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  #41  
Old Posted Feb 8, 2021, 11:42 PM
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02/08/2021 - Rescue work is now happening!

Milleanium Tower San Francisco- Rescue Work
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  #42  
Old Posted Feb 13, 2021, 2:16 AM
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Another view of the work in an attempt to stop the tilt & sinking.

Millennium Tower - San Francisco
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  #43  
Old Posted Feb 23, 2021, 8:46 PM
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Quote:
Originally Posted by Pedestrian View Post
As a general rule of thumb, in San Francisco office towers are structural steel-framed with concrete elevator cores, of course, but residential buildings are nearly all reinforced concrete like the Millennium tower. I believe it's because the flexibility and swaying that's unavoidable in steel buildings during "minor" tremors would unnerve residents in buildings full of them.
That's the case for most residential condo buildings here in NYC as well. Steel structures are designed for commercial office buildings, and reinforced concrete is designed for residential buildings.

Why is this? My main hypothesis is that it comes down to two factors:
1. Minimizing floor-to-floor height while maximizing ceiling height.
2. MEP services
3. Fireproofing concerns

First, we must understand the difference between steel structures and reinforced concrete.

Steel structures rely on the large girder beams resting on columns as the main support. Resting on top of these girder beams are joists that span the girder beams. Then a reinforced concrete floor (or more usually, a concrete over metal deck floor) rests on top of the joists and the girders. The result is that thick steel girder beams will run across the ceiling in regular intervals. For aesthetic purposes, these structural elements are usually covered up by drop ceiling panels, and the "nominal" ceiling height would be measured from the floor to under the thickest steel girders:



Whereas, for reinforced concrete buildings, the underslab joists are much thinner and built monolithically with the floor above using a single pour of concrete. Therefore, reinforced concrete structures can achieve a higher "nominal" ceiling height while minimizing actual floor-to-floor height.



The next reason (based on my hypothesis) is MEP services. Commercial offices need substantial MEP services (HVAC, electrical wiring, data lines, fire sprinklers, etc.), and the space above the drop ceilings can be used to hide these mechanical elements. Residential buildings have fewer MEP needs, so they have no need for a cavernous empty space above drop ceilings. Therefore, reinforced concrete structures are better for residential needs.

Finally, fireproofing. Steel is not fireproof (as we all have been aware for almost 20 years), and needs to be fireproofed. Steel fireproofing is fairly flimsy and weak, and often needs to be inspected and re-applied as-needed. That may be feasible in office buildings, where tenant turnover and facility renovations are fairly regular. But in residential buildings, condo owners would likely balk at regular intrusions into their space. Therefore, it's best to have a naturally fireproof structural material that does not require periodic maintenance, and reinforced concrete fits the bill (that is, as long as there are no water problems that may damage the reinforcement over time).
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  #44  
Old Posted Mar 6, 2021, 3:00 PM
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Now that I think about it, friction piles is what they use in Dubai because the bedrock is so dang deep under the sand. I remember seeing it in a Science Channel show some years back concerning the design and construction of Burj Al Arab.
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  #45  
Old Posted Jul 9, 2021, 5:50 AM
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7/8/2021 - the serious work continues to fix a serious problem.

Millennium Tower, San Francisco
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  #46  
Old Posted Jul 9, 2021, 8:01 PM
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Sound transmission is also an advantage of concrete vs. steel in residential.
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  #47  
Old Posted Jul 13, 2021, 11:53 PM
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Quote:
Originally Posted by mhays View Post
Sound transmission is also an advantage of concrete vs. steel in residential.
Concrete is definitely better than steel for sound reduction, although the science of sound reduction is rather tricky and not as straightforward as other engineering fields. In addition, the typical sections that sound travels through are the same in both building types:

- Floors & ceilings
- Walls

The floors & ceiling for both will consist of concrete of some sort, either reinforced concrete or concrete over metal deck.

The walls will be the same for both too: drywall partitions over steel stud, most likely.

So it comes down to whether the developer is willing to invest in sound reduction solutions between units and the public hallways.
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  #48  
Old Posted Jul 14, 2021, 2:14 AM
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Slab on metal deck is pretty thin...I've heard that makes it far less effective for limiting sound transmission.
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  #49  
Old Posted Jul 15, 2021, 1:20 PM
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Quote:
Originally Posted by mhays View Post
Slab on metal deck is pretty thin...I've heard that makes it far less effective for limiting sound transmission.
Yep, that's definitely true. Though reinforced concrete floors aren't that thick themselves (4-6 inches, typical), they are still somewhat thicker than slab on deck (2.5+ inch top cover).

With that said, there are acoustical treatments that can be added to reduce sound afterwards. It comes down to whether the developer or tenant(s) is willing to pay for these treatments. Sound reduction engineering comes down to the following:
- measuring to determine the most common frequencies and their respective amplitudes (sound intensity)
- designing acoustical treatments to block those common frequencies
- determining if any materials resonate at a common frequency, then dampening or de-coupling that material

It's a tricky science because one material that blocks one set/range of frequencies might not block another. So you need to mix and match materials. Heavier materials are inherently better at sound reduction, but they are no good if they resonate at a frequency that occurs commonly in the building.
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  #50  
Old Posted Aug 26, 2021, 12:07 AM
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Quote:
Repair work paused on S.F.'s Millennium Tower due to continued sinking
J.K. Dineen
Aug. 25, 2021
Updated: Aug. 25, 2021 3:42 p.m.

