I'm no civil engineer but I talked to one about this story and the concerns seem to be overblown. First, the foundation as I am aware is created by drilling to the point of refusal. Obviously bedrock is ideal, but in a place like SF where some of the land is based off old landfill, refusal is acceptable. From there a concrete foundation is laid and the building rests upon it. Concrete slabs are secure and in the event of an earthquake it will be allowed to shift and "float" if the ground underneath liquifies. Secondly, the sinking and leaning can be corrected. Engineers account for some level of sinkage as the building settles, and if it tilts, they can apply ties to the sides to straighten it. In the most severe of cases they can shore up the foundation and build more supports. I'm sure the builders are examining their options currently so it'll be interesting to see what course of action they take.
I'm in a similar line of work and don't fully agree with this. Yes everything he's said is true, but to an extent, and having these rates of settlement per year is very troubling for a structure designed to live 60-100+ years. If the issue is not groundwater related (which seems unlikely with surrounding structures largely OK) then expensive remedial work (underpinning / grouting / similar) is going to be required in the immediate future before this causes serious issues.
Admittedly I don't work in an seismically active region but I design for max settlements of ~50mm over a building's lifetime, and the Millennium Tower is experiencing that every year.
Really? That's what they design them for? That seems extremely short-sighted. What is supposed to happen in 60 years? Why were medieval structures able to last hundreds o f years but we can't do it now? Are new York's hundred year old sky scrapers due for being torn down now? I don't understand why anybody would think that's a good idea besides the fact that it'll be your grandchildren's problem and you won't have to deal with it.
Nobody lives in those 100's years old buildings; those that live in buildings say 200 years old do so at great cost, because they love the history. Functions change, tech changes, living patterns change, and retrofitting is much more expensive than rebuilding after some point. Especially for commercial buildings and downtown areas, it makes little sense to use buildings beyond 100 years, save for some of historical significance.
As a European this is just .. shocking. I was born in a city that has been settled for almost 2000 years, which has the oldest parts surrounded by walls that were constructed between the 12th and 14th century.
Even when I moved away from there to a new city the flat I bought was in a building almost 200 years old.
I know that America is "new", and that modern buildings are, by definition modern, but the idea that you don't plan for the long-term, and that buildings aren't old is very surprising.
I'm not sure what you're saying, but I'm very likely to live in a city older than yours. Buildings hundreds of years old are castles, churches or a limited number of inner city monuments. They are very hard to live in, let alone live comfortably in; they are very expensive to retrofit and in general nice to look at but a pain to live or work in.
Buildings 200 to 100 years old are more common, but still suffer from many defects; unless renovated in a way that is essentially 'build a new house inside the old one'. Which is more expensive than knocking the old one down.
Anything less than 100 years but older than say 20 can be made to 'modern' standards, for some values of 'modern'. It won't be nearly as comfortable as a new house though, again unless renovated the expensive way.
Housing needs change. If you think we Europeans build for 100+ year use, you obviously know nothing about real estate or construction, and the history thereof. If you think the fact that we have a bunch of old buildings is because they were designed that way, you are very mistaken.
I'm trying to say that while uses change that building is "effort" and if you're going to do it you should do it with the assumption that it will last.
Re-purposing old buildings is very common in the UK, and whilst sometimes that might mean essentially ripping it all out and rebuilding form the inside-out that's an extreme case. When it comes to "public buildings" and "houses" that people live in mostly the changes aren't so drastic.
I'm sure that people putting up buildings in the 1600s didn't imagine they would still be in use, but the fact that they are is a good thing. What I'm really trying to say is that planning to build something with the assumption it'll be dead/useless/retired in a hundred years seems wasteful and short-sighted. Europe is used as an example of how things have turned out otherwise. Though I appreciate there aren't any skyscrapers/tower-blocks of that age in the world. Unless you think of tenement buildings from the 17/18/1900s.
What a bizarrely hostile-feeling response. I'm not sure the person you were responding to was making the point that we are building for 100+ years of use, simply that most properties where he lives were easily as old as that and experienced little in the way of issues.
None of the 100/200+ year old houses and flats I have lived in - both within the UK and other older European cities - have had any major defects beyond what some of my other acquaintances living in modern buildings have experienced. They have all been perfectly "comfortable" as a 'new build' property.
I have read that unreinforced concrete structures can stand for thousands of years, which you confirm just by noting some Roman structures still standing.
You can build much higher and bigger with reinforced concrete, but they will only be up for at most many decades, because steel always rusts.
