With traffic transferred onto the new east span of the Bay Bridge, attention has turned to a careful demolition of its 1936 predecessor, foreground. © CEC/Silverado joint venture
A demolition team is using building information modeling and a rich historical record to dismantle the east span in the reverse order from which it was built.
December 10, 2013—Earlier this year traffic on the San Francisco-Oakland Bay Bridge was shifted onto the new east span, which includes an impressive 2,047 ft self-anchored suspension bridge that employs a 525 ft high single support tower and a cable that is nearly 1 mi long and weighs 10.6 million lb. This completed a more than $6-billion seismic retrofit of the critical infrastructure link between the two cities.
Attention then turned to the far less glamorous task of removing the old east span, a massive double-tower steel truss structure with cantilevered sections and a long causeway that opened in 1936 to great fanfare. The bridge was deemed to be a seismic risk following the Loma Prieta earthquake on October 17, 1989, a 6.9 on the open-ended Richter scale.
“As soon as we switched traffic off the old bridge onto the new bridge, I breathed a sigh of relief,” says William Howe, P.E., a senior resident engineer with Caltrans. “In a great earthquake, the old bridge wouldn’t have fared very well at all.”
The west span of the bridge underwent an extensive seismic retrofit that included the installation of 96 viscous dampers, the replacement of 500,000 of the original 1930s rivets, and the addition of 17 million lb of structural steel in the form of bracing, plates, and replaced members. A similar retrofit of the east span was deemed prohibitively expensive because the foundations were “woefully inadequate,” Howe says.
Workers are currently removing the upper concrete decking to
lighten the load on the cantilevered sections of the bridge.
© CEC/Silverado joint venture
The demolition project presents formidable challenges to the engineering team. In total, the east span is 1.97 miles long with more than 58,000 tons of steel and 245,470 tons of concrete. The cantilever truss section alone is 2,420 ft long, with 20,412 tons of steel and 12,460 tons of concrete to be removed, not including the piers, which will also be demolished.
“In the old days, we may have used more explosive means than we are using today,” says Howe. However, environmental concerns about the San Francisco Bay and the presence of lead paint on the bridge have prompted a restriction on the amount of time that work in the water can be conducted. “As soon as you say that, you are essentially booked into taking [the bridge] apart in the reverse order from which it was built.”
This creates a type of puzzle. Removing one member from a truss bridge can change the loads on other members, possibly creating dangerous spring-action effects. The team has addressed this concern in a variety of ways.
The demolition team is a joint venture of California Engineering Contractors, of Pleasanton, California, and Silverado Contractors, of Oakland. Caltrans has hired an independent contractor to develop a building information model (BIM) for the project.
“The model allows [us to] meet with the contractor and discuss where he needs to make cuts,” Howe says. “The Bay Bridge is essentially made up of plates, angles, and rivets. There aren’t any rolled shapes on the Bay Bridge. It’s really a pretty simple structure. But because there [are] a lot of plates, there are up to 12 plys of plate at the knuckles.” And at the knuckles, for example, lower or upper chord members connect to verticals or diagonal members, and those knuckles can weigh up to 127 tons, he explains.
Using the model, the team can cut each knuckle digitally and discern the effects on the structure. The model provides the ability to see through the plates for a clearer understanding of how the knuckles function.
As many as 12 plates of steel meet in some of the bridge’s
massive knuckles, the heaviest of which weigh 127 tons.
© CEC/Silverado joint venture
“Because the model is mathematically accurate, it can also calculate lift and pick weights,” Howe says. “It really can be helpful.”
During demolition, the team will monitor the state of the span via a precise system of 90 reflectorless targets installed at key locations on the bridge. As members are removed, the updated target locations will be determined and entered into a computer system. This data will be compared against the predicted locations in the BIM software. A large discrepancy would indicate a cause for concern. “It’s a really good tool to track how the demolition is going as far as safety goes,” Howe notes.
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.
The demolition will be accomplished via several contracts. The first, a $93-million contract, includes demolition of the cantilever section and the construction of the last onramps to the new span that couldn’t be constructed until the older bridge is removed. Future contracts will focus on the demolition of the array of approach spans.
Work has begun on the removal of the upper concrete bridge deck. Workers will then remove the lower concrete deck from the cantilevered portions of the bridge. Eventually this decking will be replaced by timber and the team will begin removing structural members.
“That is important because it’s a cantilever,” Howe says. “If you can lighten up the loads on the structure, you can get away with bigger picks. When they start the demotion they will be cutting the suspended span in the middle and working their way back. If they don’t have the dead load of the concrete on the upper and lower deck, they are way better off.”
Howe, who grew up in the area and remembers traversing the bridge as a child, says he has mixed feelings about seeing the bridge come down.
“It’s a grand old lady. I’m a little bit sorry to see it go,” Howe says. “It’s a really unique project because we are finishing the new bridge and taking apart the old one. It’s a rare opportunity.”