Home Forums Hunting & Shooting Hunter's Campfire How to calculate airfoil lift force on a pavilion roof with no walls?

How much lift is there on a 12x24 foot 4/12 pitch roof 10’ 2” feet high at the ridge line, in 40, 50 and 60 mph wind? A rough approximate will be more accurate than my guesses.

We are putting up a 12x24 foot roof over a patio. It is hefty wooden posts bolted to a floor of unsecured paving bricks, with a wooden rafter frame and metal roof sheeting. No walls. I am concerned about damage from windfoil lift effect. I saw wind lift the entire roof of a big 40x100 foot hay shed.

On the hay shed, the medium pitch A shaped roof lifted in one unit, like an airplane wing. It flew 25 yards to one side and set down with surprisingly little damage. It was bolted to the tops of telephone poles, but that was not strong enough to hold it down. The main damage was from twisting as it landed.

My grandson thinks I am nuts to be concerned. I have tried online and find only engineering outfits who will calculate lift if you hire them, and a few free sites with complex mathematics and symbols I don't know for input. However, my bumbling calculations so far indicate that the uplift force could range from several hundred to several thousand pounds.

I’m also researching air flow spoilers to kill the lift, however much it is.

So you got 300 sf. of wing bolted to a unsecured paving brick, why worry?

Does your grandson understand how airplanes fly?

You are setting your poles on papers and bolting them to something I could lift?
While being worried about lift?

I'm a chainsaw and sledge hammer, truck driver, of a builder.

But assuming your name reflects your location,
I would be getting solid support below the frost line as a first step in any building.

Don't know crap about building or even where you are.
But poles would either sit on footings resting below the frost line or be in a concrete filled hole resting below the frost line.


I have been accused of over engineering building projects.
More is always cheaper than nor enough.

Being cheap will cost you 10 times as much. I am dumber for actually responding because you are going to build a kite to save money.

You need a 2’x2’x2’ cube of concrete.I would use steel columns and anchor bolts. I’ve built steel structures and metal buildings for the last 26 years.

Concrete and steel columns shouldn’t be more than $2500. $200 for 4 columns, $1200 for concrete, $150 foot anchor bolts- rest is incidental monies.

Make your dumb grandson dig the piers.

Dont put it anywhere close to something you dont want smashed by flying debris!

He could successfully bracket and bolt wood timbers to the 2x2x2 concrete foundation you suggest.

It'd keep the appearance more aesthetically pleasing.

I agree. However, I like the added strength of anchor bolts tying the concrete and steel better than a bracket and a Tapcon. One could easily wrap the steel column in whatever wood you want. It’s done all the time

But if Lowes or Home Depot don't have it on the shelf it ain't happening for most.

Updated insurance policy.
Mother Nature is very good at destroying things. Some things more susceptible than others.

Osky

Both of those place sell anchor bolts, rebar and concrete last time I checked...

We're talking about steel columns vs wood

Build it and insure it.

I actually had one I built out at my Parent's ranch back in 08 fly off about 75 yards when we got tropical-storm-force winds. It was about the same size as what you're building and had a tin roof, the 4x4s I used were 3ft in the ground but had no cement. I built it out of lumber from part of a house I had demoed so it was no big loss and there was no damage to the travel trailer it was built next to.

Talk with builders that construct post frame buildings in your area. The designers of pole barns and hay sheds have solved the problem of uplift. Specific methods to embed and anchor the treated posts into the ground, and to securely connect the posts to the roof structure to prevent "Liftoff."

Very simple to remove some paver blocks to install properly embedded posts, and then reinstall pavers up to the posts.

Have a 12'x12' gable roof 4 pitch there are 4 steel post supporting the roof they are lag bolted to a concrete slab with a rat wall plate is 6"x6" with lag bolt holding it down....
I live on Lk. St. Clair several time a year we get 60 mph+ winds off the lake ....you can see a little move meant with the wind gusts.....

Actually during the winter I rap the outside to protect the boat inside it seam like the wind effects it less with the outside covered.....we get some heavy storms in the late fall and winter.....

Have you already built it? If not then you need to put concrete posts in the ground and secure your posts on top. You can get a cardboard tube at the building supply for a post form. Just dig a hole. Plumb the tube in the hole and cut it the correct height and pour with reinforced concrete. I would want the concrete post 3 feet in the ground and 18" off the ground here. There's a steel fitting that sets into the concrete post to attach your wood post on top.

If it's already built you need to seriously anchor it to the ground. Mobile home anchors might work.

There's a reason this home remained standing after a severe storm, it wasn't by chance.

The 'absent' homes that use to surround it were built to code..

Thanks for the feedback. Good stuff all. First of all, it ain’t my project though I have helped a bit in building it. There is a reason I mentioned vertical posts merely bolted to unsecured paving blocks and you all confirmed my thinking.

The building is in a small clearing in tall timber, normally quite protected from wind but we get bad swirling gusts about once a year.

Re the original question about lift: Muffin, thanks for the link on how to calculate that. The site you link is the simplest I had found and I’d already spent some hours trying to get it to work for me. It is the simplest site for such calculations and it is still complex and incomprehensible to someone who does not know engineering terms and abbreviations. It starts OK with clear inputs but soon shifts into engineering speak, confusing input terms and by the time it spits out calculations for 13 sections of the roof it expresses them in terms of no use to me. What is wrong with saying, “lift per square inch, or foot.”?

It has been a curious exercise. Airplane engineers post such statements as,”...a 747 wing can generate one pound of lift per square inch.” That is clear, useful info, and obviously over simplified. Roof engineers apparently cannot bring themselves to compose a parallel sentence such as , “A 4/12 shed roof with a 50mph wind at 90 degrees to the ridgeline can generate x pounds of lift per square x.”

Damned timing !

Betting Cash, could answer this !