Accurately applying windloads are not for the faint of heart. There are entire books dedicated to the subject on how to apply them accurately, and specialty software programs and the like. I've outlined the basic engineering procedure for determining wind load pressures below:
According to ACSE-7 (American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures), which is THE source for engineers in the US, the wind pressure is determined by:
qz = 0.00256 * Kz * Kzt * Kd * V * V * I
where
Kz is a factor ranging from 0.68 to 1.89 based on the building height and exposure (open field, in a woods, etc.)
Kzt is a factor relating to the surrounding terrain. (Is the structure on a hill, edge of a cliff, etc.) Wind speeds up going up and over a hill.
Kd is a directionality factor, based on the type of structure. For most structures, this is 0.85.
V is the wind velocity in mph, based on a three-second gust windspeed commonly seen in the new IRC and IBC building codes, not the "Fastest Mile" windspeed seen in the UBC/BOCA.
I in an importance factor. For most structures this is 1, agricultural buildings less than 1, and for hospitals/shelters, greater than 1.
After evaluating the above equation, we get the wind velocity pressure for a particular building at a paticular height.
To determine the pressure to apply to the structure, you use a series of different equations, based on the structure type, but is in the order of:
p = q * G * Cp - qi * (GCPi)
where
q is the pressure developed above (qz). (qz is based on a particular height. Different pressures can be applied to the building at different heights.)
G is a gust factor based on the rigidity of the structure (again, usually 0.85)
Cp is the external pressure coefficient, which can be a positive or negative number, depending on building shape, windward or leeward side, etc.
qi and GCPi are similiar, but are the internal pressure coefficients.
So, Mark, assuming a 20' x 20' open pavillion, in a 100 mph wind zone, making some general assumptions, qz is 18.5 psf. For a 10 degree roof slope with the ridge perpendicular to the wind direction, the uplift (design pressure, p) is 24 psf on the windward side, and 13 psf on the leeward side. For a 12/12 pitch, with would switch up to a 5.6 psf DOWN pressure on the windward side and a 11.1 psf uplift on the leeward side. And, with open structures, the uplift comes from air moving on both sides of the roof (above and below). Air must move faster over the top, since it has to travel up and down the roof, which causes a lower pressure, pulling/pushing the roof up. Essentially, same as an airplane wing.
