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post #1 of (permalink) Old 06-07-2001, 12:46 AM
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I REALLY NEED YOUR HELP (o/t)

OK guys, here's my dilemma. I know it sounds stupid, but i can't figure it out. The really sad part is that someone here WILL KNOW THIS!!

I need to build an OCTAGON. It has 8 sides, of course. The octagon must be 10 feet from one side to the other. What i need to know is this:
1) how long should each side be?
2) what angle should the joints be?

I have a few ideas...
does 4.1422 feet sound about right, and how about 135 degrees?


I wont tell you what this is going to be until my numbers have been verified.

I can fly a plane, save a life, or rebuild an engine...but this geometry stuff is above my head!

thanks a million, guys...

-mike

Give me Jeep or Give me Death!!!

(Will trade functioning organs for CJ-7 or flatfender)
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post #2 of (permalink) Old 06-07-2001, 01:01 AM
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Re: I REALLY NEED YOUR HELP (o/t)

I AM NOT POSITIVE!

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post #3 of (permalink) Old 06-07-2001, 01:04 AM
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Re: I REALLY NEED YOUR HELP (o/t)

1) 14.14ft
2)?? Maybe 130degrees

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post #4 of (permalink) Old 06-07-2001, 03:36 AM
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Re: I REALLY NEED YOUR HELP (o/t)

I assume that you are building a gazebo. . .

The process to find out the length is a long mathematical process that would take up a page of BBS and about four or five sketches too [img]/wwwthreads_images/icons/crazy.gif[/img]!

Since I couldn't sleep and it is 0325, I would double check these figures by cutting eight strings to length, form them into an octagon, and then check the length of your sides. Or if you have a concrete slab large enough, just chalk it out with your chalk box and check it. Measure twice, cut once - remember?

Your length of the sides will be 5' 3 3/8" from outside tip to outside tip and your angle (the inside angle that you cut to butt the ends of the header joists together) will be 67.5º. In fact, your angle won't change no matter how large or small your octagon is - remember that!

I would suggest purchasing a "adjustable speed square." Stanley makes a nice one that is black with yellow lettering. It is a triangle with an arm that sweeps through it. It has a little knob on the end of the arm that you can tighten to set the arm at graduated angles. It will make marking your angles much easier, much faster, and more accurate when you lay this out. You will need it when you start cutting those rafters! I used to be able to mark these angles with a regular framing square - but that was back when I had something to prove to the crusty old farts on the job site [img]/wwwthreads_images/icons/blush.gif[/img]. When I got out on my own and hired my own crew, I have all but forgot how to pick one up [img]/wwwthreads_images/icons/laugh.gif[/img].

In doing this calculation, I assumed two things: First, that you meant 10' from the outside edge of your header joists and not the inside edge. Second, that you meant 10' from the outside edge of your header joists on the flat side of the octagon - and not the corners.

Hope this helps. It should be close enough for you to tweak it from here.

Now I am going to get some sleep. Naaah. It's 0335 now - I might as well brew a pot of coffee and get busy!



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post #5 of (permalink) Old 06-07-2001, 06:31 AM
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Re: I REALLY NEED YOUR HELP (o/t)

4'-1 3/4 " sides, the corners are 45 deg, so each piece has a 22.5 deg. on each end, 10'-9 7/8" from corner to corner or 5'-4 15/16" from the center to any corner...love that autocad : )

looks like you had it right

post #6 of (permalink) Old 06-07-2001, 06:57 AM
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Re: I REALLY NEED YOUR HELP (o/t)

Or if it is a gazebo you can go to

http://www.diyonline.com/servlet/GIB...session.ccid=0

Surf the site, you may have to sign up (it's free), there is design tools and all. My friend just used this. got his plans and material lists off the web for free. The end result of buying the materials Costs a hell of a lot less than the homecenter kits do.

post #7 of (permalink) Old 06-07-2001, 07:20 AM
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Re: I REALLY NEED YOUR HELP (o/t)

Mike - My hand at autocad says that your numbers are right on. 49.7" sides at 135 degree angles.

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post #8 of (permalink) Old 06-07-2001, 08:46 AM
 
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Re: I REALLY NEED YOUR HELP (o/t)

my vote is also for the 4.142'(49 11/16") and 135º. love that autocad! matt

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post #9 of (permalink) Old 06-07-2001, 08:48 AM
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Re: I REALLY NEED YOUR HELP (o/t)

Just because I had two minutes before work, I ran it in Cad and got the same as above.

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post #10 of (permalink) Old 06-07-2001, 11:38 AM
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Re: I REALLY NEED YOUR HELP (o/t)

For the AutoCad guys:

Thanks for the correction on the length of the sides. [img]/wwwthreads_images/icons/blush.gif[/img] After a few hours of work and a pot of coffee, I see where I made my error in calculation. [img]/wwwthreads_images/icons/blush.gif[/img] BUT . . . Your program is measuring the angle of one header joist to the next - kind of like the angle of one side of the stop sign as opposed to the next. AutoCad is using this fomula: x = [(n-2)* 180 / n)] (where n = the number of sides.) Now to translate your angle into a real life cut to butt two pieces of wood together at a 135º angle, you need to divide that amount by 1/2: 67.5º. This is where your 45º comes in. The 45º is the inside angle at the center of the piece of pie.

Here is how you can see it in real life: draw a stop sign, then draw lines in from each corner to the center - kind of like cutting it up like a pie. Those corners that intersect in the middle of the pie are the 45º angles (NOT 45º CUTS THOUGH!!), and those outside corners where each slice comes together at the edge of the pie are the 67.5º angles.

Mike - just use my angles and their measurements and you should come out all right!



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