The view of San Francisco from atop the self-anchored suspension (SAS) span’s 525 ft support tower is stunning. Photo courtesy of Caltrans
The seismic retrofit of the east span of the Bay Bridge, linking San Francisco and Oakland, moves closer to completion with delivery of the final four deck sections for the dramatic self-anchored suspension span.
The $5.4-billion seismic retrofit of the east span of San Francisco’s Bay Bridge will move forward this fall with the installation of the final four deck sections of the self-anchored suspension span (SAS), the most complicated link in this comprehensive project. The enormous sections were delivered to the site in late August. Workers are scheduled to raise them into place before the end of the year, according to the Bay Bridge Information Office (BBIO), a joint effort of the California Department of Transportation (Caltrans), the Bay Area Toll Authority, and the California Transportation Authority to keep local residents informed about the project.
The SAS, which spans 2,047 ft, is the engineering centerpiece of the Bay Bridge project. It differs from most suspension bridges in that it uses a single support tower and a single cable that is nearly 1 mi long. That cable, which is 2.6 ft wide and weighs 10.6 million pounds, will anchor into the east end of the roadway on one side, pass over the tower, loop under the west end of the bridge, pass over the tower again and anchor onto the opposite side of the roadway. The cable comprises 137 strands of 127 5 mm diameter wires, according to Bart Ney, the senior communications manager for Caltrans.
The SAS will be supported by a single cable, nearly a mile
long. Illustration courtesy of Caltrans
Workers will begin to maneuver the cable into place next year with the help of orange mesh catwalks, 12 ft wide, installed 4 ft below the cable’s ultimate location. The catwalks increase in slope to reach 28-degree angles on the east side of the tower and 35 degrees on the west side. Fluorescent work lights illuminating the catwalk give residents a sneak preview of how the finished bridge will appear when the cable is installed and lighted.
“The cable will be placed, strand by strand, on the catwalk system,” Ney said in written responses to questions from Civil Engineering magazine online. “The catwalk is a work platform for the construction crew and also a place for the hardware necessary to place the main cable. A gondola system similar to a ski lift will pull each of the one hundred thirty seven strands across the catwalks.”
The 280,000 motorists a day who travel the Bay Bridge to and from work have witnessed steady progress on the SAS this year. The single support tower has risen in sections from a concrete and steel foundation 85 ft long, 73 ft wide, and 21 ft thick, anchored by 13 piles that are 10 ft in diameter and extend 196 ft below the water level, deep into bedrock, according to the BBIO.
Workers will use these catwalks to pull the strands of the SAS
cable into place. Photo courtesy of Caltrans
The tower is built in four separate legs, joined with shear link beams designed to allow each leg to move independently in an earthquake, absorbing energy without compromising the bridge. Rising 525 ft above the water level, the tower was completed this summer after workers hoisted the world’s largest double cable-saddle to the top, Ney said. The saddle will distribute the cable’s immense weight evenly over the four beams.
The Bay Bridge retrofit resulted from an extensive seismic study undertaken following the Loma Prieta earthquake on October 17, 1989. During the earthquake, a 6.9 on the open-ended Richter scale, a section of the east span, which leads into Oakland, was damaged as the bridge shifted several inches, sheering bolts, and collapsing a section of the upper deck onto the lower deck. One motorist died in the collapse. The bridge was repaired and reopened one month later. (See “Aftershocks: The Legacy of Loma Prieta,” Civil Engineering, January 1994, pages 46-49.)
The seismic study revealed that some elements of the complex bridge could be retrofitted, while other sections required replacement. A retrofit of the west span of the bridge, leading into San Francisco, was finished in 2004. (See “Replacing the East Bay Bridge,” Civil Engineering, September 2000, pages 38-4). Engineers specified the installation of 96 viscous dampers for the span, the replacement of 500,000 of the original 1930s rivets, and the addition of 17 million pounds of structural steel in the form of bracing, plates, and replaced members. The bridge was open during much of this phase of the project, which involved 1,000 workers and took five years even with 24-hour-a-day schedules, the BBIO reported.
A crane begins to hoist a section of the huge central tower
into place. Photo courtesy of Caltrans
Other elements of the bridge include a sweeping skyway, which extends from the SAS to the Oakland Touchdown section. The skyway comprises 452 massive precast concrete segments supported by 14 sets of piers, anchored by a total of 160 piles, driven more than 300 ft into the bay’s mud by one of the world’s largest hydraulic hammers, generating 1.2 million pounds of force, according to the BBIO.
The west approach in San Francisco was completed in 2009. The Oakland Touchdown, which will reroute traffic from the existing east span to the skyway and SAS, is under way. The Bay Bridge seismic retrofit is scheduled to be complete in 2013.