half inch vs three quarter inch for 24 foot 3V 5/8 dome?

[PocoJoe]

Hi - designing a dome for this year, and comparing notes on various web sites for construction guidelines.

I would like to construct a 24 foot diameter 5/8 3V dome; I calculate that there would be ~385 lb of steel if constructed of 0.75 EMT, 252 lb if constructed of 0.50 EMT.

For comparison, I calcuated that a 2V 24' diameter dome constructed of 0.75 EMT would weigh 214lbs; and 312 lbs if constructed of 1" EMT. Adding 25% estimate for the addtiional steel to make a 5/8 would make the estimates 270 lb if constructed of 0.75 EMT and 350 lbs if constructed of 1" EMT.

Comparing the amount of steel in the two designs suggests that using half inch EMT in a 24 foot 3V 5/8 dome should scale the same as using 0.75 in a 2V of the same diameter. The larger number of vertices and shorter leg lengths for the 3V would seem to make it inherently stronger.

There seems to be little use of 0.50 EMT in construction of domes of any size, but these calcs lead me to ask why.

Has anyone tried to construct a 24 foot 3V 5/8 from 0.50" EMT and can share their experience? (other than worrying every time someone started to climb on it?)

Thank you in advance-

[illy dilly]

I can't answer you question. But, I think your camp and our camp are probably building almost the exact same dome.

Our thoughts were just go with 3/4", our calculations were that we were going to need 86 sticks of EMT at about 4.4lbs is about 380lbs.
Out next thought was 'damn I thought a 24' (diameter) dome would way a hell of a lot more than that' and we didn't put any more thought towards weight.

[Bob]

Anything less than 3/4 will be crushed like a bug in the wind out there, and 3/4 is borderline for anything bigger than what you're building.

[capjbadger]

Bob is right here. Once you get beyond the "standard" 2V 16' dia dome, you need 1" (or more) EMT.

The 1/2" stuff will not hold up. Don't even think about it.

-Badger

[illy dilly]

Anything less than 3/4 will be crushed like a bug in the wind out there, and 3/4 is borderline for anything bigger than what you're building.

Thats pretty much what I've read. I read some place (I believe 'Dessert Domes') that 3/4" should be fine if none of the vertex to vertex are more than 5'. So yeah, we are pushing the limit.

[PocoJoe]

Thanks- experience beats theory every time!~

[PocoJoe]

In reading about domes, people say over and over "stick your feet at the junctions, not in the middle of the pipe". I went online to find out what I could regarding the structural properties of EMT conduit, and found an online reference regarding buckling characteristics.

http://www.kineticsnoise.com/seismicboo ... 20data.pdf

The design recs here seem to suppoprt other people's experience- assuming Force Class II (251-500 lbs) the maximum span for 1/2" conduit is 3.14 feet, for 3/4" is 5.13' and for 1" is 11.75'.

Class III loads (501-1000, this would be somebody not sitting quietly but climbing actively) has span limit for the emt of 2.22" for 1/2"; 3.63" for 3/4", and 5.88' for 1".

This sure fits with Bob and CapJBadger's recommendations.

[b]So--- Next question- since people tend to climb on the horizontal elements, has anyone made EMT domes that use 1" for the horizontals and 1/2" for the verticals?[/b] would sure save a lot of weight and money!

[capjbadger]

In reading about domes, people say over and over "stick your feet at the junctions, not in the middle of the pipe". I went online to find out what I could regarding the structural properties of EMT conduit, and found an online reference regarding buckling characteristics.

http://www.kineticsnoise.com/seismicboo ... 20data.pdf

The design recs here seem to suppoprt other people's experience- assuming Force Class II (251-500 lbs) the maximum span for 1/2" conduit is 3.14 feet, for 3/4" is 5.13' and for 1" is 11.75'.

Class III loads (501-1000, this would be somebody not sitting quietly but climbing actively) has span limit for the emt of 2.22" for 1/2"; 3.63" for 3/4", and 5.88' for 1".

This sure fits with Bob and CapJBadger's recommendations.

So--- Next question- since people tend to climb on the horizontal elements, has anyone made EMT domes that use 1" for the horizontals and 1/2" for the verticals? would sure save a lot of weight and money!

Hmm... From my experience with 3/4" EMT, a 5' span will buckle at about 250lbs (in the middle). 500lbs seems rather high.

I've never seen anyone mix EMT dia. in a dome. The problem with your theory is when climbing, you press against those "vertical" struts to climb. If you make them thinner, they will get bent.

-Badger

[illy dilly]

Hmm... From my experience with 3/4" EMT, a 5' span will buckle at about 250lbs (in the middle). 500lbs seems rather high.

I've never seen anyone mix EMT dia. in a dome. The problem with your theory is when climbing, you press against those "vertical" struts to climb. If you make them thinner, they will get bent.

-Badger

I don't know why this just now made me think to go test all this out.
We have a wearhouse with all sorts of different random lengths of different sizes of EMT. I'll go grab some 5' lengths in different diameters, lay it accross two cinder blocks and try standing on it! DUH! (I hate when I over look the most obvious ideas)

I would think that mixing diameters would get super confusing. Since in a 3V 5/8 you have plenty of each kind of strut A,B, C some of each going vertical and some of each going horizontal that it would get really confusing.

