San Francisco officials and the city’s team scrutinizing permit applications for shoring the 645-ft-tall Millennium Tower—which has settled 17 in. and tilted 4 in. over more than a decade—say the review will last several months. The fix scheme, likened to putting a bumper jack next to a flat tire, calls for drilling and jacking 52 piles—socketed into bedrock as deep as 250 ft—under the north and west sidewalks. The system would cost less than $100 million and take about 18 months to complete.
Permit applications dated Dec. 3 were submitted by the Millennium Tower Association, which consists of the condominium homeowners. “Public safety is our top priority,” said the city in a Dec. 4 statement. “We will process these permits expeditiously with peer review from experts and thorough analysis by city agencies. City departments will work with homeowners to ensure the structural fixes will protect all residents and the public.”
There is an urgency to finish the review but “not because the building is getting less safe” as the months go by, says Gregory Deierlein, a Stanford University engineering professor, who, as a consultant to the Dept. of Building Inspection, chairs the design-review team for the fix.
Based on a study by structural engineer Simpson Gumpertz & Heger and additional analyses by SGH and structural engineer LERA, the review team concurs with SGH and LERA that the settlement has not appreciably affected the seismic safety of the building, says Deierlein.
Ronald O. Hamburger, the SGH senior principal who developed the repair scheme, says the aim is to remove 20% of the building weight from the underlying clay strata by drilling reinforced concrete piles socketed 30 ft into bedrock.
The system relies on loading each pile with 800,000 lb using a permanent hydraulic jack that reacts against a new mat extension. The jacks would be housed in a maintenance access vault above the mat (see diagram, facing page). The team has applied for an easement to work under the sidewalks, which meet at the northwest corner of the block.
The 20% load reduction represents “what I could comfortably transfer from the new piles to the existing mat without major modification of the existing mat,” says Hamburger, whose client is Paul Hastings LLP, a lawyer retained by the tower’s developer, Mission Street Development. MSD is part of Millennium Partners.
The weight loss would result in a rebound of the north and west sides of the building by about 1 in. almost immediately, says Hamburger. That would remove about 25% of the tilt, he adds. “Over time, we expect another 25% to 50% of the tilt to come out through continued settlement of the south and east sides,” he says.
Under the SGH plan, 2-ft-dia piles would be installed 4 ft, 9 in. on center under sidewalks along 200 ft on the west side and 100 ft on the north side, just outside the building’s footprint.
Piles would extend up through an 8-ft-wide extension of the tower’s 10-ft-thick reinforced concrete mat—4 ft from the old mat. New and old mats would be connected by chipping into the side of the old mat and coupling new reinforcing steel onto existing rebar.
The city’s review team will “look carefully” at design documents and supporting calculations, including SGH computer models, says Deierlein. “We will open up the model and check input data and may do our own calculations,” he says.
Other members of the review team include structural engineer Marko Schotanus, geotechnical engineer Craig Shields and geotechnical-seismic hazards consultant Shahriar Vahdani.
Construction of the 58-story tower began in 2006 and ended in 2009. The structure, designed by DeSimone Consulting Engineers, is cast-in-place concrete with post-tensioned slabs above the ground level. The mat is supported by about 950 precast concrete piles, each 80 ft long, that do not go down to bedrock.
According to Hamburger, the settlement is a result of the consolidation of the upper strata of the area’s Old Bay Clays. “The load above is squeezing water out of the clays,” he says.
Treadwell & Rollo, the original geotechnical engineer, predicted 4 in. to 6 in. of uniform settlement, which was exceeded during construction. “The reason given was that the site had been dewatered more than anticipated,” says Hamburger.
Hamburger blames the settlement and differential settlement on the prolonged lowered water table caused by dewatering, including during construction of the nearby Salesforce Transit Center, Salesforce Tower and 350 Mission Street. “The lowered elevation of the water table has been a significant factor,” he claims.
Transit center owner Transbay Joint Powers Authority disagrees, saying there is no evidence to support the dewatering claim. Geotechnical engineer Gregory Axten, in an August court filing by TJPA lawyers, concluded there were “significant errors” in the original settlement calculations for Millennium Tower, according to David Satterfield, a spokesman for TJPA.
“Axten notes that if those calculations would have been done correctly, the settlement that would have been predicted ‘would have exceeded the amount of settlement that has been experienced by the Millennium Tower to date,’ ” says Satterfield.
In a statement, Boston Properties, which owns 95% of Salesforce Tower, adds, “Regarding Mr. Hamburger’s observation, Millennium Tower was known to be sinking and tilting long before construction began on the Salesforce Tower site; indeed, Millennium Tower began sinking even before Millennium’s construction was completed, and was known to be tilting to the northwest in mid-2009.
"Excavation at the Salesforce Tower site did not begin until 2015," the statement continues. "At that point in time, Millennium Tower had already settled over 14 in. and was tilting 17 in. at the top. Of all the buildings in the area, only Millennium Tower is experiencing any problems.
"Mr. Hamburger is a structural engineer retained and paid by Mission Street Development, the developer responsible for the construction of Millennium Tower. At odds with Mr. Hamburger’s views as a structural engineer, nationally recognized geotechnical and hydrological experts have found that Salesforce Tower had no effect on the sinking or tilting of Millennium Tower, rather the sinking and tilting were the result of MSD’s fundamentally flawed construction. Millennium’s attempt to blame Salesforce Tower is belied by the geotechnical and hydrological data,” the statement concludes.
There are currently eight lawsuits involving the troubled tilting tower, says PJ Johnston, a spokesperson for MSD. Defendants include TJPA, MSD, the city, contractors, subcontractors and others.
“We are involved in a confidential mediation process that has successfully produced the [repair plan] and is expected to lead to a global settlement on claims as well,” says Johnston.