Tilting tower nears stabilization
A revised plan to arrest and rebalance the sinking and tilting of San Francisco’s Millennium Tower moved closer to getting the official sign-off after an outside panel of engineers recommended it to the Department of Building Inspection. If all goes as planned, the work could be implemented by the end of the year.
The plan uses 18 instead of 52 piles, or columns, to take some weight off the building’s foundation and stop it from settling further at the building’s northwest corner at Mission and Fremont Streets. Casings are sunk into the deep rock bed below and the piles are then drilled through them to embed 40 feet in the rock.
Project engineer Ron Hamburger of Simpson Gumpertz & Heger said in an email that “all 18 structural piles have been installed” as of March 9 under the originally permitted plan to install 52.
“The Department of Building Inspection takes no exception to this scope change and has determined the proposal complies with the relevant building codes,” Neville Pereira, the department’s deputy director of permit services, wrote in an email to the city’s planning department, where the revised plan will now head for environmental review.
The original plan to use 52 piles was paused after it caused further settling. Engineers concluded 18 piles could instead could take the weight.
Each column will support 1 million pounds instead of 800,000 pounds, although they are rated to take up to 2 million pounds, Hamburger said.
“This building is heavier for its footprint than other buildings that had been constructed in that part of San Francisco in past years,” Hamburger said.
That weight has been squeezing water out of old bay mud deep beneath the surface. The piles go down past that mud into rock.
“We’re taking a portion of the building’s weight off of that (mud) and transferring it onto rock,” Hamburger said.
If successful, that would rebalance the weight of the building to correct for some of the settling and tilting that has occurred over time, Millennium Tower spokesperson Doug Elmets said in a statement.
The building could recover as much as an inch of settling, and about 4 inches of the tilting to the west and an inch to the north, Hamburger said.
The outside panel of structural experts wrote in their recommendation that in the long term, by 2060, the 18-pile plan could recover about 4.3 inches of westward tilt and 0.3 inches of northward tilt. In contrast, the 52-pile plan was estimated to recover about 5.5 inches of westward tilt and 3.5 inches of northward tilt during the same time.
According to Hamburger, the next steps would be to excavate the area around the piles driven into the rock, pour new concrete around them and use hydraulic jacks to transfer some of the building’s weight. The work could be done by the end of the year, he said.
The work is part of a $100 million plan to stop the 60-story tower from sinking beyond the foot and a half it has already settled at its worst point. The building has also rotated slightly and is tilting toward Fremont Street by about 27 inches and toward Mission Street by about 9 inches, Hamburger said.