Speaking publicly for the first time since the so-called fix of the Millennium Tower caused the high-rise to suddenly sink and tilt more, the fix’s lead designer testified Thursday that the building could lean more than six feet and still safely withstand a massive earthquake.
“As everyone in the Bay Area is aware, as we’ve been installing the new piles, there has been an increase in settlement and tilting of the building” said structural engineer Ron Hamburger before the government oversight committee of the San Francisco Board of Supervisors. “The increase is not as dramatic as the media would have you believe.”
The tower is currently leaning 23 inches to the west and 9.5 inches to the north, which includes about six inches of newly added tilt to the west just since May.
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While acknowledging he had concerns about the accelerated sinking and tilting soon after installation work began, Hamburger said engineers have since learned important lessons on ways to anchor the project with piles sunk to bedrock on two sides without adding to tilting and sinking.
Hamburger said the building experienced negligible amount of settlement during a recent test conducted of new installation methods designed to limit two suspected factors – soil lost during digging and vibration during construction.
“We have successfully mitigated the vibration,” Hamburger said, adding that crews also limited the amount of soil taken away from around the foundation in the revised digging process.
But data released this week – as work resumed with new installation of 100-foot long steel casings on Mission Street – show the high rise settled one tenth of an inch, triggering a quarter-inch of more tilt to the west. That’s roughly the same weekly rate that triggered a halt to the fix project back in August, before any measures were taken to limit ground loss or vibration.
During his testimony, Hamburger downplayed the potential that current rates of tilting pose any issue – restating his belief that the fix was not even needed to preserve the building’s safety.
He pointed to a new analysis showing the building could tilt just over six and half feet to the west and nearly three feet to the north -- and still safely survive a massive quake.
“We’ve concluded that the building is still able to safely resist this maximum considered earthquake shaking with that amount of tilting,” he said, but indicated the “practical maximum” for tilting to maintain operations for the building’s elevators and plumbing systems is on the order of 40 inches, about half as much.
Even the lower limit, he said, provides a margin of nearly a foot more of additional tilting before the “practical maximum” is reached.
Hamburger also said that 40-inch level was about maximum amount of the building could be expected to experience based on soil conditions, had no fix work ever been done.
As to the 6.5-foot tilt level Hamburger now says the building could survive in a quake, San Francisco Supervisor Aaron Peskin -- who convened the hearing after NBC Bay Area first reported about the fix problems -- was dubious.
“That sounds just ludicrous to me,” Peskin said at the hearing. “That a building can tilt six and a half feet in the muck in an earthquake prone part of Northern California and not be a seismic or life safety hazard. I would like to have a third set of eyes on that.”
Hamburger stressed it’s not likely the building would tilt that much, given that the completed fix will arrest the settlement and even reverse some of the tilting. He noted that engineers now propose relying on 42 piles, instead of the original 52, and could reduce that number further, based on the outcome of a key planned test of sinking a pile to bedrock in coming weeks.