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Calculating lift of a tarp carport #5436670 05/23/20 11:10 AM
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JHZR2 Offline OP
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Hoping for some opinions from those so inclined.

Have a Cover Logic Monarc carport for the summer. Just to keep some direct sun off of cars in the pm sun.

Set it up and of course we had a windy day (25-45 mph gusts), and it lifted up and dropped. My child observed it, and said it flew up towards the clouds, flipped and dropped back down. Good thing nobody and nothing was out there!

It will be on asphalt, which is why I hadn’t tied it down yet. I don’t think tapcons are right for asphalt.

So... lift. I know it’s somewhat straightforward, but can also be rather complex in terms of lift coefficient.

So what Id like to calculate is how much weight, and how strong of cord, I’d need to use in addition to some sort of anchors, to keep this thing from flying up and away.

I’m thinking that wind in approximation is horizontal flow, and if normal to the arc of the canopy, I guess induces lift (though it’s hollow and so doesn’t really have a different length/distance on top and bottom) as is necessary.

I was going to go along the route of this:

https://wright.nasa.gov/airplane/lifteq.html

With the simplified assumption of CI as:

Cl = 2 * pi * angle (in radians)

Am I overthinking this? Is there a simpler/better way?

Perhaps just an angle derating of the wind pressure calculation P = 0.00256 x V2, where V is the speed of the wind in miles per hour (mph)?

The unit for wind pressure is pounds per square foot (psf)... Thinking then I just calculate times the tarp area, and then * sin(assumes incident angle of wind)?

Thanks!

Stock image from Home Depot:
[Linked Image from images.homedepot-static.com]

Re: Calculating lift of a tarp carport [Re: JHZR2] #5436681 05/23/20 11:26 AM
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ABN_CBT_ENGR Offline
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The actual engineering would be structural wind loading, not lifting.

Used many of these and there is a much simpler solution assuming this is a temporary install.

Not knowing your topcoat or substrate where you want to anchor it- get 6 or 12 inch tent spikes and drive then in and secure.

Then the wind will tear it up long before it flies away.

Re: Calculating lift of a tarp carport [Re: JHZR2] #5436706 05/23/20 11:59 AM
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I had something similar on my deck years ago and did not want to take it down everytime the wind kicked up. Depending on the feet get yourself 6 5 gallon buckets. Cut the tops so they fit all the way into the bottom of the bucket. Bolt the tops onto the feet with some small hardware. Short 6-32 or 8-32 bolts worked. Set the feet with the tops bolted on into the buckets and fill with sand and wet them down. If you want to maximize the space and efficiency bolt the tops off center so the legs are held on the side of the bucket. That way the buckets are mostly outside of the cover. Cost about $25, easy to take down and stores with the buckets stacked inside each other.

Re: Calculating lift of a tarp carport [Re: JHZR2] #5436745 05/23/20 12:56 PM
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From here;

https://www.grc.nasa.gov/WWW/K-12/airplane/lifteq.html

the Variable Cl (Lift Coefficient)-complex dependencies) will be problematic since we don't know what this coefficient value will be.

For starters, I would use a Cl of 1.8 with a purely horizontal wind.


Last edited by MolaKule; 05/23/20 01:00 PM.

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Re: Calculating lift of a tarp carport [Re: JHZR2] #5436755 05/23/20 01:19 PM
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Sail effect- (parachute)

surface area, velocity and a slight collective- depends on shape and primary leading edge ( and any crosswinds) as to how it lifts and moves.

Re: Calculating lift of a tarp carport [Re: JHZR2] #5436765 05/23/20 01:36 PM
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The short answer is the rest of the structure (fabric material and frame) are not designed to withstand the minor wind forces you are describing. If you anchor it down to the level of determining all these calculations is going, you will find the next weak point.

Quick and dirty is to use large party tent type stakes and/or sandbags (like the 70lb sand tubes most hardware stores sell for traction) or the posts set in buckets and strapped in. You'd be done with that by now.

Re: Calculating lift of a tarp carport [Re: JHZR2] #5436838 05/23/20 04:43 PM
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I love the bucket and sand idea!

