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Pressure Coefficients

Internal Pressure (GCpi)

There is a force pushing on your building from the inside, not just the outside. Understanding internal pressure is key to getting wind loads right.

^
^
^
^
Positive GCpi
+0.18
Pushes OUT
^
^
^
^
Negative GCpi
-0.18
Pulls IN
Enclosed Building
+/- 0.18
Impact windows or shutters installed
Partially Enclosed
+/- 0.55
Unprotected openings present
+
Positive
Pushes outward
or
-
Negative
Pulls inward

Pressure Inside Your Building

When wind blows around a building, it does not just push on the walls from outside. It also creates pressure changes inside the building through small gaps, vents, and openings.

Think about being in a car on the highway. Roll down one window and you feel the air pressure change inside. Buildings work the same way!

This internal pressure can be positive (pushing outward on all surfaces) or negative (pulling inward). Engineers must check both cases because wind direction changes and so does internal pressure.

The Critical Plus/Minus

Why do engineers write GCpi = +/- 0.18? Because you must check both conditions and use whichever is worse for each component.

Example - Leeward wall (back wall):

External suction pulls outward: -20 psf
Add positive internal pressure: +13 psf
Total outward force: -20 + 13 = -33 psf

But with negative internal pressure:
External suction: -20 psf
Negative internal pulls inward: -13 psf
Forces partially cancel: -20 + (-13) = -7 psf outward

The positive internal pressure case is worse! That is why we check both.

External
-20 psf
Internal (+)
+13 psf
Total
-33 psf
Internal pressure adds to external suction
Net Pressure Formula
p = q(GCp) - qi(GCpi)

p = net design pressure

q(GCp) = external pressure

qi(GCpi) = internal pressure

In the Design Equation

Internal pressure appears in the net pressure equation that determines what your walls, roof, and components must resist.

The formula subtracts internal pressure from external pressure. But remember - subtracting a negative number adds! This is where the plus/minus really matters.

For walls being sucked outward, positive internal pressure (pushing out) makes things worse. For walls being pushed inward, negative internal pressure (pulling in) can be worse.

That is why wind load calculations always check multiple combinations of external and internal pressure to find the worst case.

Frequently Asked Questions

What is the Internal Pressure Coefficient (GCpi)? +
GCpi represents the pressure that builds up inside a building during wind events. It can be positive (pushing outward) or negative (pulling inward). For enclosed buildings, GCpi is plus or minus 0.18. For partially enclosed buildings, it increases to plus or minus 0.55.
Why does GCpi have plus and minus values? +
Wind can create both positive and negative internal pressure depending on where openings are located relative to wind direction. Engineers must check both cases - positive GCpi adds to outward suction on walls, while negative GCpi can reduce or reverse net pressures. Both cases must be designed for.
How does internal pressure affect windows and doors? +
Windows and doors must resist both external wind pressure and internal pressure acting together. When wind hits the windward wall, external pressure pushes in while internal suction may pull out. The component must be designed for the combined effect, which can be higher than external pressure alone.
What GCpi should I use for a house in Miami-Dade? +
For houses with impact-resistant windows or approved shutters, use GCpi = plus or minus 0.18 (enclosed building). Without protection, the building may be classified as partially enclosed requiring GCpi = plus or minus 0.55. Always check enclosure classification first.

Get Complete Pressure Calculations

Our calculator combines external and internal pressures correctly, checking both plus and minus cases. Get accurate net pressures for Miami-Dade projects.

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