Wind Load on Horizontal Roof & All Sides Open

[pflow]

I am a ME by trade but am trying to learn how to apply ASCE 7 to this particular problem:

* Corrugated sheet steel horizontal roof (angle ~ 1 degree)
* Roof supported by four posts and otherwise open.
* Spec provided for wind pressure and velocity.

I am trying to select the thickness, pitch, and height of the corrugated sheet steel roof and want to consider potential wind loading.

Since this is open, and the roof is horizontal, is there a wind load in principle? It seems like applying the wind velocity spec over the entire area (length X width) will result in an oversized plate thickness since wind flows horizontally only?

Suggestions would be appreciated.

 

[hokie66]

Not familiar with the ASCE standard, so someone else will have to advise. But in general, wind does apply normal forces to horizontal surfaces. Otherwise, an airplane wouldn't fly.

 

[JedClampett]

I'm out of the office, so I don't have my copy of ASCE 7, but I know for a fact that there is a case for a open structure monoslope roof.
And don't think that because the roof is flat, the vertical forces are trivial. They're huge. Think of an airplane wing. As I remember, depending on the Effective Wind Area (EWA, the uplift forces start at .8 times the basic wind pressure and go up from there.
You'll not only have to worry about the roof spanning between supports, but anchoring it to the supports and then anchoring the posts (or whatever) to the ground.

 

[msquared48]

Yup. Think uplift with a basically flat roof.

Mike McCann
MMC Engineering

 

[Ron]

Yes...JC and Mike have it. ASCE 7 has a provision for monoslope roofs over open structures. Use that. You will have uplift on your roof, flat or slightly sloped.

 

[pflow]

Thank you everyone for the replies - much appreciated.

 

[MiketheEngineer]

Uplift mostly - probably in the 10 - 15 psf range if you are in the "normal" US

But posts will need to be checked for uplift and some side forces....

 

[pflow]

MiketheEngineer:

I ran the numbers this morning and got 9.9 psf - close to your range. The code said to use a miniumum of 10 psf.

I did a "beam stess" calc using tabulated moment of inertia values for glavanized corrugated steel sheet. It looks like p = 2.67" X 1/2" in 0.04 thickness passes. Is this a reasonable selection?

 

[MiketheEngineer]

10 psf is minimum. What kind of beams, girts or spans are you working with??

What is the Sxx or Ixx??

Fb = M/S - keep Fb around 20,000 psi or less - usually!!!

 

[hokie66]

Don't you have manufacturer's load tables available for the steel roofing? These are based on testing, and don't necessarily agree with beam calculations, as light gauge material does some funny things. Load capacity depends on the span, which you didn't give. Also, fasteners may be the controlling criteria.

Don't know where you are, 10 PSF is not much. If you have snow, that will likely control for the roofing selection.