If you don't want to bang spikes into your asphalt, some 550 cord at a 45 degree angle, anchored up top, and secured off to the side should also hold it down.

You could also get cinder blocks and 1x6s, feed the cinder block up the leg, attach the leg to the 1x6, and let the cinder block(s) rest on the wood.

Re: Calculating lift of a tarp carport [Re: eljefino] #5436910 05/23/20 06:28 PM
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Originally Posted by eljefino
I love the bucket and sand idea!

If you don't want to bang spikes into your asphalt, some 550 cord at a 45 degree angle, anchored up top, and secured off to the side should also hold it down.

You could also get cinder blocks and 1x6s, feed the cinder block up the leg, attach the leg to the 1x6, and let the cinder block(s) rest on the wood.


One problem with plastic buckets and barrels is that they slide on asphalt or concrete. They may keep something "down", but it may still move. A sandbag of the same weight is a better option than a bucket.

Re: Calculating lift of a tarp carport [Re: JHZR2] #5436943 05/23/20 07:17 PM
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Too many non linear variables for the heavy duty math required. It will look like a log table when done for actual loads given wind speed.


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Re: Calculating lift of a tarp carport [Re: sloinker] #5437057 05/23/20 09:34 PM
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Originally Posted by sloinker
Too many non linear variables for the heavy duty math required. It will look like a log table when done for actual loads given wind speed.


That was my first thought too... But I want to get to the ROM point of how much is too much.

My driveway setup is a lot like the HD picture I posted in the beginning. So I dont want a lot of stuff out to the side, blocking access of the other door, etc. And I dont really care for a ton of concrete filled buckets or bags or cinderblocks. If it goes to that pount, aesthetics matter more to me, and Ill spend a few thousand dollars to get a legit carport engineered and built (probably the best scenario anyway).

So I really just want to get a grasp of if I need 100 lbs or 1000 lbs to keep it down in a reasonable breeze (say a gust of 45MPH.

My FIL has a similar structure to keep one of his boats under, and he is in a very windy area. He removes the tarps for a hurricane, but theyre up otherwise and it holds up fine. The difference is he has a concrete pad and has some bigger anchors holding the whole thing down. I have asphalt in that part of my driveway, and so Im not sure if anchors would work, or if they have to be really large to hold properly underneath.

My ideal is a half cinderblock (Lowes sells these near me), on each leg, and the plastic "foot" (which keeps the metal leg 1/8" off the ground) and leg through-bolted to the cinderblock.

This would make the foot only ~12x12, very low profile, and easily moved if I wanted to move it (I do want to be able to temporarily remove one middle leg to maneuver cars through it.

Thing is, six half cinderblocks at 14# each is only 84 lbs...

So if I use the uber-simple calculation, where P = 0.00256 x V2, where V is the speed of the wind in miles per hour (mph) and P is in pounds per square foot (PSF), and the tarp is 10x18' or 180ft2, then,

@ 25 MPH P=1.6psf x 180sf = 288lbs
@45 MPH P=5.2psf x 180sf = 933lbs

But that would only be appropriate if the whole thing was perpendicular to the wind. The wind was (is) turbulent, and parallel (approximately) to it.

Seems high, though my child said they saw it lift up towards the clouds, and I did find it many feet over in my yard, flipped over...

So, using the NASA tool with a geometry approximately of the carport...

[Linked Image]

Camber as percentage of chord was handy once I found this... It is 20% in my case, barring more thought or better measurements of the carport tomorrow...

[Linked Image]


Yikes!!! 1952 lbs of lift?!???! So maybe 1000 lbs isnt so far off for a non-horizontal wind... But regardless, attaching cinderblocks (or even concrete buckets) to the bottom will just result in an additional 84+ lbs of more damaging mass flying up and down...

But wind probably isnt purely horizontal either... So....?

Last edited by JHZR2; 05/23/20 10:33 PM. Reason: calculated camber and percentage
Re: Calculating lift of a tarp carport [Re: JHZR2] #5437105 05/23/20 11:05 PM
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I've never had one without sides, but even then they can get some decent lift on them. I've seen people anchor them to wooden planter boxes in the corners, a few cubic feet of damp earth per post will get you your ton of weight without looking terrible. Plus maybe they can help with sideways racking as well. There are some slick HD shade options available, cantilevered etc. we saw lots of the playgrounds in NZ have huge shade structures using fabric and they have plenty of wind there.


