Calculating pressure for a roof to be thrown 300'

[PostFrameSE]

Due to the recent storms we had a roof system (in part) take flight. How can I roughly calculate the pressure required to throw an object of known weight and surface area a specified distance? I'd like to determine what the pressures or actual wind speed must have been to create such a situation.

Thanks and Merry Christmas!

 

[BUGGAR]

Doubly hard because you have both lift and lateral forces. To be realistic, you should consider them both. If you figured forces sufficient to cause roof failure from both lift and lateral force, you would then have to get into figuring flight distances of flying roofs.

And Merry Christmas to you. This roof didn't take Santa's chimney with it, did it?

 

[KootK]

I'm not sure that this is knowable unless you can somehow estimate the flight time. If you had just enough wind speed to achieve lift off but you maintained it indefinitely, your roof might get blown to Cambodia.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[drawoh]

PostFrameSE,

Is pressure what would throw your roof? Was the roof accelerated to the velocity needed to ballistically travel three hundred feet, or did something it keep airborne at some velocity for the time required to travel three hundred feet? I am sure there are other ways to toss a roof.

--
JHG

 

[msquared48]

Add to this is the aspect of the roof with respect to the ground.

Initially it starts with uplift pressure due to the wind over the top (Wing mode), then rotates over a period of time to see a combination of vertical suction (wing mode) and direct pressure from underneath (sail mode), and varying along the flight path with their effects.

You would really need a video of what happened and a time log of the associated wind speeds to model this.

Challenging problem...

Mike McCann, PE, SE (WA)

 

[dhengr]

PostFrameSE:
I don’t know that that is something which is really calculable. But, do a good job of laying out and thinking through the chain of events which allows such a thing to happen and you probably come to the conclusion that is doesn’t take hurricane force winds to cause these roofs to take flight. If they comply, they barely comply, with the IBC, IRC, or the NDS, you know the saying, ‘save a spike or a bolt, and yu save a few minutes and a buck or two.’ They are relatively inexpensive farm buildings with little likelihood of much public safety involvement, so the codes and AHJ kinda let them slide. At first you have some wind pressure on one side/surface and suction on the other side, partially open building probably, over a large contributing area as relates to truss connections on beams or at posts. Don’t forget that pressure differentials and fluctuations are highest at eaves and building corners where the ripping apart invariably starts. Once you’ve caused/allowed the hold down connections to fail, and uplift which was probably never designed for in the basic design and detailing anyway; then it only takes a positive/uplift pressure one percent over the weight of the roof system for it to take flight. Then what happens, who knows, a slight gust of wind, in a slightly different direction, and it goes 400' instead of only 200'. That would be my explanation of what happens. If you want to go a little deeper take a look at the design load, wind load, wind pressure criteria in ASCE 7, and take the weight of the roof structure above the truss bearings and you should be able to back out a wind pressure or wind speed from their convoluted approach to wind loads to make your roof fly. Their approach has considerable testing behind it to make us much smarter about how this all happens, but it is still not an exact science. The big question will still be what forces allowed the connections to fail in the first place, and get this all started.

 

[PostFrameSE]

Thanks all. These are farm buildings but we take our design seriously, considering the 90mph winds, partially enclosed enclosure (even though it probably truly lies somewhere between pressures from partially enclosed and open), pressures greater at edges/corners, etc. I like the design and am comfortable with it at design loads. A fellow engineer is doing some inspection today and found that not all connections were quite as they should have been. That's the challenge. Did it fail because of excessive wind or less than stellar construction?

Oh what I wouldn't give to have a video showing how this happened. A lot could be learned!

I think about a sheet of 29 gauge corrugated metal setting on top of a pile loose. I agree that it wouldn't take much wind to pick it up off of the pile. However......to get it to sail 300' is another thing. As some have alluded.....different wind gusts could have occurred and kept the roof in flight much longer than one would expect.

No chimney Buggar!

Thanks for sharing your time and expertise on this Christmas Eve!!

 

[SAIL3]

the only condition in which there is any degree of certainty is at the moment of initial lift-off. One has the weight of the roof, the number and condition of the tiedowns and from that one could calculate the required uplift force for the roof to break free. With this uplift force in hand, one could calculate the wind velocity that could generate this force by working backwards. If the calculated wind velocity comes out unreasonably high, then, that may indicate that the tie-downs were not as designed or deficient in some way...
once the roof is airborne, all bets are off, as this would be an extremely complicated problem to solve due in no small part to the shape of the roof possibley changing shape in flight...

 

[Ron]

Agreed with Sail3....Lifting the roof off is going to require the higher uplift pressure. After that, everything is unpredictable but the loose piece can then be tossed about in any direction and for as long at the wind blows on it!