Revisions Proposed for Millennium Tower Fix
A reduced scope for the stabilization of the Millennium Tower in San Francisco was recently approved by the project’s engineering review team on behalf of the city’s Department of Building Inspection.
The revision, which involves reducing a 52-pile design to just 18 piles, now heads to city officials for final approval from the San Francisco Planning Department.
According to reports, the 645-foot-tall residential condominium, composed of 58 stories, has sunk a total of 17 inches since its completion in 2008. The building’s tilt has also been reported to have worsened since construction to mitigate the issues launched in November 2020.
Opened in 2008 at the cost of $350 million, the Millennium Tower was designed by Handel Architects, with structural engineering by DeSimone Consulting Engineers. Webcor Builders was the general contractor on the job, according to the Concrete Reinforcing Steel Institute.
Almost a decade after completion, in August 2016, the San Francisco Chronicle reported that Millennium condo owner John Eng had filed a lawsuit against both the building’s owners, Millennium Partners, and the Transbay Joint Powers Authority, whose nearby construction may have caused the tower to begin sinking and tilting.
In the complaint, the plaintiff called the building “defective,” and claimed Millennium Partners “knew that the Millennium and Subject Homes were not of marketable or habitable quality.” The suit noted that the building sits on a manmade “mud fill” in an area that was once underwater, and asserts that the decision to build on a concrete slab and 80-foot piles instead of piles anchored into the 200-foot-deep bedrock was made “to cut costs.”
The suit also stated that “over time the building most likely will sink an additional 8 to 15 inches into the landfill” over which it is built, and the tilt, currently measured at about two inches toward the northwest, could also get worse. According to the suit, cracking and buckling were also visible in individual units and in the building’s commons area.
Friday Morning Constitutional: Revised Millennium Tower Fix Gets Sign-Off https://t.co/JdSnrj4SVu— usa share news (@usasharenews) June 19, 2022
By February 2017, a government oversight committee began conducting hearings to determine what went wrong to cause the building to sink 16 inches and tilt 2 inches. According to reports during this time, Millennium Partners also hired Sage Engineers to investigate the foundation issues and potential solutions.
That spring, the tower saga grew as the building’s homeowner’s association sued the developer and others for more than $200 million in damages to the residential structure. At the time, the lawsuit was the only one in the litany of litigation that sought a permanent fix.
Continuing into the summer, by mid-July, the NBC Bay Area Investigative Unit reported another issue with the Millennium Tower—gaps in the curtain walls of at least one unit, ultimately posing a fire hazard. Should a fire start in the unit below, it was predicted that it would easily spread upward, increasing the chances of causing various smoke damage.
Within the same month, the tower reportedly sunk another inch, but officials had finally reached a possible fix. According to the LERA firm and DeSimone Consulting Engineers, the problem could be both stabilized and somewhat reversed by adding 50 to 100 new piles down to the bedrock, beginning from the building’s basement. Each individual pile would be 10 to 12 inches in diameter, noted the Chronicle. The estimated price tag for the repair was reported to be between $100 million and $150 million, although some experts feared it would exceed $1 billion.
However, by April 2018, another proposed fix was introduced: drilling holes down to bedrock to stabilize one side of the tower while letting the other side sink down, allowing the building to level itself out. By this time, the tower had sunk a total of 17 inches and tilted 14 inches to the west and 6 inches to the north. Estimations for this solution ranged from $200-500 million.
In September, several months after a retrofit repair for the Millennium Tower was proposed, another issue came to light: cracks had appeared in a 36th-floor window, sparking concerns over whether the damage was local or symptomatic of a more widespread issue.
According to reports, the issue could have stemmed from structural issues, a material defect or be a one-off event. Architecture and engineering firm Allana Buick & Bers were hired by skyscraper residents to help determine what caused the damage.
Over time, the tower’s saga has seen roughly 146 lawyers involved in nine different lawsuits filed regarding the structure’s various issues.
In September 2019, an independent four-person panel—hired by the city and headed up by Stanford engineering professor Gregory Deierlein—approved the shoring fix after meeting 11 times with design teams from engineering firm Simpson Gumpertz & Heger for the proposed changes.
In a letter addressed to the City and County of San Francisco Department of Building Inspections at the time, the panel—made up of Deierlein, Shahriar Vahdani, Marko Schotanus and Craig Shield—announced that by adding new piles to the structure, chances of future sinking should be reduced, in addition to improved seismic performance.
