Monoslope roof wind load

[vmirat]

I'm designing a canopy structure composed of open web steel joists with roof deck on the top chord and soffit panels on the bottom chord. The canopy would be an open building, monoslope free roof per ASCE 7-10. I need to figure out the wind pressure on the soffit panels. I noticed that ASCE 7-10, Figure 30.8-1 has a note which states that Cn is net pressure with contributions from the top and bottom surfaces. How do I figure out the wind pressure on just the bottom surface? Would it be 50% of the total?

 

[JedClampett]

Yes, wind pressure is the suction on the top plus the pressure on the bottom of a surface. But to split those up is risky as I don't know (maybe someone does) the percentages assigned to each. I'd put the full pressure on the soffit and not take any chances.

 

[Deker]

The commentary states that for open building roofs with a top and bottom surface, the pressure coefficient should be separated for the effect of top and bottom pressures, or conservatively, each surface could be designed using the full pressure coefficient. I don't know of any published guidance on how to separate the pressures, but here is how I have been doing it:

1. Calculate the wind pressure using the "Clear Wind Flow" pressure coefficient. This represents the net pressure acting on the top and bottom surface.
2. Calculate the wind pressure using the "Obstructed Wind Flow" pressure coefficient. The code says that obstructed wind flow denotes objects below roof inhibiting wind flow (>50% blockage). Since this could be applied to a condition with 100% blockage, I interpret this pressure to be acting on the top surface only.
3. The difference between the pressures calculated in steps 1 and 2 is the pressure acting on the bottom surface.

Each surface should be also be checked using the +/- 16 psf code minimum. If the canopy is adjacent to a building, I would apply the wall pressure to the bottom surface of the canopy for some edge distance based on judgement.

 

[ajh1]

I'll start off by saying that I certainly don't have any definitive answers, but wanted to give another tack to the process in comparison to Deker's approach. My take on the obstructed flow is that it restricts the flow under the roof but tends to compress the wind near the roof. ASCE 7 commentary for section 27.4.3 seems to lend credence to that: "Restricting the flow can introduce substantial upward-acting pressure on the bottom surface of the roof, thus increasing the resultant uplift load on the roof."
I tend to view this more in a vein similar to how the coefficients on canopies has been done where you are working with the basic external pressure on the roof, just as if the building was enclosed, then adding an additional component for wind on the underside. In the case of MWFRS for canopies, that underside value is either 0.7 or 0.8 depending on whether you are looking at the directional or envelope procedure. C&C is built into the coefficient tables, although taking the difference between the main roof and overhang coefficients where both are shown, would give an indication of the magnitude of the underside force.

 

[SAIL3]

I agree with ajh1 and would question the #2 assumption stated by Deker....

 

[Deker]

Thanks for pointing that out, ajh1. It looks like I was wrong to assume that obstructed wind flow acts only on the top surface.

 

[vmirat]

I received some help from ASCE on my question.
“Based on the question, it seems that the designer is only asking about the soffit which suggests that he has been able to calculate roof load on the top surface. While there may be some ground effects that will reduce the flow separation under the bottom surface of the structure relative to the flow over the top surface, the clearance is enough in our opinion that he should design the soffit for the same wind loads as the roof surface.”

The following disclaimer was added to their response:
"Please note this information is the personal opinion of the subcommittee members that reviewed your question and is not an interpretation of the ASCE 7-10 standard."

 

[JGard1985]

Go big or go home....

Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[JedClampett]

ASCE should of said, "Listen to Jed, y'all"