Hexayurt Wind Loads - Why Your Tie-Downs Matter

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Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 2:45 pm

At various times, people have asked just how strong does their yurt have to be? Can we use less strong tape? Do we have to tie it down so well? Do we really need such large/good rebar/pegs/etc.? How much force will the wind put on my yurt?

To have a figure to use as a yard stick, I've taken the two extremes of wind pressure on a vertical wall from a 75 mph wind and applied them to a simple profile for the four most common hexyurts. The graph shows the total resulting wind force in lbs. against the simple profile for 14 lbs./sq.ft. (the force in an open field) and for 28 lbs./sq.ft. (the force possible with wind channeled by surrounding objects/shelters).

For those browsing and not signed in to see the graph, the calculated values of a 75 mph wind on a simple profile are:
8' hexayurt, 96 sq.ft., 1,350 lbs to 2,700 lbs.
12' hexayurt, 160 sq.ft., 2,250 lbs. to 4,500 lbs.
6' hexayurt, 40 sq.ft., 550 lbs. to 1,100 lbs.
6' stretch hexayurt, 64 sq.ft., 900 lbs. to 1,800 lbs.

In practice, the total force on a hexayurt will be less than these values as the hexayurt shape allows for the wind pressure to escape around the sides and the top. I haven't calculated how much escapes, but you can see from the simple profiles in the attached graph that shapes where roof is a greater percentage of their profile will have less vertical wall and should shed more of that pressure.

So while this simple profile method doesn't tell us the actual force we'll experience on the playa, it gives a ball park figure to respect that the forces of wind on a hexayurt can be significant, and that anyone cutting corners by doing less that what has been successful on-playa in a strong wind is taking a risk of their hexayurt being damaged or blowing away a la Wizard of Oz. Some have experienced watching their yurt blow apart and away, while others have arrived back to their camp site to find their hexayurt is gone, with a few scraps of the tie-downs left (and perhaps a few scraps of duct-tape...).

While some of your neighbors will be entertained watching this dis-assembly, the ones down-wind will not be happy - particularly if your building materials at velocity injure anybody or damage or take out their shelter. And those expecting to shelter in your yurt - and particularly anyone inside your yurt during this dis-assembly - will definitely NOT be happy campers.

Please carefully consider following the recommended methods and materials for constructing and securing your hexayurts, and other shelters.


Hexayurts - why your tie-downs matter - 75 mph wind loads.png
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 3:07 pm

Bi-Filament tape (220 lbs | 91 oz.) Holding Strength

Bi-Filament Tape (220 lbs), Holding Strength by Length of Contact, to 12 inches.png


Contact lengths of 39" provides a holding strength that matches the strength of this tape.

Bi-Filament Tape (220 lbs), Holding Strength by Length of Contact.png
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Last edited by Canoe on Sun Jul 21, 2013 3:16 pm, edited 1 time in total.
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 3:11 pm

Bi-Filament tape (220 lbs | 91 oz.) Holding Strength across Hexayurt panel joints, butt & 1" gapped

It's very easy to end up with the tape applied with a 1/4” offset. And even by a 1/2” offset. And if you're rushing on playa, a 3/4” offset is not unheard of. As this reduces the width of the contact strip on one side:
  • that contact strength is reduced and it becomes the limiting strength of the taped joint, and
  • the load on the foil under the tape increases.


FOUR foot panel joints

Bi-Filament Tape (220 lbs), Four foot joint length.png



EIGHT foot panel joints

Bi-Filament Tape (220 lbs), Eight foot joint length.png
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Last edited by Canoe on Sun Jul 21, 2013 3:23 pm, edited 1 time in total.
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 3:12 pm

Different tape widths have different loading on the foil surface of the panels.

Bi-Filament Tape (220 lbs), Loading of Foil Surface under Tape.png
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Last edited by Canoe on Sun Jul 21, 2013 3:14 pm, edited 1 time in total.
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 3:13 pm

Pro-Graff Tape (48 lbs), Holding Strength
Lengths of 12.8" provide a holding strength that matches the limit of this tape's strength.

A good choice for adding strength across a joint where you'll want to remove this tape before disassembling your yurt, particularly if it's implemented with folding tape hinges.
Too narrow to be recommended for joints along the panel width; golden recommendation is still the 6" bi-filament tape.

