MBMA (Metal Building Manufacturer's Association) had information in their old manual for open conditions. For wind that was blowing parallel to the frames, generally on the sidewalls of the building they stated the following:
For open buildings, a coefficient of +1.05 shall be applied to the covered areas of zones 1 and 5 (windward wall surfaces); a coefficient of -1.05 shall be applied to the covered areas of zones 4 and 6 (leeward wall surfaces).
For wind blowing perpendicular to the frames, generally on the endwalls of the building they stated the following:
A coefficient of 1.8N (N = number of frames) shall be applied to the solid area of the largest frame projected onto a plane normal to the ridge.
Additional information is available but is too voluminous to type here. If you will post your email address, I will email the appropriate pages. Note that these coefficients are based on the old fastest mile wind speeds, so may not be totally correct for use with newere 3-second gust wind speeds, but should get you close.
use a factor of 1.05 on the windward surfaces and a factor of -1.05 on the leeward surface for any