCAT Lynx, Amtrak, Paratransit, and the planned high-speed rail link to Los Angeles will all operate out of the new terminal designed by Pelli Clarke Pelli Architects.

The Transbay Transit Center stretches five blocks, image via Pelli Clarke Pelli

The five-storey structure contains one million square feet of space stretching five city blocks. With retail at ground level vertically bookended by a 2.2-hectare rooftop park, the complex serves more than just commuters. In addition to its functional application, the structure's architectural expression is mapped with an undulating translucent facade which masks some of the engineering secrets of the building from afar, but upon closer inspection, much of the building's structural features come into focus. Its massive steel frame provides architectural interest while performing the very important task of resisting earthquakes. A large light column spans the height of the structure and ensures an ample amount of sunlight fills the lower levels of the transit center. The design institutes 3.5 million pounds of steel castings spread over 75 cast node geometries.

Transbay Transit Center, image via Pelli Clarke Pelli

Toronto-based CAST CONNEX has provided their unique expertise and services in bringing these architecturally exposed cast steel nodes to life. SkyriseCities recently sat down with Carlos de Oliveira, Chief Executive Officer and Principal Structural Engineer of CAST CONNEX, who shed some light onto the project's intricate form and the challenges his company faced throughout the realization of the development.

Transbay Transit Center construction, image courtesy of CAST CONNEX

"It’s such an important project for the future of San Francisco. Not only is it going to be a bus and transit terminal, but it’s going to be the future hub for high-speed rail into the city," said de Oliveira. "There’s tremendous activity and boom in San Francisco and all of the politicians at state, federal, and municipal levels are behind the development. Like any big project, it’s had its challenges with respect to budget. But the biggest challenges related to budget really have nothing to do with an over-lofty design or anything like that, it’s just that the San Francisco construction industry is in such a boom right now that the prices are getting higher because the contractors are so busy. They can turn away work. Every contract that has gone over budget did so because the local construction industry is so hot."

Cross section of the TTC, image via Pelli Clarke Pelli

De Oliveira praised the funding model that was used to finance the project, hoping to see similar ideas take root in Canada. "When they decided where they were going to build, obviously there’s a number of considerations, but one of them was the potential for new development in and around the transit terminal. Basically the city set aside property around the future transit terminal and allowed the Transbay Joint Powers Authority to sell parcels of land with the revenue funding the project," said de Oliveira. "One of the biggest parcels that they sold was for the Transbay Tower which is going to be the tallest tower in San Francisco. It’s refreshing. They captured the value created by the project and invested it into the creation of the transit terminal. I think it was a brilliant funding model."

The Light Column situated within the 36-metre Grand Hall, image via Pelli Clarke Pelli

De Oliveira credits the project with spurring much of the construction focused downtown today. He also explained that because of the way the TTC has been designed, commuting will be significantly smoother. "A lot of people can’t afford to live in San Francisco, so people who actually work in the city live outside. A lot commute in across the Bay Bridge on buses and these buses get stuck down in local traffic and it takes forever to get in. From 3 p.m. into the night it’s just complete traffic chaos to try to get out of the city," said de Oliveira. "The TTC will have dedicated lanes across the Bay Bridge. There’s going to be new bridges that will go straight into the building, drop people off, and then go straight out and never touch the street." He explained that dedicated parking structures are also being constructed to house buses for AC Transit during weekday off-peak hours when the buses are not transporting riders between San Francisco and the East Bay. "It’s going to be a really incredible space with respect to transit and it created an entirely new neighbourhood."

The Light Column from the roof, image via Terri Boake

De Oliveira credits Pelli Clarke Pelli for envisioning a design that showcases the inner workings of the building. He likened the interplay between the facade and the steel frame to a forest. "It wears the structure on its outside, everyone can see the seismic system, and it’s built out in these really elegant looking trees. It’s going to be a centrepiece and a crown jewel for the city," he said. De Oliveira also pointed to the rooftop park and its 1000-seat amphitheatre, cafes, and lush green space which provide additional public uses for the site. "It will even have these really cool water jet systems designed by Bay Area artist Ned Kahn that run the length of the structure and which will be triggered by sensors that respond to the vibrations of the flow of buses on the deck below. So when a bus pulls into the second storey and rides along it, occupants of the roof park level will be able to experience the transit activity through the choreographed effect on the water."

