SAN FRANCISCO—A fix for San Francisco’s Millennium Tower has been approved. Simpson Gumpertz & Heger will be the engineer-of-record for the fix, according to Building Design+Construction Magazine issued online on July 29.
According to Simpson Gumpertz & Heger(SGH), although the original geotechnical report predicted 4 to 6 in. of total settlement, by 2014 the building had settled more than 14 in. Litigation was initiated in 2015 when homeowners alleged that the building did not meet code, was unsafe, and required a $300 million retrofit consisting of installing more than 400 piles extending through the 10 ft thick foundation mat to rock, located approximately 250 ft below grade.
SGH analyzed that the fifty-eight-story Millennium Tower is founded on piles that have been driven through soft, compressible clay soil deposited by the San Francisco Bay along with bedrock more than 200 ft below street level. It extends into a dense sand layer overlying ancient marine deposits of clays, silts, and sands. Consolidation and lateral displacement of the soils under the building’s weight as well as adjacent construction activity have caused the tower to settle more than 17 in. and tilt 4 in.
The mediation settlement includes a voluntary foundation upgrade wherein 52 piles will be installed along the building’s north and west sides. The upgrade is designed to arrest settlement, allow gradual recovery of tilt over time, and improve the building’s seismic performance. Permitted as a voluntary seismic upgrade, the projected construction cost is $100 million and is slated to commence in November 2020, according to SGH. The fix will prevent any future settlement and reverse the current tilting over time.
San Francisco’s Millennium Tower was developed between 2005 and 2009. The 56-story tower was designed to be the city’s premier residential address. A list of residents past and present includes football great Joe Montana, the Golden State Warriors’ Steph Curry, and the San Francisco Giants’ Hunter Pence. With a development cost of $600 million, units sold for a total of $750 million, including the most expensive unit which sold for $13.5 million in 2009.