Caltrans photo. Dismantling trusses is a dangerous job.
The 77-year-old east span of the San Francisco-Oakland Bay Bridge is being taken down, piece by piece, as part of a Bay Bridge seismic retrofit project.
In fact, the new bridge was designed to be a lifeline in a large quake. It will be used to transport food and emergency supplies to San Francisco or Oakland in an emergency.
The new east span of the bridge opened in 2013. The self-anchored suspension bridge was built at a cost of $6.4 billion.
The east span being removed is located .5 mi. (.8 km) east of Yerba Buena Tunnel to 0.8 mi. (1.28 km) west of the bridge toll plaza. It extends 1,400 ft. (426.72 m) or 1.97 mi. (3.17 km) and is made of steel frames called trusses. The cost of removal is set at $512 million.
The bridge stands 382 ft. (116.43 m) above water at the highest point of cantilever.
The truss section is composed of three main parts: the anchor span, the cantilever span and the suspended span. Each keeps the other in balance.
Dismantling of the old bridge started after traffic was moved onto the new structure. The first job was to remove the steel superstructure. Then dismantling of the bridge foundation began. Complete removal of the old east span is expected sometime in 2018.
Funding for the project is provided by the Toll Bridge Seismic Retrofit Program, made possible by Assembly Bill 144 passed in 2005. No federal funds are allocated for the project.
The project was developed through a partnership between the Bay Area Toll Authority, California Transportation Commission and Caltrans.
“Caltrans directs the project,” said Leah Robinson-Leach, chief of media relations and public affairs for Caltrans in the San Francisco Bay area.
The contractor for the project is California Engineering Contractors Inc./Silverado Inc. Joint Venture. California Engineering Contractors is based in Pleasanton, Calif., and Silverado is based in Oakland.
The joint venture bid $69 million. Its duties include removing five 504-ft. (153.6 m) truss spans, 14,288-ft. (4,355 m) double-deck truss spans and 13 steel towers, Robinson-Leach said. Separate contractors handle removal of the underwater foundations.
Pieces of the old east span are being removed in the reverse order from which the span was built.
The first part of the demolition process was to cut the suspended span of the cantilever truss at its center in order to begin removing it.
In order to cut the suspended span, workers had to neutralize the stresses and pull the suspended span on each end toward the rest of the bridge. Otherwise the two halves of the suspended span would have collapsed and fallen into the water when cut.
Once the suspended span was gone, then workers began removal of the two halves of the cantilever. They also started dismantling the truss span section that stretched east of the cantilever to the Oakland shore.
Recent demolition included a pier implosion in November 2015. The pier, known as E3, was the largest of the underwater concrete piers supporting the old Bay Bridge east span.
Many people in the Bay Area watched in person, online or on television as hundreds of small charges collapsed the concrete and steel structure into its own foundation.
Also in 2015, workers removed the two-level, five-lane concrete deck on the east span.
More recently, workers cut the first of five 504 ft. long, 1,950-ton (1,769 t) truss sections of the old structure free from its piers and lowered it 180 ft. (55 m) to barges below. The truss sections are called “504s.”
The process took place over two days in early February. A second section is scheduled for removal in mid-April.
Removal of a 504 section is immediately followed by extraction of the marine or underwater foundation beneath it.
“Lowering these spans will represent the largest sections that will come down during the demolition of the eastern spans,” said Steve Whipple, principal construction manager in a Caltrans release just before the removal.
The five sections extend about a half mile.
Once the 504s are all removed, crews will begin a similar process of removing 14 288-ft. (88 m) span sections called “288s” that extend to the Oakland shore. Again the sections will be lowered onto barges, but these barges will be designed to handle the shallower water where the 288s are located.
At completion of the project, more than 58,000 tons (52,617 t) of steel and 245,000 tons (222,260 t) of concrete will have been removed.
Project Presents Serious Challenges
Dismantling trusses is a dangerous job. Removing one member from a truss bridge can change the loads on other members, possibly creating a spring-action effect.
To determine the best way to remove high-tension pieces, engineers are using a 3-D finite element computer program model based on structural analysis and historical records that show how the forces are distributed.
During demolition, crews will monitor the state of the span through a system of 90 retro-reflector prism targets installed at key locations on the bridge. As steel members of the bridge are removed, the updated target locations will be determined and entered into a computer system.
Environmental Protection is Part of Demolition Project
Although the new east span is up and running, the environmental work connected with demolition, continues. Monitoring and mitigation efforts are scheduled to continue well into the future.
The environmental team will work alongside demolition crews to ensure that environmental laws are followed and negative impacts on water quality and fish life are avoided.
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