The 62-ft-tall, 100-ft-dia dome, along with one façade of the old store, will be incorporated in a $410-million, 1-million-sq-ft addition to San Francisco Center, a retail complex owned by Westfield America Trust and Forest City Enterprises.
Project officials opted to jack the dome rather than dismantle and store it to mini-mize the impact on the historic component, especially the decorative metal inside, says Barry Widen, senior project manager for the developer, which is acting as its own general contractor.
|FLYING HIGH Landmark dome in San Francisco is supported by a temporary tower.|
The domes structure is made up of steel ribs tied together at the base by a 16-in.-deep ring beam. To prepare for jacking, subcontractor C.E.Toland & Son, Benicia, Calif., diagonally braced the dome and replaced loose rivets with bolts. Splice plates also were added to the ring beam, which served as the domes primary support during lifting, according to Michael Kalin, project manager for Los Angeles-based structural engineer Nabih Youssef & Associates.
In early April, Sheedy Drayage Co., the jacking systems local designer and builder, assembled a lifting platform in the basement of the store with a 70-ton rough terrain crane. The cross-shaped platform consists of four 36-in.-deep beams on 44-ft centers, says Archie Schachle, Sheedy vice president.
On top of a new 5-ft-deep mat slab, Sheedy then assembled the 90-ft-tall, 40-ft-sq temporary steel tower. A 100-metric-ton strand jack was secured to the top of each of the towers four columns. After jacking the platform approximately 58 ft, Sheedy bolted eight 14-in.-deep, 15-ft-long beams to the top of the lifting platform and welded them to the ring beam. Connections between the store and dome then were detached.
On May 5, the dome was jacked 30 ft. The team closely monitored the four-hour process to ensure that the structure remained within 1 in. of level. Additional movement could have redistributed loads that would have strained fragile joints, Kalin says.
On May 18, after making the temporary tower taller, Sheedy jacked the dome another 30 ft. It will remain in that position while the store is demolished and an eight-story atrium space is built below. Contractors expect to lower the dome another 2 ft to sit on its permanent structure next year. Costs were not disclosed.
For the project team, one significant challenge remains: removal of the temporary tower from the new building. Since the crane will no longer be on site, the massive tower will have to be disassembled piece by piece, says Widen.
(Photos by Susan Lohwasser)