Copyright (2011) California Department of Transportation, Photographer: Bill Hall
Ironworkers work to remove cable from shackles used to lift 1,100-ton shafts for the new Oakland-San Francisco Bay Bridge suspension span’s 530-ft-tall tower.
Photograph © Joseph A. Blum
signifi cant seismic events so that the roadway would remain open for emergency vehicles. T e entire Bay Bridge crosses from San Francisco to Oakland via
several connecting parts including a tunnel and two bridge spans. T e western suspension bridge was updated with extensive seismic retrofi tting. T e eastern section was completely replaced with the innovative Self-Anchored Suspension (SAS) Bridge. “What makes a SAS diff erent from the average suspension bridge is that the main cable used to uphold the bridge is anchored into itself creating a type of structure like that of a hanging basket,” says Nadar. Geological scientists predict that there is a 60 percent chance for
a major earthquake to hit San Francisco within the next 30 years. Each part of the SAS Bridge, the decks, the towers, and the wire ropes will all move in an earthquake, but are designed to not fully
24 MARCH-APRIL 2013 WIRE ROPE EXCHANGE
break apart. Some concrete pieces are designed fail and then be easily repaired after the event without having to shut down the bridge’s traffi c. Caltrans takes this project very seriously. “T e construction team has a photo of the 1989 bridge failure in every construction trailer as a reminder of why they are doing the project,” says Jordana Jackson, a representative of Caltrans.
THE BRIDGE’S ENGINEERING FEATURES T e SAS has just one tower and spans 2,047 feet across the eastern part of the Bay Bridge expanse, making it the longest self-anchored suspension bridge in the world. T e singularly-long cable stay is anchored underneath one side of the bridge and then anchored into the roadway on the other side. 200 suspender ropes then hang off the main cable stay and fully support the roadway sections below. T e tower has four independent legs that rest on soft soils rather than on stone as most other bridges that have a more solid foundation. “Soft soils off er very little resistance naturally for the foundation. T erefore, it was critical to design the bridge to move and fl oat during the shaking as diff erent pieces. We designed the bridge by looking at every microsecond of how the structure would react during a seismic event through a computer program,” says Nadar. When the tower supports move in an earthquake, there is a fl exible connector that will be able to cushion the tower supports so that they will not break apart. Likewise, there are pins holding together pieces of the roadway so that the roadway can also fl ex back and forth, up and down during the seismic event.
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