**Ask a monkey a physics question thread**

[kingnat]

I'd love to see a back-of-the-envelope-type calculation for this: http://www.viralnova.com/backyard-igloo/ that gives me reasonable confidence that something this size, with blocks this small has a very low chance of failure.

[wufwugy]

I'd love to see a back-of-the-envelope-type calculation for this: http://www.viralnova.com/backyard-igloo/ that gives me reasonable confidence that something this size, with blocks this small has a very low chance of failure.

If they were to fall, where would they fall?

IIRC the earliest examples of stable stonework above empty space is the arch. It works. Because it has no where to fall to

[MadMojoMonkey]

I'd love to see a back-of-the-envelope-type calculation for this: http://www.viralnova.com/backyard-igloo/ that gives me reasonable confidence that something this size, with blocks this small has a very low chance of failure.

A dome is an incredibly stable structure, allowing for a lot of tolerance in deviation from the ideal shape.

Consider a hemi-spherical soap bubble on the surface of the dish water. A bubble is perfectly stable under the load of it's thin shell up to a certain size. Comparing the diameter of a large soap bubble to thickness of the bubble's shell is a pretty large number.

Since there's no pressure loading on the igloo shell, it all comes down to supporting its own weight, which it would do, even if it was much thinner than the 4 inches it looks to be.

So it comes down to loading. It's a practical aerodynamic shape, adjacent to the ground. My guess is that it would stand up to extreme winds in excess of 100 mph. Perhaps twice as high.

Additionally... I think a lake only needs to be frozen 6" thick to play hockey on it. This link seems to confirm that notion. According to this, at 4 inches of ice, a frozen lake is thick enough to ice fish.

If 4 inches with a transverse load of a human is safe enough to encourage the public to walk out on to it, then a 4 inch thick igloo that is adequately mortared should be able to support a person standing and maybe even jumping on it... maybe not even a thin person.

The snow-crete is a very nice mortar for this structure. If it's done right, it has just enough moisture to make a material bond with the ice blocks it's holding in place. Even if it's a little weak, you can add water and more snow in the joints and shore it up. It's a very strong structure because you're effectively welding the ice blocks together.

***
I can break out some calculations for thin shell structures, but they're in the context of pressure vessels, and so the loading isn't the same. Nonetheless, thin hemispherical shells are amazingly efficient structures when pressure loaded. An inverted, weighted caternary (a la the St Louis Gateway Arch) is designed to have equal stress in all sections. If you built the igloo in this manner, you could make one the size of a large house, I'd wager. It's not like ice is weak in compression.

[kingnat]

A dome is an incredibly stable structure, allowing for a lot of tolerance in deviation from the ideal shape.

Consider a hemi-spherical soap bubble on the surface of the dish water. A bubble is perfectly stable under the load of it's thin shell up to a certain size. Comparing the diameter of a large soap bubble to thickness of the bubble's shell is a pretty large number.

Since there's no pressure loading on the igloo shell, it all comes down to supporting its own weight, which it would do, even if it was much thinner than the 4 inches it looks to be.

So it comes down to loading. It's a practical aerodynamic shape, adjacent to the ground. My guess is that it would stand up to extreme winds in excess of 100 mph. Perhaps twice as high.

Additionally... I think a lake only needs to be frozen 6" thick to play hockey on it. This link seems to confirm that notion. According to this, at 4 inches of ice, a frozen lake is thick enough to ice fish.

If 4 inches with a transverse load of a human is safe enough to encourage the public to walk out on to it, then a 4 inch thick igloo that is adequately mortared should be able to support a person standing and maybe even jumping on it... maybe not even a thin person.

The snow-crete is a very nice mortar for this structure. If it's done right, it has just enough moisture to make a material bond with the ice blocks it's holding in place. Even if it's a little weak, you can add water and more snow in the joints and shore it up. It's a very strong structure because you're effectively welding the ice blocks together.

***
I can break out some calculations for thin shell structures, but they're in the context of pressure vessels, and so the loading isn't the same. Nonetheless, thin hemispherical shells are amazingly efficient structures when pressure loaded. An inverted, weighted caternary (a la the St Louis Gateway Arch) is designed to have equal stress in all sections. If you built the igloo in this manner, you could make one the size of a large house, I'd wager. It's not like ice is weak in compression.

