How to build a dome ???

TristanX · Post by **TristanX** » Sat Apr 26, 2008 1:07 pm

I am going to build a 3/4 conduit dome this year. I already have my 10 foot 3/4 conduit and I'm double checking the instructions before I start cutting.

I think I've gotten myself confused.

I've been told that the best dome for 10 foot conduit is 17 feet wide (8.5 radius).

I've been to the desert domes page, and I thought I understood everything .. until I saw the TIPS page

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http://www.desertdomes.com/tips.html

Here it says ...

Step 4 ... The magic numbers for a 2v dome cut from 10' conduit pieces are A = 5.3', and B = 4.7'. These numbers allow for holed drilled Â¾" away from the ends. 2v people may now skip to step 5

But in step 5 it says .. Cut the tubes according to the strut factors plus 1Â½".

So is A = 5.3 plus another 1.5 ? or is A just 5.3 ?
So is B = 4.7 plus another 1.5 ? or is B just 4.7 ?

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I found another page

http://www.weasel.com/dome_16.html#intro

Here it says:

As conduit usually comes in 10 foot lengths, I picked a radius of 8.3 feet, because that gave me pieces of 5.1 and 4.5 feet. (These are center-of-hole to center-of-hole distances). When you add in 1 inch (center-of-hole to end of pipe) at each end (there are 4 ends), and allow .2 inches for cutting, you end up with exactly 10 feet. I allowed a little bit extra room between the hole and the end of the conduit so that the hole wouldn't weaken the conduit.

But I don't see how 5.1 + 4.5 + (1 inch times 4) + .2 (for cutting) equals 10 feet ????

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I would appreciate ANY help !!

Thanks

gyre · Post by **gyre** » Sat Apr 26, 2008 5:16 pm

You may be able to get full lengths of conduit.
That would be 21 feet.
I checked on this, but I can't remember my results.
I did find aluminum conduit.
I ended up buying fence post pipe myself.

MikeVDS · Post by **MikeVDS** » Sat Apr 26, 2008 6:02 pm

A = 5.3', and B = 4.7' are the total length with the added bit so you can drill holes. Step 5 is not considering the tip it gave in step 4, it's again assuming that you're designing your own dome, not using their "magic" length. So for most people you have to add extra on what you calculated, if you're using their numbers, ignore that part of step 5.

5.3 + 4.7 is 10 feet. If you look at the actual values they are rounded off a bit. This doesn't matter. Basically you end up with negligibly larger angles. A nice math tip is that the SIN of a small number is about equal to 0. So trying to be really picky about the numbers won't change anything.

But I don't see how 5.1 + 4.5 + (1 inch times 4) + .2 (for cutting) equals 10 feet ????

1 inch + 0.2 inch for cutting is equal to 0.1 foot. (remember 12 inches in a foot)
5.1 + 0.1 + 0.1 (I added 0.1 twice, one for each end) = 5.3
4.5 + 0.1 +0.1 = 4.7
4.7 + 5.3 = 10

TristanX · Post by **TristanX** » Sat Apr 26, 2008 7:21 pm

Thank you .. I knew I must have been not seeing the forest through
the trees !!

MikeVDS · Post by **MikeVDS** » Sat Apr 26, 2008 8:39 pm

They do word things in a confusing way. Like 0.1 inch 4 times. It took me a minute to figure out how a strut had 4 ends, then I realized they meant per piece of conduit. I didn't figure out what they meant until I was half way through my post. That is why I prefer to do the numbers myself, if you actually know the details, then you won't mess up because you misread something someone else worded poorly.

TristanX · Post by **TristanX** » Sun May 11, 2008 10:22 am

Just to confirm (ie, measure twice, cut once .. lol)

4.7 (56.50 inches) for one pipe
5.3 (63.50 inches) for the second pipe

Crimp the ends 1.5 inches on each end

Drill a hole .75 from each end

56.50 - .75 - .75 = 55 inches (div 12) = 4.58
63.50 - .75 - .75 = 62 inches (div 12) = 5.16

so from hole to hole = roughly 4.5 (actually a bit "more" but that's ok)
so from hole to hole = roughly 5.1 (actually a bit "more" but that's ok)

Correct?

MikeVDS · Post by **MikeVDS** » Sun May 11, 2008 11:46 am

I calculated all the numbers just to double check for you. Everything looks correct.

To be nit picky there are a couple irrelevant errors. I assume you did them on purpose because they are irrelevant but it also shows I actually did the math too.

4.7 (56.50 inches) for one pipe
5.3 (63.50 inches) for the second pipe

4.7 feet is 56.40 inches. I would have assumed you rounded except you added the extra 0 after .5, which makes it technically an error.

Same with 5.3 feet but it's 63.60 inches.

so from hole to hole = roughly 4.5 (actually a bit "more" but that's ok)
so from hole to hole = roughly 5.1 (actually a bit "more" but that's ok)

That's correct. Desert domes assumed the loss was from the metal loss from the cut. Even if you didn't lose any it's close enough to not matter. You can be pretty sloppy without having problems. Some people on certain designs reduced the number of different struts because some were within an inch of each other so they just made them all the same for simplicity and had no problems.

TristanX · Post by **TristanX** » Sun May 11, 2008 12:07 pm

Yes, sorry my math in the hundredths position was a bit sloppy .. but thank you, you've confirmed the general idea I had going that my calculations are "essentially" correct (with the obvious small variations)

Thank you so much !!