Building the octagon....

Ideas, advice, tips, and tricks regarding shelter, shade, tents, and camping. Yes, this includes RV's too.
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BetaBox
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Building the octagon....

Post by BetaBox » Thu Feb 05, 2009 4:44 pm

So I want to build a nice STURDY tent. Here is my scale model.
Wood dowels=2x4
straws=10ft 3/4" emt
rope=used 8ml climbing rope or paracord or something.
Walls are five feet high by 10 feet long. there is a center pole that is 10ft high. Fabric is not too much of a concern for us (10.1 canvas).
The cover will be held down on the EMT with snap clamps and some with grip clamps distributed.
J stakes or the equivalent will hold down the bottom ring of EMT as well as an anchor point for the neighboring post.

My question is, do you think these types of designs can handle the 80mph winds. It's like a cross between a circus tent, medieval tent, and a desert tent. I'll post more when i have done more work on the real thing or the model cover.

The square footage will be about 480 ft^2. Less than a dollar a square foot for a tent this size is not too bad in my book.
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penguin
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Post by penguin » Thu Feb 05, 2009 5:44 pm

I'm not a structural engineer, but IMHO I'd say you two potential points of failure are the fasteners pulling out of the ground or the 2x4s snapping. I'd hate to have you build it my way and have it blow away (or fall down) but from what I see with your model I'd probably replace the 2x4s with galvanized fence post, bolt the EMT to the posts and drop all of the criss-cross ties (but keep the bottom EMT stapled to the ground as well as the out side guy lines (which could be extended to the center pole through the galv pipe).

Like I said, not an expert by any stretch.

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Sensei
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Re: Building the octagon....

Post by Sensei » Thu Feb 05, 2009 5:45 pm

BetaBox wrote:My question is, do you think these types of designs can handle the 80mph winds.
I'm having trouble imagining ANY tent with vertical walls enduring 80mph winds. I think the gusts usually top out at around 50mph+ or so.

Dog help us if Black Rock City were to be hit by 80mph winds - poor ravers would be flyin' everywhere! Just imagine the sensation of getting hit in the head with a 2' disco ball moving at freeway speeds!

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ygmir
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Post by ygmir » Thu Feb 05, 2009 5:47 pm

you might check out do it yourself yurt designs.....they're pretty good in wind.........
there's probably a page with dimensions and everything.
or,
maybe someone here is an expert.......

or,
put a pickup on the roof.
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Bob
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Post by Bob » Thu Feb 05, 2009 6:40 pm

Not entirely sure what you're proposing, but I suggest building one wall out at the beach or on open ground and seeing how it performs in the wind.
Amazing desert structures & stuff: http://sites.google.com/site/potatotrap/

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Post by mdmf007 » Thu Feb 05, 2009 8:02 pm

Bob wrote:Not entirely sure what you're proposing, but I suggest building one wall out at the beach or on open ground and seeing how it performs in the wind.
Another way to do the math for sail force is here -
http://www.sailingusa.info/cal_wind_load.htm

50 square foot panel with 70 knots (84mph) =1055.95 pounds of Sail Force, on that particular panel. Thats in the open at a perpendicular angle and does not take into account any sort of vortex forces from other objects.

To more accurately figure Sail force on your structure, determine the entire cross section of your structure. Since your building is symmetrical simply draw the cross section to scale on graph paper, with a side view of the roof angles (if any) and figure the area - then plug that figure into the calculator above, with the desired wind force.

I would guess 70 knots would be the absolute maximum I have seen out there, and that would be when the dust devils pass immediately over a single area.

Its the mid sized dust devils you have to worry about.
The little ones are too small (see video) even though they are fast.
The huge ones (see video) cover a large area, but have slower winds overall. But the medium ones, have high speeds, and can cover a whole structure.

Me Roundhouse Kicking a small Dust Devil:
[youtube][/youtube]

and someone elses clip of a huge Bm DustDevil:
[youtube][/youtube]
Whatever you build, anchor the hell out of it - you can never have to many anchors.

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SonOfUgly
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Post by SonOfUgly » Thu Feb 05, 2009 8:55 pm

maybe make the upper "straws" a little shorter so the top slants in abit to help with the winds :?:

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Post by Toolmaker » Fri Feb 06, 2009 10:14 am

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Post by robotland » Fri Feb 06, 2009 11:33 am

Good luck finding drinking straws that big! The point of failure is obviously the bendy part....
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gyre
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Post by gyre » Fri Feb 06, 2009 3:15 pm

You can deal with wind by flexing or being rigid.
The canvas becomes the stress point with a rigid frame.
If you can at least allow the canvas to move over the frame and avoid wear, it will help.

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Oldguy
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Post by Oldguy » Fri Feb 06, 2009 5:39 pm

Check out the DPW blog on this site that shows setting the King Posts in CenterCamp. Cables are strug. square and triangular sails are hung on the cables with plastic ties. The cables carry the structural loads. The sails can be replaced if damaged. There is sufficient space between sails to limit wind loads...Your octagon is very similar to the King Posts.
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Oldguy
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Post by Oldguy » Fri Feb 06, 2009 6:02 pm

On a side note, the ancient Romans used masts around the Coliseum to hoist out sails with pulleys and block-n-tackles to create shade. Look it up.

