America

San Francisco’s ’tilt’ skyscraper tilted by 3 inches last year

January 13, 2022
Henry Club

written by Jackie Palumbo, CNN

According to the engineer who fixed it, San Francisco’s spectacular Millennium Tower is slowly sinking—and as a result, the 58-story skyscraper was tilted by 3 inches on both its north and west sides last year.

Opened in 2009, the estimated $350 million project comprises two buildings, the larger of which is home to 419 luxury apartments, including a $13 million, 5,500-square-foot penthouse. The uneven settling of the tower, the city’s tallest residential structure, has caused cracks in the surrounding sidewalks and the basement walls of the adjacent smaller, 12-story sister building.

Although engineers are now working to stabilize the skyscraper, a city hearing last week revealed that the tilt would continue for several more months. experts have blamed Nearby construction projects and a process known as “dewatering” weakened the soil beneath the tower.

In 2008 the San Francisco skyline received a new addition with the 58-story Millennium Tower (pictured right). Credit: Santiago Mejia / The San Francisco Chronicle / Getty Images

The $100 million fix, announced in October 2020 after years of lawsuits and hearings, is expected to be completed later this year. According to the stabilization project chief engineer Ronald Hamburger, who spoke at the hearing, the building will continue to tilt at a rate of 2.5 to 3 inches north and 0.75 to 3 inches west during 2021, he confirmed in an email to CNN.

“We begin this new year 2022, just as we ended last year and many other years, with the sinking and demolition of the Millennium Tower,” city supervisor Aaron Peskin said at last week’s hearing. according to NBC.
According to Hamburger, in recent years, the rate of inclination has decreased as the building settles. But there have been ups and downs, including an increased slump from September to December 2021, part of which coincides with a two-month halt in construction suggesting that stabilization efforts could worsen the problem, according to local media outlets. Hamburger said that as of December 31, the tower had leaned a total of 24 inches to the west and 7.9 inches to the north. It has settled in the ground about 18 inches deep.

But the tilt is expected to worsen through September of this year, Hamburger said, as engineers install new support piles — concrete columns encased in steel — that extend more than 250 feet into the base below to anchor the building. Will reach to.

The construction involved new piles to stabilize the two buildings, anchored to a base deep below the ground, which would eventually bear the weight.

The construction involved new piles to stabilize the two buildings, anchored to a base deep below the ground, which would eventually bear the weight. Credit: Gabrielle Lurie / The San Francisco Chronicle / Getty Images

“Once these piles are installed … using hydraulic jacks, some of the building’s weight will be transferred to these new piles,” Hamburger explained. “This will reduce the excessive pressure on the Old Bay clay material that caused the original settlement, prevent settlement and allow the building to recover some of the tilt.

“Retrofit is not going to stop the ‘sinking’ unless the pile is pushed into the cornerstone and attached to the foundation,” he said. “Since stabilization is still under construction, it hasn’t worked to help the building yet.”

leaning tower

Leaning towers are not unheard of when it comes to vintage architecture. Structures famous for their leanings in Pisa, Italy and Nevyansk in Russia have become tourist attractions, while London’s Elizabeth Tower (the clock tower with the famous Big Ben bell) has a slight but increasing inclination. But in one of America’s most expensive cities, the tilted modern skyscraper is a costly and controversial dilemma due to unstable ground.

“They’re relatively rare, but I think you need to be extra careful when you’re working with (leaning) towers because they manifest themselves in a way that a lower building wouldn’t,” said Civil Engineer John Burland said CNN in 2017 interview. “A lesser building may settle a little higher at one end than the other, but most of that can be taken up within the structure of the building.

“The trouble with a tall building is that you can see it tilt, which people don’t like. It may be safe, but the owners of the building will not be happy at all,” he said.

A Brief History of the World’s Tallest Buildings

Millennium Tower is the fourth tallest building in San Francisco at 645 feet tall. Designed by Handel Architects and developed by Millennium Partners, the tower’s properties will continue to be sold as engineers work on the fix. In December, the one-bedroom, two-bathroom apartment sold for $1.16 million, while the one-bedroom, one-bathroom sold for $765,000, according to Zillow.

Hamburger said the building was and will remain safe for residents.

“Since we anticipate that the rate of inclination will continue to decrease over time, and will eventually stop, we do not anticipate that the building will ever tilt enough to be unsafe,” he said. “I’ve done analysis that indicates the building can withstand at least 70 inches of inclination to the west and 30 inches to the north, before compromising its ability to resist earthquakes.

“Even if this amount of tilt was exceeded, the building would still be safer than most existing buildings in San Francisco, but would not be safe according to current building codes,” he said.

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