Round foundation help

[Neil2]

Tony

Try this:

http://www.cleavebooks.co.uk/scol/callipse.htm

So for your 21x18 (axis 42x36), the foci = 21.6.

Measure over to the middle, 21 inches, then put a nail in at 21.6/2= 10 3/4 inches each side on the base line. These are the ellipse foci.

Put a nail in at one end of the 42 inch line. To get the loop length, tie a cord loop tight around this this nail and the farthest of the two foci nails. Now, take the cord loop and loop it around the two foci nails. Now put a pencil inside the loop and draw the ellipse.

Note that the height you have chosen, 18 inches, is 85% of the radius. This is at the most aggressive end of the 85-90% "rule of thumb" ratio. At 90%, the height would be 21x.9= 18 7/8. I would go for about 18 1/2 inches. Take a look once you have drawn it.

Note also that because your soldiers are 4 1/2 inch vertical, you have to "catch up" to the ellipse over the first 2-3 courses, see below (this sketch is for a smaller ellipse - on a larger one it is easier to "catch up").

[tony boca]

Neil,

How does this look? I put the top nail 18.5" from the base line. Place the two base nails (foci) 27" from each other. If I place the full soldier on the perlite without an angle cut, the pen goes right to the top of the soldier. But this will also this will give me a mortar joint of 1.75" between the soldier and the first course. What's your opinion? Do you think the mortar joints will be too thick?

[tony boca]

I changed the foci to 23" apart. This gives me a 16 degree angle to cut for the soldiers. This also decreases the mortar joint significantly. The next course should have a 1/2" mortar joint. I'm going to play with these measurements in a dry run and see what happens.

[Neil2]

That 23 inch foci ellipse looks good Tony.

It helps to mock it up and figure out what "looks right"

[fxpose]

Hi,

I'm also interested in achieving a lower ceiling height, but at the same time use the "indispensible too" brick jig/guide posted over at tools and techniques. The tool seems fool proof by people who have used them.
Anyway, I posted sketches of a variation of this tool (post #51) which I think might work to achieve a low ceiling.

One question I have is, must ellipse domes follow certain profiles or would most any low arc profile work?

[tony boca]

fxpose,

I would start by using the elipse that gives you the height you want and the tallest soldier possible. I kept going back an forth with the foci to find what worked. My problem now is that i pitched the oven 1/2" higher in the back to keep the rain out. Now my soldiers sit below the floor even more now. Back to the drawing board.

[Neil2]

"One question I have is, must ellipse domes follow certain profiles or would most any low arc profile work?"

I think any low arc profile will work but an ellipse is the "theoretically" best shape.