Open Letter to SF Board of Supervisors from Howard Wong, Architect, via email
Hello Supervisors: Appreciate the thorough investigation. Current estimated structural status: 16″ settlement, 23″ tilt to the west, 9.5″ tilt to the north. Although some owners have sold condos at a loss (others got settlement payments), owners obviously want to minimize financial hits. Their engineer says the building can tilt 6.5 feet and still resist an earthquake—which is true since steel towers sway in earthquakes and wind (high winds can be very powerful). But such sway is a temporary state.
Also, “resist” doesn’t mean no structural failure. Even well-engineered buildings can have extensive damage at connections and frames, rendering the structure uninhabitable—not to mention occupants’ trauma from the rocking/ rolling and windows breaking. Millennium Tower’s tilting and sinking exacerbates potential problems—by additional stresses on its structure.
The big problem is the soil and geotechnical conditions. The entire tower sits on a 10-feet thick concrete mat foundation—and it has dished 14″, not just tilted. For a 10-feet thick slab to bend, the forces must be immense. Since the mat is designed for the tower’s vertical loads, differential structural loads may be due to subsurface voids and/ or varying/ substandard soil conditions. Who knows?
What we do know is that digging/ drilling/ excavating has consequences. Perhaps, it’s better to facilitate sinking the opposite corner of the building—which was the fix for the leaning Tower of Pisa.
BTW: The tallest building in the world, the 163-story Burj Khalifa in Dubai also has a mat foundation—but compacting homogeneous dense sand (existing for thousands of years) is different from San Francisco’s heterogeneous dumped fill.
BTW: As an architect, I work with structural engineers and minored in structures in college. But much building performance is somewhat intuitive—when one palpably sees nature’s forces at work.
Best, Howard Wong, AIA
RELATED:
NBC: Fix Designer Says Millennium Tower Could Tilt 6.5 Feet., Still Withstand Major Quake https://www.nbcbayarea.com/investigations/fix-designer-says-millennium-tower-could-tilt-6-5-feet-still-withstand-major-quake/2719044/#:~:text=on%20Fix%20Thursday-,The%20tower%20is%20currently%20leaning%2023%20inches%20to%20the%20west,the%20west%20just%20since%20May.
STRUCTURE: Stabilizing San Francisco’s Leaning Tower https://www.structuremag.org/?p=17838
CIVIL ENGINEERING: Stabilizing the leaning Tower of Pisa https://www.ice.org.uk/what-is-civil-engineering/what-do-civil-engineers-do/stabilising-the-leaning-tower-of-pisa
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