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Free-standing column of stones with permeable sides as a thermal mass

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Posted by David Delaney on May 11, 2005, 12:44 am
I spent some time thinking about how to build a free standing thermal
mass with permeable sides.  The eventual goal is a yet another thermal
mass design for entirely passive solar heat collection for 100% solar
heating in cold climates. How about the construction method described
and pictured  at

Suggestions for improvement?

It would seem that the structure described would be obviously stable
and robust up to a ratio of height to diameter of 2:1.  Any thoughts
about  3:1? How about 4:1?

David Delaney, Ottawa

Posted by Gary on May 11, 2005, 1:39 am
David Delaney wrote:

This is not very analytical, but having built a couple fences and some
gabions from the heavy wire mesh fencing material, I think the
following would work OK, and be less work:

Cut a length of wire that is 2 ft longer than the perimeter of your
cylinder.  Form it into a cylinder that is the diameter you want.
This will result in 2 ft of overlap.  Wire the two feet of overlapping
fence together with short pieces of the same wire.  You can cut one
end of the fencing just short of one of a vertical wires -- this
leaves wire tails a couple inches long sticking out.  Use these wire
tails to secure it to the vertical wire at the end of the splice zone.
I suppose that if you wanted more resistance to side forces you could
add a piece of vertical wire fence across the diameter, and wire it in
as well.

I'm probably missing something on your diagram, but why would the air
circulate through the pile, rather than just following the open
channels around pile?


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Posted by David Delaney on May 11, 2005, 2:33 am

Hi Gary

This procedure produces much more variability on the tension in
individual horizontal wires than the tension pipe method. It also puts
almost the whole horizontal load on the welds in a single vertical
wire. You might distribute your ties among many vertical wires, but
each horizontal wire would still be stressing a singe weld. You might
use a very large number of ties, many near each horizontal wire,
distributed over the whole overlap zone, but then the amount of work
is grows to be greater than the tension pipe method, and would in any
case lack its simple comprehensibility.   The pigtail burried in the
mass experiences resistance to the outward pull of the peripheral
tension of the skin being applied by stones protruding into almost
every one of its mesh openings, never mind the friction of the
horizontal wires against stone.

It would just follow the open channel if it were forced. This
structure won't work with forced convection. With free convection, air
won't descend lower than its height of neutral
bouyancy.  When the air heater is hot, the pressure difference from
top to bottom inside the enclosure will be greater than the pressure
difference from top to bottom in the air heater, with the pressure
differences around the circuit summing to zero due to flow resistance
pressure differences through the openings in the enclosure. There will
be a net flow around the circuit, with cool air falling out of the
lower part of the mass, drawing warm air into the upper part of the

David Delaney, Ottawa

Posted by Gary on May 11, 2005, 4:03 am
 Hi David,

I agree that the method you propose is easier on the wire.  I just
wonder whether you need to be that easy on the wire?
If you assume that the rocks act like a fluid (very conservative),
then the pressure at the bottom of the 5' dia, 8' high cylinder would be
(8ft)(120lb/ft^3)/ 144 = 6.6 psi, and the hoop tension in the wire
would be Pr = (6.6 lb/in^2)(2.5ft*12in/ft) = 200 lb/in or 200 lb per
wire for 1 inch pitch wire.  Maybe thats enough to require more than
just the simple wire ties -- not sure -- I guess you could test a weld
and see :-)


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Posted by David Delaney on May 11, 2005, 12:20 pm


I also have in mind the possibility of fatigue at welds and other
stress concentrations due to thermal stress and earth tremors.

Earthquakes will occasionally double the hoop stress.

I'd like to be able to convince very conservative people that a stone
column is a safe thing to have embedded in the structure of a house.


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