New monitoring data shows that while the $100 million Millennium Tower fix appears to have stopped further sinking and tilting, the building is not reversing the lean at the rate predicted by engineers.
Back in June, the troubled tower became fully supported on the north and west sides by a total of 18 piles sunk into bedrock as part of the building’s retrofit project. Monitoring data confirms the fix has successfully prevented further sinking.
That’s a major accomplishment, according to an internationally recognized expert.
“I would say the building has stalled from the designers’ viewpoint -- that was their primary objective,” said tall building foundation engineer Harry Poulos.
Get a weekly recap of the latest San Francisco Bay Area housing news. Sign up for NBC Bay Area’s Housing Deconstructed newsletter.
But, he says, there is also bad news.
“Their objective of correcting that tilt to a significant extent – I fear that expectation has not been met.”
To predict how the building would behave following jacking – that’s the process of shifting some 18 million pounds of weight onto the new support piles – Millennium fix engineers relied on an elaborate computer model.
The simulation analysis predicted as much as four inches of offset in the tower’s tilt, occurring at a continuous rate during the first six months after jacking.
But the recent monitoring data shows that after the early progress of three quarters of an inch of tilt improvement over nine days, tilt improvement has stalled in the more than two months since.
Poulos suspects tilt recovery peaked right after jacking was done.
“Unless you were magically able to jack the building up further,” he said, “we are still left with a significant tilt of the building.’’
If the tower does not right itself any further, the high-rise could be left permanently tilting 29 inches as measured at the northwest corner of the roof.
While Millennium fix officials acknowledge there has been less tilt and settlement improvement than the computer model predicted at this point, they stress that: “given the many assumptions and simplifications inherent” in such analyses, they see the “match between measured behavior and the analysis as excellent.”
Officials with the city’s Department of Building Inspection credit the fix for having stopped the tower’s tilt troubles from getting worse.
“Any recovery is welcome news but is not the primary objective of the retrofit and would be expected to occur gradually over time,” DBI said in a statement.
If it turns out the tilt doesn’t improve more, experts say that 90-foot-high underground wall under the foundation’s east side may be to blame.
The wall – built to protect the underground parking structure next door – is buried beneath the 10-foot thick mat foundation, right under the higher side that needs to sink if the tower is going to straighten out even a few inches.
Millennium officials stress that the computer model accounts for that wall – predicting the underground structure itself will sink along with the foundation over time, as the fix shifts some of the tower’s weight to the east.
