Seismic Innovations & Enhancements on the East Span
Hinge pipe beams are not the only innovation in the SAS. The single, 525-foot tall tower is made up of four separate steel legs connected by shear link beams, which allow the legs to move independently. The beams are designed to absorb most of the seismic energy during an earthquake and protect the tower from catastrophic damage. The damaged beams can later be removed and replaced.
Two massive marine foundations provide solid support for the tower and eastern end of the SAS, while a land-based foundation on Yerba Buena Island supports the western end. The tower’s foundation includes 13 drilled shafts that extend 196 feet below water to anchor into bedrock. The 16 piles in the eastern support reach down nearly 340 feet to reach bedrock, while the western support’s foundations extend down 80 feet through the island’s solid rock.
At the other end of the East Span is the Oakland Touchdown, where much of the seismic innovations can be found in its foundations. Columns are pinned at the top to reduce severe motion from being dispersed to the foundations and to the road-decks. The foundations utilize flexible vertical piles to absorb seismic forces more effectively. The OTD also features hinge pipe beams where it connects to the Skyway.
The existing East Span’s seismic upgrade began with an interim retrofit of the original span that echoes much of the permanent work on the West Span. Original rivets were replaced with high-strength bolts, and viscous dampers, steel plates and steel lateral braces were added. At the section that collapsed, during the 1989 earthquake, expanded hinge seats were added to allow greater movement in that area.