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# Calculating Post Placement for Circular Fence

#1
01-09-13, 05:55 AM
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Calculating Post Placement for Circular Fence

For his Eagle Scout Service Project my son is building a fenced in area for the local SPCA to be used a dog training obstacle course. It is a very large area. The hitch is the fence must be in a circle so one dog would never be able to "corner" another dog in the event of a problem with the animals.

How do I calculate the spacing of the posts so the fence can run a large circumference of a circle? I don't think I can use the standard 8' unless I somehow curved the cross pieces of the fence slightly (which seems far too advance woodworking for these kids). I don't want the posts too close and waste a bunch of lumber. Is there a standard formula for this calculation?

#2
01-09-13, 06:59 AM
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The circumference (distance around the perimeter) is the diameter (a line straight across the circle, though the center, from one side to the other) times pi (3.1416). So, if you have a field that is 100 feet square and want the circle to totally fill it the circle would be 100 feet in diameter. Multiply that times 3.1416 (pi) and you get 314.2 feet circumference or fence length. Then you can divide that 314 by whatever until you get an even number for your post spacing.

I would not bother with curving the fence rails. Just use straight boards. Once you get more than 8 or 10 sections it will stop looking like an octagon and more like a circle.

#3
01-09-13, 07:16 AM
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Thank you for the quick response. Yes, I knew how to find the circumference. The circumference will probably be well over 600 feet (although I don't have the exact measurements yet). My concern was how to to attach the fence rails flush post to post and make the curve. I guess I can just angle cut the ends of the rails to get flush to the next post. I thought maybe the posts needed to be closer together than normal to make the curve, but I guess I am over-thinking this. Thanks for your response.

#4
01-09-13, 07:29 AM
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I think you are worried too much about it looking like a circle. At 600', even straight boards will meld into a circle, like Dane said. You won't even be able to tell the boards are straight, optically.

#5
01-09-13, 07:46 AM
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And, I would not bother mitering the fence posts. If your circumference is close to 600 feet the angle between the boards and post will be quite shallow and not worth the trouble of cutting angles.

#6
01-09-13, 12:14 PM
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Fence

If you are concerned about the angle between the stringers and the posts, you could use round posts and mitre the ends of the stringers 2.4 degrees to get a good fit where two adjacent stringers meet at the post.

You will need to use the radius of the circle to locate each post.

#7
01-10-13, 07:30 PM
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Actually, wirepuller, his stringer cut angle (for a circumference of 600') works out to 2.398421334 degrees, or (2 degrees, 23 minutes, 54.3 seconds). Meaning 1/32" taper-cut for typical 3/4" stock.

Reminds me of a memorable experience from my early DOT surveying layout days. We were staking the location of concrete drop inlets on a curved street for my first urban bridge job, and (as a newbie, wet-behind-the-ears EIT) I was doing my best to steady the plumb bob while tugging on the steel tape at exactly where the centerline of inlet belonged. The grizzled old project engineer walked up, yanked the tape and plumb bob from my hands, and yelled at the waiting excavator operator "Put it here!" as he paced off the distance from the reference hub and pushed the paving pin into the base course. And then he yelled at me "We aren't building a wrist watch here!"

#8
01-11-13, 04:25 AM
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Accuracy

Thanks for the help, BridgeMan45. My slide rule will not give me that degree of accuracy. I leave the desired degree of accuracy to the builder. This project will get interesting if the ground is not perfectly level.

#9
01-11-13, 05:58 AM
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What, you don't cut to .000000001" accuracy with your chop saw? I suppose it would be difficult to get your saw and the lumber into an electron microscope to see if you're lined up on the mark.

#10
01-11-13, 12:09 PM
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Hey, Dane, I have a friend (metallurgical engineer) who sends me his household project cad drawings out to 10/1000ths of an inch. Pulleeessse, I'm a nail driver. 1/32" is super fine with me.

#11
01-11-13, 03:53 PM
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I just went to the eye doctor for a new glasses prescription because I couldn't see the 1/32" marks and 1/16" has been getting tough. I guess that makes me a 1/8" or a 1/4" person.

#12
01-11-13, 07:11 PM
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Slide Rule?

Wirepuller,

You really need to get out more. They now have these new-fangled gadgets called calculators--for less than \$10, you can pick up a TI-30XA with all of the fancy trig functions, natural and naperian logs, multiple roots, etc., etc. They even add, subtract, multiply and divide if you know which keys to push.

Just for kicks, a while back I took my Post Versalog (slide rule) to an inspection class I was teaching for my former company--when I held it up and asked if any of the young engineers or technicians knew how to use it, one of the engineers timidly raised his hand and muttered he had once seen a picture of one, but never had seen one in the flesh. It was touch-feely for the next five minutes as it made the rounds, with everyone wanting to play with it.

#13
01-12-13, 04:15 AM
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Accuracy

Your points are well taken, guys. I was just kidding about the slide rule. I have not used one since 1967. My only concern was to enable the builder to be as accurate as he cares to be. I would personally tilt my circular saw blade a couple of the degrees an go.

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