Determining Uplift Pressures

P = Ce x Cq x qs x Iw

P = design wind pressure

Ce = combined height, exposure and gust factor coefficient as given in table 16-G (of the UBC).

Cq = pressure coefficient for the structure or portion of the structure under consideration as given in table 16-H (sometimes referred to as factor for areas of discontinuity: eaves, rakes, ridges, corners).

qs = wind stagnation pressure at the standard height of 33 feet as set forth in table 16-F. (Basic uplift pressure can also be determined by: velocity squared times .00256).

Iw = importance factor as set forth in table 16-K (Table 16-K is an occupancy category table. For wind, most buildings will be 1.0 Notable exceptions are hospitals, police and fire stations which will be 1.15).

Example: a school building located over 1/4 mile inland from the coast on open terrain, with a basic wind speed of 110 mph, average height 20 ft., 4/12 pitch.

Basic wind speed: 110 mph
qs = 31 psf (110 x 110 x .00256)
Ce: 1.13 (Table G - 20 ft. height, Exposure C)
Importance: 1.0
Cq for typical roof elements (field) is 1.3
Cq for eaves, rakes, ridges is 2.6 (Cq taken from table 16 H)

Field:1.13 x 1.3 x 31 x 1.0 = 45 psf
Eaves, rakes, ridges: 1.13 x 2.6 x 31 x 1.0 = 91 psf