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Comment by physcab | Hacker News Reader

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.

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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.

> designed to live 60-100+ years

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.

>but in a place like SF where some of the land is based off old landfill, refusal is acceptable

For a steel-frame building, yes. For a concrete high rise you're playing with fire. I'm surprised the building code allowed this.

That's cool. I wouldn't buy a condo there though. Would you?

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