How strong is a dome? (trapeze rigging question)

waivej · Post by **waivej** » Fri Apr 06, 2007 1:42 pm

Hi,

I'm working on a quasi-geodesic dome project for use as an aerialist rig and performance space. Does anyone have advice on what size tubing would be appropriate?

I've built a scale model 1/2" conduit 1V Icoahedron, but am really shooting for 25' of height - and it needs to be able to handle a reasonable amount of weight from 1 or 2 points. (2000 pounds for safety?)

Have you built something similar? How much weight do you think it could suspend? Also, I've been impressed with the way an "icosahedron" can be covered with straight peices (no wrinkles). Do you know if it is stronger/lighter to go with a "higher V" geodesic?

-John

PS: Sorry for double posting this... I accidentally posted it in the share resources page but didn't know how to move it.

capjbadger · Post by **capjbadger** » Fri Apr 06, 2007 1:50 pm

A 1v's struts are going to be too long to be strong for the size dome you want. I'd go with a 2v or even a 3v dome with minimum 1" struts. I think Thunderdome is a 3v, 1 1/4" EMT dome and that supports alot of people climbing all over it plus the people fighting inside.

Badger

Dork · Post by **Dork** » Fri Apr 06, 2007 2:00 pm

I've heard the Thunderdome has smaller conduit inside the 1 1/4" (I've also heard it was rebar) for extra strength. They had problems with buckling when people stood in the middle of sections before, but with the current setup even my fat ass standing in the middle of a beam it barely flexes.

You could get the strength you need with a 1V, but the beams would have to be huge and you'd be limited to where you could attach your rigging. It will be much easier to climb as well, which is nice even if you don't plan on letting your audience climb. You'll want to get up there to attach covers, rigging, tighten bolts, etc.

waivej · Post by **waivej** » Fri Apr 06, 2007 2:19 pm

Badger and Dork,

That's great. I was curious about Thunderdome's specifics. The rebar trick sounds familar. And, that certainly holds enough weight. Making it easy to climb makes a ton of sense.

-John

waivej · Post by **waivej** » Fri Apr 06, 2007 2:26 pm

PS: Any ideas on how to test the feasability of modified dome designs? One of the performers liked the way a 3V Icosahedron model "looked" like a circus tent. (basically just a 3V dome with 1 size strut.)

Gravity Mike · Post by **Gravity Mike** » Fri Apr 06, 2007 3:17 pm

It also depends on where on the dome the force is applied. In T'dome, folks are climbing all over it. If you're talking about fastening trapeze and the like to a dome, you attach at the joints and the extra rebar is likely not needed.

Our dome is 24' diameter, 16' high in the middle, 3 freq, made of 3/4" conduit. We've had 6 people climbing on it at once, quite strong, but you have to step not in the middle of the struts.

Gravity

Toolmaker · Post by **Toolmaker** » Fri Apr 06, 2007 8:32 pm

The type of dome will be one important decion to be made before claculating your needs. As mentioned a 3V is much better than the 2V for holding the weight. The choice of material for your struts is where you get alot of your strength. If you used solid bar stock or rebar it'll handle a ton more than with emt. The machinery handbook and a few other books have most of the formulas you need to work with when you decide on your strut material. Some wiki to get ya started..

http://en.wikipedia.org/wiki/Strength_of_materials
http://en.wikipedia.org/wiki/Tensile_strength

A welding thread here in case you want to weld your own stuff. Might be some other stuff on a few of the websites in there engineering related.
http://eplaya.burningman.org/viewtopic.php?t=17123

gyre · Post by **gyre** » Fri Apr 06, 2007 11:23 pm

If you can find a copy of Domebook it might be useful too.
I have one somewhere.
I should have gotten Fuller to sign it.
It didn't occur to me.

Bob · Post by **Bob** » Fri Apr 06, 2007 11:52 pm

Thunderdome started with a 3/4" EMT conduit dome (35' dia) and upgraded to 1" conduit a few years ago. Contact Pacific Domes in Ashland, Oregon.

robotland · Post by **robotland** » Mon Apr 09, 2007 5:33 am

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

-Especially the Dome Calculator. I've used that rebar-in-conduit trick myself....DON'T use smaller-diameter conduit scraps. You are VERY wise to build a model when considering alterations to the basic dome shapes, given that radically different struts will create uneven distribution of load. I've done a number of experiments with removal of struts to create doorways and merging domes with spheres, and would even go so far as to suggest a LARGE model or even full-scale prototype, to be tested off-playa.
One way that you could alter the shape of the structure without losing the basic integrity would be to add non-load-bearing struts. Pick any three vertices and bolt struts to them, joining into a pyramid. Or two struts connected vertically, and then repeated around the circumference and stabilized with long horizontal struts or cable.

waivej · Post by **waivej** » Mon Apr 09, 2007 7:49 am

Gosh,

Thank you! That list of welding links is downright amazing. I've been welding for years, but learned things by reading through that post about them. DesertDomes.com is awesome too. I based my 1/2 scale model on reading Tara's great information(desertdomes.com). It was made of 1/2" conduit and was strong enough to do pullups, but I wouldn't dare climb it.

Right now I am leaning toward some sort of egg or silo shape. Getting 25 feet of height becomes a daunting challenge for a dome. It just gets huge and expensive. But it was extremely inspiring to hear that the 3/4" conduit dome held 6 people at once (and how you want to climb it to maintain). That's exactly the sort of real world experience I was excited to hear about.

