Ah the almightly dome

[LionsNzebras]

So we are still digging and researching into domes.
Have not settled on a frequency or size.

We were at home depot yesterday, and took a look at local EMT, its a great material vs the PVC they carry and I think that is the one decision we have made. While PVC is light weight, its just not strong enough to even think of hammocks or climbing.

EMT is the way to go for us...

Now with that said, how large can my dome be before I have to go up in frequency?
As an example, a 10 foot 3/4 EMT wont hold a 200 pound man, I would crush that thing like a fat kid on a smarty. My son who is only 60 pounds was able to flex it quite badly when we placed it between the isles only 4 feet off the ground. (yeah I let him climb it in the store muahahhaha)

We did the same test with a 10 foot piece of 1 inch and he could not move it, but it did flex under my 250 pounds.

With that said, I highly doubt that we would EVER use a 10 foot strut (the dome would be massive hehe)

Using the calculator at domeorama.com I have come up with 2 ideas.
v3 4/9 dome with a 16 foot radius. 4 strut types, longest being roughly 7 feet. (EMT costs at 1 inch = 90*11.98 = 1078.2+tax)
OR
v4 5/12 dome with a 16 foot radius. 6 strut types, longest being roughly 5.2 feet. (EMT cost at 1 inch = 1054.24)

a 7 foot 1 inch strut and a 5.2 foot strut are 2 completely different beasts, in the way they will react under stress and load.

The plan is to hang 2+ hammocks or have some type of net at the top to hold gear, but that will also depend on the height of the dome.
The above domes are 13 feet and 11.5 high respectively.
The only way to make this higher is either to go bigger dome, or to change the shape. HOWEVER....
We are trying to minimize the amount of stuff we bring out to the playa.

If we go higher, I will need more than an 8 foot step ladder, and traveling all the way from Canada, they are going to look at me funny at the border. Even with 2 8 foot ladders it might be an interesting crossing.


So here are my questions.

1. How do I choose my EMT diameter with strength in mind? From what I understand the shorter the strut the stronger and rounder the structure becomes, but it also becomes a much more complicated design as you go up in frequency. (e.g would be putting a tower on the top of a dome, or allowing people to sleep in a net at the top of said dome, or hammocks, etc etc)

2. We want to minimize weight, and building time, and ladders. But in reality, strength and a plan is more important.

3. At some point we want to add in 2-3-4 domes connected to a much larger dome in the center, do we have to go with the same frequency to build appropriate tunnels between domes?

So far we are thinking a 1 inch EMT, but might depend on size of dome.


I would love the opinions of others on this.

MY LOCAL COST OF EMT:
(Edmonton Alberta, Canada)

1 inch: 10 foot length, 11.98 each
1 1/4 inch: 10 foot length, 18.98 each
1 1/2 inch: 10 foot length, 22.98 each
(you can see why I lean towards 1 inch)
I did not take prices of the 3/4 or the 1/2 as I felt they were too flimsy.
I did not get the wall thickness

Best Regards,
Wally & Christa

[FIGJAM]

If I were planning to hang anything in a dome, I would use ring bolts at some of the junctions where the struts all come together.

Then you would have a ready place to hang whatever.

[LionsNzebras]

Ring bolts are a fantastic idea, thanks!
Never even crossed my mind. I could pick up some climbing beaners too and make it all clip type.

[GreyCoyote]

What you want are ring NUTS.

[Sunbeam56]

If Figjam had ever built, bought, or used a dome, I'd think about it... Otherwise, ignore...

[GreyCoyote]

Now with that said, how large can my dome be before I have to go up in frequency?
As an example, a 10 foot 3/4 EMT wont hold a 200 pound man, I would crush that thing like a fat kid on a smarty. My son who is only 60 pounds was able to flex it quite badly when we placed it between the isles only 4 feet off the ground. (yeah I let him climb it in the store muahahhaha)

We did the same test with a 10 foot piece of 1 inch and he could not move it, but it did flex under my 250 pounds.

With that said, I highly doubt that we would EVER use a 10 foot strut (the dome would be massive hehe)

The longest strut you really want in the Real World is about 5 feet if you plan on assembling it without a crane or hoist. Most vertices in an icosahedron-based dome have a total of six struts (or 5 struts for a very few joints), and holding all of that metal (that's 30 linear feet!) in the air, while inserting a single bolt, and trying to start a nut, while holding yourself up in the air gets **hairy**! Or at least that's my experience.

