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In the mountains they build things strangely. Their houses are made of interconnected hollow spheres. Although I really like the aesthetic, I can't figure out why they do so. In theory, a hollow sphere is harder to build. So, what is it about the mountains that makes the people who live there want to build that way?

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    $\begingroup$ Why do Eskimos build hemispherical igloos? (If you are building the houses out of ice, stone, concrete etc. and you have no steel or wood to make strong beams to hold up your ceiling and roof, a hemispherical vault is much easier to build than a flat roof.) $\endgroup$
    – AlexP
    Nov 23 '20 at 12:45
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    $\begingroup$ do you mean hemispherical, spherical buildings will not stay on a mountain. $\endgroup$
    – John
    Nov 23 '20 at 20:52
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    $\begingroup$ @John I'm open to suggestions. The way I have drawn them is spheres with the bottom cut off so yeah, I guess they aren't strictly spherical. However, I can also envision them being spherical inside with foundations that aren't spherical. I'm open at the moment, but the key thing is roundish all over except possibly the floor. $\endgroup$ Nov 23 '20 at 20:53
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    $\begingroup$ Is their widest point at ground level, or do they come in some? i.e. did you just cut a bit off the bottom of a sphere, or did you cut off the whole bottom half, or even more? (The "even more" would make a low dome, like you might expect to find on a very large scale over a mars colony. Half way would make a hemisphere.) $\endgroup$ Nov 25 '20 at 6:33
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    $\begingroup$ @PeterCordes: I envision them cut off below the equator, so more than a hemisphere $\endgroup$ Nov 25 '20 at 10:54

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A sphere is, among all the solids, the shape that minimizes the ratio between surface and volume. Since heat is dissipated through the surface, it makes the house more energy efficient, which in mountain region is surely a benefit.

Which is why also animals living in cold regions tend to be more rounded, to waste as little body heat as possible.

Just to give you an example, let's compare a sphere and a cube with the same volume, $V$: since for a sphere $V=4/3\pi r^3$ and therefore $r=($$3\over 4$$1\over \pi$$V)^{1/3}$

the sphere will have a surface of $S_{sphere}= 4\pi r^2=4\pi($$3\over 4$$1\over \pi$$V)^{2/3}$$\approx 4.8 V^{2/3}$

while the cube will have a surface $S_{cube}=6V^{2/3}$.

As you see the sphere has less surface than a cube with the same volume, thus all the rest being the same would lose heat at slower rate than the cube.

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    $\begingroup$ That said spheres can’t share any faces with the floor, which tends to be a constant temperature and can’t draw heat from the house via convection. Couple this answer with a compelling reason to stay off the floor (mountain dogs? Angry bears?) and it makes perfect sense though. $\endgroup$
    – Joe Bloggs
    Nov 23 '20 at 13:20
  • $\begingroup$ @JoeBloggs, the ground on a mountain can be pretty cold. Constant temperature but very low. $\endgroup$
    – L.Dutch
    Nov 23 '20 at 15:00
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    $\begingroup$ Even if you're in an area with permafrost the ground is going to be warmer than the air (or cooler, if you're somewhere hot), and it's much easier to warm up and keep warm. Still, that's only an argument for hemisphere vs suspended sphere, and might not even be that if there are predators and/or a lack of flat ground. $\endgroup$
    – Joe Bloggs
    Nov 23 '20 at 15:17
  • $\begingroup$ To be clear: I like your answer! $\endgroup$
    – Joe Bloggs
    Nov 23 '20 at 15:18
  • $\begingroup$ I've accepted this answer because I can only choose one, and this one has the best base from which I can incorporate the others. Thanks everyone else for their ideas, which can be worked in :-) $\endgroup$ Nov 23 '20 at 15:35
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Access to a building material which makes spherical constructions easier than angled ones.

Like a tree which tends to grow curved trunks. Or very large animals with curved bones (although large land animals are uncommon in mountain regions - they are usually much better adapted to plains).

Or shrubs with long and thin but flexible branches. They lack the compressive strength for load-bearing pillars or walls, but can be woven into arcs. When you have nothing but arcs to build with, then spheres are really the only thing you can build (tubes would also be an option, but the openings might be weak-points).

