ASCE 7-22 · C&C · FENESTRATION
Window & Door Design Pressure
Design Pressure (DP) is how you specify a window or door for wind. The unit's DP rating must meet or exceed both the positive and the negative C&C pressures calculated for its location.
THE TWO DIRECTIONS DP MUST COVER
One unit, two pressures
Wind pushes inward (positive) on the windward face and pulls outward (negative suction) on side and corner zones. A DP rating is symmetric: DP-50 means the unit is rated for +50 psf inward and −50 psf outward.
POSITIVE vs NEGATIVE DESIGN PRESSURE
Both must be satisfied
Positive (inward)
Windward pressure pushing the unit into the building. Driven by external coefficient +1.0 (Zone 4 @10 sf) minus the enclosed internal pressure −0.18.
GCp +1.0 @10 SFNegative (suction)
Outward pull that usually governs DP selection. Strongest at corners (Zone 5, GCp −1.4 @10 sf) versus the wall field (Zone 4, −1.1).
ZONE 5 −1.4 @10 SFHOW TO PICK A DP RATING
Calculate → cover → select
Net C&C pressure is p = qh[(GCp) − (GCpi)], with qh = 0.00256·Kz·Kzt·Kd·Ke·V² and Kd = 0.85. The worked numbers below use an enclosed building (GCpi ±0.18) at a corner window, with a site velocity pressure qh = 40 psf.
1 · Calculate
Positive: 40×(1.0−(−0.18)) = +47.2 psf. Corner suction: 40×(−1.4−0.18) = −63.2 psf.
+47.2 / −63.2 PSF2 · Cover both
The DP demand is the larger magnitude of the two. Here suction −63.2 psf governs over +47.2 psf.
DEMAND 63.2 PSF3 · Add margin
Round up to a rated DP step so the unit meets or exceeds demand — never specify a DP below the governing pressure.
ROUND UP4 · Select DP
63.2 psf → specify DP-65. It is rated +65 / −65 psf, covering both the +47.2 and −63.2 demands.
DP-65 ≥ DEMANDEFFECTIVE AREA & ZONE
Bigger units & corners change the number
GCp is a function of effective wind area. A larger unit (bigger effective area) sees a lower GCp magnitude than the −1.4 / −1.1 / +1.0 values tabulated at 10 sq ft. Zone 5 (corners) always demands more suction capacity than Zone 4 (field). Always check the unit's actual area and its zone before fixing the DP.
HVHZ · MIAMI-DADE & BROWARD
Impact rating is also required
In the High-Velocity Hurricane Zone, a sufficient DP rating is not enough on its own — fenestration must also carry impact approval under the Florida Testing Application Standards.
SPECIFY WITH CONFIDENCE
Get the C&C numbers your DP must beat
Calculate zone-by-zone positive and negative C&C pressures so you can specify the right DP rating for every opening.