re: San Francisco Landmark, Luxury High-Rise Millennium Tower Is Sinking Fast

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How far below New Orleans do you think you will find bedrock?

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Concrete piles, stretching more than 200 feet down, are what allow One Shell Square and other buildings to float above it all.

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LINK

Not 80 feet. I'm no engineer and that jumped out at me as a red flag. I guess I've done enough work in this area, though.

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It's the only way to build in New Orleans, but even here, I'm pretty sure high rise pilings are typically deeper than 80 feet.

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well they could use displacement piles instead of non displacement piles to solidify the soils around the structure. I have not looked at the plans but i believe this is what was done for the Dome.

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Condominiums in the 58-story building have price tags as high as $10 million.

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Well, they used to be.

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Not 80 feet. I'm no engineer and that jumped out at me as a red flag. I guess I've done enough work in this area, though.

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It's all about the soil supporting it. Generally speaking the deeper you go, the sturdier the soil is (think of water pressure in a pool). However, a lot of times sturdier soil can be found relatively close to the surface.

For example here in New York bedrock is found within 10 feet of grade, or even at the surface in a lot of places. Or you can find places like JFK airport (which was built on marsh) where the blows/foot are in the single digits and you need a substantial foundation to hold everything up.

If there's an error here it sounds like it's on the geotechnical side of things rather than the structural. BTW shear walls are standard practice in high rise construction.

For more info, here's a link put out by the Concrete reinforcing steel institute on the millennium building.

LINK

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Millennium Tower stands 645 feet and is the tallest reinforced concrete structure situated in a seismic zone 4 region, as well as the 4th tallest structure in the city of San Francisco. The $350 million project is comprised of a 59-story tower and a 12-story building connected by a 3 level podium structure, The above ground structure contains approximately 900,000 sf with an additional 250,000 sf of parking and support space located below-grade. This project required a 75-ft deep excavation, which is one of the deepest in San Francisco. The tower's immense height posed many challenges and required the creative use of technologies and cutting edge innovation. Concrete, while a unique choice for this high-rise tower, is utilized most effectively to make this project a financial success for the developer. Millennium Tower demonstrates that concrete is a safe and secure building material, even in seismically challenging situations.

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Eta:

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In addition to significant innovations above grade, the tower's below grade solution is also challenging. A tower of this height creates a higher bearing pressure below the core (14,000 psf). To provide support for such a high concentration of force, 950 14-inch square precast piles were used. While overturning due to earthquake is a significant factor, the number of piles was governed by gravity design. In order to provide a high-confidence against a possible foundation shear failure, vertical shear reinforcing is added in the 10 ft thick pile cap.

The project's five-story basement extends 75 feet below grade, nearly 45 feet below the site's water table. This open cut required special waterproofing. A unique shot-crete mixture containing Caltite is employed at perimeter walls, eliminating the need for a conventional adhered waterproofing membrane.

The foundation supports two very different types of structures. Overtime, the building is expected to settle as much as 5". The two towers are separated by a horizontal joint, and hinge slabs connect the two structures allowing for additional settlement to occur. The area between the towers, supports only three levels above grade and requires tie-downs to resist the hydrostatic uplift at the bottom of this deep excavation.

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Again it sounds like soil, not structural failure. 14 ksf bearing pressure is high.

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I know a contractor who only works o condos valued $25,000,000 or more in Chicago

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There's not one property for sale in Chicago north of $20MM ( LINK). I think your contractor friend has a loose grasp on high-end real estate prices.

Do you live in Louisiana? If so....glass houses my friend. Earthquakes cause destruction and death you know kind of like flooding.

I bet the price is still higher than purchase price. Sold in 2009....that was a low point in our market.

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I'm no engineer and its been a while since I've stayed in a Holiday Inn Express but I was thinking the same thing. Bedrock moves during an earthquake so drilling pilings to sit on a bedrock in a fault zone seems unwise....but I really have no clue what I'm talking about.

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I wonder if those exact words were spoken during the Millennium Tower planning meetings.

Do you and your house mate have a condo there?