What really happened at the Millennium Tower?
practical.engineering
practical.engineering
Considering the crazy SF prices, especially for new condo developments, the future is especially important when buying an apartment in SF.
So it is perfectly rational for people to refuse to buy based on an engineering statement that the building is safe but may not be safe a couple of years from now.
I'm not buying the "we didn't know people would build and drill nearby" argument. It's downtown San Francisco for God's sake.
"By that time, the building had settled another inch and was out-of-plumb by more than a foot or 30 centimeters. That’s not something you could notice by eye and was probably only discernible to the most perceptive residents, but it’s well beyond the 6 inches allowed by the building code."
Twice the amount allowed by the building code sounds like a problem.
Also what people don't realize that the swing at the top floor of such a tall building due to wind may be actually about also another foot at worst case.
For things like wind deflection, more than 2" per 100 feet is considered excessive.
It's not the same engineers. The first ones may well be wrong and the later ones right.
But given that most people buying a home are looking for a low-risk proposition, I get why prices dropped. Especially people buying condos; a big part of the point is that a lot of the hassles are taken care of for you.
I sometimes wonder about that. For what it costs in ongoing condo fees, you could pay someone to provide exactly the same level of worry-free maintenance on a regular house.
Though I'm skeptical of the terms you'd get on a loan secured by a badly damaged home.
Plus, if you sell the house with a bad roof, a smart buyer would insist on paying market value minus cost of roof replacement. If you replace roof, you’ll get market value. Either way, you’re going to lose the roof replacement cost.
i'd subtract whatever your water/trash/sewage bills are from "typical" HOA rates in your area, and subtract typical difference in home vs HO6 insurance rates, then get to net to compare.
Plus, you don't need to be concerned with a HOA saying you can't rent your place out on whatever platform you choose.
It seems this is a chaotic environment that may defy the ability of experts to make accurate predictions.
> Bur another (third?) set of engineers was also wrong, not anticipating the impact that their work to fix the situation would have on actually accelerating the problem.
https://blog.architizer.com/inspiration/collections/ten-famo...
Downtown San Francisco is NIMBY central, and it is quite surprising people were actually able to build something new there. It's a reasonable assumption that the zoning etc gridlock continues, and very little new construction happens nearby.
Also, I've noticed that we (computer professionals here on HN, roughly) don't always think that the general public is particularly rational in their choices about computers. Perhaps most professional civil engineers have similar feelings about the public being rational in their own area of expertise?
0. https://www.extremetech.com/computing/294907-why-moving-the-...
Hah, how’s that for an understatement. Most devs definitely see the general public much as Joe Wilkinson sees Fabio.
So whilst they try and determine who pays, these properties are now basically worthless, banks won't give mortgages, no one will buy them.
Last I heard the modified drilling procedure didn't help and the accelerated sinking hadn't fully stopped so right now there is no plan to resume.
When the pile drilling started lean was 17". In less than a year of that work it's now 22". By building settlement standards that's blindingly fast and makes me think the soil on that corner is in much worse shape than anyone assumed... possibly bad enough it won't hold in a large earthquake. It may be a more complicated interaction than just compression due to the load.
I'm not sure what they can do at this point. Have a bunch of crews get the piles installed as quickly as possible and hope you can do it before the sinking becomes dangerous? Given it is a concrete building rather than steel they probably can't do much to reduce the weight but moving everyone and everything possible out might help a little. To counteract the leak maybe they can drill on the other corner (that's always been part of the long-term fix) to get it to settle at a faster and thus more even rate?
Definitely a difficult problem.
There's nothing stopping the builders to get the bad column out and try it again. And again. Yes it's expensive as fuck, but this is still the foundation we're talking about. This (and the load bearing core) is where cost must come after safety.
Yes, naturally safety needs to be quantified and that just gets us back to cost again in a sense, but that's why there's a minimal safety codified (in the building code).
It's all about cost. There are many ways to get those nice columns in the ground, it's possible to drill them and put annular rings on top of each other (just like with regular wells) then fill it with concrete. More expensive than the bentonite slurry method? Sure, but it's still simple.
This is all outlined in the article, and it's frustrating to see people commenting when they haven't read it.
They are really simple, but help me better understand the world we live in.
I think the channel is well worth subscribing to and watching in full, the transcripts are useful to come back to afterwards, but it’s not 30 seconds of content being stretched out to 15mn so all the ads can fit in.
He's leaving out a big factor that any condo owner understands: maintenance fees. I wouldn't take a unit in that building if it were free, because for the lifetime of the building there will be ongoing excess maintenance associated with every system because of the settling. Imagine the extra wear on the high speed elevators running out of true vertical. Imagine the differential stresses placed on pipes and boilers and tanks that expect to sit vertical. Imagine the effect of continually reseating the building's connections to local services like sewer and electricity. And imagine the effect of the HOA spending money on grand fixes while routine maintenance gets deferred.