Generally speaking, a 5-foot long strut needs to be 1" nominal EMT. Note that even at 1 inch trade diameter, while the dome may technically be "climbable", you will want to keep your feet within 12 inches of the vertexes. At a 5-foot span, and a 225 lb load mid-span, you should using 1-5/8 thinwall tube or larger. This is commonly known as "fence rail". Go any less and you're going to bend some struts.

Another point that should be discussed: things like hammocks are really sneaky force multipliers. A hammock that is stretched taught horizontally prior to putting weight in it, puts a whopping big load (far beyond the basic weight of the human) on the attachment points. There are riggers charts online that show the various multipliers, and you would be well-informed to check them out. What they all say however, is the more "sag" in the hammock, the easier it is to support as the force vectors translate from being inward "pulls" to downward moments in the plane of the vertical compressive, which is where domes excel. Conversely, the tighter the ropes, the more the downward load is translated to in "inwards" load, which is difficult for the structure to resist.

Maybe a better, shorter answer for you is this: if you're going much over 30 feet in diameter, and definitely if you're going longer than 5 feet long in any strut type, you need to do some actual engineering. Up to 30 feet diameter is pretty easy to "eyeball", but things get rapidly more complex (and expotentially more dangerous) as you grow much beyond this. You would be shocked at the difference in stresses between a 3V 35 foot dome, and a 4V variant of the same size.

Finally, a note about domes generally: Domes are strange, counter-intuitive structures for the non-engineer. For any compressive force, the end result is a tension (expansion) on the base ring. Conversely, for any uplifting force, the result is compression on the base ring. In a wind, they tend not to "flip-over", but to "slide". The enemy isn't the "pressure" on the face of the dome, it's the "suction" on the lee side. In other words, domes may react differently than you may initially think is "common sense". The ONLY good way to assure your design, and the attachments thereto, will behave in a safe and controllable manner is to model it (or to use someone elses design that has already been modeled).

Anything bigger than 30 feet, or anything that will be used to support people, really needs to undergo this modeling process for safety. And note that safety isn't just about structural issues. You also need to address things like grounding for lightning protection. A dome can be a very effective Faraday cage providing virtually complete protection from a direct lightning strike, or it can be a death trap for its occupants. The difference is how the grounding system was engineered. (Scary bit: I bet not one dome in 20 at Burning Man 2014 was grounded, which is a glaring omission considering the weather we had in 2014!).

This stuff isn't rocket science, but finding-out the hard way your design is fatally flawed in the middle of a storm can ruin your whole day.

[some seeing eye]

Hey welcome to the Burningman DIY world! We are not structural engineers and we are not responsible for any of these suggestions whatsoever in any country. OK!

A common strategy is to size the diameter and frequency to get 2 struts out of a 10 foot piece of pipe. There are also 20 foot metal tubes, but rarely in EMT. EMT is a mass market product with minimal structural spec. Same with fence pipe. Note that the tower dome people on ePlaya used structural tubing and a structural engineer to design their crows nest.

16 foot radius is quite large. I think 12 foot radius is large. Do you need that, or have other uses throughout the year for it? Some rent their domes for various purposes several times per year.

Most people do 5/9 to get the most standing room. As you note, that means you need wheeled scaffolding to erect.

Your tunnels to sub-domes are going to be a pentagon or a hexagon, unless you are crawling through a triangle. You could design your sub domes to use different size pipe remnants from the big dome.

You might need to up your struts around the pentagon or hexagon on the domes where the center of the pentagon or hexagon is missing. The structure is going to be needed to be supported by the domes not the tunnel. The tunnel should be considered non-structural, unless you want to get a structural engineer with software involved. You might seriously look into lock washers at every layer of every vertex if you are taking struts out. Good to check your torque throughout the week on every vertex.

If you are rigging weight from verticies, it is possible, and I have seen, dome sections go from tits-out to tits-in, just from a few inches of rain on the cover. The whole smashed pipe and a bolt design is hugely prone to that. Mirroring Mr Coyote's advice, which is always smart.

You can search my old posts. Your cover is something to think about before committing. Covers with tunnels is non-trivial. A huge consideration is the weight of the cover. How to get it on and off.

You can get a price estimate for covers from commercial sites. You can calculate the square feet of cover against the cover material costs. You can make your own with heavy duty sewing machines, a large space and assistants.

[LionsNzebras]

I am starting to think that the dome design is not a good idea without some practice.