Displacement like this seems like not such a big deal, especially when worded as it is in this article. You see "...several CM of displacement per year..." and you immediately are lead to believe that this displacement is equal over the entire footprint of the structure. That is usually not the case.
Instead, you end up with differential displacement over the entire footprint of the building. Some areas will sink 1cm, while others will sink 3cm. This "imposed displacement" puts the foundation under constant force, causing some tremendous force to be placed on the structure itself, as the foundation is now behaving as a beam of sorts.
Interesting that the lead photo[0] also shows Salesforce East (formerly Salesforce Plaza, 350 Mission) also sinking at a similar rate. I'm wondering what all of this will end up meaning for Salesforce Tower (the large hole in the ground to the left of the Millennium Tower), since it's close to topping out and will be one of the two tallest buildings west of the Mississippi River.
Also of note: you can actually see the buckling of the sidewalk and curbing immediately adjacent to the Millennium Tower. I'll see if I can get a photo to share today. There's certainly quite a bit of heavy construction equipment traveling on Fremont/Mission, but I can't see how it would cause buckling sidewalks.
"The Sentinel-1 satellites have shown that the Millennium Tower skyscraper in the centre of San Francisco is sinking by a few centimetres a year"
I'm no engineer - but to me "a few cm a year" seems very significant, and would be easily detectable by current surveying technology? You would probably even visually be able to see it at the entrance to the building after 1-2 years?
I've learnt from previous discussions on HN that it is indeed a major source of concern for everyone involved, especially since it's also tilting and since it's a seismic area. They build the sidewalks again a few times a year. The tower is quite new and the builders are probably liable. The use of the satellite is probably rather to highlight that the Millenium tower isn't the only one.
You can certainly measure this with current surveying techniques. What the satellite InSAR technology allows is the measurement of millions of points at the same time, over thousands of square kilometres. And with the Sentinel-1 satellites providing free images every 6 or 12 days, the measurements can be updated frequently.
You can certainly measure this with current surveying techniques.
Not easily. And compared to what? The classic USGS survey markers don't have their height measured accurately enough, and they move with the land. The USGS does have precision "vertical control point" markers, and there are about ten of them along the northeastern coast of San Francisco. The highest precision vertical control point nearby is a GPS station on top of Building 1 at Fort Mason, and its height is valid only to 1.35cm accuracy at 95% confidence.[2] Absolute elevation is tough to measure at those scales.
Relative elevation, relative to the average of all ground points for, say, a kilometer radius, is something a radar satellite can do well.
Authoritative third party records that establish the period over which a building or land started to move is also potentially useful in resolving disputes over who and what might be responsible for the movement
(nb: I'm currently working with startup that consults on satellite/legal issues)
I don't know what your exact definitions is of "cheap", "millimeter-level", and "every few hours", but your comment does sound a bit misinformed.
What you are possibly referring to is called dimensional control in geomatics and civil engineering, and there are various measurement techniques that can be applied, including GNSS-based.
In a way, the hardest part of it is the planning phase, where you have to decide which technique is most appropriate to meet precision, accuracy, timeliness, and cost constraints. All those factors kind of fight against each other. Finding the sweet spot is non-trivial.
Okay, tell me where I'm wrong: AFAIK you can buy a few hundred to few thousand dollar receiver that tracks the carrier phase, let it sit around for a few hours, and then with aggressive post-processing you should be able to figure out the right phase and know your location within a few mm.
Edit: And worst-case, you should be able to do an exact survey once, and then you can remember that and know which phase to use in the future. (Barring sudden multi-cm shifts, but in that case you have bigger problems.) You still have atmospheric fluctuations to deal with, but that's why you're measuring all day.
Indeed. Just to anchor this with current state-of-the-art numbers, I took a look at the CSV data file linked to by the page referenced in my above comment:
The quoted horizontal standard errors of the daily position measurements are ~1.5mm. The vertical standard errors are ~5-7mm. (Not as bad as a factor of 10, but certainly worse than 2x.)
The vertical position measurement responds well to averaging of daily errors to beat down the RF propagation effects that cause them.
It has to do with the geometry: GPS is measuring the differences between the distances to satellites which are mostly "overhead". In a sense it's similar to how our stereoscopic vision can resolve movement from side to side more accurately than movement towards or away from us.
If you're at the top of a skyscraper you can mitigate this somewhat by looking at satellites closer to the horizon as well -- this solves the geometry problem -- but then you run into increased noise from the larger amount of atmosphere you're looking through.
