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Skyscrapers are a lot more to a city than just real estate. They represent financial prosperity and engineering acumen. They transform the identity of a city, emerging as a new shape that must be depicted in skyline silhouettes. When tourists arrive, they flock to the Empire State Building or Willis Tower for ten-mile views and selfies.
For the city of San Francisco, skyscrapers are being built quickly and steadily, with 14 over the last 14 years and another 10 expected by 2019. San Francisco has a complicated history with skyscraper development, seeing a spurt of buildings over 200 ft. between 1890 and 1930, another era of upward expansion between 1955 and 1972, and yet another that kicked off shortly after the turn of the Millennium.
There are several reasons for this boom-or-bust building cycle in the City by the Bay, ranging from a very low building height limit (areas in yellow on the map have a limit of just 40 ft.); overprotective laws against ‘Manhattanization;’ laws that ensure community areas such as parks and plazas aren’t oppressively shadowed; historical ordinances; and advances in seismic engineering that show tall buildings are safer than smaller buildings, as they are constructed to cope with some lateral deflection anyhow.
Millennium Tower, opened in 2009 in San Fran’s South of Market neighborhood, has been the source of some controversy lately. Millennium Tower has sunk 16 inches and now tilts 6 inches, and the sink could double before the building finally settles. In a city with major earthquake concerns, the poor structural integrity of the building is generating unease for its residents and neighbors.
Now people are trying to figure out who to blame.
Is it the tower developers? Millennium Tower includes a concrete mat-slab foundation and 950 friction piles, each 60 to 91 feet long, that are drilled into the underlying soil composed of mud and fill after the 1906 earthquake ruined much of the neighborhood. For friction-type foundation piles, the load capacity is solely based on the soil’s ability to provide friction against the shaft of the pile. Historically, buildings on friction piles have done quite well in San Francisco beforehand, although there has never been one as heavy on top of such squishy soil.
Is it the tower’s new neighbors? Across the street, four buildings are being erected by Transbay Joint Powers Authority, including what will become the city’s tallest tower. Each is being constructed with end-bearing piles, which are driven 200 ft. into the ground to the Franciscan Assemblage bedrock. Millennium Tower developers and engineers accused the new construction of dewatering the soil beneath the Millennium Tower, resulting in the soil compressing and destabilizing the tower’s foundation. The authority denies the dewatering accusation, and says Millennium is simply a case of poor engineering.
 Is it the fault of the city? The city had previously rejected a similar tower in the area in 2004, under grounds that friction piles wouldn’t be enough to support the project. Apparently the same questions weren’t raised this time around, because the city didn’t have the engineering expertise to evaluate the soil integrity of the project, and didn’t have the ability to make Millennium’s builders do the evaluation. Instead the city relied on computer models provided by the developers.
Who is to blame is ultimately up to the courts to decide. Now engineers have some options for ‘fixing’ the tower.
· Remove the top 20 floors
· Reinforce soil underneath building
· Balancing the lean with a heavy building on the other side of Millennium Tower
· Doing nothing, as Millennium Tower promises the building is still structurally sound
· Do nothing, and move residents if/when the tower approaches a tipping point
Fun fact: Millennium Tower received nine awards for excellent design and engineering. Today, the project looks like it will eventually be condemned.
Who do you think is to blame? And how do you think Millennium Tower could be rescued?
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Re: The San Fran Skyscraper with a Six-Inch Slant
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