Construction continues on the massive Transbay Transit Center in San Francisco designed by Pelli Clarke Pelli Architects (PCPA). At 1,425 by 171 feet, the immense footprint puts this 75-foot building in the groundscraper category. As San Francisco's answer to New York's Grand Central Station, the project will be a hub for 11 local and regional bus and rail transportation systems, with retail and office space available, and a 5.4-acre public park on the roof. 

Rendering of Transbay Transit Center with Salesforce Tower and 181 Fremont visible in background, image by Pelli Clarke Pelli

Construction on the site began in 2010, after PCPA won a 2006 competition for the new San Francisco transit hub. The firm also designed the neighbouring Salesforce Tower, with support from the updated Transit Center District Plan which removes density caps in the area and built in some exceptions to the city's height restrictions to create a ring of highrises around the Transit Center. In all, 640,000 cubic yards of sand and mud were excavated from the 1,650 by 182 foot site. Not only did this excavation turn up thousands of 1930s timber piles, crews also discovered an ancient human skeleton and a mammoth tooth.

Transbay Transit Center construction site, image courtesy of Cast Connex

The site's proximity to the San Francisco Bay and the San Andreas fault pose particular structural challenges. Ground water can be found just 10 feet below the surface, and so the building must be literally anchored against flotation. As an essential facility, the transit center also needs to be available for immediate occupancy in the case of a magnitude-8 earthquake. 

3d section with structural details, image courtesy of Cast Connex

In order to weather such a severe earthquake, the building depends on a series of custom cast steel moment frames spanning the width of the building. Each tree-like frame bifurcates at the ground floor and bus deck levels, calling for more than 75 individual cast node geometries where different columns meet.

Base of the Transbay Transit Center Light Column, image by Cast Connex

About a third of those unique node geometries were required for the building's 150-foot light column - a design element that will bring natural light into all levels of the project. In the construction photo above, you can see a steel ring made from cast steel nodes that have been welded together to form the heaviest loaded ring in the light column.

Rendering of the Light Column at Transbay Transit Center, image by Pelli Clark Pelli Architects

Once construction of the steel structure is complete, crews will begin work on the rooftop park. With 500 trees requiring two to six feet of soil depth, a 1000-foot long fountain, and a 1000-seat amphitheater, the park materials are expected to load an additional 56,000 tons onto the building. The roof will also include wetlands for storm and grey water pretreatment, which will contribute to the LEED Gold certification that the project anticipates. Phase one of the Transit Centre is expected to wrap up in 2017, with a second phase to extend the underground Caltrain line conditional upon funding.

Rendering of rooftop park, image by Pelli Clarke Pelli Architects

For more renderings, check out the dataBase entry for this project that is linked below. Let us know what you think about this project in the comments and associated Forum thread.