I have no use for a dome other than this, and perhaps a single local event, so to invest thousands of dollars for these events seems pointless, unless its going to be long term.

You guys have both very valid points. I am not an engineer, not even close, Sure I can change my own oil but that does not make me a mechanic by any means!
Perhaps I should build a small one in my back yard so my wife can grow plants or something, just to get a "feel" for it.

my EMT I was going to have bent by a local company that I have ties with, these guys are engineers, but I did not want to spend the 1700 bucks they quoted me to design my dome, they did offer their services and advices at 35/hr for the whole project...Its still an option though.

The one thing I know about "smashing metal pipe" is that a straight smash is a bad idea, a curved smash is a much smarter design.

I did know about the stress on vertices with hammocks, pull vs push, but did not think of a "taut" vs "hanging", which is why I decided to go with a larger pipe, but after doing some reading, pipe size is of little relevance when most of the stress is placed on the "smashed pipe" side and into the bolt itself. Sure the pipe takes some stress but not as much as the connection points, I think that is the entire point to the design is it not?

"Quality" seems to be most important when talking about the materiel itself. a 3/4 inch pipe out of EMT, vs scaffold materiel for example. (sorry don't know the name for the scaffold materiel) and this makes sense.

As for dome Size, my kitchen is 15-16 feet across \ wide, we laid 4 adults on the kitchen floor, used rubber maid containers to simulate space for gear, and decided that it was enough space for a small camp of 4 plus gear and a little eating area. Basically everything was inside the dome, we planned on a little 5x5 space outside to sit the 4 of us, with a little table. and a very basic sunshade secured off poles and the dome itself (have to rethink this) The hammock idea came from an afternoon napping idea, which lead into a larger dome design to sleep 6-8 with gear. (not going to happen now though)
One thing that I just realized is that when I put my calculations into the dome calculator is it says "radius" which is half of a circle. so a 16 foot radius makes a MONSTER dome. 32 feet across, its these types of errors that make this a bad idea hehe. Just have to be careful with the design, have some people say "hey that's a bit big, what do you need 800 square feet for"
I think a 12 foot radius would be more than big enough, thats still 24 feet. and 500 sq feet based on the "spherical radius"
One thing that I am still not understanding is the slope or "usable space" inside a dome, dome calculators use "spherical radius" to calculate square footage. I guess its like a tent, you cant put too much on the edges, except back packs, coolers etc etc?

I have been looking at duck canvas for the covering, and had an idea to fold it in half, secure one half then "pull" the other half over the top with 2 ropes and people at the same time, like a rolling effect. I think it should cost me about 1000 bucks or so to cover the dome, but I have been thinking of air flow and stuff too, something that GreyCyote mentioned is the "suction" of the dome and I totally forgot about how the door creates a draft which literally sucks the dome off the ground and into the air, seemingly to float then flip over and cascade down the field in a tumbleweed fashion, mowing down would be participants along the way. (this sounds horrible by the way, but its obvious some of you have seen it with your own eyes)

I will continue to google and ask what some people may think are silly questions, but if I don't ask, we may have to medivac my ass home and we dont want that.

OR, ill join your camp, and bring my tent hehe. (which honestly is not a bad idea at this point, sometimes I tend to bite off more than I can chew out of sheer excitement)

Best Regards,
Wally

[some seeing eye]

A popular shelter are carports / car canopy. Good secondary market and more uses around home. Check other threads for info.

[AntiM]

A popular shelter are carports / car canopy. Good secondary market and more uses around home. Check other threads for info.

Standard carports usually cannot support a hammock without bending the poles. Just a note.

[LionsNzebras]

The testing has started! yeahhh heaw baby!

2 hunks of 1 inch EMT 10 feet long showed up at my desk at work today, people looking at me wide and full of wonder, they eyed my massive pipes (no not my arms!) I just smiled and said personal project....to which, I got a wink, and a WTF are you doing now......to each their own I replied....they know better than to ask me about my personal life...Dirty adult party fun I told one girl, she said, Yeah I hear you there (as the other guy walked away), "got room for one more" she asked, So I winked back, and said perhaps yeah, just have to have a quick chat with my wife...she giggled and walked away....So I sit here with a grin on my face at 11:07am looking at my test pipes, taking up way more than the space of my cube, amongst the cube farm.....When people walk past my cube I get the oddest looks, no idea why though, THIS SHIT IS NORMAL!