The work, which was slated to begin in November, involved drilling and jacking 52 concrete piles socketed more than 30 feet into bedrock under the north and west sidewalks. These piles would support a new mat section, or collar, tied into the existing mat.
The piles are reported to be 2 feet in diameter and weigh 140,000 pounds. The process also planned to relieve stress on the compressed soil located underneath the north and west sides of the building.
The tower sits on 950 reinforced concrete piles that have been driven up to 90 feet deep into bay mud.
After the collapse of Miami’s Champlain Towers South in June, worries were noted to increase about the leaning (and sinking) Millennium Tower. As structural and inspection concerns spread throughout Florida, professionals were forced to take another look at San Francisco’s high-rises.
In September 2021, upon measuring an accelerated settlement on the Millennium Tower, the city advised homeowners to wait until a revised pile installation method was approved before resuming the high-rise's foundation retrofit.
Senior Principal Hamburger, of Simpson Gumpertz & Heger, was reported to have paused work on the retrofit “out of an abundance of caution” in August.
According to reports, an acceleration in the structure’s settlement and tilt were witnessed following the installation of both 36-inch-diameter pile casings and 24-inch-diameter piles. In working to develop a new plan for retrofit, the project’s design team met with the San Francisco DBI on a weekly basis, in addition to working with contractor Shimmick Construction Co. Inc. to revise installation methods.
Earlier that month, O’Riordan sent a letter to the Millennium Tower Association requesting that the group refrain from resuming pile foundation retrofit efforts until department and city engineers could review the forthcoming updated construction approach for the pile and casings installation.
DBI and its design team further reported in its letter that they were in line with Hamburger’s assessment, agreeing that the building settlement does not present a “structural concern and believe that Mr. Hamburger’s hypothesis that construction activity was producing the accelerated rate of settlement was likely accurate.”
Shimmick Construction Co. successfully installed the 36-inch pilot pile casing utilizing a less-disruptive construction method in October, according to reports.
The casing was driven 106 feet below ground over two days, said Patrick Hannan, Communications Director for the DBI. Building settlement was 0.001 feet, the smallest amount readable with the method used. Measurements 80 feet below the ground surface indicated that vibration was “negligible.”
It had been reported that since August, 33 36-inch pile casings and six 24-inch pile casings had been installed.
Most recently, in November, it was reported that Shimmick successfully installed a second outer 36-inch casing after minor equipment issues had disrupted the project’s next steps.
Latest Revised Design
In a letter written June 15, the engineering design review team detailed how the Millennium Tower could be successfully retrofitted in a revised proposal for an 18-pile upgrade plan.
Replacing the original 52-pile design, the team reports that the reduced-scope stabilization plan would avoid further settlement and tilt of the 645-foot-tall building until 2060—the project’s goal. In addition, the team also estimated that the new plan would recover 0.3 inches of settlement, 4.3 inches of westward tilt and 0.3 inches of northward tilt that has accumulated over time.
“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.
According to Hamburger, the 18 piles have already been installed, as they were included in the previously approved 52-pile design. After careful review and structural analyses that looked at the building’s response to gravity loads and earthquakes, the engineers found that the load could be safely increased by 20% to 1 million pounds per pile.
As a result, the project would only require the 18 piles and, in turn, would reduce the amount of work required on the building, as well as the amount of shoring and excavation.
“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,” Hamburger said.
Currently, Shimmick is reported to be in the process of installing excavation shoring so they can excavate 25 feet below grade on two sides of the building.
Once this is completed, crews intend to tie the existing foundation to the new piles with rebar coupled onto the existing reinforced steel for a new 10-foot-thick concrete mat on each of the two sides that can be tied into the existing mat. After this step is finished, crews can then jack the load onto the installed piles.
The day after the team submitted their recommendation for the change, San Francisco DBI Permitting Services Deputy Director Neville Pereira wrote in a letter to city Planning Dept. officials the DBI “takes no exception to this scope change” and would recommend permit approval contingent on the Planning Dept. approval of the environmental impact review.
“The reduced-scope design will enable rapid completion of construction and stabilization of the building,” said Spokesperson Doug Elmets in a statement.
No timeline has been released as to when officials expect to release a formal decision. If approved, however, Hamburger estimates that the team could complete the remaining work before 2023.