Hmm, I still don't see it. The comparison to stereoscopic vision seems misplaced: we judge side-to-side motion by an entirely different (and non-stereoscopic) mechanism than depth, and neither mechanism is mathematically very similar to that of GPS. And my understanding is that GPS receivers compute their distance to various satellites, which means if all the satellites were all nearly overhead, I would expect the vertical component to be the best understood.
You judge depth not from a single image but from the difference between two images. Similarly, GPS works not by measuring signal arrival times, but by measuring the differences between arrival times.
Good point, but I'm afraid the analogy still isn't spelled out enough for me to actually make sense of it. Like why does that make a horizontal/vertical discrepancy?
If you move horizontally, you're getting closer to one satellite and further away from another -- the changes in GPS signals add, just like if something you're looking at moves from side to side. If you move vertically, you're getting closer to or further away from all the satellites at once; but since GPS relies on the differences between signal arrival times, that's much harder to pick up.
Thanks! It's unfortunate that the diagram on wikipedia is actively misleading since it corresponds to known distances rather than known differences of distances.
I wanted to point out that, as you mention, with aggressive post-processing, you can routinely get mm vertical accuracy from GPS measurements. This has been done for several decades now, but it's not an off the shelf result. (Not a "cheap GPS device")
The post processing technology was pioneered by NASA among others. In the western US there is a network of over 1000 such monitors, anchored very deeply, that use these methods. Here is the time series from one in the Bay Area: http://www.unavco.org/instrumentation/networks/status/pbo/ov...
In that plot, you can see lots of interesting effects. First, the virtually continuous lat/lon velocity. Second, the annual vertical trend which usually has to do with groundwater. Additionally, smaller and larger perturbations from all kinds of sources from nearby construction to large scale seismic effects like distant earthquakes.
GPS of this kind is a complementary technology to the fantastic InSAR measurements featured in TFA. Both are widely used in geodesy to measure seismic deformations and deformations due to groundwater extraction. The two approaches offer different temporal, spatial, and accuracy trade offs. For a building like this, if you really wanted to measure subsidence, GPS would yield more temporal information than InSAR.
If I recall correctly, the fundamental limit to the GPS accuracy is uncertainty on tropospheric propagation of the radio signal.
I don't know if this technology is readily available for one-off commercial applications, but it wouldn't be surprising. The required accuracy is clearly within reach, and the software to do the post processing is widely licensed (https://gipsy-oasis.jpl.nasa.gov).
The article points out this can cover huge swaths and millions of buildings at once. It speaks of covering a continent with subsidence alerts. Afixing gps to each building might not be competitive.
> But that's not what they chose to focus the article on.
The first few paragraphs are about the Millennium Tower, but that's just the hook. The rest of the article discusses the wide-area applications, including SF, elsewhere in the Bay Area, and Oslo, Norway.
Somewhat unrelated, but for me this is another argument in favor of renting vs. buying. Sure, most people won't buy a super expensive condo in a super expensive city that happens to be sinking into the ground, but if you taken that $1.5m (they go for much more[0]) and put it into index funds instead, you could pay ~$5k/mo in rent forever and just leave when problems arise.
Someone buying a $1.5m condo only has to put up $300k for the mortgage, so it's not correct to say that the alternative is to just put $1.5m into the stock market: the buyer often doesn't have that much.
Also, it's more precise to use the S&P dividend yield (2%) or 10-year Treasury yield (2.4%) to gauge how much someone with $1.5m could spend in rent, assuming they put all their interest into rent. They'd be renting something at more like $3k/month.
Now take the fact that home prices are, at the higher end, about 25x the yearly rent of the same place. So this person with $1.5m is essentially renting a place worth $900k, whereas the buyer (who perhaps only has $500k) is living in a nicer place that cost $1.5m. And that's not considering the fact that rents could rise further.
There are taxes and maintenance to consider, the risk of putting a lot of money into a volatile asset, and being tied to one place. Financial leverage in general is risky. But it's not a clear-cut decision like most people try to make it.
That Treasury is vastly safer than a condo, which fluctuate significantly more than a standalone structure and often has dramatic increase in fees over time. Further renters can move to a new building every few years for minimal costs.
4% return above inflation and taxes is a bit optimistic long-term, but otherwise this is correct. However where you stand depends on where you sit. Many other factor have to be "priced in" and the prices are dependent on what's important to you.
Eg, factor in equity (after 15-30 years of mortgage you own a condo), valuation (in 15-30 years that condo will be worth more than 1.5M), risk (...unless that condo sinks into the mud!), finance (you can borrow 1.5M to buy a condo but you can't borrow 1.5M to invest and rent), and flexibility (you can move out of a rental more easily then selling a condo).
