Partially Enclosed Structures Explained
Understanding whether a building is partially enclosed is a critical step in wind design. This classification determines the internal pressure coefficients (GCpi) used in ASCE 7 and directly affects the loads applied to components such as windows, doors, roofs, and wall systems.
Definition
(ASCE 7-16 / FBC Reference)
A structure is considered partially enclosed when:
- The total area of openings in one wall (A₀) exceeds the sum of openings in the remaining walls and roof (Aₒᵢ ) by more than 10%:
Aₒ>1.10×Aₒᵢ
- The total area of openings in that dominant wall (A₀) is greater than 4 ft² (0.37 m²) and less than 20% of the gross area of that wall
Aₒᵢ /A₉ᵢ ≤ 0.20
Worked Example
Consider a structure with five sides (four walls and a roof), having a gross area of 20 ft². The openings are shown in blue below:
Conclusion: Both conditions are met → The structure is Partially Enclosed.
Condition 1
(Side A)
Aₒᵢ = 3+2+2+0 = 7 ft
1.10×Aₒᵢ = 7.7ft
Condition 2
A₉ᵢ = 4×20 ft+20 ft= 80 ft
Aₒᵢ /A₉ᵢ = 7/80 = 0.0875 ≤ 0.20
ASCE 7-16 Guidance on Enclosure Definitions
According to ASCE 7-16 Section C26.2, the definitions of enclosed, open, and partially enclosed buildings are specifically intended to guide the selection of the correct internal pressure coefficient (GCpi).
All other structures that do not clearly meet one of these three definitions are classified as partially open by default. These include buildings with large openings on multiple walls—such as parking garages, where wind can pass freely through but the structure does not satisfy the strict “open” or “partially enclosed” criteria.
For such partially open structures, the internal pressure coefficient (GCpi) is typically ±0.18, and these pressures are applied to the solid portions of the walls and roof.
Importantly, if a building meets both the “open” and “partially enclosed” definitions, ASCE 7 requires it to be classified as “open.” This ensures that the more accurate (and typically less conservative) internal pressure assumptions are applied in design.
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Last Update: December 2, 2025