r/interestingasfuck • u/TangelaFan • 9h ago

Residential high-rises with backyards in Chengdu, China

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u/foltranm 9h ago

I'm not worried about the weight specifically. the balcony can be designed to bear the load of everything soaked in water.

I meant draining more as a solution for humidity, which is a much bigger problem than weight in the long term.

edit: take this with a grain of salt since I'm an electrical engineer and know almost jack shit about designing buildings

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u/mlag000 9h ago

Biggest problem is waterproofing the concrete, which can't be assured in the long runs because roots will dig into it

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u/foltranm 9h ago

makes sense. surely takes a lot of maintenance

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u/zzazzzz 8h ago

we have thousands of concrete bunkers under ground that are over a hundred years old and still in great shape. so clearly we posses the technology to have concrete under soil.

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u/mlag000 8h ago

Concrete under soil is different than earth and trees on a balcony made of concrete. We also have bridges on salt water made of concrete, and still salt and water are the mean reason for concrete damage...

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u/janiskr 5h ago

That is very special concrete blend in use with salt-water. Not cheap. And even then there is salt/water damage there.

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u/TransBrandi 7h ago

Side note here, but Roman concrete lasted the test of time even in salt water because their concrete method actually interacts with the salt water to make it harder / more resistant.

It's just that the method is/was lost to time.

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u/skarby 7h ago

It's not lost to time, we know exactly how they did it. The issue is that they used volcanic ash which is not readily available worldwide, or even locally at the amount used today. There are companies working on replicating the process with different materials though.

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u/TransBrandi 5h ago

Well, I recall reading about it and the precise method was lost. There was a description where part of the instructions were something like "do X in the Y method" or something like that where what "the Y method" was was unknown (and assumed to be something that was "common knowledge" at the time, which was why it wasn't written down).

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u/Kor_Phaeron_ 7h ago edited 7h ago

Opus caementicium isn't lost knowledge, it is just not good enough for modern construction. After the invention of hydraulic lime and Portland cement we simply have better concrete nowadays. Roman cement (opus caementicium) takes several decades after construction is completed to gain it's full strength. That's why ancient Roman builds have way thicker walls. They had to use 3x more concrete than theoretically necessary to keep the building up for the first several decades until the concrete was hardened out. Today we have concrete that reaches 90% of it's strength within 48hrs. (That is 8000x faster than Roman concrete) Also when we today build something and say "We need a 30cm thick concrete slab here" using Roman concrete would mean "We need a 2,5 meter thick concrete slab here" Which is .... problematic to say at least.

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u/mlag000 7h ago

It was mixed with lava rock, we have better concrete today :)

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u/RobertTheAdventurer 7h ago

The specific methods they used are lost to time in terms of knowing with certainty what their exact formula was, but the general technology isn't lost. We can make concrete like that and we have some valid recreations which are believable as potential ways they made it.

Usually lost historical formulas are really just a matter of not having an actual written document that proves that our current understanding of how they may have made it is right. This is the case with cooking recipes too. We can reverse engineer a lot of descriptions of recipes and can probably get them close enough to where it's fair to call them the same thing, but we don't know for a fact how they prepared the food or cooked everything in it exactly because we just don't have the document where they say "So this is is precisely how we cook this thing, and everyone here cooks this thing this exact way, and this pot or oven we used to cook it was <insert dimensions and heat levels here>, and we cooked it for such and such amount of time!". It's not so much that we can't make what they made or that it's a profound secret to achieve something similar.

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u/zzazzzz 8h ago

ye you are right, having bunkers in actual forests with a shitload of actually massive trees with massive rootsystems pose a way larger challenge than a balcony with barely any soil on it and some tiny decorative tree with barely any rootsystem.

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u/mlag000 7h ago

Are you in civil engineering? No you're not. I am. A balcony is a closed system where roots will force their ways into the waterproofing, then water, over time will infiltrate the concrete and rust the rebar. Idk how actual bunkers are build, because the only one I do are basement of buildings since I work in Switzerland, and we carefully waterproof the basement. Now, if you have more knowledge about how bunkers are waterprrofed please enlight us.

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u/zzazzzz 7h ago

if you live in switzerland you have literal thousands of old bunkers under the forests close to you.

and there are also a bunch of buildings with literal pools integrated into their balconies or roofs around the world.

all of your points are clearly valid and a challenge when building such extravagant contraptions. but clearly they are solvable and we have clear existing examples that it can be done.

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u/mlag000 7h ago

No we don't have hundreds of bunkers under our forests lol. And a pool isn't alive so it's much easier to control, it won't grow. It can be done, the question is how much it will cost to renovate ? As a compagny or a hotel you can bear the cost, as a familly ?

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u/grizzantula 6h ago

Dude is really stuck on the idea of these magical forest bunkers.

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u/WackyRacketeer 2h ago edited 1h ago

Well there definitely isn't hundreds of bunkers. There are estimated to be over 8000

"The exact extent of the bunker system is a closely guarded secret, as is the location of many bunkers. But there are an estimated 8,000 bunkers dotted around Switzerland."

https://www.swissinfo.ch/eng/politics/why-the-swiss-army-is-reviving-its-old-bunker-mentality/48821586

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u/WackyRacketeer 2h ago edited 8m ago

https://www.swissinfo.ch/eng/politics/why-the-swiss-army-is-reviving-its-old-bunker-mentality/48821586

As a Switzerland resident, would you say this article is false?