Yeah I had it delivered, free delivery from the local electricians, gota love resources. If and when I order, its going to be interesting with 70 lengths showing up at my desk, I cant wait. Perhaps I should build a dome over my desk made of 1x1 wood, then use a nurf gun to launch an all out attack on my co-workers!

The bend testing will begin in the next day or two, we will form a die for a 5 ton hydraulic press during testing to make sure that bends are exactly the same every single time. Might take a few presses and die changes to get that all sorted out. My first fray into anything mechanical, other than being on the pit crew of a nascar team changing tires and putting on stickers, yeah that was me....(need electronics and stuff done, I am your guy, but dont ask me to grind, cut or chop anything....)

Once the die is set (and paid for), and we have the pressing down, we will test some hole drilling, jig building, then it all gets moved to my house, and preparations start to build endless streams of struts, and other goodies over the -30c winter months.

First few parts of building and testing should be done in my garage this winter, just something simple to test bend radius-es (shit is that even a word) and what not.
First full scale build will happen on my drive way, in April or so. Modifications and other fun stuff designed and built.
First full real world test is transport to FreezerBurn, spending a week living what we have learned on eplaya and ironing out further details.
More modifications and perpetration for the burn in 2015.

Started looking at Duck canvas and what not, from the recommended website, I think that may get ordered in March or so, depending on testing.
They recommended the army canvas, but I am unsure, more research to do.

IT's a rough time line, but looks good, ill document it all along the way and share what I find with everyone.
Be it good or bad, its the plan.

I want to thank everyone that has helped me, guided me, slapped me and pushed me in the right direction.
(FigJam, GreyCyote, Kat, and Remi)

[torrey.smith]

GreyCoyote is a great resource here

Here are some of my build videos for a general sense of dealing with epic amounts of tubing (also some random fun).

2014 fun recap:
2013 fun recap:

Annealing strut ends:
Night annealing:
Flattening struts with a press brake:
Making a waterjet fixture for struts:
Waterjetting struts:
More waterjetting:
Countersinking the holes:
Letter Punching struts:
Making a protractor on the Mill:
Bending to angle:
Waterjetting 18 ft tubes:
Complex Bends:
Tower Extension:

Misc
Covering Stakes with tennis balls:
Proper Medical care on the Playa:
Sextant Camp: https://www.facebook.com/sextantcamp

[GreyCoyote]

Just a few quick thoughts before I crash in an exhausted heap:

1). 5 tons may be marginal for the press. FWIW, I use a 20-ton set at about 7 tons for most of my work. It's nothing more than a Harbor Freight upright 20-ton press with a retrofitted 20-ton air-over bottle jack ram. (Shop air does all the work! No more hand-pumping! And you can dial the pressure to whatever you need and get uniform crimps every time).

2). No need to spend money on dies. Use some thick, scrap plates and make your own. The only real requirements are a depth stop (so you can get standard length crimps every time), keep the plates parallel to each other, and have a radius to the back of the bend for best strength. Note that shorter crimps will be somewhat stronger than longer ones, so use the smallest crimped area possible consistent with cramming 6 struts onto a single bolt.

3). Army duck (also called shelter duck) is definitely what you want for an all-season cover. Ask the folks at BigDuckCanvas.com for a sample of the 10.10 oz with fire retardant. It's perfect for what you're doing.

4). Jigs are critical, but they dont need to be expensive or elaborate. A simple length of kiln-dried 2x4 dimensional lumber cut into sections with a bolt on one end and a place to drill holes on the other works great. Attach this to your drill press and you can get darned near perfect center-to-center measurements all day long. Note that in a freq 4 dome you REALLY want to keep the center-to-center hole tolerances very tight, so spend the time to do it right. (A 4V hemispherical dome has 20 struts on the base ring. Imagine if each hole is off just 1/8th of an inch. The resulting error will be more than 2 inches, meaning the dome will NOT assemble, even with massive persuasion and hydraulics.)

Take a long look at Torrey's links, too. That stuff is just pure gold. It'll be time well spent!

[LionsNzebras]

GC,

1) The press in question for testing is an 80 ton press, they have some levers and gauges on it, with 2-3 different leavers to pull, once its set its all automatic and you better get out of the way. (we have been talking about having it punch the holes for us too but I dont have access to this press without paying. SO...the testing will be just that, a few ends to make sure the die works perfectly.
I will have to do all the work manually in my own garage or another that has access to a good press.
Was looking at a 1 ton press from princessauto.com but I dont think its going to cut it with 1 inch EMT, I was also looking at a 20 ton press, but I have zero use for it after this project that I know of, so we are going to try and see if we can use a car guys press and do all the work in a single day.