In the near term, the effect of the tilt is negligible, but buildings like these continually suffer from imperfect, discontinuous maintenace under the best conditions; the Millenium Tower will forever be marked by every realtor and owner as maintenance-expensive (meaning vastly higher monthly fees) compared to all the other buildings around.
Depending on where the connections are in the building, they may also not have suffered as much settlement as the most affected corner of the building.
Grady makes great civil engineering videos.
There was another one of these here just the other day about processing cocoa from bean to chocolate. The written text kept saying things that implied a visual reference, but no images to support it. It wasn't until the end of the transcript before the video itself was embedded.
It amazes me that ANYONE would prefer to watch a video over reading, I literally cannot understand why that would be the case.
It would be great if prefer-video people could write something up for video-never people like me on why they prefer it! Then I could try to make a truly terrible video explaining my perspective. ;)
A transcript, while definitionally broad, is commonly used with the implication that someone has spoken in free-form (ie not reading from a script), and the transcript is the writing down of their speech.
You know this how?
I'm surprised the piles don't break due to the sinking on only one side? Can it not happen?
There’s a pretty in-depth discussion thread on this forum with a lot of people who seem to know more than me and my SWE friends: https://www.eng-tips.com/viewthread.cfm?qid=470048
We live on the corner of the building that is sinking the most, so I'm not sure why we don't see any tilt. Perhaps we need to be higher up to observe it?
I have no idea whether this is feasible in San Francisco. One difference is that London is not prone to large earthquakes.
1. "London's skyscrapers float on rafts embedded in the clay." https://en.wikipedia.org/wiki/London_Clay#Engineering
That's effectively what the Millenium Tower did - their engineers determined that friction piles to the sandy layer would be the cheapest acceptable option. Then, due to factors they may or may not have been able to predict, it turned out to not be an adequate solution.
If they had known beforehand, the next cheapest option would have been chosen - likely end-bearing piles to the bedrock, like they are now retrofitting. Floating raft foundations are truly a last resort due to their expense.
Where there's enough money to be made, you'll find shady business people and corrupt politicians willing to risk people's lives and livelihoods for a buck.
Plus, the federal government will keep giving people enough money to rebuild on flood plains, so it makes financial sense to keep building in places that keep getting washed away.
Though at some point I think code enforcement returned, especially for large buildings.
The landlady would always talk about how the house held up great during the '89 earthquake. But that didn't impress me considering the epicenter was in Santa Cruz.
As a contractor with a foundation that settled it's nerve racking.
That said, very few foundations that settle need to be repaired. The average homeowner thinks the opposite though.
I stopped my settling by draining mine, and the neighbor's roof downspouts to the street.
Even without that fix, I doubt my house would have settled further.
It's true that most buildings/houses settle.
When I see modern townhouses slab foundation instead of proper basement (which Archimed law favors as soil is just a very slow liquid in which building is floating like a ship until of course it's piles are driven into bedrock) I cringe at that cost cutting though it is ok for a 3 story building. I never imagined that it would be done to a skyscraper - no basement and no bedrock reaching piles. The story of Millennium tower reminds those buildings in Siberia on permafrost where builders cheated and did shallower than needed foundation - you can notice those buildings from a distance by twisted pagoda style roofs, etc.
A mat or raft slab foundation - such as the kind used in the Millennium Tower - can be just fine for a small structure such as townhome. A mat/raft foundation is different than a regular slab on grade foundation in that it is thicker and has steel reinforcement running throughout, not just in the perimeter footers. [1] Its strength is due to the way the steel reinforcement distributes loads across the subsurface.
My 1600 square foot house is built with such a foundation and I got a close-up look at it during a recent major structural renovation and it is incredibly robust, even over-engineered.
Just because it was a bad choice for a skyscraper doesn't make it a bad choice for far lighter structures.
1. https://www.finehomebuilding.com/2019/12/30/mat-slabs-an-inv...
There are other SF high-rises using slab, yet their piles reach the bedrock. Also Millenium Tower is reinforced concrete instead of typical for such high-rises steel - it was more "economical" - and due to the leaning/sinking stress the concrete there is cracking.
It sounds like they still don't have a solution to fix the leaning tower of San Francisco.
The issue is getting it to settle evenly?
So you dig down on the sides and then use something: a long drill, a water jet, to open up small 1 or two inch channels. Are there high power lasers that can do this?
With the stresses, the building will crush out the small open space and... settle?
So the initial modelling was badly wrong, but new! improved! modelling show it's safe. The dumb public is being completely irrational. /sarc
Well, not most. Yet. And sure, possibly ever.
(Go read the article / watch the video ... and a few others of Grady's. He really does do excellent work.)
It is also not most buildings.
(It is the most expensive building, however.)
My "ever" referred to whether or not most buildings might ever be constructed in a vacuum. Clearly, some are, and it's all but certain that more will be.
It might have been topped by "the Great Mosque" (~100 billion USD): https://en.wikipedia.org/wiki/Masjid_al-Haram
In laboratory terms, that's an ultra high vacuum which only specialized facilities on earth can achieve. I think that calling it Vacuum is perfectly reasonable, unless your standard for vacuum is deep interstellar space?
