What is internal pressure coefficient (GCpi)?

Internal pressure coefficient (GCpi) measures how wind pressure builds up inside a building. When wind hits a building with openings, air rushes in and creates pressure from the inside pushing out. This adds to the external suction, making windows work harder. Enclosed buildings have GCpi of plus or minus 0.18, while partially enclosed buildings jump to plus or minus 0.55.

What makes a building 'partially enclosed'?

A building becomes partially enclosed when openings on one side are much larger than openings on other sides. Specifically, if openings on one wall exceed both 4 square feet and 1% of that wall area, AND exceed 10% of total openings on other walls, the building is partially enclosed. This can happen during hurricanes when one window breaks.

How much does internal pressure increase wind loads?

Switching from enclosed (GCpi = 0.18) to partially enclosed (GCpi = 0.55) adds about 0.37 to your pressure coefficient. On a typical Miami-Dade HVHZ building, this can add 15-25 PSF to your design pressure. That is why impact-rated windows are so important - they prevent the building from becoming partially enclosed during a storm.

Can I design for enclosed if I use impact windows?

Yes, this is exactly why Miami-Dade requires impact-rated glazing. If all your windows and doors are impact-rated and tested to maintain integrity during missile impact, you can design assuming the building stays enclosed. This results in lower required design pressures and potentially smaller, less expensive structural members.

Inside vs Outside

When Wind Gets Inside

Imagine blowing up a balloon while someone squeezes it from outside. That is what happens when wind enters your building. One broken window can triple the force on every other window. See exactly how it works.

Pressure Increase

0.55

GCpi (Partial)

PSF Added

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GCpi = +/- 0.18

Enclosed - Minimal internal pressure

The Big Difference

Enclosed vs Partially Enclosed

Same building, same storm. But one broken window changes everything.

Enclosed Building

All impact windows intact

GCpi = +/- 0.18

Internal PressureLow
Window StressExternal only
Typical Added PSF~6 PSF
ClassificationDefault for HVHZ
RequirementImpact glazing

Partially Enclosed

One or more openings breached

GCpi = +/- 0.55

Internal PressureHIGH
Window StressInternal + External
Typical Added PSF~18 PSF
ClassificationWorst case
ResultCascade failure

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Base Velocity Pressure (qz) PSF

External GCp (typical wall)

Enclosed (GCpi = 0.18)

-62

PSF Total

Partially Enclosed (GCpi = 0.55)

-79

PSF Total

One broken window adds:

+17 PSF

The Cascade Effect

How One Window Destroys a Building

Hurricane Hits

Wind pushes on the outside of every window and wall. Inside, pressure is normal - maybe a little lower as air leaks out through small gaps. Your building is "enclosed" and handling the storm fine.

Debris Strikes

A piece of flying debris - maybe a roof tile, a branch, a piece of gravel - hits a window on the windward side. If it is not impact-rated, it shatters. Now you have a big hole facing the wind.

Wind Rushes In

Hurricane-force wind pours through the opening. In seconds, the interior pressure skyrockets. Your building just became a partially enclosed balloon being inflated from inside while squeezed from outside.

Cascade Failure

Every window now faces external suction PLUS internal pressure pushing out. The load on each window just increased by 30% or more. Windows that were handling the storm fine now fail. More openings, more pressure, more failures.

Roof Lifts Off

The roof experiences the same pressure spike. External uplift plus internal pressure pushing up. The roof structure, designed for enclosed conditions, cannot handle the combined load. The roof fails, and now everything inside is exposed to the storm.

Common Questions

Internal Pressure Explained Simply

Think of it as measuring how much wind gets trapped inside your building. When wind hits, some pressure builds up inside and pushes OUT on windows from the inside. For a sealed building (enclosed), this pressure is low - GCpi of 0.18. But if a window breaks, wind rushes in and that internal pressure jumps to 0.55 - three times higher. This internal push adds to the external pull, making every window work much harder.

Your building becomes partially enclosed when openings on one side are way bigger than openings on other sides. The code has a specific test: if openings on one wall are more than 4 square feet AND more than 1% of that wall, AND more than 10% of total openings on other walls, you are partially enclosed. One broken window during a hurricane can easily meet this definition.

Going from enclosed (0.18) to partially enclosed (0.55) adds 0.37 to your pressure coefficient. On a typical Miami-Dade building with 45 PSF velocity pressure, that is about 17 PSF added to every window - roughly a 25-30% increase. That is the difference between windows that survive and windows that fail.

Impact windows are tested to survive debris strikes without creating an opening. If debris hits and the window holds, your building stays enclosed. That is the whole point of HVHZ impact requirements - not just to stop debris, but to keep internal pressure low. This lets engineers design for the lower enclosed pressure, which means smaller structural members and lower costs overall.