Monoslope roof wind load
Monoslope roof wind load
(OP)
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?






RE: Monoslope roof wind load
RE: Monoslope roof wind load
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.
RE: Monoslope roof wind load
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.
RE: Monoslope roof wind load
RE: Monoslope roof wind load
RE: Monoslope roof wind load
“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."
RE: Monoslope roof wind load
Jeff
Pipe Stress Analysis Engineer
www.xceed-eng.com
RE: Monoslope roof wind load