Many financial planners assume 4% or even 5% annual returns above inflation. Heck, I forget which state it was, but I remember reading about some boneheaded assumption of 8% annually written into the laws for its pension fund.
Can you clarify on why you think 4% is optimistic, but then say 4-5% appears to be a reasonable 30 year expectation?
I guess with these things I always think of past performance not being a guarantee of future performance, but on the flip side I'd like to be able to live off my investments when I retire, so making some sort of informed assumptions is probably better than not.
The expected return of a year is not independent of the year before. Rudimentary models that many financial advisors use don't account for that. Even the more sophisticated folks don't account for it enough as I think is appropriate. They think they've observed dozens of years and the evidence says it's nearly impossible for the market to go down (as a whole) over a ten-year period. I think they've observed only a handful of decades and therefore should have very low certainty.
I'm still heavily invested in the stock market. I just think you should be very cautious in your assumption of how much money you need to live comfortably on interest/dividends in perpetuity.
Historically, 4 or 5% appears to be a reasonable expectation over a 30-year timeframe after 1940 if you consider each year as an independent random variable and expect that the future will look mostly like the past.
Moreover, you could feasibly buy insurance for such things which would be the financially appropriate way to think about it in terms of risk.
Tack on the cost of insurance to the cost of ownership to make the calculation.
Owning in SF has been smarter than renting for the last 20 years in almost all cases.
All the surpluses in the Valley have gone into real-estate.
In fact - given the failure rate of startups ... if you had $10 M in 1990, the best investment you could possibly make would be simply to buy real-estate, assuming you don't have access to the top tier VC funds as an LP.
By buying real estate - you're basically 'investing in tech' indirectly, across the board. And there's considerably less risk.
> you could pay ~$5k/mo in rent forever and just leave when problems arise.
We saw our apartment kept raising our rent, and yet the housing market in the area was still recovering from the crash so we decided to buy.
> but if you taken that $1.5m (they go for much more[0]) and put it into index funds instead,
Granted most people don't have $1.5 probably. Going to the bank and asking for $1.5 to invest in the stock market might not work as well as going and asking for a home loan.
Also there interest paid on the home mortgage is tax deductible. That's another incentive.
Given that simply getting a $1.5 loan from a bank to do whatever investment doesn't work, most people think in terms of "what do I pay per month and what do I get for it". In some markets what people pay per month on a mortgage and a large house is less than what they'd pay in rent in a smaller apartment . And also rent could go up at any time. A fixed rate mortgage will stay the same.
People who buy $1.5m condos in cash usually don't care that much about rate of return. Or are you implying borrowing $1.5m at mortgage rates and investing that? Not happening.
More along these lines, these condos are veblen goods not "the only place you can afford" type residences. If you have $1.5M in cash laying about to invest, you're probably not going to be happy in place that rents for $5K a month.
it boggles my mind that the developers were not required to sink the foundation piles all the way into bedrock. how unusual is this for tall buildings in the city by the bay? there is so much wealth in SF, this is a luxury tower, it's all a bit strange.
There are bedrock piles, and there are friction piles. The latter rely on friction of the material they're plunged into to stay put. They are effective and commonly used.
Despite how comforting the word bedrock is to our ears, neither one is "superior" to the other.
Obviously something is going wrong here, but it's not a case of "oh the developers cheaped out and decided to just build a giant skyscraper with no foundation!"
SF Chronicle had a good overview[0] of what could be going wrong. From the article:
The tower was built using a concrete frame instead of steel. Which is much heavier - it puts about 4.75x the amount of pressure on the soil below the building (as compared to other steel framed buildings of similar size).
It appears that some of the sinking could be due to weight and changing soil composition (less moisture and nearby excavation).
I'm curious: How many other buildings over 20 stories in the SOMA neighborhood use friction piles? Why would anyone use bedrock piles if they are more expensive and have no "superior" advantages? How many buildings with bedrock piles in SF have tilted this much as they have settled in?
You could say whatever you like, but none of the other tall buildings in SF are sinking noticeably, which suggests your theory may itself lack a solid foundation.
Depends. Some are but but in recent years it's been more common to use steel structures which (being lighter) don't require that.
Noise might have something to do with too. The last building that I recall having bedrock-depth pilings involved loud shuddering bangs through the financial district for nearly a month. Like you could feel it through the ground 3 blocks away every time they dropped the hammer..
that's why you go down to the bedrock, because the earth in between is not strong/stable enough. also, while much of eastern SOMA is indeed landfill of the bay, this particular building is far enough inland and would be within the original SF coastline.
