Difference between revisions of "Roof Support"

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[[Image:ArchDorm01.jpg|frame| Architecture as Mole Hills, Standard dorm room]]
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[[Image:ArchDorm01.jpg|frame| [[Architecture as Mole Hills]], Standard dorm room]]
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==Introduction==
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Structural support in lunar habitats will be quite different from earth based buildings. One primary difference is that, due to the need for lunar habitats to maintain a sizable internal pressure, most of the standard operating stresses will be internal rather than external.
  
One serious lunar problem will be your roof falling in on you.
 
  
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== Safety considerations ==
  
==Using the roof for radiation and heat shielding==
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Most proposals for long term lunar habitats call for the use of a thick blanket of lunar regolith to be piled atop the structure, providing protection from temperature swings, meteorite impacts, and cosmic radiation for the inhabitants, as well as any electronic equipment (see [[Architecture as Mole Hills]] and [[Architecture as Tent  City]]). Most of the thickness called for in this blanket is to protect against cosmic radiation.
  
The need for radiation shielding for lunar settlement occupants  means that there will be significant mass on the roof of all buildings used for long term occupancy. Much of the electronic equipment will need shielding too.  This shielding could be in the form of lunar regolith as suggested in [[Architecture as Mole Hills]] and [[Architecture as Tent  City]] or material brought from Earth, but it must be provided.  
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Most of the experience mankind has accumulated concerning radiation deals with nuclear radiation. The cosmic radiation encountered beyond earths magnetic field is much less understood, as it can only be replicated on earth by means of a particle accelerator. As such, the question of how much regolith is needed to protect the inhabitants is currently little more than educated guesswork, and will likely remain so until more thorough field study is performed.
  
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One common estimation is to provide enough regolith to equal earths atmosphere in shielding potential. This is based on the fact that the earths magnetic field has collapsed several times in geologic history, and said collapses did not seem to have any major effect on the life on earth. As such, it is believed that the mass of earths atmosphere is by itself sufficient to guard against cosmic radiation. Assuming that air and lunar regolith have similar shielding properties according to their mass (as they do for nuclear radiation), this gives several different thicknesses, depending on what altitude is used for comparison.
 
 
Also, if your massive roof falls in on you, you will be in real trouble.
 
 
 
 
 
==Inflatable Housing==
 
 
 
Since the interior living space must be at considerably higher air pressure than the exterior, it is only logical to use the force this pressure exerts on the outside walls to help support the weight of the roof.  The rooms then become like balloons getting much of their strength from their internal pressure.  The amount of mass an inflated roof will support is directly related to the internal pressure.
 
 
 
A major problem comes when you lose the pressure and the massive roof falls in on you. Loss of pressure, or blow out, is a real possibility due to meteorites, ejecta, landing accidents, or industrial accidents.  Even if you can get into an environmental suit, you will have to get to safety before your air supply runs out.
 
 
 
 
 
===Pressure Considerations===
 
 
One of the most important considerations in the design of a lunar settlement will be the internal air pressure.
 
 
 
====High Pressure====
 
 
 
High air pressure makes the living space more Earth like.  People and plants accommodate easily and food is easy to cook.
 
 
 
But, spacesuits must be at low pressure for the joints to work with acceptable amounts of efforts. For the body to accommodate from normal air pressure to low spacesuit pressure can talk as long as three hours.
 
 
 
 
 
====Low Pressure====
 
 
 
Low air pressure, with adequate oxygen content, allows accommodation to spacesuits in only a few minutes or just seconds in an emergency.
 
 
 
The long term health effects of low pressure are not fully known for people or plants.  Any agricultural areas may need additional CO2 and nitrogen.  The human body will simply never fully accommodate pressures below about 30 kPascal.
 
  
At low pressures you can use 100% oxygen which greatly simplifies the entire life support system.  Testing this type of system is very dangerous and has resulted in two serious fires. 
 
 
Water boils at such a low temperature that cooking is difficult.  You simply cannot get things hot enough to really taste right.
 
 
Also low air pressure will not support as thick a layer of regolith over inflated buildings.
 
 
====Supporting Regolith====
 
 
Here are some of present ideas for lunar settlement air pressures and how much regolith they will support:
 
 
 
 
{| border=1
 
{| border=1
! Pressure !! Pressure !! Boiling Point !! Regolith Shield !! Regolith Supported !!Comment
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! Location !! colspan="2" | Atmospheric Pressure !! colspan="2" | Regolith Needed to Equal Atmospheric Shielding !! colspan="2" | Miniumum Internal Pressure Required
|-
 
| kPascal || PSI || C || Meters || Meters ||
 
|-
 
| 101.3 || 14.2 || 100 || 5.4 || 32 || Earth standard, ISS body
 
|-
 
| 84 || 12.17 || ?  || 4.5 || 27 || Denver, a high altitude city
 
|-
 
| 81.4 || 11.74 || ?  || 4.3 || 26 || Mexico City, a high altitude city
 
 
|-
 
|-
| 74.0 || 10.2 ||   || 4.0 || 24 || Open airplane, ISS ports
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| || kPa || psi || m || f || kPa || psi
 