2) the die will cost me 35 bucks Canadian for them to machine, it has 2 parts, a top and bottom with springs to separate on its own, very simple. Because we own the shop, they are using scrap metal so I dont have to buy it, they are doing the design during work time (that's where the 35 bucks comes from) and they are doing the machining at lunch for me, for free

We have been talking about adding sides that have a screw or two, that we can adjust for size of materiel. 3/4 up to 2 inch. This might cost me much more though as it requires a lot more precision machine work. I have not decided if I want to spend the money. They quoted me 4 hours total on this, including the cad drawing.

One thing I want, is precision, ends each and every time, regardless of the size of dome I choose to build, or materiel used.

3) Yup army duck from bigduck is what I was looking at, they were extremely helpful. If my dome is 14 foot round, I was thinking of getting a 16-17 foot square piece.

4) The jig we were building is out of 1x2 that fits the current press, it will need adjustments for a garage setup. A couple of horses, a flat sheet of plywood with the 1x2 attached. This attaches to the press base with C clamps.

Thanks guys, you kick ass!
W

[LionsNzebras]

One thing I forgot to ask is about bolts.

So I have access to socket head cap screws and bolts of all types, my cost is very low as we buy them by the hundreds of thousands anyway.

We were looking at "socket head cap screws" last night, with a grade 8 washer, grade 8 nut and grade 8 locking washer for our design.
One thing that sticks out in my mind is the rubbing of the canvas, and it splitting open if you use a regular hex bolt, that would be bad.


While I do realize that grade 8 may be over kill, the cost is not much more for me between 5 and 8.

The "socket head cap screws" (sorry I dont know if this is the right name) are black in color, I was told they would rust over time and its not a big deal.
Carriage Bolts are all fine and dandy, but they have that square on the underside of the bolt, which will press into the metal and possibly strip it, I do like the idea of a round head though.

Thoughts?

Oh the die design is done (cad only). but I dont know how to put images here yet hahaha. There is an image button on the top, just not sure how to use it.

[FIGJAM]

For anything I'm making that has to be assembled and disassembled, I like wing nuts!

[LionsNzebras]

For anything I'm making that has to be assembled and disassembled, I like wing nuts!

Fig, I dont think wing nuts on a dome are a good idea, especially if its going to support PEOPLE or more structure like the tower design, platforms whatever.....
This is one of the reasons we wanted to use a lock washer on the underside.

[FIGJAM]

Depends on the size bolt you use.

With the wing nut and lock washer on the inside, wouldn't it be easier than having to use 2 wrenches on every joint?

I'm lazy, so domes seem like a lot of work to me.

[LionsNzebras]

I figured out the attachment thing:

This design has no springs to open it after press, there is no space for them, this would be a direct attach system to the press, its also a fixed design for 1 inch EMT.

Were looking at building some pockets to allow for 4 springs, and working out the additional cost, but this design will always be for 1 inch materiel.
Were also looking at building in a punch system to make the hole as well, should not be a big deal.

The Die in this picture is 35 bucks with no materiel costs.

EMT_Flat_Side.JPG

EMT_Flat_Front.JPG

EMT_Flat_Closed.JPG

[LionsNzebras]

Depends on the size bolt you use.

With the wing nut and lock washer on the inside, wouldn't it be easier than having to use 2 wrenches on every joint?

I'm lazy, so domes seem like a lot of work to me.

I thought about the wrench thing, I guess it would be a wrench and a socket.
The wing nut would be a lot quicker I think as well, let me talk with the engineer and see what he says.

[FIGJAM]

With regular bolt and nut, I would have one person just put the bolts, lock washers, and nuts on, followed by the "wrencher" who would do nothing but tighten them up.

This would save a lot of time, and the "bolt person" wouldn't have to fumble with the wrenches.

[GreyCoyote]

All of the bolts in a dome are in shear all of the time. The only thing holding that dome together and straight is the torque on the bolts. So they are important, engineered pieces.

Wingnuts are not designed to be used in this type of application. If you consult any of the engineering references you will see that wing nuts are a very poor fastener for structural reasons that those references will fully explain.

As to the apparent necessity of having two wrenches, this isnt an issue. I use smooth-faced carriage bolts and nuts. To assemble, a drill motor fitted with a deep socket is used. If you do your washer stack correctly, the carriage bolt simply grabs and the head does not spin.

