The engineering design review team looking into revised plans to stabilize the Millennium Tower in San Francisco on behalf of the city’s Dept. of Building Inspection has OK’d an 18-pile upgrade plan in place of the earlier 52-pile design.
The retrofit proposal would avoid further settlement and tilt of the 645-ft-tall building until 2060—the project’s goal—members of the review team wrote in a letter detailing their findings June 15. Additionally, they estimated the design would recover 0.3 in. of settlement, 4.3 in. of westward tilt and 0.3 in. of northward tilt that has accumulated over time. The structural analyses looked at the building’s response under gravity loads and earthquakes.
“Subject to continued monitoring of the building settlement and tilt, through construction and following completion of the 18-pile PPU, we see no technical reasons to withhold approval of the proposed revisions to the building permit for the voluntary foundation retrofit,” the review team members wrote.
The 18 piles have already been installed, as they were included in the approved 52-pile design, too, says Ronald Hamburger, a senior principal with Simpson Gumpertz & Heger and the engineer-of-record on the project. Now the contractor, Shimmick Construction, is in the process of installing excavation shoring so they can excavate 25 ft below grade on two sides of the building. Then they will tie the existing foundation to the new piles with rebar coupled onto the existing reinforced steel for a new 10-ft-thick concrete mat on those two sides that will be tied into the existing mat. Once that’s done, they can jack the load onto the piles.
The revised design reduces the amount of work required on the building, the amount of shoring and the amount of excavation needed. Hamburger says they looked at installing as few as six new piles before settling on 18.
The 52-pile plan would have put a load of 800,000 lbs per pile, but Hamburger says the engineers are confident they can safely increase the load by 20% to 1 million lbs per pile.
“We're not taking as much load off the existing foundation as we were originally but we're taking sufficient load off to accomplish the project objectives,” he says.
The review team received the plans in January and spent about six months analyzing the design. Hamburger and Doug Elmets, a spokesperson for Millennium Tower, both said they expect city officials would sign off on the plan soon following the review team’s approval.
Following the review team's recommendation, San Francisco DBI permitting services Deputy Director Neville Pereira wrote in a letter to city Planning Dept. officials on June 16 that DBI “takes no exception to this scope change” and will recommend permit approval contingent on Planning Dept. approval of the environmental impact review. Officials didn’t say how soon they expected a formal decision.
“The reduced-scope design will enable rapid completion of construction and stabilization of the building,” Elmets said in a statement.
Hamburger says the team hopes to complete the remaining work this year.