|-
 
|-
| 59.1 || 8.3 ||   || || 19 || ISS spacesuit
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| Sea Level || 101.3 || 14.2 || 5.4 || 17.9 || 16.9 || 2.4
 
|-
 
|-
| 33.5 || 4.7 ||   ||  || 10 || Apollo spacesuit
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| Denver, a high altitude city || 84 || 12.17 || 4.5 || 14.8 || 14 || 2
 
|-
 
|-
| 30.6 || 4.3 ||   || || 9.9 || Shuttle spacesuit
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| Mexico City, a high altitude city || 81.4 || 11.74 || 4.4 || 14.3 || 13.5 || 2
 
|-
 
|-
| 26.0 || 3.65 || 69.0 || 1.4 ||   || Top of Mount Everest
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| Open airplane || 74.0 || 10.2 || 4.0 || 13 || 12.3 || 1.8
 
|-
 
|-
| 10.0 || 1.5 ||   ||   || .|| 1/10 Atm, 16,000 m, unconscious in 10 sec
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| Top of Mount Everest || 26.0 || 3.65 || 1.4 || 4.6 || 4.3 || 0.6
 
|}
 
|}
  
The Regolith Shield column shows how much lunar regolith is needed to provide radiation shielding equivalent to the air above your head at these locations on Earth.  The Regolith Support column shows how much lunar regolith can be supported on the Moon by that level of internal pressure.
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(assuming a regolith density of 1.9 grams/cm^3 and lunar gravitational acceleration of 1.63 m/s^2)
 
 
These calculations are based on the following parameters:
 
 
 
{| border=1
 
| density of packed regolith || 1.9 || g/cm^3 || Used for this calculation
 
|-
 
| density of loose regolith || 1.5 || g/cm^3 || just poured in a pile
 
|-
 
| Lunar gravity || 1.63 || m/s^2 || about 1/6 Earth
 
|-
 
| Human body temperature || 37.0 || C || 98.6 F
 
 
 
|}
 
 
 
 
 
It is important to note that the internal pressure inside a living area has plenty of force to support a substantial thickness of lunar regolith above it for radiation and thermal shielding even if low pressures are used.
 
 
 
==Safety rule==
 
 
 
The roof cave in problem will require a strong safety rule that will greatly affect the design of lunar buildings. One possible rule is this:
 
 
 
;Roof Support Rule:  With the internal building pressure completely lost, a person wearing an environmental suit must have enough clearance to crawl to safety even if a second person is lying immobile in the evacuation path.
 
 
 
 
 
===Rule's effects===
 
 
 
Such a rule, combined with the very high cost of bringing mass from Earth, will make large open rooms very rare on the Moon.  Rooms and even halls will require internal columns or bearing walls to hold up the roof with a loss of pressure.  Most rooms will have to be narrow in at least one direction and will have to have central support for the roof.
 
 
 
 
 
What will be very difficult to build will be very large areas such as meeting rooms and mess halls.  They will probably need internal supports.
 
 
 
The absence of large rooms will make living in a lunar settlement even more claustrophobic. This effect might be countered with panoramic vistas of open space on external monitors, via [[periscope windows|pariscope style windows]], or with viewing doom rooms.
 
 
 
 
 
----
 
 
 
 
 
[[Image:Under Roof Sketch 18 Sept 2007.png|thumb|320px|Elevated roof concept]]
 
 
 
 
   
 
   
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As seen from the chart, the minimum habitat pressure level needed to support the regolith shield on pressure alone is quite low, even for sea level equivalent shielding. Even if a structure was only pressurized to equal the peak of mount everest, it would be able to support sea level radiation shielding and still have a net outward force. The reason for this is that, though the regolith piled atop the structure is the same mass per unit area as earths atmosphere would be, lunar gravity is only around one sixth of earths, resulting in much less force pushing downward, but the same pressure pushing upward. If the same structure was constructed in earth-like gravity (but still in a vacuum), the pressure balance would be quite different.
  
==Hard Roof==
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As a result, a lunar habitat functioning under normal parameters would not require any internal support for its main structure, as the internal forces of pressure are greater. A lunar habitat could therefore be essentially a giant, reinforced balloon, covered in lunar regolith.
 
 
 
  
If a practical lunar cement can be developed, then we can avoid pressure supported roof problems completely.
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===Depressurization===
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One safety consideration for such a structure is to plan for operation in case of pressure loss, also known as a blow out. Some mechanism of coping with a blow out would be required.
  
Earth cements cannot be used as they are water based and the minerals that they are made from are not available on the Moon.
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One method of dealing with a blowout is to make the outer structure sufficiently strong that it will not only keep in the pressure under normal conditions, but will also hold its own in a depressurized state. As seen in the chart, a structure capable of supporting a sea level equivalent regolith shield under depressurized conditions would be subject to just under 3 psi of pressure across its roof. This is approximately equivalent to designing an earthbound structure capable of maintaining standard atmospheric pressure six feet underwater in an emergency, as well as containing twice atmospheric pressure under the same conditions normally. A [[steel]] or [[titanium]] structure could be made sufficiently strong to withstand these forces, as could a properly designed [[Sintered Brick Construction|lunar brick]] structure reinforced with steel cable.
  
