Crossposting - Colony Wall Loadings

> > How much water, dirt and buildings can we put on them
> > per square foot before chunks start pushing out. Esp
> > since that dirt and water is going to go where the maximum
> > acceleration from the spin is concentrated.
That of course depends on how thick and strong the hull is. One
important observation that can give you some more gut feeling (always
important!) is that the atmospheric pressure on the Earth near sea level
corresponds to ten meters of water, or about the weight of three meters
of rock.
So if you design your habitat for, say, half the atmospheric pressure
of the Earth, you'd need to add a meter and a half of rock, or more than
that of dirt, to double the load upon the hull. Since the Earth's
atmosphere is more than seventy percent inert nitrogen, you could remove
most of that and still have something easily breathable but with just
half the total pressure.

> Very good question. My mind has been pondering that as well. I think
> that the structural design of the habitat will have to be designed
> specifically for any extraordinary structures within the the
> habitat. The ground of the habitat will include dirt, small hollow
> hills, lakes, forests, rivers, etc. We can probably estimate the
> loading that they would place on the structure beneath them. For
> multistorey buildings, all of that weight will have to be designed
> for.
I agree with that. If you build a skyscraper in a space habitat,
and the "ground" just there weren't strengthened for that, you'll have
one skyscraper rocketing out of a hole in the hull, taking a lot of
people and atmosphere with it...

> I've even toyed with the idea, within a cylinder habitat, of having
> a building near each endcap, that has a ground floor on each of the
> three land panels and each of the three buildings rise to join in
> the center. The buildings would have variable gravity and join at a
> minimal-g recreation center. From there, further construction could
> progress along the zero-g axis. But as I've been considering this
> plan, I've turned a lot of attention to the load factors that all of
> that building mass would place on the ground footprint. My gut
> feeling is that if these building are designed from the outset, then
> the structural design of the habitat could be such as to spread the
> load of the building mass.
If you build that building such that it supports itself, you won't
have to face that problem to a very high degree. Say you build an
axial-symmetric building, consisting of several cylindrical floors.
They would not need to rest heavily upon the "ground". They would only
have to built together with the "ground" so that they wouldn't rattle
around against it.
Still, I'd like at least one end cap as not a building, but a sloping
park. As the slope increases, weight is reduced, so even when the slope
goes very steeply upwards, the hike up won't be as difficult as it would
on Earth.
Of course, by the time the slope is 45 degrees, your weight is still
reduced only by less than thirty percent.

> This whole issue of loading has led me to give a long hard look to
> concrete construction for the habitats. Concrete can take impressive
> compressive loads, but to directly addresss your question - I'm not
> sure at the moment how thick the "floor" would have to be.
As has been mentioned, the habitat hull would have to take tensile
loads. A concrete foundation can withstand high loads that seek to
crush it. A habitat hull will have to withstand loads that seek to rip
it apart.

> My seat of the pants approach for the moment is to work with the
> following assumption. Six feet of concrete for the outer shell. Then
> a 20 foot high service level where the mag-lev transport is, where
> the light industry, shopping, offices, etc are. The interior is
> reserved for habitation and parks. Above the service level is
> another 4 feet of concrete/dirt/lake etc. For some areas where deep
> water is required, we can cut the shielding down to the minimum
> required to support the weight of the water above it, i.e. 29 feet
> of water, 1 foot of structure. For parks with trees and grass, 3
> feet of dirt, 1 foot of concrete support. Is that enough depth for
> tree roots? For densely populated city centers, the service level is
> minimized and much of that volume is consumed by thick concrete
> support structures, effectively giving up to 30 feet of concrete
> below the taller structures.
The waste heat radiators need have approximately the same area as the
land that is illuminated by sunlight. I made the calcs on the
sci.space.policy NG some time ago. It agrees with my gut feeling, of
course: the Earth radiates from roughly (within an order of magnitude)
the same area as it receives sunlight on. But if you have a certain
land area that receives about 800 W per square meter in the habitat's
daytime (this corresponds to a warm day on the Earth), and you reradiate
this heat from waste heat radiators that are about 25 degrees Celsius,
24 hours a day, and they are the same area as that certain land area,
then those radiators will be enough.
Where do you put the waste heat radiators?
If you simply pump air from the habitat to external coolers, these
coolers need be thick-walled enough to withstand the same air pressure
as the habitat hull. Or you can somehow make the hull itself the waste
heat radiator.

Also consider this: assume a cylindrical habitat with a single metal
hull, and a meter of soil on top. Assume the cylinder axis is pointed
towards the Sun, so that the hull, except for one endcap if that endcap
isn't in the shadow of some other structures, is in perpetual darkness.
Then the outer surface of the hull will be cold indeed. The bottom
layer of soil will be about one degree warmer than the outer surface.
Near the bottom of the soil you will have dry ice temperature.
Twenty centimeters below the topsoil you will have permafrost. Not a
good idea if you try for a habitat with subtropical flora and fauna.
Three options present themselves to prevent the land from becoming
summertime tundra: heat the outer surface of the hull (such as by
shining sunlight on it), add an insulating layer below the soil, or make
a double hull. If at least the outer hull is of metal, you can then
remove waste heat from the hab by pumping air through the interstitial
between the inner and outer hull. I never calculated the approximate
necessary airflow, but I did calculate that if the airflow is big
enough, this would be enough to remove all the waste heat from, say, and
Island Three type cylinder.
Probably you would want to pump air from near the axis at one end
cap, through the interstitial, and out near the axis at the opposite end
cap.
Also useful would be the fact that the air, cooled by heat loss and
then further cooled adiabatically as it rises towards the outlet near
the axis, would precipitate some of its moisture. This could be the
main source of potable water in the colony. Of course, any cooling
surfaces provide this.
The combined thickness of the double hull would not be much different
from the thickness of a single hull. A single hull would need to
contain the load from both the air pressure and the weight of the soil
and structures. With a double hull, the outer hull would contain the
air pressure and the inner hull the weight of soil and structures. Air
pressure and weight would be vectors in the same direction, and hence
can be added just like real numbers.

Of course in the interstitial between the hulls you could build
structures, such as subways and offices, as you proposed above, at least
if the outer hull is thick enough to shield against radiation. And if
it needs to contain air pressure, and the inner hull only needs to
contain a meter of soil and some buildings, the outer hull will be the
thickest one anyway.

Jon L. Beck.