As to using high-grade fasteners, this is an unnecessary expense except in certain very highly stressed members where a single failure would be catastrophic (ie, a zipline tower support). For the basic dome connections in an F4, there is massive load spreading and redundancy. When modeled you will find the tubing fails long before any Grade 3 bolt. Use them if they are handy, but they are unnecessary for 99% of the connections.

Finally, bigger bolts are NOT better. You need to balance hole size and peripheral margins. This too is common engineering practice.

[LionsNzebras]

All of the bolts in a dome are in shear all of the time. The only thing holding that dome together and straight is the torque on the bolts. So they are important, engineered pieces.

Wingnuts are not designed to be used in this type of application. If you consult any of the engineering references you will see that wing nuts are a very poor fastener for structural reasons that those references will fully explain.

As to the apparent necessity of having two wrenches, this isnt an issue. I use smooth-faced carriage bolts and nuts. To assemble, a drill motor fitted with a deep socket is used. If you do your washer stack correctly, the carriage bolt simply grabs and the head does not spin.

As to using high-grade fasteners, this is an unnecessary expense except in certain very highly stressed members where a single failure would be catastrophic (ie, a zipline tower support). For the basic dome connections in an F4, there is massive load spreading and redundancy. When modeled you will find the tubing fails long before any Grade 3 bolt. Use them if they are handy, but they are unnecessary for 99% of the connections.

Finally, bigger bolts are NOT better. You need to balance hole size and peripheral margins. This too is common engineering practice.

GC I would agree here 100%, we looked at the shear forces of Grade 5 and Grade 8 bolts, as well as the EMT tube, the Grade 5 bolts are a little more than double the shear forces of EMT.
Grade 8 bolts are just crazy talk for the expense. Here is a copy of our test data. Bolt data was from (https://www.boltdepot.com/fastener-info ... chart.aspx)

Bolts:
Grade 5 Yield Tensile 92,000 120,000

Grade 8 Yield Tensile 130,000 150,000



EMT: Yield Str (Psi) Tensile (Psi)
TYPICAL TEST REPORT EMT UL 797 45,000 – 52,000 52,000 – 60,000

The engineer seems to think that carriage bolts will "rip" (his word) the EMT holes open under stress\load, as the dome flexes. He was talking about the hammocks swinging inside and wind load at the same time causing the stuff to flex and rub, I glossed over at that point, but will discuss it with him again. I have to google half the words he uses ahhahaha.
He recomended the "socket head cap screw, G8 washer, G8 nut and a G8 lock washer). I dont have the costs back yet, but will ask him to let me know what the difference is. I dont think its that much. But if we can save even 25 bucks, thats a good thing.

[LionsNzebras]

I just sent an email for the prices on carriage bolts, nuts, and 2x washers. I also asked him what size he was recommending.

I also remember a conversation we were having last night about the washer size, and I think he is making the washer size = to the edge of the strut (flattened side), something about spreading out load, and that is exactly the point of said washer...so I am told. He also said something about sticking to standard SAE washers as they are built to spec, and not getting into some custom washers due to costs. And that makes sense.

[LionsNzebras]

What about these to hang my hammocks on ?

http://www.sourcingmap.com/12mm-female- ... 42424.html

[GreyCoyote]

I *really* like this engineer. Sounds like a plain-speaking old-school type who doesnt suffer fools. Good!

He is entirely correct. A naked carriage bolt would punch through the EMT under load. We solved that problem with two washers under the bolt head, and a washer under the nut. With those mods the entire thing works nicely. I will post pictures of the bolt and strut stack so you (and the engineer) can see what I am talking about.

One thing (as your engineer will tell you), is it is NOT so important to design a structure for maximum strength, and in fact it is an egrecious error many times to do so. It is however VERY important to design a structure that will fail in predictable and hopefully safe ways! We call this "crashing by design". In our dome, we want the struts to buckle first. This lets the load spread out to the nearby redundant strut members and hopefully prevent, or delay, a catastrophic collapse. Failure of a bolt causes a ripple effect that can produce a catastrophic and uncontrolled failure.

For the record, I am very pleased you have a real engineer working on this with you. There are any number of well-meaning "internet experts" out there whose ideas need to be challenged with some cold, hard numbers from the engineering world. This project is one of those where details really do count, and wild-assed "common sense" speculation can get people hurt or worse. When he takes issue with ANYTHING, please post it. Raw.

