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The San Fran Skyscraper with a Six-Inch Slant

Posted September 28, 2016 9:05 AM by HUSH

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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#1

Re: The San Fran Skyscraper with a Six-Inch Slant

09/28/2016 1:00 PM

A thousand Quatloos on doing nothing!
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#2

Re: The San Fran Skyscraper with a Six-Inch Slant

09/28/2016 4:37 PM

If it's not leaning enough to be a tourist attraction,, I wouldn't worry about it....

At 6" lean = 0.0417°

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#3

Re: The San Fran Skyscraper with a Six-Inch Slant

09/29/2016 12:25 AM

The reason is simple.

"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."

No miracle occurred during the ensuing 5 years to stabilize the ground.

But, "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".

"Trust us", the developers said, "We've modeled it".

Nothing will be done until after the developers have left town or declared bankTrumpcy and walked away to start over under a new name.

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#4
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Re: The San Fran Skyscraper with a Six-Inch Slant

09/29/2016 12:37 AM
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#5
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Re: The San Fran Skyscraper with a Six-Inch Slant

09/29/2016 1:22 AM

That depends on how much $kin they have in the game.

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#6
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Re: The San Fran Skyscraper with a Six-Inch Slant

09/29/2016 1:47 AM
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#7
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Re: The San Fran Skyscraper with a Six-Inch Slant

09/29/2016 11:11 PM

The answer is simple. Outlaw the future construction of tall, narrow buildings supported on a small foot print.

Instead, construct square or rectangular buildings all the same size. Land formerly used as streets, parking lots or open areas can be utilized in the building design.

Open areas can be designed similar to the way modern cruise ships utilize open pent roof space.

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#13
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Re: The San Fran Skyscraper with a Six-Inch Slant

10/01/2016 2:54 AM

Re"the city didn’t have the engineering expertise to evaluate the soil integrity of the project"

I think we could start a whole new post on the real cost of cost-cutting public services. In this one example of many, we're seeing the real cost of government getting rid of their own experts who are focussed on looking after the interests of their employer (and the community in general) for short term gain. Someone probably got a fat bonus for that decision - false economy!

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#15
In reply to #13

Re: The San Fran Skyscraper with a Six-Inch Slant

10/04/2016 12:53 PM

Actually, the owner whose building the building should do that. All the city has to do, is have it meet a set of standards.

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#16
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Re: The San Fran Skyscraper with a Six-Inch Slant

10/05/2016 10:24 PM

Fair comment - just so long as someone actually does a proper and unbiased job of ensuring the standards are met. My basic point is that you can (normally) rely upon the public service to do a proper and unbiased/impartial job and not be influenced by the chance to make a quick buck and move on – conflict of interest. I’m not casting aspersions on the whole private sector here, but there is no doubt there is an unscrupulous element within it. I see the city requiring “engineering expertise to evaluate the soil integrity of the project” to make sure that unsafe buildings are not built as analogous to the need for police and armed forces to keep communities safe. The reason it's in the city's interest is that it'll probably have to deal with the consequences if the building has to be demolished etc. and everyone else walks away from their responsibility.

My comments are based upon my simple reading of the OP that:

  • The initial application was rejected by the city’s engineers due to soil integrity concerns
  • The re-submission of the same application was accepted by the city when it no longer had the expertise to evaluate the soil integrity aspect of the proposal i.e. I understand the post to be saying there was ignorance by the city with respect to the soil integrity concerns (or possibly that they had other, in hindsight, questionable advice)
  • The soil conditions hadn’t changed between submissions
  • The city reduced its payroll by no longer having those engineers
  • There is concern that the building may have to have extensive and costly work done or be demolished
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#8

Re: The San Fran Skyscraper with a Six-Inch Slant

09/30/2016 2:00 PM

S.F.? I'm betting it leans left...

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#9
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Re: The San Fran Skyscraper with a Six-Inch Slant

09/30/2016 2:35 PM

Probably goes either way....haha

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#10

Re: The San Fran Skyscraper with a Six-Inch Slant

09/30/2016 2:51 PM

What's the actual problem? do the front doors still open? If so, get over it.
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#11
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Re: The San Fran Skyscraper with a Six-Inch Slant

09/30/2016 6:33 PM

As long as the furniture doesn't slide to one end of the room, you're OK.

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Re: The San Fran Skyscraper with a Six-Inch Slant

09/30/2016 10:19 PM

It's all about the money....the stink is driving the property values down....

http://sf.curbed.com/building/433/millennium-tower

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#14

Re: The San Fran Skyscraper with a Six-Inch Slant

10/04/2016 12:51 PM

Don't these buildings of this size actually sway at least that much due to wind loads.

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#17

Re: The San Fran Skyscraper with a Six-Inch Slant

10/26/2016 8:39 AM

I thought I had read that the building is also sinking. 16" so far!

I'd be concerned that my tub might not drain properly ..or

How much could grout slurry foundation remediation help?

I could guess that it might.

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#18

Re: The San Fran Skyscraper with a Six-Inch Slant

10/27/2016 8:13 PM

6 inches of tilt and 16 inches of subsidence is what you can expect to get when you only pay for a low-bid geotechnical investigation...

... and maybe if the (designer) adds a dozen more stories after the Final (?) Plans are approved by the City (?)...

... or the vibration from adjacent pile-driving lessens the grain friction and/or promotes micro-fissuring in the underlying soil strata...

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#19

Re: The San Fran Skyscraper with a Six-Inch Slant

12/04/2016 7:10 PM

You build on a garbage dump, you get ..... And to think "Silicon Valley" is 30 miles down the road! Kinda like computers, garbage in→∝ garbage out ⇓

Caitlyn talking to Joel: Where were you during that 5.9 Earthquake?

Joel: "I was swinging both ways in my penthouse. And, now I have a street level door!"

The real question is, who's to blame? There is plenty of history about San Francisco's liquefaction during an Earthquake, so why are "they" allowing 56 story hi-rises in an area where they know for a fact that it's landfill?" Besides $$$$$ ?

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#20
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Re: The San Fran Skyscraper with a Six-Inch Slant

01/25/2017 7:40 PM

Theoretically, an (elastoplastic-cement ?)-slurry could be pumped under the lowest level, and being a fluid, the vertical pressure would then tend to re-direct the centroidal axis back towards verticality...

Practically, however, it could be very hard to prevent the slurry from then squeezing through any available openings, and then, be forced to rise up to (flow) out at street-level...

Under the pressure of the entire building, any such slurry might well shoot out of the surrounding ground, or simply buckle the lowest concrete floor slab, without further reinforcement than the lowest floor level already has...

And, deny all occupancy until the entire (slurry-wedge) completely hardens...

Hmmmm...

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