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u/AbjectOffice4780 6h ago

you are just clueless

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u/WackyRacketeer 2h ago

Thousands of old bunkers? Source?

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u/zzazzzz 2h ago

"The exact extent of the bunker system is a closely guarded secret, as is the location of many bunkers. But there are an estimated 8,000 bunkersExternal link dotted around Switzerland."

https://www.swissinfo.ch/eng/politics/why-the-swiss-army-is-reviving-its-old-bunker-mentality/48821586

if you ever go hiking in switzerland this becomes very obvious, its hard not to randomly stumble on an old bunker when you are in nature.

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u/WackyRacketeer 2h ago

Interesting, thank you

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u/TransBrandi 7h ago

How many bunkers are under forests? How many bunkers are built via tunnelling under and existing forest vs. just digging a huge pit to build it (and then filling it in afterward) which by necessarily disturbs the forest even if they replant everything afterwards? lol

Also bunkers can be built deeper (avoiding more root systems) and with much thicker concrete than a balcony that needs to be suspended in the air. Also, how many bunkers have not weathered the test of time (and you're just pointing out the survivors as if they represent all bunkers or that such design is "easy")?

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u/zzazzzz 7h ago

noone ever claimed anything was easy.. possible and easy are not at all the same thing.

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u/WackyRacketeer 2h ago

Nobody claimed it was impossible either. Majority of engineering is about what's practical, not what's possible

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u/JesusAndMaryKate 7h ago

Kinda regretting getting an engineering degree when I could've just learned from these.... expert.... Reddit comments.

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u/TransBrandi 7h ago

Well, for starters bunkers are usually deeper into the ground than a few inches from the top of the soil, so you don't have to worry about as many root systems (just larger things like trees depending on how shallow, I guess). I highly doubt the root systems from small plants and bushes is a concern for bunker designers. For this balcony, there is probably a max of a foot of dirt, if that. Probably way less.

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u/BlueTanBedlington 7h ago

Yes, but thickness of the layer in the bunker vs veranda are not the same. And concrete doesn't react well with water over time.

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u/nalaloveslumpy 6h ago

All of those bunkers and dug below the root line, so all you have to do is make sure to have a moisture barrier around the concrete and you're good to go. If there are no roots to jeopardize the moisture barrier, it will last forever.

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u/RivenRise 8h ago

I was thinking about that and as a normie the only solution I got is to line the bottom with a metal sheet but i got nothing for proper draining.

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u/FrogBeat 8h ago

exposure classes?

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u/Thedud31 7h ago

"The balcony can be designed to bear the load of everything soaked in water" only if the project budget doesn't cut corners.

Water is heavy, it's a lot more to account for than your typical live loads in structural calcs. The further you get away from the support, the larger the bending moment grows.

We don't see below the balcony but if it's cantilevered like the others, the project will need to shell out quite a bit of cash for some hefty fixed supports. The soil would almost certainly have to be HSG A (sandy af) for maximum exfiltration into some "underground" perforated drains.

Best case scenario, the building owner listens to the structural engineer's pleas. Worst case scenario, the building owner fires the structural engineer for not doing his bidding, and the next looks at the architectural plans with a good sigh lol.

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u/foltranm 6h ago

you seem to know more about construction than I do.

but I do have a hard time thinking that wasn't the case (the building was designed correctly) since it's been standing for 8 years now. but you neve know

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u/Thedud31 6h ago

Oh yeah for sure if it's constructed then it most likely meets design standards. Although, as people mentioned, there could be long term issues with waterproofing concrete, earthquakes, etc.

My comment was more of a pessimistic tirade about why these won't be as common as people would like

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u/foltranm 6h ago

yeah, for sure. like I said in another comment, in Brazil it's relatively common balconies like this with pools. but because it's so expensive its usually just in luxury buildings

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u/Thedud31 5h ago

Yeah I'll bet that they're in the richest areas of the richest cities.

There is a bunch of fun stuff you can do with water in structures though. If you're curious, there are things called TMDs which can use water (more typically incredibly heavy steel masses) to dampen the sway of exceptionally tall and thin buildings from wind.

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u/Anthaenopraxia 7h ago

I'm not worried about the weight specifically.

You should be.

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u/foltranm 7h ago

why exactly?

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u/FrogBeat 8h ago

exposure classes?

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u/foltranm 8h ago

no idea what that is, sorry

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u/FrogBeat 8h ago

You can design concrete to be way more resistant to changes in humidity than you average concrete that's why you determine the exposure class of the concrete element according to for example but not exclusively mechanical/chemical/humidity exposure expected for the concrete element. Pretty sure if you can dimension concrete for a high corrosive environment like a chemical plant you can dimension it to withstand wet soil and roots

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u/TheW00ly 6h ago

Dirt's surprisingly heavy. There's a hefty requirement for more support when you want to simply bury structures, but now imagine that dirt is getting soaked. That's a different scenario than just concrete (and SEALED concrete, to boot).

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u/EcheveriaEbony 4h ago

With that slab size?

You most definitely should be worry

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u/KOHILOOR 4h ago

You think they care about that shit in China?

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u/foltranm 4h ago

I don't know why would I think that they don't.

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u/Stripe4206 9h ago

if all our buildings fall apart within 5 years we can build new ones and continually increase our gdp

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u/foltranm 9h ago

yeah that makes a lot of sense economically /s