[PocoJoe]

Hi Illy- great idea! let us know- thanks

[some seeing eye]

This is a strange thread - It's like someone trying to convince themselves to do the wrong thing and everyone is telling them it's the wrong thing, but they won't take no for an answer. What's the difference in price - $150? 130#? 130# is a person and a half water supply. Whatever. But consider, when you go to sell your dome, after a few years of BM, who is going to buy or even want free a janky mixed tubing dome?

Dome frames are easy it's the cover that's hard, unless you use a parachute.

[PocoJoe]

I agree the thread is strange! I didn't realize conformity was so highly valued. When I look under my house, there are 2x4, 2x6, 2x8 elements. I have to wonder if the only reason all the struts are made of the same diameter conduit is a) it is prettier, or b) because it is assumed all the loads on all the stuts are the same. When people start sitting on the thing, the loads are no longer along the long axes of the struts and it is going to fail by buckling at some point. It makes sense to me to beef up the parts that are likely to fail.

[illy dilly]

What's the difference in price - $150? 130#? 130# is a person and a half water supply. Whatever. But consider, when you go to sell your dome, after a few years of BM, who is going to buy or even want free a janky mixed tubing dome?

Dome frames are easy it's the cover that's hard, unless you use a parachute.

From my calcs the cost difference is about $260 bucks. And the weight difference is about 215lbs. Thats all numbers rounded pretty generically. Are either of these numbers gonna make or break the bank in the end... until we've paid for our cover and know exactly how much its gonna cost us.. it might.

I can't imagine that anyone is actually considering mixing pipe sizes. Thats just nuts!

[some seeing eye]

http://www.byexample.com/library/calculators/geodesic_dome_calculator/?domeRadius=12&domeUnits=feet

Desert Domes is also a great resource! They make dome frames showing you the value of the labor for making your own.

For a 24' diameter 5/8 3V the lengths line up with 10' pipe lengths, vertex to vertex - the OP has that down. That's ~83 lengths of 10' pipe and some spares. Home Depot online $180 1/2", $359 3/4". About 2x difference. Save costs by ordering as a contractor at an electrical supplier.

EMT weights 30 and 46# per 100 ft for 1/2 and 3/4. So 260# and 399# total assuming the tubes extend 1.5" beyond the vertex. OP calcs are similar.

Builders can do what they like, but I am going to be more inspired on the playa by an innovative cover and even more so by great creative people. than mixed pipe.

[Bob]

Urm, EMT is designed for pulling wire. It's not designed to be structural.

[PocoJoe]

Found a cautionary tale in the archives of a camp that had a 30ft 4V half inch conduit dome that went for a ride in the wind - apparently a sail and no anchor is part of the story. Six different chord lengths, all under five feet in length- half inch conduit. Something like a hundred and fifty sticks of EMT- a really big project.

viewtopic.php?p=258504

Yikes! Glad no one was hurt.

The conduit is structural but in a limited sense- it is designed to carry the weight of wires inside it. For the 24 footer in quesition, the 86 sticks of conduit are designed to carry 700# of wire for 3/4" and 385# for 1/2". Not the same as a live load of wiggling humans, but a guideline.

Tarps and windload are huge. For 80 mph winds, the load is 30 lbs per square foot- that is a very large load. For that 30' dome, there is 350 sq feet of wind load in cross section- at 30 lbs per sq ft, 10000 lbs of wind load. I don't care if it is half inch, three quarter inch, or one inch conduit- that is a huge load.

In any case, glad I wasn't on top of Radio Free Valhalla Dome when it went for a ride on the wind!

[some seeing eye]

I have personally seen a 1/2" EMT dome crushed by a windstorm on the playa. I think it was about 9 or 10 and about C in 2008 or 2009. The beauty of Pacific Domes is that they are structurally engineered. An advanced engineering student with access to the right software is another approach.

[illy dilly]

Desert Domes is also a great resource! They make dome frames showing you the value of the labor for making your own.

For a 24' diameter 5/8 3V the lengths line up with 10' pipe lengths, vertex to vertex - the OP has that down. That's ~83 lengths of 10' pipe and some spares. Home Depot online $180 1/2", $359 3/4". About 2x difference. Save costs by ordering as a contractor at an electrical supplier.

EMT weights 30 and 46# per 100 ft for 1/2 and 3/4. So 260# and 399# total assuming the tubes extend 1.5" beyond the vertex. OP calcs are similar.

Builders can do what they like, but I am going to be more inspired on the playa by an innovative cover and even more so by great creative people. than mixed pipe.

Some Seeing Eye, we used the Desert Domes Calculator too! I agree great tool!
My calcs came out a little different. But not by a ton. Also, I added a bit of cost to the Home Depot costs.
I work for a low voltage (telephone, MATV/SMATV, A/V, Broadcast cabling) contractor here in Denver. And I feel that, Home Depot has pretty competitive prices on EMT, with out having to fill out a credit app and set up an account, and buying in numbers under 1000'. Depending on where you are electrical suppliers may be willing to give whole sale rates with out tax IDs, credit apps, and smaller quantities. Our wholesale rates are about $38.85/1000' for 3/4" and $69.30/1000' for 1".


Are my calcs, with the weight and price marked up a bit from Home Depot. Though, I can't seem to figure out how to rotate that image. On my photobucket it shows it horizontal, but keeps loading it on here vertical, hum?