Keep pumping this engineer for help. In fact, show him this entire thread and the pictures I will post shortly. When he goes "Hmmm....yup" and smiles, you have a winner. When he laughs out loud, well.... Not so much.

[GreyCoyote]

What about these to hang my hammocks on ?

http://www.sourcingmap.com/12mm-female- ... 42424.html

Yup. Thats the critter. Likely you dont need 12mm, but its the right style.

The best way to hang a hammock with these is to put a carabiner through the ring nut, and put the hammock lines through the 'biner. This makes for a less prone-to-fray interface and lets you take the hammock down in seconds.

[Jovankat]

A naked carriage bolt would punch through the EMT under load. We solved that problem with two washers under the bolt head, and a washer under the nut.

Are you saying the head of a bolt could pull through the hole in the flattened strut under load?
But washers would stop that?

That boggles my mind. Enough that I'm not sure I've understood correctly. But if that is what you're saying I'll believe you because you certainly seem to know what you're talking about but wow. I just.. Wow.

For the record, I am very pleased you have a real engineer working on this with you. There are any number of well-meaning "internet experts" out there whose ideas need to be challenged with some cold, hard numbers from the engineering world. This project is one of those where details really do count, and wild-assed "common sense" speculation can get people hurt or worse.

Yeah this is why I was a little dubious when aronsewell started the thread about building a dome with a tower for his first burn, there's working out how to make a dome and there's working out how to make a dome that will stay a dome. I figured there'd be more to it then was immediately obvious to the layman but it's been really interesting seeing just how much there is. Someone I know was keen to build a dome from scratch for their camp this year, in the end her boyfriend brought a frame and she just made the cover and even that didn't last the week so with every post in here I'm more and more glad she didn't try to build the frame herself.

When he takes issue with ANYTHING, please post it.

Yes please do, this is super interesting, thanks for this you guys. I'm going to go back to watching in awe now.

It is however VERY important to design a structure that will fail in predictable and hopefully safe ways! We call this "crashing by design". In our dome, we want the struts to buckle first. This lets the load spread out to the nearby redundant strut members and hopefully prevent, or delay, a catastrophic collapse. Failure of a bolt causes a ripple effect that can produce a catastrophic and uncontrolled failure.

Oh, I wanted to ask about hanging a hammock off a eye-nut like posted above as opposed to the struts. How does that fit with the idea of having the struts fail first? Wouldn't adding a hammock to the nuts and bolts at the join add to the stress they are under? And if that were to cause a fail it would be the bad kind you described above yeah? Whereas if you hung off the strut and it was too much the strut would just bend wouldn't it, and that's the better kind of failing yes? Or is the idea that as long as you're sure the hardware can support it it's fine?

[GreyCoyote]

A naked carriage bolt would punch through the EMT under load. We solved that problem with two washers under the bolt head, and a washer under the nut.

Are you saying the head of a bolt could pull through the hole in the flattened strut under load?
But washers would stop that?
That boggles my mind. Enough that I'm not sure I've understood correctly. But if that is what you're saying I'll believe you because you certainly seem to know what you're talking about but wow. I just.. Wow.

Yep. Steel is tough, but it's not hard to pull a bolt head through a piece when it's loaded-up. It's also hard to really visualize the magnitude of the forces involved until you've seen it first hand. Remember that bolts that are loaded radially to the centerline are just levers, and as a lever they are a natural force multiplier. Pull sideways on a 2-inch long bolt and the force on the head contact area multiples 20, maybe 30 times. When the unit pressure finally exceeds the strength of the metal, the metal either flows or tears. Carriage bolts are somewhat notorious for this behavior as the head works like a rocking punch.

Yeah this is why I was a little dubious when aronsewell started the thread about building a dome with a tower for his first burn, there's working out how to make a dome and there's working out how to make a dome that will stay a dome. I figured there'd be more to it then was immediately obvious to the layman but it's been really interesting seeing just how much there is. Someone I know was keen to build a dome from scratch for their camp this year, in the end her boyfriend brought a frame and she just made the cover and even that didn't last the week so with every post in here I'm more and more glad she didn't try to build the frame herself.