Pro-Graff Tape (48 lbs), Holding Strength by Length of Contact.png
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 3:50 pm

To simplify:
  • that 6" bi-filament tape is the strongest
  • you can apply it mis-aligned by 3/4" all along an entire length between two panels, and the result is still stronger than 4" wide bi-filament tape applied perfectly equally on each side.
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Elderberry » Sun Jul 21, 2013 3:52 pm

WOW. You've been busy. I have a question on the Gaffer's tape. Do you know if the strength/adhesion is effected by heat? I didn't see anything on that when I visited their website.
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 4:24 pm

Not that much work. I've been sitting on these for the better part of a week, trying to figure out the best way to present and publish. As we're approaching the shipping time limit for people to obtain their materials in time for this year, I decided to put this out as is, even if it's not in an optimum form.

jkisha wrote:...Do you know if the strength/adhesion is effected by heat?...

I don't have any feed back for use of pro-gaff tape in on-playa temperatures.

But you raise a very important point, and one I've been struggling on how to write up. We need to caution that for on-playa use, we're not going to get the full holding strength from tapes. Different tapes will have different strength retaining properties under playa heat. Like most of the 'duct-tapes' letting go under heat and wind load, or some let go just under heat, curl up and blow away (some swear they hear them whimper... or maybe that was their camp mates standing behind them). Some "professional" duct-tapes have succeeded on-playa, and some have not. If you're going to count on any tape, you have to have reliable feedback that they held up under both the heat and the winds of the playa.

The adhesion specification provided by the manufacturers is for adhering to stainless steel or clean steel in lab conditions at room temperature. That's all I can find published.

Between playa dust (or at home, dust or panel cutting debris) on the panel or blown onto the tape just before it's applied, old dirt or finger grease on the panel surface and elevated playa temperatures, we're not going to get the same level of adhesion as is reported in labs. So on a practical basis, we can't know what the actual holding strength will be on the panel's foil surface, but we can see how much less strength various tape widths or tape choices will provide.

So, once again, the tried & true Playa-Tested©)'( trumps everything we can calculate.

Beyond a yardstick between width and tape choices, it can also show us why the six inch wide bi-filament yields the most successful on-playa results. Like it's tolerance to mis-alignment when applied (meaning you don't have to get it aligned perfect to get good strength) and we can expect the same for losing a little bit of square inches of holding due to a little bit of dust. It is interesting to note that a 75 mph wind delivers up to 28 lbs./sq.ft. for 450 lbs. of force on a 4'x4' panel, or 900 lbs. of force on a 4'x8' panel. This 6" wide bi-filament tape, even when mis-aligned by up to 3/4", still calculates as strong enough to take that entire load on a single length of joint, and in practice, that force on the panel will be shared among three joints. So with the decreased holding strength due to the factors noted above, we can reasonable expect that 6" wide tape to do the job. And reports from the playa support that.

To ball park it:
  • the 6" wide bi-filament tape yields strengths exceeding the 75 mph wind maximum loads,
  • the 4" wide shows numbers in the range of those loads, and
  • the 3" wide tape is below those loads.
Which pretty much reflects what people report as what happened on-playa:
  • 6" wide = almost always good (application dust & errors unknown)
  • 4" wide = usually o.k.
  • 3" wide = often not o.k.
And that's with the 220 lbs. | 91 oz. bi-filament tape.
Use weaker tapes and you've got poorer results.
Last edited by Canoe on Sun Jul 21, 2013 4:45 pm, edited 5 times in total.
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Re: Hexayurt Wind Loads - Why Your Tie-Downs Matter

Postby Canoe » Sun Jul 21, 2013 4:39 pm

I was surprised to see how much holding strength was lost in a joint by mis-alignment of tape during application. And how much more significant this was for narrower tapes.

It has an interesting implication for hexayurts made as folding designs, with tape hinges applied and assembled at home. You have the time and clean location to be able to clean and dry the panels, and to make sure they're dust free before applying the tape for your hinge. Both for the primary tape hinge and the tape you put on the back to prevent the primary tape from peeling off. And you can take your time to ensure the tape is applied equally across the gaps for an equal sized contact width on each side.
You should have the maximum possible strength for the tape you chose to use.
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