Building Classification

Partially Enclosed Buildings

When wind breaks through your building, everything changes. Internal pressure skyrockets and loads increase dramatically. Here is why enclosure classification matters.

Enclosed Building

+/- 0.18

Internal Pressure Coefficient (GCpi)

Partially Enclosed

+/- 0.55

Internal Pressure Coefficient (GCpi)

When is a Building Partially Enclosed?

A building is partially enclosed when BOTH conditions are met:

1. The total area of openings in one wall exceeds the sum of openings in all other walls
2. AND the opening area exceeds 4 sq ft OR 1% of that wall's gross area (whichever is smaller)

Low

Enclosed
Low internal pressure

HIGH

Partially Enclosed
High internal pressure

The Balloon Effect

Imagine a balloon. When the opening is small or closed, the pressure inside stays low. But blow air into it, and it wants to expand - pushing outward on all sides.

Buildings work exactly the same way. When wind forces its way through an opening in the windward wall, it pressurizes the interior like inflating a balloon.

This internal pressure pushes outward on every surface - walls, roof, windows. It adds to the suction already pulling on the leeward side, making total loads much higher.

That is why a broken window during a hurricane can lead to roof failure - the internal pressure was never part of the original design.

The Numbers Tell the Story

Look at the internal pressure coefficients:

Enclosed: GCpi = +/- 0.18
Partially Enclosed: GCpi = +/- 0.55

That is three times higher! When you multiply this by velocity pressure (qz), the internal pressure component of your load calculation triples.

For a Miami-Dade building with 75 psf velocity pressure:

Enclosed internal pressure: 75 x 0.18 = 13.5 psf
Partially enclosed: 75 x 0.55 = 41.3 psf

That extra 28 psf pushing outward on every wall and the roof is enormous. It can mean the difference between a building that survives and one that fails.

Enclosed Building

13.5 psf

Partially Enclosed

41.3 psf

Based on qz = 75 psf (typical Miami-Dade at 30 ft)

🛡️

Impact Glass

Not considered an opening when properly rated

🚪

Hurricane Shutters

Approved protection keeps building enclosed

💥

Standard Glass

Considered potential opening in HVHZ

🌀

No Protection

Must design as partially enclosed

Protection is the Answer

In Miami-Dade's High Velocity Hurricane Zone, there is a simple rule: protect your openings or face higher wind loads.

When windows and doors have approved impact-resistant glazing or hurricane shutters, they are not considered "openings" for classification purposes. This allows your building to be designed as enclosed.

Without protection, you must assume those openings will fail during a hurricane. The building must be designed as partially enclosed with much higher internal pressures.

This is why Miami-Dade requires impact protection on all glazed openings. It is not just about keeping rain out - it is about keeping the building structurally sound.

Frequently Asked Questions

What makes a building partially enclosed? +

A building is partially enclosed when one wall has openings larger than the sum of openings in all other walls combined, AND the opening area exceeds 4 square feet or 1% of that wall area. This typically happens when wind breaks a window or door on the windward side.

Why does partial enclosure increase wind loads? +

When wind enters through a large opening on the windward wall, it pressurizes the building interior like inflating a balloon. This internal pressure adds to the external suction on other walls and the roof, dramatically increasing total forces - sometimes by 50% or more.

How do I avoid partially enclosed classification in Miami-Dade? +

In the High Velocity Hurricane Zone, all glazing must be impact-resistant or protected by approved shutters. When windows and doors are properly protected, they are not considered openings, allowing the building to be classified as enclosed with lower internal pressure.

What is the GCpi value for partially enclosed buildings? +

Partially enclosed buildings use GCpi = plus or minus 0.55, compared to plus or minus 0.18 for enclosed buildings. This higher internal pressure coefficient means walls, roof, and connections must resist significantly greater forces.

Calculate Wind Loads for Your Enclosure Type

Our calculator applies the correct internal pressure coefficient based on your building's enclosure classification. Get accurate loads for Miami-Dade projects.

Calculate Wind Loads