BTW...

It's 12 ft in diameter, i.e., fucking huge compared to the original.

[MadMojoMonkey]

BTW...

It's 12 ft in diameter, i.e., fucking huge compared to the original.

I feel compelled to point out that an "igloo" is a cartoon invention, as far as I can tell. I would LOVE to see a link to any indigenous people using an igloo as a normal part of their daily lives. I found lots of variations on the practice of making a temporary shelter out of ice/snow, but none of them are what I'd consider an igloo.

Also, having made and used a snow shelter in Boy Scouts, I can tell you that if you can't keep the wind out, you've wasted your time. If you're going to sleep in it, you'll want a (at least mostly) sealed door, and a small roof vent so you can put a camping stove or small campfire inside. This will cause the walls to slowly melt, but so slowly that it's not relevant. However, the water runoff will ruin your comfort if you don't prepare for it. 1) The door to the igloo goes downhill. 2) carve a fist-sized groove in the floor around the wall on the inside and out the door. 3) Never put anything in the groove.

[kingnat]

I feel compelled to point out that an "igloo" is a cartoon invention, as far as I can tell. I would LOVE to see a link to any indigenous people using an igloo as a normal part of their daily lives. I found lots of variations on the practice of making a temporary shelter out of ice/snow, but none of them are what I'd consider an igloo.

Also, having made and used a snow shelter in Boy Scouts, I can tell you that if you can't keep the wind out, you've wasted your time. If you're going to sleep in it, you'll want a (at least mostly) sealed door, and a small roof vent so you can put a camping stove or small campfire inside. This will cause the walls to slowly melt, but so slowly that it's not relevant. However, the water runoff will ruin your comfort if you don't prepare for it. 1) The door to the igloo goes downhill. 2) carve a fist-sized groove in the floor around the wall on the inside and out the door. 3) Never put anything in the groove.

I was talking to friend about this exact issue of wind. I was thinking about making a wall on the interior to serve as a tunnel and then have a cover for the entrance. As it is, we made the exterior hole, WAY too big.

[chemist]

I feel compelled to point out that an "igloo" is a cartoon invention, as far as I can tell. I would LOVE to see a link to any indigenous people using an igloo as a normal part of their daily lives. I found lots of variations on the practice of making a temporary shelter out of ice/snow, but none of them are what I'd consider an igloo.

Also, having made and used a snow shelter in Boy Scouts, I can tell you that if you can't keep the wind out, you've wasted your time. If you're going to sleep in it, you'll want a (at least mostly) sealed door, and a small roof vent so you can put a camping stove or small campfire inside. This will cause the walls to slowly melt, but so slowly that it's not relevant. However, the water runoff will ruin your comfort if you don't prepare for it. 1) The door to the igloo goes downhill. 2) carve a fist-sized groove in the floor around the wall on the inside and out the door. 3) Never put anything in the groove.

Shelter
The Inuit were semi-nomadic. On winter hunting trips they built igloo out of compacted snow. Two men could build one igloo in an hour. Igloo let light in, kept warm inside and kept the wind out. In summer tents were built from walrus skins.

http://ecx.images-amazon.com/images/...L._SL1500_.jpg
Is this the link you would love to see?

[MadMojoMonkey]

Shelter
The Inuit were semi-nomadic. On winter hunting trips they built igloo out of compacted snow. Two men could build one igloo in an hour. Igloo let light in, kept warm inside and kept the wind out. In summer tents were built from walrus skins.

http://ecx.images-amazon.com/images/...L._SL1500_.jpg
Is this the link you would love to see?

It's a good pic, but really grainy. I can't tell if the shape(s) on the right are separate "iglooyas" or the tunnel entrance I'm not seeing. I'm specifically fixated on the tunnel entrance to an ice shelter... I'm willing to accept snow for ice. The reason is that, I think, whenever I say the word "igloo" to a random person, they imagine that tunnel entrance... not a snow/ice wigwam.

When I was young, someone told me the tunnel was where the sled dogs slept... but I've heard nothing similar in 20+ years, so I don't know if it's worth believing.

The caption explicitly states that an igloo is a permanent structure, made of stone.