- Historic Photos and Computer Imaging Will be Used to Overcome Safety Challenges
Demolition of the old Bay Bridge began in November, with crews beginning the process of chipping away concrete and cutting steel on the 77-year-old structure. The first phase demolition contract was awarded to a joint venture of California Engineering Contractors of Pleasanton and Silverado Contractors of Oakland. The demolition will be done roughly in the reverse order of the construction of the bridge. Silverado and CEC will handle demolition of the cantilever section just east of Yerba Buena Island, with two separate contracts to be awarded this year for the rest of the span to the Oakland Touchdown. The demolition is scheduled to be completed by the end of 2016, at a cost of $281 million.
Upper Deck First
The first phase of the demolition includes the removing the concrete from the upper deck, to be followed by removal of the lower deck from the cantilevered sections of the bridge.
Crews will remove 2,125 tons of concrete, 374 tons of rebar and 1,300 tons of steel deck supports, then start the cutting and dismantling of the steel on the upper and then lower deck.
An article on the American Society of Civil Engineers website noted that the demolition will be complicated because, “Removing one member from a truss bridge can change the loads on other members, possibly creating dangerous spring-action effects.” Caltrans will be able to safely demolish the bridge by using Building Information Modeling (BIM). Caltrans senior resident engineer William Howe told the ASCE, “The model allows (us to) meet with the contractor and discuss where he needs to make cuts.” Howe also said, “Because the model is mathematically accurate, it can also calculate lift and pick weights.” The BIM system will enable engineers to monitor how the bridge structure is holding up as steel members are removed.
Historical Photographs Used
The ASCE article also noted that, “The demolition strategy is also informed by the rich historical record documenting the Bay Bridge construction in the early 1930s. An extensive collection of photographs denotes how and where the builder braced certain members during construction. Additionally, the as-built documentation has proven to be extremely accurate.”
Bay Bridge spokesman Andrew Gordon said that during the next phases of the demolition, workers will take apart the rest of the cantilever section, remove the piers and pilings down to the waterline, and then take out the foundations underwater down to the mud line. Gordon said some work will be done simultaneously on different sections: “While they’re working on the eastern half of the cantilever, there could be crews working on the foundations underwater.”
The demolition is also complicated by the need to protect the Bay waters from pollution, including lead-based paint and other materials. All of the debris will be hauled away and most of it recycled or re-used, or re-purposed. Silverado has extensive experience in bridge and transportation demolition work, including separating out waste material to be recycled or reused. The contractor performed the demolition of sections of the Bay Bridge at Yerba Buena Island in 2007 and did the demolition work required to replace the 300 foot long section of the bridge in 2009.
Retrofit Work Complete
Caltrans also reported that in December it had completed the installation of the saddles on the new East Span of the Bay Bridge to seismically retrofit the section where broken rods were discovered last March. The saddles sit on top of the base of the shear keys, with steel tendons inside the saddles spreading down either side of the concrete cap beam. The shear keys prevent the decks from moving too much during an earthquake. Bolts holding the shear keys in place were fractured but could not be replaced because they are embedded in concrete and there is not enough clearance to remove them. In May, engineers selected custom-made steel saddles as the fix to exert that same clamping force as the original bolt design to hold down the shear keys. The saddles were selected because, despite requiring more detailed fabrication, installation was easier and required less drilling of the concrete cap beam.
Caltrans reported that, while work related to the functionality of the saddles is complete, other work will continue for the next couple of months, including painting the saddles, covering rod heads with caps and removal of the scaffolding beneath the bridge. The final cost of the saddles, which includes design, fabrication and installation, will be approximately $25 million.