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

Damned timing !

Betting Cash, could answer this !

Why??? Problems were addressed in replys 2,3, and 4. With the definitive answer in post 4 from me.

But whatever- Someone always has a better answer here😂

Why??? Problems were addressed in replys 2,3, and 4. With the definitive answer in post 4 from me.

But whatever- Someone always has a better answer here😂

Thanks for the feedback.

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.”

You're both wrong.

The OP merely asked for mathematical formulas to help strengthen his view point with his grandson.

The OP isn't doing the construction.

My sarcasm in referencing wings and airplanes may have inadvertently lead him to the information he needed to inforce his stance.

OK hero, put up the formula.

Don't get ya panties in a wad, Nancy !

Not being a structural engineer, how in the fuct would I know that you had, possibly given the definitive answer ?

As to my above post, I know because he & I traded favours. I helped him with some auto stuff & he helped me with some load bearing & shear construction stuff.

Get over yourself, tool !

You’re a diipsheit. I’ve been doing this for 26 years. However, I had my 23 year old Civil Engineer daughter run calculations for local codes and she said I was overkill by 1/3 yard per pier.

No one on this website can answer the question with precisely the exact answer. Wanna know why dummy?

OP didn’t provide where he lives and his design criteria- which should’ve included IBC Building Code, reactions on each column, seismic stability factors, wind speed design, collateral loads, and location factor correction values.

All of which I knew to ask, but Emma (23) wanted you to know Karen.

Go dig his piers with his dumbass grandson dippy.

You were Post #5
Pretty good answer.
Pier footings for the posts would be the way.
Tube steel columns are nice but could be done with 6x6 wood posts for the size of the structure.

Simpson CB66 type cast in place post bases and through-bolted to posts.



Then the same class of hardware for the overhead work :



Then Hurricane clips for all the rafters to complete the Continuous Load Path all the way up the structure.
H1 clip is a nice clip

You’re a diipsheit. I’ve been doing this for 26 years. However, I had my 23 year old Civil Engineer daughter run calculations for local codes and she said I was overkill by 1/3 yard per pier.

No one on this website can answer the question with precisely the exact answer. Wanna know why dummy?

OP didn’t provide where he lives and his design criteria- which should’ve included IBC Building Code, reactions on each column, seismic stability factors, wind speed design, collateral loads, and location factor correction values.

All of which I knew to ask, but Emma (23) wanted you to know Karen.

Go dig his piers with his dumbass grandson dippy.

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.

a 747 wing can generate one pound of lift per square inch.

12x24 roof @ 4/12 pitch = roughly 300sf.

300sf = 3600 square inches.

3600 square inchs = 3600lbs of potential lift.

How much lift can your recommend Simpson H1 clip bare?*

























* 363lb

I hope you're not employed in a position that requires much in the way of math skills. Your calculation is off by a factor of 12. There are 144 square inches in a square foot, not 12. The correct product of the equation is 43,200 square inches, not 3600 square inches. It does make a difference.

OK hero, put up the formula.

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.”

I do this for a living. Licensed PE. There isn't a place in our lower 48 that has a design wind load of only 60mph - and there's no need to design to a lesser load. Use worst case loading. I think the lowest is 90mph...and that depends on what code you are using. Generally we use what's called a 3-second gust value and most design loads are 3-sec gust values. The basic wind pressure formula is 0.00256 x (wind speed in mph)^2. So a 90mph wind has a pressure of about 20.7psf. The exact wind calcs are PIA (which is why no one will do them for free) and the pressures/uplifts vary on different parts of the roof, e.g. corners, edges, center. Suffice to say, if you use 30psf as your uplift value, that should cover about 90% of the USA's wind loads and roof designs.

That uplift load is distributed based on tributary area of the structural elements. I don't know your rafter spacing, but assuming four equally spaced posts, you'll have 2160lbs. of uplift at each post. (288sqft x 30psf) / 4 = 2160lbs. Concrete is 150lbs/ft^3 so you'll need 14.4 cubic feet of concrete at each corner and a bracket at the top and bottom of each post that is good for 2160lbs. in tension.

Sorry if the intended humor of my replying to a question that was asked with the OPs early findings went over your head.
It was asked with a wink and a smile and replied to likewise.