Circles and Squares revisited

[Marcel]

(M) There are clearly loyal adherents to both basic designs but I would like to explore the possibility of considering "the best of all possible worlds" (aka The Candide approach):

(M) Given the same internal volume, same ratio of door height-width to dome height, same insulation, same materials, .... is there any essential difference(s) in the performance of an oven with a rectangular footprint versus one with a circular footprint?

(M) Thanks for your feedback.

Ciao,

Marcel

[dmun]

You will find attached a drawing of a three foot circle, superimposed upon a square of the same area (about 6.9 sq.ft.). Given the same height, the volume of the ovens with the same footprint would be similar. You will see that the square is 2' 7 1/2" across. Now this wouldn't matter if the corners were of any use, but unless you are baking multiple loaves in pans, they aren't.

We're trying to get a space up to a high temperature with a minimum of fuel and time. Having every part of the oven the same distance from the initial central fire will help do that: there won't be any cold, distant corners.

David

[Marcel]

(M) Dear David,

You wrote, in part,

(D)"You will find attached a drawing of a three foot circle, superimposed upon a square of the same area (about 6.9 sq.ft.). Given the same height, the volume of the ovens with the same footprint would be similar."

(M) I must not understand your meaning because when I respond intuitively, it seems that with the same area size foot print, a "hemisphere" would provide significantly *less* volume than a barrel vault. Have I missed your meaning, or is my intuition way off?

Thanks,

Marcel

[Robert Musa]

Originally Posted by Marcel

I must not understand your meaning because when I respond intuitively, it seems that with the same area size foot print, a "hemisphere" would provide significantly *less* volume than a barrel vault. Have I missed your meaning, or is my intuition way off?

david probably should have said "useable area" instead of volume. also, a hemisphere which has the same floor area as a rectangular box has less exterior surface area, which means less bricks to bring up to cooking temp. (not to mention greater structural stability).

[Marcel]

(R)

Originally Posted by Robert Musa

david probably should have said "useable area" instead of volume. also, a hemisphere which has the same floor area as a rectangular box has less exterior surface area, which means less bricks to bring up to cooking temp.

(M) That seems evident, and desireable.

(R) (not to mention greater structural stability).

(M) But are you saying here that a brick fitted dome ( as opposed to a precast one, of course) has greater structural stability than a "barrel vault" where the there are no internal gaps between the bricks?

(M) Hope I'm not putting too fine a point on this I don't want to open the proverbial Pandora's Dome. ;-)

Thanks,

Marcel

[Robert Musa]

Originally Posted by Marcel

(M) But are you saying here that a brick fitted dome ( as opposed to a precast one, of course) has greater structural stability than a "barrel vault" where the there are no internal gaps between the bricks?

imho, absolutely! (now watch my dome collapse tomorrow...)

[Marcel]

(R) imho, absolutely! (now watch my dome collapse tomorrow...)

(M) I need to preface all of what I next write with capital "IMHO":

(M) Robert, though you are probably correct when looked at from every point on the dome, our concern is with our "dome collapse tomorrow". In that regard, our worry is that vertical pressure from brick weight will overcome the horizontal resistance at the base.

(M) If our dome curve at the highest point of our igloo is the same curve as that of any of the identical curves along the vault, then that vertical pressure on the vault should be the same as that on the igloo.

(M) If we have vertical front and back walls, as in a bread box design, all of the force there would be vertical, and "safe", so from the standpoint of specific resistance to verical pressure, a vault would seem to have an advantage.

(M) My concern is not that a car could hit the side of my igloo, but that a tree limb might fall on it's top ;-) Since you were an attorney, I better rest my case.

Ciao,

Marcel

[dmun]

I was afraid that I was going to have to do some actual math, but once again the web is to the rescue:

http://www.1728.com/diam.htm

If you enter a diameter of "3" into the sphere calculator, you will get a volume of 14.137 cubic feet.

If you enter a diameter of "2.622" and a height of "2.622" height into the cylinder calculator you get a volume of 14.158 cubic feet.

Of course both of those figures are halved, since we are dealing with a half dome and a half cylinder.

The halfsphere and the half cylinder have volumes in proportion to the area of the footprint.

David

[Robert Musa]

Originally Posted by Marcel

If we have vertical front and back walls, as in a bread box design, all of the force there would be vertical, and "safe", so from the standpoint of specific resistance to verical pressure, a vault would seem to have an advantage.

well, i seem to recall that either jim or james have actually built and then collapsed a barrel vault oven (i looked for their post on this but couldn't find it). so they're the experts on oven destruction...

however, i think the difference is that with the dome, the downward force is distributed uniformly around the dome whereas with the barrel vault the downward force is pushing out to the left and the right only and not distributed to other areas of the oven.

i know that with my dome i could begin removing bricks from random locations and the removal would not cause the collapse of the dome or even of adjacent bricks in most cases, until i had removed quite a few of the bricks. with a barrel vault, on the other hand, i would bet that significant portions of the dome would begin falling much sooner as i removed bricks from random locations.

[Alf]

Personally I fell your way off the mark even trying to work out the maths required to decide if a small dome will collapse. These ovens have been built for thousands of years in all sorts of shapes and curves by folks that just got on with it and then started baking. On my last oven-building course we built a 1-meter demonstration oven using brick and a 10 – 1 weak lime mortar (the mortar wont set) on a sand form. At lunch time on the second day we removed the sand and the oven stayed up. We then had a bet as to how much weight (8 kilo concrete blocks) the dome could stand, I reckoned one block, as the mortar was still very soft. Lost the bet as the dome collapsed when the third block was placed on top (it cost me 24.00 Scottish pounds in drinks)

Alf