The Millennium Tower in downtown San Francisco’s South of Market district just can’t escape bad publicity. The luxury condo tower, designed by Handel Architects in collaboration with structural engineer DeSimone Consulting Engineers, was completed in 2009 and has been sinking and tilting ever since.
The problem was first identified in 2016, and a $100 million solution developed by engineers Simpson Gumpertz & Heger promised to stabilize the foundation of the tower, which is the tallest residential building in San Francisco at 645 feet. Work began on structural remediation in May, and, it turns out, the building has rapidly dropped an inch since then, leading to a halt in construction.
As reported by NBC Bay Area, the Millennium Tower Association informed residents of the issue on Monday, August 23, but stressed that no material harm had come to the building and that it remains fully safe for occupancy. However, the proclamation has done little to calm nerves just months after the tragic collapse of Champlain Towers South in the Miami–Dade County community of Surfside.
The 58-story tower’s foundation currently consists of 10-foot-thick concrete and nearly 1,000 reinforced concrete piles reaching down close to 90 feet into a layer of soft clay. The practice is not uncommon in the Bay Area, and the design team contends that the present structural issues stem from the construction of the adjacent Salesforce Tower, which is the tallest building in San Francisco.
The stabilization plan was developed over several years and, if completed, will see 50 new steel-and-concrete piles drilled approximately 250 feet into bedrock; they measure 24 inches in diameter and weigh a whopping 140,000 pounds. Once installed, the piles would reduce the compressive force on the clay soil to the north and encourage leveling out to the east and south. The construction crews had just started drilling the first dozen casings for the piles before work was called off.
Unfortunately, the Millennium Tower is not an anomaly within San Francisco. As noted by the San Francisco Chronicle, stretches of the downtown area are sinking at a startling rate of approximately three-quarters of an inch annually. Increasing sea levels are expected to further degrade the clay soils the city is built atop, and exacerbate the rate of sinking.
Let’s hope the good times last.