Need Suggestions Re: Fastening Canopy to Dome

[melodiousdirge]

I'm building a 3v 5/9 dome for the 2011 burn. The skeleton is done, and now I'm working on a canopy. The canopy is made of 16oz white billboard tarp cut into polygons and seamed together into a geodesic shape to fit inside the skeleton. Yes - inside. I figured it would be easier if I could spread the 100+lb canopy out under the skeleton and then pull the vertices up via fasteners and hook them onto the skeleton (rather than try to heave a canopy over the top).

My mind's eye vision for fastening the canopy to the dome right now is to install a heavy duty 1/2" grommet at each vertex in the canopy. On the inside of each grommet I'll put a 2.5" diameter disc, cut from 3/4 inch plywood. The disc is mainly just to spread the load out over a larger area than just the grommet. Now originally I envisioned a 4" long bungee cord, knotted inside the dome, on the inside of a hole drilled through the plywood disc. On the outside end of the short bungee would be a hook that went through an eyebolt at each vertex.

So that was my vision, but It's been hit with some roadblocks
1) 3/8" Diameter Eyebolts are anywhere from $1-$3 each, so this is a bit pricey. If it ended there it wouldn't be so bad, but:
2) 4" long bungee cords are not that easy to come by. I considered buying the cord and making the bungees myself, which would be tedious; but I can't find the bare hooks to use to make the cords.

I looked at bungee balls - packs of 100 on amazon are dirt cheap, and they would not be supporting much load each, but if I put the ball on the inside of the canopy and I have a loop on the outside, I'll have to either use hook screws at the vertices (even more expensive than regular closed eye screws) or else use nylon zip ties to hold the loop to each vertex (serviceable, but pretty crude).

I bought 100' of rope to use just in case, but this is also crude, and tedious. I also would like to have the anchor points to have a slight give to them if possible to allow the canopy to distort a bit in the wind if it needs to (this is really just engineering masturbation though, I'm sure a solid fixation will be just fine).

Anyone have some thoughts I haven't already gone over? I'm thinking maybe I can get some S-hooks and make all the bungee points from bulk bungee cord, knotting one end around the S-hook. Or even S-hooks with bungee balls; though I'd worry about these flapping loose in the wind.

[Igneouss]

You have the plywood circles with a hole in the center. You have the gromets in the dome cover. It would take two people but simply run a zip tie through the plywood and gromet. It goes around the external dome frame. Back through the holes. Inside, simply but anyhting through the zip tie loop. Nails are cheap. A bolt would work. A piece of dowl. Then snug up the zip tie.

Bring a small pair of wire cutters for dis-assembly. Snip the zip ties.

Done deal.

[some seeing eye]

Good design, on playa pics! Our 24' dome cover was about 500# and a &*# to get on and off top down. A lot of hardware is poorly made today. But some is professionally rated, like item A http://www.webriggingsupply.com/pages/c ... ngs.html#A and item E http://www.webriggingsupply.com/pages/c ... ngs.html#E. Both would work to join the loop end of the ball bungie to the machine screw eye vertex bolt. Quick on and quick off. You will save a lot of money from year to year by cleaning dust off your hardware to prevent it from rusting up.

I am also a huge user of zip ties on playa, and they can be doubled or tripled for strength. The bungies have the advantage of absorbing dynamic loads, like when the wind gusts to 60.

[melodiousdirge]

http://www.webriggingsupply.com/pages/catalog/hardware/chainfittings.html#A

Hah! I forgot about that type of spiral hook, those might just be perfect! I was thinking more about it last night and I'm thinking I'll just buy 100' of stretch cord and cut it into 12" lengths. Double it over and knot at the free end. This would give me a heavy duty bungee-ball like tether. I'd stick the loop through the plywood and grommet and knot it again so it couldn't fall out. Then with either a modified S-hook or possibly those spiral hooks, hook it over the vertex. Still thinking about it though

Igneouss: Thanks for your thoughts; I'd kind of run down that pathway with it a little bit, and it seems like it would work OK, but it would be annoying to put together and take apart, and I don't have a lot of patience for that. I'd much rather spend 10 hours now making it easy to put up than 5 hours on playa messing around with it.