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  • $\begingroup$ Interesting. I was thinking along these lines (among the others in the answers) $\endgroup$ Nov 23 '20 at 15:33
  • $\begingroup$ Another possibility might be the stomach of a large animal which hardens after death to a level that can support a significant amount of weight. In that case you would: a) kill your animal and extract the stomach, b) inflate it and tie off the ends, c) wait for it to harden, and finally d) cover it with mud or whatever for extra strength and insulation and cut entrance holes. $\endgroup$ Nov 25 '20 at 1:48
  • $\begingroup$ Looking at round huts in Africa, it was clear that the reason they are round is because they make the roof by attaching poles together to make a cone. This is so much easier if the poles are all the same length and thus the underlying walls will turn out round to match. $\endgroup$
    – RedSonja
    Nov 25 '20 at 11:46
  • $\begingroup$ Snow saw into truncated-pyramid blocks by the Inuit is also an option (you end up with domes) $\endgroup$
    – ikrase
    Nov 27 '20 at 3:13
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Round, almost hemispherical houses have been popular for thousands of years in dangerous climates like Skara Brae in the Orkney Islands.

enter image description here

No reason why the same arguments for comfort (minimum surface area for warmth) and low wind resistance wouldn't apply in mountainous regions just as much as in windswept sub-arctic islands.

It's a reasonable question for architects from southern climates to ask though - they probably aren't used to low drag coefficients as a design goal!

Wikipedia has a little more on Skara Brae including this plan of the main houses

enter image description here showing they were further packed together, and a picture of some of the 5000 year old fitted furniture...

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Balloons. Balloons are round. Whatever they use to make their houses, they have to blow them up like a balloon. Maybe stiched animal hides? Something like paper mache built up over a bladder structure? Maybe a mixture of bird feathers, straw, fluff, and tar, like birds make their nests out of, but with a roof - like a wrens' nest.

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Although the knowledge is lost in the mists of time, the mountain is really a volcano. After being nearly wiped out, ancestors built spherical houses so that they could quickly escape an eruption by simply rolling down the slopes. The volcano hasn't erupted for a thousand years and the inhabitants have forgotten why they build in this way but it is their heritage so they continue to do it.

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Snow. Sometimes they get extreme snow fall, a near-sphere is the best shape to resist this load.

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Circular cultures

There have been plenty of "circular" cultures in history. It seems not too many steps from there to spherical houses, which I'm pretty sure have existed as well. The circular or spherical shape can help deflect water, snow and mud streams that they might get in contact with, so the house can take much more pressure from nature. Although they aren't as efficient in all situations compared to circular houses, it is certain that square houses simply aren't as efficient as well. So the cultural choice for spherical interconnected houses is a good choice. Spherical houses also minimise the amount of snow or other debris that can collect on a roof, making it less susceptible to collapse.

In addition, @L.Dutch-ReinstateMonica♦ makes a good point of less heat lost through the surface, as a sphere is the smallest surface to volume ratio.

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  • $\begingroup$ Interesting. It would be an interesting point of philosophy - did the houses become circular because the culture reveres the circle, or did the culture revere the circle because the houses (and other things) made more sense to be built that way, and then it just passed into mythology from engineering $\endgroup$ Nov 23 '20 at 15:35
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Hazardous gas and liquid mitigation - and structural integrity.

These underground settlements are full of nasty surprises, pockets of hydrogen gas, pockets of methane, pockets of propane from unattended stoves (you didn't say what year this was set. Bottled LPG is frequently used as a heat and cooking source), pockets of water, pockets of carbon dioxide, all sorts of nasty things which both rise and sink in the air.

By having a dome ceiling and a hemispherical under-floor cavity in every room, it is much less likely for a nasty gas or liquid to spread throughout the entire complex, rather it is contained to one area. It also tends to be contained above and below the living area, allowing people to survive random gassings or floods that occur from time to time.

They initially tried to dig it out with square corners but soon realised a sphere is stronger - each room is basically an arch rotated around a circle. They make each room a sphere so that there are zero (or at least minimal) weak points.


The city design I referenced in a recent question about a hexagon layout city was actually originally designed to be underground spheres arranged hexagonally - Suburban living at the centre level, nice dome over the top with a projected horizon, all retail/school/medical/post/etc at the centre, and under-surface subway line connecting each to adjacent suburbs. Also under the surface level was automated life support systems, automated manufacturing, and stockpiles of everything needed. In the fiction I was working on it was how a large western country survived run away climate change in ~100 years by moving underground.