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Re: Calculating lift of a tarp carport [Re: JHZR2] #5437157 05/24/20 01:33 AM
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Originally Posted by JHZR2

...My ideal is a half cinderblock (Lowes sells these near me), on each leg, and the plastic "foot" (which keeps the metal leg 1/8" off the ground) and leg through-bolted to the cinderblock....


This is the exact setup (close to exact, i used heavy landscape blocks) i made for a canopy. It lasted exactly 2 days before the wind destroyed it. The legs and the plastic feet were the weak links. It might be that my canopy used cheaper support legs than you have.

The blocks and the masonry bolts that attached the feet to the block did fine.

Re: Calculating lift of a tarp carport [Re: JHZR2] #5437190 05/24/20 05:14 AM
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Granted its your project but you are not only way overthinking this but you are employing the wrong methodology.

If you will (presuming it will be on level ground)

What is the substrate and top this will be mounted on ( say concrete over dirt, asphalt over dirt etc.)?

Permanent install or seasonal?

Do you desire it to be re-locatable?

How close is it going to be to any wall/structure that will reflect wind?

Actual make/model would be nice to get an idea of structural strength of the members ( don't want it to twist either)

This is basic construction, not designing an airplane

Re: Calculating lift of a tarp carport [Re: ABN_CBT_ENGR] #5437455 05/24/20 01:02 PM
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Originally Posted by ABN_CBT_ENGR
Granted its your project but you are not only way overthinking this but you are employing the wrong methodology.

If you will (presuming it will be on level ground)

What is the substrate and top this will be mounted on ( say concrete over dirt, asphalt over dirt etc.)?

Permanent install or seasonal?

Do you desire it to be re-locatable?

How close is it going to be to any wall/structure that will reflect wind?

Actual make/model would be nice to get an idea of structural strength of the members ( don't want it to twist either)

This is basic construction, not designing an airplane


Level ground near sea level

Asphalt driveway over sandy dirt

Tarp is seasonal; not likely to remove poles, but one pile in middle needs to be removable temporarily.

I want to be able to move it if/as desired.

The setup is more or less identical to the picture in the original post. Right up against the one bay of a two car garage.

Respectfully, this is an airfoil, note the parameters I selected for camber, cord, span, and AoA, as well as thickness, align to essentially a tarp, sail, or kite in a horizontal wind. My suspicion is that wind isn’t really horizontal though.

If there’s a better methodology to calculate lift on a structure like this, I’m all ears.

Just want to understand approximately how much mass I need to counter lift from wind. Given that the thing lifted 10’ and flipped midair, there certainly is some force there...

Re: Calculating lift of a tarp carport [Re: JHZR2] #5437471 05/24/20 01:25 PM
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Well in structural engineering and for my PE, it was basic wind loading against a structure but some of the math is similar but a structure doesn't act or function like a wing. ( for the exact reason you state and no corrections for degrees of freedom plus air at ground level has different densities due to terrain and a whole lot more) all that aside and with respect..

Get an auger and do 6-8 inch holes about 18 inches deep- 1 per foot

Get about a 2 inch pipe and drill 2 holes in it for a dowel to stick through ( biased and close to the bottom)

Put a threaded connector on it- bury that even with the asphalt and stuff something in the connector to keep the cement from filling that area

Pour that full of cement, pour and dry- that puts your threads even with your asphalt- put a pipe plug in it and you can drive over it if needed and keep it pristine.

The adapter is a close nipple with a coupling ( big enough for the selected canopy leg to fit in)- use Loctite to make sure they don't unscrew

Drill it where it fits the canopy leg of choice and put a bolt through it.

That way you can leave it or move it any way you need to any time you need to.

I can also say with confidence a CAT-V Hurricane will not pick that up ( it will shred the tarp and bend the legs but it wont move)

That will last a lifetime and cost around $100 for everything but an auger rental

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