[Igneouss]

http://www.webriggingsupply.com/pages/catalog/hardware/chainfittings.html#A

Hah! I forgot about that type of spiral hook, those might just be perfect! I was thinking more about it last night and I'm thinking I'll just buy 100' of stretch cord and cut it into 12" lengths. Double it over and knot at the free end. This would give me a heavy duty bungee-ball like tether. I'd stick the loop through the plywood and grommet and knot it again so it couldn't fall out. Then with either a modified S-hook or possibly those spiral hooks, hook it over the vertex. Still thinking about it though

Igneouss: Thanks for your thoughts; I'd kind of run down that pathway with it a little bit, and it seems like it would work OK, but it would be annoying to put together and take apart, and I don't have a lot of patience for that. I'd much rather spend 10 hours now making it easy to put up than 5 hours on playa messing around with it.

No worries. There's always lots of good ideas - ways to skin a cat.

[Zang]

Zang here, structural engineer for Suspended Animation. We have a lot of experience in tying things to other things, and in particular I have a lot of experience in structural rigging -- including helping other camps tie off their tarps so they don't shred.

Your biggest problem won't be how to secure the grommets. Your biggest problem will be avoiding what's known as static rigging. Static rigging is where you connect to the grommet and then make a single straight line to your structure. What this does is create a yank point on your tarp, and it will slowly degrade and ultimately shred. Using shock cord (a.k.a. bungee) helps with this, until some serious wind comes up and it hits the maximum stretch on the cord, and then you have a yank point again.

The solution is to borrow from nautical rigging, and go what what's know as running rigging.

Imagine you have a tarp with 6 grommets along the edge. The static rigging solution is to connect six individual lines, one at each grommet, and then connect that to your structure. The running rigging solution is to use a single line, tied off to the structure at one end. Then, run the line through a grommet and back again to the structure; run it around a pipe in your structure, then back to the next grommet, then back around the structure, and so on. The result should look something like this: WWW

It is important that you don't tie off the line at any point. You simply want to run the line through the grommet and back again; then a half loop around your structure and back again. This allows the line to shift a bit which is exactly what you want. When you tie off all the edges of your tarp this way all the force will be equally distributed across your grommets, and the shift in the line will help absorb the shocks. Put plenty of tension on the lines when you tie them -- slack in the lines will allow your tarp to flutter and it will yank free.

I strongly recommend paracord for this application. It's cheap, strong, light, and nice and stretchy.

Two final thoughts:

1. I've seen structures use both bungee & ball ties, and running rigging. The bungee & ball solution failed catastrophically; the running rigging stayed solid. A lot of people swear by bungee cord but years of experience on the playa have proven the running rigging solution to be far superior. Trust me.
2. Trying to explain this in text is a little hard, so if you're not 100% on how to do this drop me a line at zang (@) suspendedanimationcrew.com and I'll gladly explain in person.

[melodiousdirge]

Now THAT is some good information; thanks a bunch Zang. I'm a mechanical engineer myself but I don't have much experience with rigging.

What you've said is great info and I can think of many professional products that use the running rigging method you've described (trampolines, tinted welding barriers, military sun shields). The issue I have is that my canopy will be made up of many polygons, cut and seamed together to form a geodesic shape. This fabric geodesic dome will be suspended inside a steel skeleton by tie points at each vertex. I envision the finished product to look similar to this:



So you can probably see, using running rigging might be possible (I'll scratch my head on it for a while, maybe I can find a way to weave the rope through the canopy) but it's unlikely to be easy. The reason for making the canopy this way is for ease of setup (don't need to heave a 100+lb canopy over a 10+ foot tall dome).

Actually, now that I think about it, I could thread eye-screws into the plywood circles, and possibly use running rigging through the eye screws and around the vertices... hmmmm, you have given me much to consider.