The rooftop park, image via Pelli Clarke Pelli

San Francisco's building codes require structures to meet certain seismic standards, which the TTC exceeds. "It’s pretty well the most modern designed seismic force resisting system there is. It was designed using a performance-based design approach and so it’s beyond what is typically code-based. If you just follow the building codes, you’re designing a building so that it doesn’t collapse in a design level event," said de Oliveira. "There’s some protection with respect to the maximum considered earthquake, but this structure has been designed for a significantly larger event. It’s a structure that has to withstand the largest earthquake which is conceivable in the area so that if people are looking for shelter they’re going to be able to go into this structure. The seismic force resisting system is an eccentrically braced frame configuration that wraps the perimeter of the structure so you can see how it works," he said.

November 30 webcam capture from the site, image via transbaycenter.org

De Oliveira expanded on the specific elements of the building: "On the roof level, the beam that runs the perimeter of the structure, the spandrel beam, there’s a little gap of unsupported girder, and in the event of an earthquake, what will happen is each one of the architecturally exposed steel trees rocks back and forth, and that short segment of steel girder is intended to yield in shear and flexure. The trees are comprised of those steel pipes and steel castings and they remain predominantly elastic, so that the girder yields up and down and absorbs the energy input into the building by the earthquake."

The cast steel nodes comprising the Light Column, image courtesy of CAST CONNEX

He explained that there are girder sections spanning the entire structure, so its perimeter has been designed to resist an earthquake. While seeing the structure flex during California's next earthquake would certainly be fascinating, de Oliveira pointed out that would be unlikely. "To get to the point where those deformations would be perceivable to the eye would be a mega earthquake. This is really heavy structural steel construction."

November 30 webcam capture from the site, image via transbaycenter.org

By using cast steel as opposed to conventional fabrication, the project takes on a softer, more flexible look than the traditional rigid, industrial appearance of plates and bolts. "The connections are complicated. If you imagine that connection at the bus deck level, the first branch comes up from the ground level and bifurcates into two. So you’ve got the three tubular members that are framing into it, then you’ve got two beams at the sides and two from the back. So that one connection point is accommodating seven structural members from all different sides and angles," said de Oliveira. "You can imagine in an earthquake, that link beam which is yielding and shearing, is transmitting a huge amount of force down through the tree right through this joint that is carrying gravity load as well as transmitting these massive inertial forces down to the foundations. The connection is getting absolutely punished and crushed from every side and direction. So you have a geometrically complicated connection which is extremely heavily loaded. Already you’re into a point where castings can simplify that type of construction and achieve those types of performance metrics."

Closeup detail of cast nodes in seismic force resisting system, image courtesy of CAST CONNEX

He explained that castings can be designed to carry significant forces and address complex geometry. "Then throw in the fact that Cesar Pelli wanted these trees to be visible to view. Because it’s going to be exposed to view in a finished condition, it has to look good, and that’s also where castings provide significant improvements over conventionally fabricated connections," he said. Referring to the bolts and plates of conventional connections, he explained: "You wouldn’t typically want those types of connections to be exposed to view in the finished condition, you would probably cover them up with some sort of cladding. You can fabricate conventional connections to look good but it becomes very costly. When you start throwing all of this in — architecturally exposed, extremely heavily loaded and geometrically complex — castings just make a lot of sense over conventional fabrication." De Oliveira stated that the TTC implements the single largest use of steel castings in one structure. "The scale of the project is bigger than we had ever done before in terms of the number of unique geometries and the weights of the castings. It helped us hone our practice, but it wasn’t all too new to us."

President and CEO Carlos de Oliveira; Executive Vice President Michael Gray; Production Administrator Lisa Munt; Project Engineer Tarana Haque

While cast steel has been used in the construction of countless infrastructure projects in Europe and Asia, its application is still relatively new in North America. "The challenges we faced in this project included assisting the contractors in understanding how to work with castings and how to integrate the castings into their fabrication methodologies," said de Oliveira. "There’s a lot of assistance and support we provide to the users of our designs. Skanska USA had the steel fabrication and erection contract but they subcontracted all of the fabrication to a number of fabricators."

De Oliveira notes that despite the massive size of the project, completion remains on schedule for 2017. The second phase will include accommodations for high-speed rail and the underground extension for the Caltrain rail line. When complete, the "Grand Central Station of the West" will serve up to 45 million passengers per year and add a new world-class downtown neighbourhood that is now emerging from the construction dust.

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