I suspect they are undervalued, but I wouldn't live there. Not because of the tilting, which appears to be on its way to being fixed, but because it strikes me as a soulless sardine can.
https://www.highrises.com/san-francisco/millennium-tower-con...
If the bedrock is now supporting a significant share of the building, does it make it more or less able to cope with earthquakes?
Freezing could be used as part of a perm fix too. I bet there’s a way to sequentially freeze subsoils in such a way to use frost heave to actually push the building back into original position to augment the foundation. Or create a frozen layer in the mid-foundation and then pump in high pressure concrete grout below the frozen layer. Freezing water is a powerful thing…
Armchair civil engineering is fun
Extending the piles down to bedrock level was quoted at $500m, so they ditched the idea for this project.
When I was homeless, I would have preferred that to living in my car. Even tho it was a roomy car.
Well, yeah... Living in a tall sinking/tilting building would give me huge anxiety no matter what experts say
The explanation that makes sense to me is that settlement is a problem that could be very costly and disruptive over a long period of time, and it's expensive and complicated to prove otherwise. So in the case of the Millennium Tower, they did in fact attempt to prove that the settlement was not going to cause major long-term problems, but that seemingly had no impact on the price behavior. I think with such an ingrained pattern, there isn't really a way to force buyers to think unconventionally; everyone "knows" that you don't buy in a building with settlement, and no one "knows" that there are circumstances where it's OK to buy anyway.
but if they had this great model, which you would entrust your life to, why isn't it used to predict this settling in the first place?
Conspicuous by its absence in the article is any mention of how much additional lean can be tolerated before occupancy of the building becomes hazardous.
especially, since the same experts said that it will barely sink at all.
Later, thoroughly on notice that things were screwed up, they designed a solution to fix the problem. But they couldn’t predict that the vibrations from drilling would cause more sinking.
Now we’re supposed to believe they can accurately predict what the vibrations of an earthquake will do?
If this building is a pile of rubble, somewhere geotechnical engineers will be saying that it couldn’t possibly be predicted.
Here’s hoping the big one doesn’t hit while this exciting experiment in foundation installation is ongoing!
Someone confidently modeled the original construction too
It reminds me of that famous French comic about a (false) superhero called "Superdupont" in charge of protecting all things French. In one story the enemies of France steal the standard meter, and as a result all buildings become askew. I can't find it online right now but it's extremely funny.
I guess I'm not going to see something like 4.8 KN/m^2 in my life time in daily writings in the US. //sigh...
Before the current conventions (I am old enough to have studied in those times) pressure on soil would have been expressed in (still metric) in kgf/m2 or kgf/cm2 or - simplified - in kg/m2 that way (IMHO) it is easier to "visualize" pressure.
The 5 kpa would have been (roughly) 500 kg/m2, i.e. 5-6 people standing very near to each other within 1 square meter (to give you a quick term of comparison slabs in buildings are calculated to bear between 250 and 400 kg/m2, some particular ones up to 600 kg/m2).
If you imagine the soil as a liquid, with a density of around 2,000 kg/m3, double that of water, when you remove (dig) 1 meter of soil the surface below can surely carry (as it had done undisturbed for years) some 2,000 kg/m2 (usually much more), if you dig some 3 meters, you have surely 6,000 kg/m2 (that is the typical depth if digging for any "ordinary" building with a basement or underground garage).
The 11,000 pounds per square foot correspond to (roughly) 54,000 kg/m2.
This is a lot but seemingly not 100 times the "average" house, or - maybe better - the average house has a very high safety factor impolied.
I’m not suggesting that a “mass per area” convention is bad, rather that pounds is so interchangeably used as a force and as a mass that it’s not immediately obvious to the layman (especially international and not familiar with imperial units) which is being used here, because both could reasonably fit in this particular case when using area as a denominator.
A Pascal by itself is not easy to visualize, a number of kg on a square meter (or for those used to it a number of pounds on a square foot) are IMHO easy to imagine, the 500 kg/m2 we were talking about can be visualized as 5-6 people standing one close to another or as a tank 1 m x 1 m filled with 50 cm of water.
As a side note, there is no conversion between kg and kgf, they are essentially the same thing, the 9.81 is already included, and it comes into play when you start using the (BTW proper SI) Newton:
I was pleasantly surprised that an American article had metric units with sensible conversions in most cases.
The situation is very bad.
Around 2 weeks ago a city engineer for the first time mentioned "possibly dismantling the building", because logically you just can't have a skyscraper that's unsafe in the middle of downtown SF. It threatens numerous other tall buildings.
Based on videos I've seen of the Kobe earthquake, liquefaction would destroy the building even if tilting didn't decommission both the elevators and plumbing first. I wouldn't occupy anything in its shadow.
I wouldn't occupy anything in its shadow.
I also did briefly look to see if I could find any reference to a city engineer proposing dismantling the building, but couldn't find anything there.
And your plan only works if you can actually get insurance on a unit at this point.