-John

PS: An engineer friend is analyzying the forces. He seems to think 1V presents a lot of load on the top "ring" of 5 segments. But he'll let us know what size tube to use to make it strong enough. He also liked the idea of the egg shape as it would transmit alot of force down. Though that seems to make the surface alot harder to create.

robotland · Post by **robotland** » Mon Apr 09, 2007 9:58 am

I actually created an eggshaped geodesic while fooling around...uh, I mean, experimenting with a 6' 2V sphere. I just used slightly longer struts around one "pole" of the sphere. Probably changed the dynamics, but not VERY much.
As long as you can keep people off of the struts and on the vertices, a 2V (my personal default) will hold LOTS of weight- My current project consists of two conjoined 6' spheres stacked on a 9' dome with all of the weight on the five points of the dome's top pentagon. (Not six points, since that pentagon is inverted to give more room in the middle sphere.)

waivej · Post by **waivej** » Tue Apr 10, 2007 12:37 pm

Robotland,

That's cool! I liked the image of the egg shaped dome and the triple dome that you are working on.

It sounds like a 3/4" 2V dome stretched a little vertically might be strong enough for what I am trying to create. Hmmm...

-John

robotland · Post by **robotland** » Wed Apr 11, 2007 5:44 am

You've got me intrigued. It sounds like you'll be hanging a trapeze in it, which certainly presents unique problems...

waivej · Post by **waivej** » Wed Apr 11, 2007 9:15 am

Robotland,

Yeah, a trapeze, lyra, fabric, spanish web, etc. Most only require a single rigging point in the center. Though a trapeze can benefit from two points about twice shoulder width apart.

A "real" portable rig starts at $1200 for a basic good one (25 feet tall). But, we're looking to create a space that has shade, protection from mosquitos (here in CT), and something interesting to look at, etc. (It could end up being just a "tent" over a real rig.)

On the practical side. My girlfriend is in a professional track school for aerialists and there seem to be lots of people with a budding interest but few venues/rigs at BM. (There were maybe 4-5 that we knew of last year.) She's also certified to teach and we keep seeing people talking about "bringing their trapeze" hoping to find a rig and other aerialists at BM (on tribe, etc.).

-John

gyre · Post by **gyre** » Wed Apr 11, 2007 9:18 am

There are aerialists at burning man.

robotland · Post by **robotland** » Wed Apr 11, 2007 4:55 pm

waivej wrote: Yeah, a trapeze, lyra, fabric, spanish web, etc. Most only require a single rigging point in the center. Though a trapeze can benefit from two points about twice shoulder width apart.

Perhaps a miniature deck up by the ceiling of your proposed dome could serve as an "anchor loft" for various rigs...Suspending it from the topmost six points* would offer more loadbearing capability.

(*Five vertices of the top pentagon, and the top "polar" point.)

Trying to remember who it was that had the BIG dome with the aerial gear last year...Cirque Flambe'? A couple of the BIGGEST domes are made from lovely, ultralight aluminum irrigation pipe. Mmmmmm.

gyre · Post by **gyre** » Wed Apr 11, 2007 8:45 pm

What gauge is that?

waivej · Post by **waivej** » Wed Apr 11, 2007 9:30 pm

Hi Robotland,

That's a cool idea! I'd really love to have a little "crow's nest" to hang out up high. Maybe the center could be a rigid steel "bracket"? I've heard that rock climbers avoid "horizontal" rope angles to anchor points because a mechnical advantage pulls them together with tons of force.
Separate "single" and "double" mounting points could avoid this and save some height.

Do you remember the general area for that big dome? I saw four rigs last year - one huge dome (multicolored bars and no cover) near 2:30 and Esplanade... "circus camp" (red nose district) near the other end, and a couple of open rigs closer to center camp. I heard of 1-2 others...but I didn't see them myself.

-John

PS: I had a chat with my GF and she said that wind is the biggest problem with most of the rigs at BM. I guess the cover could control that...?

PPS: Do you have any experience with sewn nylon? The "earth" dome (39') from last year contracted with a Chinese company to sew a cover out of multicolored nylon (it's on ebay right now). I don't think it was that expensive. My prototype uses shade cloth. It's nice and cool in the sun but lets water through quite easily.

PPPS: I bet the aluminum is expensive...? So far I've priced out galvanized conduit at $4 per 3/4" 10' section at Home Depot and $6 or so for 1".

robotland · Post by **robotland** » Thu Apr 12, 2007 5:34 am

waivej wrote: Do you remember the general area for that big dome?
Hmmm...No, not exactly. One was on the Esplanade, next to where Miracle Grow and the Fear Trap parked...The big irrigation-pipe job was one that I observed during the Parade Of Domes 2005.

PS: I had a chat with my GF and she said that wind is the biggest problem with most of the rigs at BM. I guess the cover could control that...?

If you can wrangle a complete cover, and ways to anchor it, then it should handle the wind problem. But that, my friend, is no easy task. I've never taken on a challenge of that magnitude, but have observed the effects of windload on my own structures.

PPS: Do you have any experience with sewn nylon? The "earth" dome (39') from last year contracted with a Chinese company to sew a cover out of multicolored nylon (it's on ebay right now). I don't think it was that expensive. My prototype uses shade cloth. It's nice and cool in the sun but lets water through quite easily.

Shadecloth won't stop the wind, but the wind will have less to get a hold of. In the past four years there hasn't been enough rain to make the ground dark, so dealing with watershed shouldn't be a high priority.

PPPS: I bet the aluminum is expensive...?

I'm assuming they got it as scrap, which fluctuates between a buck and two bucks per pound these days. Around here, anyway. The irrigation pipes were amazingly light and tough, and could have been of a costlier alloy.

So far I've priced out galvanized conduit at $4 per 3/4" 10' section at Home Depot and $6 or so for 1".

That's a pretty good price, really...I've been seeing more like $5 for the 3/4" lately.