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How many more degrees of tilt can the Millennium Tower in San Francisco withstand before it becomes structurally compromised/unsound?

"...An examination in 2016 showed the building had sunk 16 inches (41 cm) with a two-inch (5.1 cm) tilt at the base and an approximate six-inch (15 cm) tilt at the top of the tower.[30] The building is leaning toward the northwest,[30][31][32] and has caused cracks in the building's basement and the pavement surrounding the tower.[33] As of 2018, the sinking had increased to 18 inches (46 cm) with a lean of 14 inches (36 cm).[34]..." Wikipedia, https://en.wikipedia.org/wiki/Millennium_Tower_(San_Francisco)

"...The latest data – including the four days that the test pile was installed from Nov. 15 to Nov. 19 – shows a quarter inch of new tilt, as well as a tenth of an inch of settlement at the time the test installation occurred. At the same time, there was marked fluctuation of water pressure below the foundation on the Mission Street side of the structure..." December 7, 2021. https://www.nbcbayarea.com/investigations/sf-millennium-tower-tilts-quarter-inch-in-four-days/2750189/

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    $\begingroup$ If they built it as well as the masons who constructed at Pisa… $\endgroup$
    – Solar Mike
    Dec 31, 2021 at 22:55

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Modern building failure due to tipping is rare but did occur, usually under strong earthquakes and loss of foundation (soil and structure), as the tilt will shift the gravity center of the building that induce a tremendous amount of stress on the foundation piling and the soil mass surrounding the piling.

So, how much is too much for a building to handle (remain functional)? As the building code generally does not specifys limit on inclination other than service considerations (floor levelness, shaft plumbness...), the answer can only be provided by the structural designer who knows the building structure, the soil strength, and has a handle on soil-structure interaction.

But for a rought estimate, H/400 - H/500 are the most recommended limit on "drift" due to wind, deflection beyond such limit is believed to cause severe structural, and/or foundation damages. From that, you can do your math by assuming H/500 is the absolute maximum a building can tilt without failure (Note, the safety factor for wind load is quite large, that can approaching or beyond 2).

Note, another building failure mode is settlement, however, it creates more of service issues and localized structural damages, especially in the case of uniform settlement. So it is not addressed here.

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  • $\begingroup$ That is a good point you make about the structural engineer(s) who designed the building being the best one(s) to answer this question. Also, I hope that no earthquake hits San Francisco between now and the time when the reinforcing pilings have been installed. I think that even a minor quake would dramatically increase the tilting of the building. $\endgroup$
    – user57467
    Jan 2, 2022 at 16:49

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