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    $\begingroup$ It's not clear to me how the spherical shape really helps with the first point. A domed ceiling doesn't isolate rooms any better than a square room, so long as the doors don't reach all the way to the ceiling. Similarly, square basement rooms that are not directly connected cannot flood one another. I don't see any compelling reason for the vaulted ceiling or under-floor cavity to be hemispherical - in fact, for the same footprint/depth/height, a square chamber can contain more fluid. This would be less effective than a regular basement. $\endgroup$ Nov 23 '20 at 20:44
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Because this is a desert and you need cooling

Higher ceilings in a building give a very useful passive cooling effect. A couple of years ago, I went on holiday to Gambia, staying at an eco-lodge. The lodge owner had built round mud-brick houses with high round roofs. In spite of the high daytime temperatures outside, the temperature inside was always pleasantly cool.

These are similar buildings constructed in the Sahel.

This wasn't the normal local building style, but it worked very well. It seems quite possible that your desert dwellers could have evolved this to deal with their environment. Like the design of Roman villas which served a similar purpose, all it needs is for one person to really nail something to fit a need, and everyone else will quickly adopt it.

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Not a Sphere but close

enter image description here

This is one building from the TV series "Raised by Wolves"

The design can be made from uncut stacked stone and doesn't require timber beams of which could be hard to get in a mountainous region while stone is plentiful. It doesn't require timber and doesn't require mortar, just plenty of rock.

Here's the ones from Star Wars enter image description here These are actual houses on a remote island located off the coast of Ireland called Skellig Michael.

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Orthogonic (right angles) architecture has not been the norm for most of human existence. It became predominant with the invasions and spread of imperial building technologies, which then violently eradicated the indigenous vernacular architectures. The reason that round and curved types of organic dwellings seem to be common in highland mountain areas is that these places have frequently been ignored or devalued by colonizers. These building styles were once the norm.

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I submit for your consideration, ancient mapmaking and land navigation.

If you have a visually distinctive landmark of a known fixed width, you can cut notches into a stick and hold it at the correct length, and get a surprisingly good idea of your range to a target based on how wide it currently is in your field of view. If you can get ranges along two bearings that are sufficiently separate, you can now accurately place yourself on a map in relation to those two landmarks. Geography permitting, one could arrange to have three or more groupings of these houses in separate locations around the mountain range - now this method of range finding can be used to reliably locate points of interest that they haven't had time or resources to beat a path to, or over terrain that cannot easily be marked or changed.

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Because they build with giant bladders and concrete. You need a source of pressurized air and a bunch of water. Ideally, you also have some sort of shellac.

The mountains probably have a large chasm that has a strong, regular wind. It probably also has natural spring very high up. For the cement, it benefits from having lime and volcanic ash, but there are plenty of other cement recipes if that doesn’t work for you. It would also be convenient to have shellac (made from rendering bugs), but that’s optional.

What you do is sew up a giant spherical bladder out of whatever materials work for you. Cloth, hides, whatever. You also set up a giant wind-catching tube that goes from your wind-chasm to your giant bladder, which can likely be reused for multiple houses. Essentially, you’re making a giant bouncy-castle. If you have shellac, you shellac the bladder, let it dry, then remove the wind tube. If no shellac, or need the wind throughout the construction process. Either way, you now have a giant, hollow form. Start pouring very liquid cement over it. You’re imitating shotcrete here, just without the pressurized application. Continue until you have a good, structural layer, let it dry, then repeat until you have thick enough walls.

Cut yourself a door and make yourself at home. If you need another room, cut a hole and repeat the process with another bladder attached to the hole.

—— As an aside, this is an entirely realistic technique. I looked into buying a house that had been built very similarly to this a few years ago (price was too high, but the house was fine).

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Transportation

A spherical house is easier to move than a square one. Assuming smallish houses made of light materials, you could just roll them to some other place. One reason would be winter migration. There could also be reasons to move house constantly: high winds, rock/mudslides or other dangerous natural phenomena.

It could also simply be a building materials issue: instead of schlepping over tons of whatever is used to build the things, just build it someplace where the materials are abundant and then roll the house to it's final destination. Or commerce: you have one tribe/village/people who are excellent at building these things and have the materials at hand. They make their living by building houses and rolling them right to their customers.

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