On Feb. 5, demolition crews in San Francisco completed a delicate, two-day operation to remove a 504-ft-long, 2,500-ton bridge truss in one piece. Joint-venture contractor CEC-Silverado lowered the 96-ft-tall by 80-ft-wide steel assembly—once part of the nearly 2-mile-long East Bay Bridge, which, in 2013, was replaced with a $6.4-billion self-anchored suspension bridge—onto a pair of barges on the San Francisco Bay.
The second phase of a three-phase demo project, the approximately $79-million effort will disassemble five identical trusses—known as “504s”—and fourteen 288-ft-long “288” trusses that extend more than 1.25 miles to the Oakland shore.
During the $93-million first phase, which was completed by the same contracting team at the end of 2015, crews worked hundreds of feet above the bay to cut apart the 80-year-old bridge’s 2,420-ft-long cantilever section (ENR California 6/8/15 p. CA25). Workers wore full-hood respirators to contend with the thick lead paint coating the steel.
The same dangerous paint covers the 504s, but, instead of using torches to disassemble the steel bit by bit, CEC- Silverado and owner Caltrans are mitigating environmental and worker risk by keeping intact each complete span section.
The strategy also is reducing project risk and environmental impacts to the bay by eliminating the need for piers or falsework. “Foundations are typically the source of greatest uncertainty, which leads to the greatest amount of claims,” says Brian Maroney, chief bridge engineer with Caltrans. “One good way to make a contract come in on time and under budget is to minimize the geotechnical work.”
Several months prior, to lighten the load, work started to remove the two-level, five-lane concrete deck and any other extraneous material. Crews shortened each end by about a half panel by cutting the floor beam at the portal so that the truss could clear the steel tower piers during lowering. As a result, the contractor had to retrofit the truss in several spots.
Four strand jacks located in each corner handle the heavy un-lifting. The jacks sit on steel horizontal Y-struts, which are affixed to the top of each tower pier and act as giant balances. The final moments before the lowered truss makes contact with the steel supports anchored to the barges are fraught with risk due to impact loads, Maroney says. This process is mitigated by careful selection of a calm-weather and tidal window to perform the operation, along with a quick-release system that could disconnect the cables from the truss immediately in the event of a large wave or other problem.
Once the truss safely rested on the barges, four tugboats maneuvered the assembly to the Port of Oakland, where crews are now disassembling the steel. The barges and steel retrofitting soon will return to prep the next 504 for lift-off, in mid-April.
Plans are in place to remove the 288s in a similar manner, using low-draft barges to contend with much shallower water.