The $100 million fix meant to shore up San Francisco’s famously sinking and leaning Millennium Tower has been put on hold for up to a month while engineers try to figure out why the building has settled an additional inch during the current remedial construction.

In a statement, a spokesman for the Millennium’s homeowners association said that the latest monitoring of the building has “indicated an increased rate of settlement associated with pile installation” . . . .

“Out of an abundance of caution, we have placed a two- to four-week moratorium on pile installation while we try to understand better the mechanisms associated with the increased settlement rate and available means of mitigating this,” said Doug Elmets, spokesman for the building. “There has been no material harm to the building and it remains fully safe" . . . .

A 2020 confidential settlement not only compensated homeowners for value lost due to the construction issues but also included a $100 million fix: 52 concrete, 140,000-pound piles that will anchor the building to bedrock 250 feet below ground. The two-year project was billed as a way to relieve stress on soils that have compressed beneath the building, contributing to its unwelcome and unanticipated sinking.

“This plan halts settlement along Fremont and Mission streets while allowing the building to level itself over time,” said project engineer Ronald Hamburger of Simpson Gumpertz & Heger, at the time construction started.

While the building has continued to sink during construction, Elmets said the outlook is still positive.

“Once pile installation is complete and load is transferred to the new piles, the building will experience substantial improvement and begin to recover some of the tilting that has occurred over the years,” he said.
https://www.sfchronicle.com/sf/artic...r-16411876.php

Well, maybe.
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  #51  
Old Posted Aug 26, 2021, 1:44 AM
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In Chicago also as a rule offices are steel w a concrete elevator core - residential are re-enforce concrete. I had thought it was primarily a cost issue.
The more expensive steel being used when there is a demand for large open spaces - most of ours start out with the entire floor open except the elevator core.
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  #52  
Old Posted Aug 26, 2021, 5:17 AM
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I would have left that building a long time ago! I just read the news from the Chronicle tonight & was like, What?!
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  #53  
Old Posted Aug 26, 2021, 1:20 PM
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Quote:
Originally Posted by harryc View Post
In Chicago also as a rule offices are steel w a concrete elevator core - residential are re-enforce concrete. I had thought it was primarily a cost issue.
The more expensive steel being used when there is a demand for large open spaces - most of ours start out with the entire floor open except the elevator core.
Both concrete and steel are expensive. Custom and larger wide-flange steel beams and girders are, of course, more expensive.

But IMO, the main reason is that steel structures offer more flexibility and larger spans between columns, which allows office tenants to customize the space to their preference and needs. One of the negatives I mentioned is that the intermediate beam supports between girders tend to eat up ceiling height space, which isn't as much of a concern for office spaces because they often use this "lost" ceiling height to hide HVAC, plumbing, and electrical equipment and connections above drop ceiling tiles.

Concrete is used in residential spaces for a few reasons:
1. It's naturally fireproof, so there's no need to regularly go into residential apartments to inspect fireproofing as you would need to do with steel structures.
2. The HVAC, electrical, and plumbing mains mostly remain fixed. And there's no need to run them along the ceiling or permit much customization beyond minor branches because apartments are smaller spaces that are mostly occupied long-term in the same configuration.
3. As such, concrete buildings typically have larger ceiling-height to floor-to-floor-height ratio compared with steel buildings. Therefore, concrete allows developers to build more floors at a shorter building height vs steel.
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  #54  
Old Posted Oct 14, 2021, 10:00 PM
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10/14/2021 - Channel 7 news on Tuesday says that "crews installed a 36-inch steel cylinder 100 feet into the ground on Fremont Street.

Engineers believe the steel cylinder will reduce erosion and vibrations from construction and vehicles."

Desperation! The building has sunk one inch since the work began to stop the sinking. One engineer says that he predicted that the work would cause more problems.
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  #55  
Old Posted Oct 23, 2021, 8:47 AM
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10/22/2021 - NBC news said that the building tilted an additional quarter of an inch during the week of the first test of the troubled fix. Can this building be saved?
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  #56  
Old Posted Oct 25, 2021, 1:43 AM
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At what point is it too far gone? There's got be a number of inches sunk or degrees tilted or good money after bad that is crossing a line of no return. I hope this plan works. The Millennium Tower website should be updated because 645 feet is not the height anymore.

I haven't been able to find any information about at what degree of tilt failure would occur.

Last edited by kingkirbythe....; Oct 25, 2021 at 2:43 AM.
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  #57  
Old Posted Oct 25, 2021, 2:46 AM
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funny

Quote:
Originally Posted by kingkirbythe.... View Post
At what point is it too far gone? There's got be a number of inches sunk or degrees tilted or good money after bad that is crossing a line of no return. I hope this plan works. The Millennium Tower website should be updated because 645 feet is not the height anymore.

I haven't been able to find any information about at what degree of tilt failure would occur.
I though about what you said about the height for a moment and then got a good chuckle! Funny!
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