Domes are basically very simple structures as long as you're using pre-canned formulae to calculate the strut lengths and you keep it under 4V. Starting at 4V, things start to need a little more precision, and this trend continues as the frequency increases. (Ironically, the higher the freq and the *smaller* the dome, the more you have to watch tolerances). Putting a tower on a dome isn't really that hard as long as you understand the forces involved and the limits of your materials. It can, and has, been done many times. But that way lies madness if you're of the "that looks about right" mindset. It needs to be carefully engineered. Some do the math, and others just get lucky. But it's been my experience that physics eventually catches-up with the "lucky".

Oh, I wanted to ask about hanging a hammock off a eye-nut like posted above as opposed to the struts. How does that fit with the idea of having the struts fail first? Wouldn't adding a hammock to the nuts and bolts at the join add to the stress they are under? And if that were to cause a fail it would be the bad kind you described above yeah? Whereas if you hung off the strut and it was too much the strut would just bend wouldn't it, and that's the better kind of failing yes? Or is the idea that as long as you're sure the hardware can support it it's fine?

You ideally want the hammock to apply the load directly to the bolt. That bolt is the center of the struts local universe. All six struts joined at this vertex will share the load fairly evenly when it's done this way. If the bolt is properly sized for the load, it shouldn't fail.

If you were to apply the load directly to the struts (lets say you hook a cord around a couple of struts near the vertex and attach the hammock to this cord), you are heavily loading just a couple of struts while all the other struts coast. And you're *really* working on a strong and asymmetrical shear on the closest bolt, which is likely to fail when the structure moves or you start boinking in the hammock.

If a bolt shears in a heavily loaded dome, it's pretty much game over. The strength of the vertex falls to zero and the dome risks collapse (and you'll be on your ass wrapped in a mesh baggie that used to be your hammock!). But if a strut fails, it simply bends, and in the process much of the load that was on that strut is now transferred to the nearby struts. They rather "support each other" under mild deformation. They may still ultimately fail if the load isn't normalized, but you don't typically get a "zipper-like" running failure. With broken bolts, under the right conditions, this is not only possible, but likely, and the dome is toast.

So. Clear as mud?

[LionsNzebras]

Without getting into a crazy volume of detail, think of a dome as a simple egg shape.

Have you ever seen a full grown man stand on an egg? Yes its possible, but if he even twitches the egg fails, because the force of load shifts off of the CENTER of the egg and crushes it. The bolt design is roughly the same thing.

If your bolt is sized properly, and its spreading load over the 5-6 struts evenly it should never fail, but if you shift said load either right or left, the entire dome picks up the slack to to speak.

Now magnify this with a tower and say a bit of wind, or 4-5 people doing some dancing, the force your talking about is thousands of pounds. Granted bolts are very strong and have 100,000 pounds of shear force but its like a magnifier glass, if your load shifts right, and say its a 250 pound man, by the time that force hits the bolt your talking about 2,500 pounds or 3,500 pounds of force. Take a tower weight and a few people and now your talking tens of thousands of pounds.

With this in mind and some other data I have posted in here, you can clearly realize that the EMT metal will fail long before a properly sized bolt. With that said...a carriage bolt has a square head, this is resting on top of a round hole, when you squeeze them together, the square head bites into the round hole. Now add on say 50,000 pounds of force and that bolt will rip through that metal quite simply.
GC mentioned he solved that issue with an additional washer (and I have not even built a real world design yet, just did a little reading and calculations) The washer spreads the load out further in a ring shape and said load gets moved out further, not to mention a standard SAE is so much stronger than EMT metal which the specs are sketchy at best.

We are using EMT because its available at any store out there, and with a proper dome design, bolt and washer setup, anyone can build a dome safely.

If you take said hammock, and place it on a strut, just off center from your strut, your taking the force and it mostly gets applied to the strut itself, yes you are absolutely right, the strut will most likely bend, but with that bend, you have now compromised the entire EGG shape of your dome, at that point, a properly applied wind could crush it.
Our idea here is for these struts to fail first, to give you time to "get out" of a failing dome, this dome would then stay where it was, if anchored properly and not get blown across the playa and injure people in the process.

One of the reasons I decided to make a Die (which for most people is way way overkill) is that I wanted to understand the strut itself, and work with the engineer to do so. I am a computer guy by trade and dont understand a lot of this stuff. Its people like GC that have guided me and let me do my own reading and to bounce questions off off. He is a invaluable resource to this community when it comes to dome design.
The reason that CUP is in the die design is for additional structure on the strut. If your struts are flat all the stress goes into the smashed part, and we all know what happens then

I hope to continue on with this project, but so far, its been a great learning experience, and a way for a burgin to connect with the community.