Dome Covering Question

[alexcaste]

Hi,

I am constructing a 10' radius (20' diameter) 3v 5/8 dome and am going to cover it in aluminet. I would like the covering to be as snug as possible on the dome. I am trying to figure out what pieces and how big of pieces I need to cover the dome. I'm horrible at math so I thought I would ask here in hope that either someone is better at math or someone has done this before.

Alex

[Ugly Dougly]

Are you planning to cut and sew aluminet?

[some seeing eye]

No need to cut the Aluminet, it stretches over the dome and can be zip tied or rebar tie wired to the struts. Get a ball, like a soccer or basketball. Measure 5/8 of a sphere and draw a line. If your dome is 10' radius at the midline and a basketball is 4.5", your scale is 1' on the playa = .45" or ~29/64" on the basketball

Use 7' width Aluminet that is about 3.15" or 3 5/32" at basketball scale. So make strips that wide and start wrapping. Then you can measure them for an estimate. We unrolled the fabric on the Playa and cut it to size as we went. Leave extra since the great circle wraps will have a triangle of unused material where it meets the ground.

Suggest one wrap of Aluminet around the base, leaving a gap for the door. Then wrap the Aluminet in great circle strips, overlapping double on the arcs facing the sun path. With no doubling you will need about 1300 square feet. Aluminet is about $.25-.32 per square foot.

You could also build a scale model with chopsticks and hot melt glue. For that diameter you could use 1' per 1" or 2" scale.

Your math may vary and mine is not guaranteed.

[Sunbeam56]

Leave room for expansion.
We have a canvas dome- originally designed to be a tight fit, but with first timers syndrome, not so much.
But it expands and contracts radically.

[alexcaste]

@ Ugly Dougly:
We can cut the aluminett there but we're just going to zip tie the aluminett to the frame.

@ some seeing eye:
I think making the scale model to figure out the pieces is a great idea. Thanks.

@ Sunbeam56
The dome expands and contracts? How much is radically?

[Ugly Dougly]

Grommets?

[Sunbeam56]

I dont have empirical data, but it goes from being drum-head tight to loose enough to climb on.
This is canvas, not aluminet.
I would think that any cover would have an expansion factor in the extreme temperature changes in the desert?

So far, our dome cover has gone from 34 degrees. F to 80 ish degrees F.

[theCryptofishist]

I would think that any cover would have an expansion factor in the extreme temperature changes in the desert?

I wouldn't say that. Canvas is a lot stretchier than you'd think. Based on the "net" part of the aluminet name, I'm guessing there's a lot of give... I'd also say that you want aluminet to fit looser and denser rather than pulled taught. More shade that way.

[alexcaste]

@theCryptofishist:
good point. I don't need it to be skin tight I guess I'm looking for more form fitting then taught.

[GreyCoyote]

Different types of canvas exhibit different characteristics. The determining factors as to how the canvas will move are the thread count, coatings, and previous shrinkage. The water content of the fibers then determines to what degree shrinkage occurs within those limits. FWIW, saturated canvas tightens-up, while dry canvas goes slack.

In regards to thread count, you need to look at the number of threads both in the warp and in the weft. These numbers can be different, and if they are, then the canvas can shrink more in one dimension than in another. Another factor is how densely the fabric is woven. Denser fabrics, overall, have better wear and water repellency, but have greater shrinkage issues.

As to coatings, there are any number of UV-stabilizers and fire retardants and fluorocarbon water-repellants that will determine how much, and in which direction, the canvas will shrink. Raw canvas (untreated) is the most dynamic, with additional coatings helping to stabilize the fabric a bit. Note that some makers of types of canvas will treat the individual threads prior to weaving, while others will treat the completed yardage. If you have asymmetric thread counts in the warp/weft, or the threads in one direction are better treated than threads in the other direction, the fabric will not shrink/expand in both axis the same amount. This can be a real problem for fabric you are trying to fit to a compound curve (ie, a dome).

Previous shrinkage is a factor too. Some canvas comes "pre-shrunk", but this still doesn't mean it's stable. Pre-shrunk generally means the first "bite" out of the shrink budget was taken when the fabric was treated after production. It doesn't mean it won't shrink/expand in normal use. Also, older canvas will start to creep more and more as the waterproofing/fire-proofing coatings deteriorate.

Ultimately, it is the water content of the canvas that will determine if it is taut or slackens. Weather drives this process very strongly. Canvas is hygroscopic by design. Wet canvas threads swell (this is what makes canvas naturally water resistant) which serves to draw the weave tighter, thereby shrinking the overall size of a given piece. Dry canvas lets those same threads contract, which allows the opposite effect.

As to how much canvas can change dimensionally, you would probably be shocked: The numbers quoted for the "pre-shrunk" and treated shelter ducks run about 1.25 to 2.0%. Untreated canvas with an asymetrical warp/weft count can easily double that. Putting this into practical terms, the pattern that is used to make a quality fitted cover out of dry 10 oz/yard shelter duck on a 20 foot dome frame will need to be 16 inches longer in the perimeter just to account for full shrinkage when it is wet. On a 30 foot frame, this means the difference between wet and dry measurements will be 24 inches. Go to a non-treated canvas, and you could double those numbers. What this means in practice is an "all-weather" canvas cover that is setup in the desert will be floppy, but that same canvas cover will be snug if it rains. If you're making a close-fitting cover for burning man, you will need to take this into account and change your patterns accordingly.

Surprisingly, if you hold the humidity constant, and just change the temperature, there is less than a 0.5% change in length between 50 and 100F. I found that really odd, figuring cotton fiber being "natural" would be most sensitive to temperature, but that wasnt the case. Humidity is the big dimensional driver.

Finally, as to stretch, canvas shelter duck isn't like jean fabric. Jean fabric is engineered to stretch and continue stretching, whereas duck isn't. When duck is tight, it's reached its limit, and additional loading beyond this point will quickly cause shredding of the seams or a tear in the fabric.

Likely way more than anyone wants to know, but there ya go.

[alexcaste]

Thank you to everyone who replied here. =)