Old Salts can teach us about fourmasted schooners, and the complex tackle and line work involved. The info is out there. I'm tripping on an image of a cluster of masts with traveling yardarms that can be manhandled thru the day to get max shade...The Beagle' s Yard of Yardarms.
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Post by LeChatNoir » Fri Feb 06, 2009 6:02 pm

I don’t think I’d do away with the cross bracing within each panel. Relying only on horizontal and vertical supports alone could allow the structure to rack, meaning the top rotates and spirals down. Pitching the side walls out would help deflect the winds. Lots of triangular bracing with guy lines would help if the walls can’t pitch out.

I like the idea… seems like it would break down and more or less fit in the back of most pickups.
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LeChatNoir
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Post by LeChatNoir » Fri Feb 06, 2009 6:07 pm

SonOfUgly wrote:maybe make the upper "straws" a little shorter so the top slants in abit to help with the winds :?:
Heeeyyy... now that's a clever notion!

I had just pictured anchorig the canvas sides out a bit, but this would allow for more privacy and shelter if desired.
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Zulegoona
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Post by Zulegoona » Fri Feb 06, 2009 6:37 pm

People in my camp do diagonal bracing with ratchet straps, on their car port, it enables them to adjust the tension easily

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mdmf007
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Post by mdmf007 » Fri Feb 06, 2009 7:09 pm

Zulegoona wrote:People in my camp do diagonal bracing with ratchet straps, on their car port, it enables them to adjust the tension easily
Ratchet straps are dirt cheap too - you can get a 6 pack for like 20 bucks. Looking at your picture as well -

I just noticed that you will need something more substantial at the top instead of 3/4 inch EMT (the straws). With the diagonal tension and the sail loading the EMT is going to buckle.

Dynamic tension is what your looking for with the diagonal straps, or turnbuckles or whatever. The earth holds the bottom in place and the EMT is supposed to keep the tops apart. That length of 3/4 EMT will buckle. I would use 4x4's all the way around, for both the posts and top frames or a more substantial pipe 1.5in black iron would kick ass and you can get all the fittings you need to simply screw it together, but 4x4s are cheaper.

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gyre
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Post by gyre » Fri Feb 06, 2009 7:30 pm

What kind of straps can you get for that?
Where's a good place to buy?
I pay about $14 to $20 for my good straps.

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Post by mdmf007 » Fri Feb 06, 2009 7:31 pm

gyre wrote:What kind of straps can you get for that?
Where's a good place to buy?
I pay about $14 to $20 for my good straps.
And you will keep paying that for GOOD ones - I was talking about the crappy bubble packed ones you buy at Wally World, or at Home Depot when you forget the good ones at home.

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Post by Zulegoona » Fri Feb 06, 2009 7:32 pm

With proper stakes and guy wires on the posts he might not need anything but rope for the horizontals, avoiding as much heavy stuff up high as possible just seems like a good idea.

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gyre
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Post by gyre » Fri Feb 06, 2009 7:38 pm

mdmf007 wrote:
gyre wrote:What kind of straps can you get for that?
Where's a good place to buy?
I pay about $14 to $20 for my good straps.
And you will keep paying that for GOOD ones - I was talking about the crappy bubble packed ones you buy at Wally World, or at Home Depot when you forget the good ones at home.
I need something in between sometimes.
The cheap ones I see usually have no strength at all.
The heavy ones don't flex enough for my light loads.

I want something between the toys and the heavy ones.
A good price would be nice too.

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Post by robotland » Tue Feb 10, 2009 1:51 pm

gyre wrote:
mdmf007 wrote:
gyre wrote:What kind of straps can you get for that?
Where's a good place to buy?
I pay about $14 to $20 for my good straps.
And you will keep paying that for GOOD ones - I was talking about the crappy bubble packed ones you buy at Wally World, or at Home Depot when you forget the good ones at home.
I need something in between sometimes.
The cheap ones I see usually have no strength at all.
The heavy ones don't flex enough for my light loads.

I want something between the toys and the heavy ones.
A good price would be nice too.
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Post by ygmir » Wed Feb 11, 2009 5:52 am

gyre wrote:
mdmf007 wrote:
gyre wrote:What kind of straps can you get for that?
Where's a good place to buy?
I pay about $14 to $20 for my good straps.
And you will keep paying that for GOOD ones - I was talking about the crappy bubble packed ones you buy at Wally World, or at Home Depot when you forget the good ones at home.
I need something in between sometimes.
The cheap ones I see usually have no strength at all.
The heavy ones don't flex enough for my light loads.

I want something between the toys and the heavy ones.
A good price would be nice too.
if you can tie a "truckers hitch", you don't need ratchet straps, and, also, can pick the size rope you use........

just a thought...........
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BetaBox
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Post by BetaBox » Wed Feb 11, 2009 6:23 am

A truckers hitch is exactly what i was going to do. I figure i can get a lot of tension on those ropes with that knot, and it allows for easy fine tuning. I was going to start building it this weekend, but my *possibly* dying 1.5 year old cat just racked up a $700 vet bill last night, so this project might have to wait until next month.

I really appreciate all the comments so far, and I'll write a little more on the subject this weekend.

peace.....

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Dork
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Post by Dork » Wed Feb 11, 2009 7:15 am

Vertical walls are fine - I built a square, flat topped structure the year before last using 1" EMT, silver tarps, economy canopy fittings, and ball bungees. It was rock solid in the wind while everything else got ripped up.

I think you could do without the bottom ring of EMT if you can figure out a way to securely stake down the 2x4s.

You might bump the sides up to 6' or more for greater usability.

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