Deck Uplift

[XR250]

I have a residential client who wants to build an inexpensive deck by laying 4x6 PT sleepers down on a bed of gravel and simply attaching cedar decking to them. My only concern is the possibility of wind picking this thing up in a hurricane. It probably will only weigh 7 PSF. This is a 90 MPH, Exp B zone. Is this a reasonable concern? I guess, technically, there is no wind speed at ground level and very little at 4" above that.

Thoughts?

 

[JAE]

If there's any irregularity around the perimeter - there will be uplift on it. I would be concerned.
Also concerned that the treated lumber would deteriorate eventually, which it will I would think.

 

[XR250]

Thanks JAE. It is ground contact rated lumber - will rot at some point, but they are aware of that and it really can't go anywhere except 4" down.
Any idea on shape factors for this thing?

 

[TDIengineer]

What about simply setting a few small fence post footings, 1 each corner, and just tie it down if you're concerned about it lifting and moving.

Would be fairly cheap..
It just needs a little resistance so it can't pick up and then grab more wind load..

 

[XR250]

@TDI;

They don't want to use any concrete. We haver talked about mobile home tie-downs. Just trying to get an idea of the uplift I am dealing with. It is 12'x40'
Even fence post footing don't have significant mass so I don't know how effective they will be.

 

[ztengguy]

I would get some 4' pickets and drive down though the sleepers and anchor to the wood. Not sure how big you will be, maybe put these around the edge.

 

[JAE]

Not sure what shape factor to use.

I input a 6" high 30 ft. x 30 ft. "building" into our wind spreadsheet calculator using V = 90 mph, Exp. B wind.
This is C&C wind with At = 100 s.f.

The attached is the results - about 13 to 15 psf uplift. Not sure if I buy that but it at least gets you a semi-rational number.

 

[Ron]

JAE....that's reasonable. Using the ratio of 90mph to the typical 120mph in my area, you're pretty much right on target. In my area, uplift rarely gets above 40 psf on low structures except for solid surfaces like a roof corner or edge....commonly in the 25 to 30 psf range. Since the deck will be wind-permeable, even the corner and edges shouldn't be much of an issue.

I would put a 4x4 PT in each corner, penetrating about 3 feet in the ground and backfill with wet soil. It takes a lot of force to extract those posts, so would easily hold down a deck.

 

[XR250]

13 was a number I was thinking as well. I like the 4x4 idea, but hitting rocks may be an issue.
FWIW, in exposure B, ASCE7 has Kz the same up to 30 ft., so my 6" tall structure does not make a difference as far as uplift (and ASCE) is concerned.
Thanks to all that replied

 

[steellion]

I think a wind event can certainly pick this deck up and make it a hazard. It would be wise to do something to tie it down into the ground. I like Ron's idea.

 

[XR250]

@Steelion;

When I said my 6" tall structure does not make a difference, I meant it has the same uplift as a 30ft. tall building - not that it had no uplift.

 

[msquared48]

A few 4' long, 3/4" diameter drift pins will work well too driven through the predrilled 4X4 sleepers.

Mike McCann
MMC Engineering