 +
A structure which was supported only by internal pressure would require additional mechanisms to ensure the safety of its inhabitants in case of a blowout. One way of coping with this is to compartmentalize the structure, designing it so that separate areas could be sealed off if needed, so that a breach in one area would not affect the entire habitat. Multiple air bladders around the outsize, also compartmentalized, would also aid in reducing the chance of a blowout. An additional safety measure that could be installed is for each section of the habitat to have its own backup gas tank, which would open in case of a pressure loss, feeding the leak until an patch job / evacuation could be achieved. Since the minimum pressure that needs to be maintained is quite low (as discussed previously), this approach could be utilized with a modestly sized backup tank.
  
----
 
  
  

Latest revision as of 19:37, 17 September 2011

Architecture as Mole Hills, Standard dorm room

Introduction

Structural support in lunar habitats will be quite different from earth based buildings. One primary difference is that, due to the need for lunar habitats to maintain a sizable internal pressure, most of the standard operating stresses will be internal rather than external.


Safety considerations

Most proposals for long term lunar habitats call for the use of a thick blanket of lunar regolith to be piled atop the structure, providing protection from temperature swings, meteorite impacts, and cosmic radiation for the inhabitants, as well as any electronic equipment (see Architecture as Mole Hills and Architecture as Tent City). Most of the thickness called for in this blanket is to protect against cosmic radiation.

Most of the experience mankind has accumulated concerning radiation deals with nuclear radiation. The cosmic radiation encountered beyond earths magnetic field is much less understood, as it can only be replicated on earth by means of a particle accelerator. As such, the question of how much regolith is needed to protect the inhabitants is currently little more than educated guesswork, and will likely remain so until more thorough field study is performed.

One common estimation is to provide enough regolith to equal earths atmosphere in shielding potential. This is based on the fact that the earths magnetic field has collapsed several times in geologic history, and said collapses did not seem to have any major effect on the life on earth. As such, it is believed that the mass of earths atmosphere is by itself sufficient to guard against cosmic radiation. Assuming that air and lunar regolith have similar shielding properties according to their mass (as they do for nuclear radiation), this gives several different thicknesses, depending on what altitude is used for comparison.

Location Atmospheric Pressure Regolith Needed to Equal Atmospheric Shielding Miniumum Internal Pressure Required
kPa psi m f kPa psi
Sea Level 101.3 14.2 5.4 17.9 16.9 2.4
Denver, a high altitude city 84 12.17 4.5 14.8 14 2
Mexico City, a high altitude city 81.4 11.74 4.4 14.3 13.5 2
Open airplane 74.0 10.2 4.0 13 12.3 1.8
Top of Mount Everest 26.0 3.65 1.4 4.6 4.3 0.6

(assuming a regolith density of 1.9 grams/cm^3 and lunar gravitational acceleration of 1.63 m/s^2)

As seen from the chart, the minimum habitat pressure level needed to support the regolith shield on pressure alone is quite low, even for sea level equivalent shielding. Even if a structure was only pressurized to equal the peak of mount everest, it would be able to support sea level radiation shielding and still have a net outward force. The reason for this is that, though the regolith piled atop the structure is the same mass per unit area as earths atmosphere would be, lunar gravity is only around one sixth of earths, resulting in much less force pushing downward, but the same pressure pushing upward. If the same structure was constructed in earth-like gravity (but still in a vacuum), the pressure balance would be quite different.

As a result, a lunar habitat functioning under normal parameters would not require any internal support for its main structure, as the internal forces of pressure are greater. A lunar habitat could therefore be essentially a giant, reinforced balloon, covered in lunar regolith.

Depressurization

One safety consideration for such a structure is to plan for operation in case of pressure loss, also known as a blow out. Some mechanism of coping with a blow out would be required.

One method of dealing with a blowout is to make the outer structure sufficiently strong that it will not only keep in the pressure under normal conditions, but will also hold its own in a depressurized state. As seen in the chart, a structure capable of supporting a sea level equivalent regolith shield under depressurized conditions would be subject to just under 3 psi of pressure across its roof. This is approximately equivalent to designing an earthbound structure capable of maintaining standard atmospheric pressure six feet underwater in an emergency, as well as containing twice atmospheric pressure under the same conditions normally. A steel or titanium structure could be made sufficiently strong to withstand these forces, as could a properly designed lunar brick structure reinforced with steel cable.

A structure which was supported only by internal pressure would require additional mechanisms to ensure the safety of its inhabitants in case of a blowout. One way of coping with this is to compartmentalize the structure, designing it so that separate areas could be sealed off if needed, so that a breach in one area would not affect the entire habitat. Multiple air bladders around the outsize, also compartmentalized, would also aid in reducing the chance of a blowout. An additional safety measure that could be installed is for each section of the habitat to have its own backup gas tank, which would open in case of a pressure loss, feeding the leak until an patch job / evacuation could be achieved. Since the minimum pressure that needs to be maintained is quite low (as discussed previously), this approach could be utilized with a modestly sized backup tank.


Hazards