I worked in business litigation for a few years and I obviously can't go into any specifics but you'd be shocked/surprised at the number of buildings that the plans say X but somehow Y happened....
Owned a condo. Code and plans said to install a drain clean out in the main sewer line. Cost to builder? Maybe $20. Done wrong and closed up, no one knew so he got away.
Few years later, maybe 10k in damages and 5k to fix. All so the builder could save $20.
Things like this x 100 are everywhere. This is why building codes are nice, although people complain about them.
I had a friend whose bathtub drain was attached to nothing at all. It just spilled out into the space below the apartment. When they opened out up, they're was a giant subterranean pond hollowed out by years of water pouring out onto the ground.
Yah there is that. In the US, it is usually mandated that you get something called "title insurance". There are different levels depending who it protects (lender vs buyer). If down the line it turns out you don't own the land your house is on, the title insurance company does lawsuit magic or somehow pays to make the lender whole.
End up costing ?? $1,000 or so per real estate transaction. 99.9999% of the time pays back zero. I suppose nice to have for the last .0001%.
We had to use ours. What happened is that the title people didn't realise the guy selling us the house had two properties and they paid off the wrong house.
Then the guy sold his (newly paid off) house and disappeared.
The first time we leaned anything was amiss was when we had an eviction notice posted on our door. The title company didn't bother telling us until it got that far.
It caused a whole bunch of problems and the company kept dragging their feet hoping someone else would pay. It took a lawyer and threat of a suit to get them to finally pay out.
The value add is saying that codes are important, but enforcement is just as important. Perhaps moreso because if you skip the latter, the former doesn't mean anything at all.
If the builder skimped on the $20 cleanout, it should have been noticed by the building inspector.
And the building code means the minimum code required not the maximum. A builder can make it better if they want to when they are building a structure.
I don't know. It sounds bad, but if you aren't a professional engineer lots of stuff sounds bad. "Hang a roadway from cables anchored at the ends? Madness!".
Unless you're a civil engineer, your comment is rather low in information value.
I find it amazing how civil satellite can be kept in orbit, be it geostationary or not, and monitor "millions of points" spread to some great km and be able to detect (or provide image with enough resolution) so that a move of few cm can be detected.
Block III and newer K-11 platforms are 20 meters in length and weight 20,000 KG.
Also the dry mass of Himawari is 1300 kg, it's based on the MELCO DS-2000 bus which is meant for communication satellites, it doesn't have an 8M telescope assembly.
I would be infuriated if I purchased a condo there. It seems like every month since last year, there are articles talking about the project. I haven't heard any solutions to the problem and I don't know anything about construction. Is this the type of problem that can be fixed, or will they have to destroy the building and rebuild it?
The main issue here is understanding what is the actual cause of the sinking:
1) extensive dewatering of the soil under the building and around it
2) some mistake in the calculations of the project and/or some foundation work made in difformity from the project
3) a combination of the two
IF it is only #1 (this is what the builders say) then stopping the water drainage or even (in certain case) re-watering appropriately the soil would stop the sinking (at a relatively low cost).
IF it is #2 or #3 it is certainly possible to "better" the soil underneath or "add" some (underground) supporting structures (at a much, much higher cost).
And of course someone will have to pay the bill.
Techically, IF the cause is #1, it is called "subsidence":
Too soon to tell. They need undisputable facts about the building shifting positions, and exactly what the risk is. High risk means the governing authority can condemn it, get people out, and sort out liability later.
No doubt determining liability will involve a lot of insurance negotiations along with various court cases. Maybe there's a code violation, but plans showing an inevitable violation were approved? Who is liable for that? Etc. There are many possible sources, even distributed sources, for a mistake like this. But central is determining the scope of the damage.
It's not discussed in the article but I'm very curious about the brightest spot in the SF Bay Area in the survey image. It's not Millenium Tower (which I work next to so that's pretty interesting too). It's actually the land just west of Oakland airport, the part of Alameda built up in the 80s called Bay Farm. As I understand it the area was marsh up until then -- that and a garbage dump that has since been covered.
Does anyone know if the land there really is sinking so fast? It's not on a fault so I'm fairly sure it's not moving much laterally.
This is from memory, and I have not been able to confirm it with a cursory search, but FWIW: In most of central London, the bedrock is far below (which allows for deep Tube tunnels), so high-rises do not rest on bedrock, and are at least partially supported by the buoyancy of large rigid basements.
Of course, London does not have the seismic risk of San Francisco. I would guess that buoyancy might be an asset with regard to support, but possibly a liability with regard to stability, in the face of liquefaction.
