Replies

1. 
   Guest_ | Oct 12, 1999 05:31pm | #1

   *
   JR,

   There's an excellent article in one of the back issues ( I'm not sure which # ) of Fine Homebuilding by Scott McBride in the Q & A section dealing with this topic. He takes you through the entire process from beginning to end. Someone else may be able to identify the number of this issue for you.

   After reading the article you may want to change your mind about making it 2 feet deep and instead make it three feet deep ( half the minor axis of the ellipse, the minor axis being 6 feet in your case.) if there is room in the dead space above the dome.

   This small change will greatly simplify the construction for you, as the arches, or ribs, that will support the drywall will be circular, each one having a smaller radius as you move from the center of the ellipse out towards the ends.

   But do yourself a big favor. Find this article and read it. It's very well written and easy to understand.

1. 
   Guest_ | Oct 13, 1999 04:52am | #2

   *JR,These can be tough......The eliptical part is the bitch. You're dealing with two different radius points here just at the plate line. The two foot height also makes the run an elipse because you are not going half the distance up that you are wide in the middle of the oval. We did it once, about the same size as yours, but it was years ago. It had a skylite in the apex. Fortunatly the rafters for this ceiling were not the roof on the backside. We installed it against a flat floor above. The skylite part was a piece of lexan that was in the floor above......I digress.The drywall guy will cuss you no matter what. His cuts will be eliptical too. Quarter an orange, peal off the skin and lay it flat and you will see what I mean. You cut it straight, but it don't lay flat that way.I havn't been much help here have I. Buy some tramel points. Make the ribs out of plywood.....the closer together the better. I don't have the formula for elipses on me, but I'm sure someone else here will have it handy.Good luck,Ed. Williams

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   Guest_ | Oct 13, 1999 04:53am | #3

   *Joe,That's round.......not oval.Ed. Williams

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   Guest_ | Oct 13, 1999 07:01am | #4

   *Thank you, Joe! That picture is worth a thousand words. It may save me a thousand bucks in labor, too. And it's so clean. Lunch is definately on me. P.S."Government is at its best state a necessary evil, at its worst an intolerable one."---Thomas Paine

1. 
   Guest_ | Oct 13, 1999 03:04pm | #5

   *I hope you have a good (read: the best in the state). Did a dome a couple years ago. Drywller screwed it all up. Lots of agrivation to get it right.Rick Tuk

2. 
   Guest_ | Oct 13, 1999 07:28pm | #6

   *JR,That's another nice Cad drawing by Joe. If you decide to build it by using circles which run perpendicular to the major axis, each circle, as I previously mentioned, will have a smaller radius as you move from the center towards the ends. But they are easy to determine.Find a nice flat open area in the house and lay down 4 sheets of plywood that are in good condition, all butted tightly to each other. Snap two lines that are square to each other to represent the major axis and the minor axis of the ellipse. The point where the two axis lines intersect, is the center of the ellipse.The foci of an ellipse whose major axis is 9 feet and whose minor axis is 6 feet, is 40 1/4". Measure out from the center this distance each way on the major axis and mark the two foci. Tack a small finish nail at each foci. Measure up 36 inches on the minor axis and tack another finish nail.You are now ready to draw the ellipse, but for one that is this large, I would NOT reccommend that you use string. It streches and gives only a "fair" result. Try to find some picture frame wire and use it instead. It doesn't stretch and is very flexible, unlike other types of wire. You'll need a single piece about 10 feet or longer.Make a loop over one of the foci nails, pull it over the top of the nail on the minor axis and back down to the other foci nail and loop it off. ( if you were to remove it at this time and stretch it out in a straight line, its measurement would be equal to the length of the major axis, or 108" )Remove the nail on the minor axis and replace it with a pencil. Keeping constant pressure against the wire, move the pencil from side to side to trace out one ha;f of the ellipse. Move the wire to the other side and trace out the other half.Now that you have the ellipse drawn, it's easy to determine the radii of each of the circles that you will cut from plywood. We already know the radius of the center circle, it's just one half of the minor axis, or 36". Now, move 12" away from it and snap a line parallel to it to represent the next circular arch. This line will be less than 36", say possibly 34". Whatever it turns out to be, half of it is the radius for the next plywood arch. Keep moving out towards the ends keeping the lines 12" O.C. to find the remaining radii. (if you want to put the plywood arches closer than that just snap out the lines on whatever O.C. measurenment that you choose.For anyone who is reading this and would like to know how to find the foci of any ellipse, here is one way to do it.Take half of the major ellipse measurement and square it1/2 x 108" = 54" 54 x 54 = 2916Do the same for the minor axis1/2 x 72" = 36" 36 x 36 = 1296Subtract the smaller from the larger2916 - 1296 = 1620Take the square root of this number to find the foci measurementsquare root of 1620 = 40 1/4"Hope this is helpful to you JR, and others also.

3. 
   Guest_ | Oct 13, 1999 08:44pm | #7

   *JR and others,One correction to my last post regarding finding the radii of the circular arches. When you move 12" away from the minor axis and snap a line parallel to it, this new line will represent the diameter of the next circle and will be somewhat smaller, perhaps, 68". In any case, whatever it turns out to be, half of it, 34", will be the radius for this next plywood arch.

4. 
   Guest_ | Oct 14, 1999 05:02am | #8

   *Thank you.Ed. Williams

5. 
   Guest_ | Oct 14, 1999 05:41am | #9

   *Joe,Thanks for pointing out a simpler method for laying out the ellipse, ( A METHOD, BTW, THAT I POINTED OUT IN AN EARLIER THREAD, IF YOU RECALL, "LAYING OUT AN ELLIPSE" ) which is probably by now, in the Archives.As you may have noticed, I made it clear in my post that it was "just one way of determining the foci points". Since the math involved is quite straight forward and only requires an understanding of the Pathagorean Theorem, I thought it might be interesting to other folks to see how to locate these points from a mathematical basis. BTW, since the math IS so easy, I personally don't feel that one method is superior, or necessarily, easier, than the other. ( unless you like to be "calculator independent", as you say, which in my mind, is a giant step backward into the dark ages. )And regarding your opening statement about "since one has to go through all the trouble of putting all those sheets of plywood together to lay out the ellipse", I can only comment that sooner or later, at some point, JR will have to define this shape anyway. It's not like I'm asking him to do anything extra.The shape of the ellipse is not going to appear on his ceiling as if by magic. He is going to have draw it out at some point, by whatever method he chooses. Always nice to exchange thoughts with you Joe. And I do mean that seriously. Like it or not, we make each others lives more interesting by our exchange of thoughts through this forum. I only hope that you see it this way also, and that others who have to wade through our bullshit, can benefit from our thoughts also.Good night. I look forward to your response in the morning. It gives me one more reason to get my butt out of bed!

6. 
   Guest_ | Oct 14, 1999 08:06am | #10

   *Thanks all!!! I am unstuck and ready to make noise. I must concede that the least amount of math is usually the best approach, however, fewer steps = faster results, and math saves steps. The depth is not critical here, a single ellipse pattern will serve....Goodnight---JRNicholson