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(Negligible Internal Forces) |
Partially Open (Moderate Internal Forces) |
Partially Enclosed (High Internal Forces) |
Enclosed (Moderate Internal Forces) |
Wind pressures can destroy a structure in more ways than just blowing it over. When wind is allowed to flow through or around a building, then the pressures generated can be relatively moderate and be limited to pushing and uplift. However, if the conditions are just right, wind can blow into a structure, inflate it like a balloon, and burst it from the inside.
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(Image from Simpson-Strongtie – Maybe re-work this ourselves to fit our narrative above?) |
When calculating pressures on a structure, it is important to first determine its enclosure classification. As defined in ASCE 7-16, Chapter 26.2, these 4 classifications – Open, Partially Open, Partially Enclosed, and Enclosed – help determine the severity of pressures that may accumulate in and around the structure during a storm.
Classifying a structure may require a bit of calculation and critical thought:
Open & Enclosed Structures
Open and Enclosed structures are the easiest to identify, as long as you remember that the entire structure must satisfy the classification:
An Open structure is one where each wall must be at least 80% open (allowing wind to flow into and through it).
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The structure is “Open” when Each Wall is at least 80% open. Internal pressures are negligible |
Open structures may have short walls or windows as long as only 20% of each side’s area is solid |
However, if even one wall fails the 80% open test, the structure is NOT OPEN |
A structure may be considered Enclosed when no wall (including the roof) has any openings larger than 4 sq ft (or 1% of the wall’s area). Note that a structure may still have windows and doors, but as long as they stay closed during a wind event, they are till considered part of the wall.
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As long as all windows and doors stay shut, the structure can be considered Enclosed |
Openings must be smaller than 4 sq ft and 1% of its host wall to still be considered Enclosed |
Damage from a storm may cause an enclosed building to allow air in, dramatically increasing internal pressures |
Partially Enclosed and Partially Open Structures
Wind being allowed to flow into a building can create a dangerous buildup of internal pressure. When a building is just porous enough with a opening on one side, the building envelope can act like a balloon, internal pressures can as much as triple, and weaker components such as windows or doors can be at much higher risk of failure until the wind is allowed to escape.
When a building is in danger of this buildup of internal pressure, it is considered “Partially Enclosed”. The structure has just enough open space along its walls and to allow wind to flow in, but not enough for it to flow out. This “Partially Enclosed” categorization can be calculated directly by satisfying both of the following two conditions (per ASCE 7-16 Ch 26.2 – Building, Partially Enclosed):
1. The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls and roof) by more than 10%
This condition describes the ballooning effect. When the openings on the wall which the wind hits directly (a wall that receives positive external pressure) is bigger than (exceeds…by more than 10%) the rest of the openings the wind wants to flow out of (the sum of the areas of openings in the balance of the building envelope), then the structure is at risk of inflating like a balloon.
2. The total area of openings in a wall that receives positive external pressure exceeds 4 sq ft or 1% of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%.
This condition is written to confirm that the balloon described in condition 1 has a significant enough opening (exceeds 4 sq ft or 1% of the area of that wall) and that the rest of the walls don’t have relatively huge openings themselves (openings in the balance of the building envelope does not exceed 20%). If the wind couldn’t easily flow in, or would have too easy a time flowing out, the structure wouldn’t act like a balloon at all.
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The structure is most likely Partially Enclosed when wind has exactly one way in and no way out. |
The main opening may be large. This structure will act like a sail rather than a balloon, and the internal pressure is still very high |
When wind is allowed to flow through the structure, internal pressurization occurs, but dangerous ballooning will not. |
Anything that does not fit into the above categorizations is classified as a “Partially Open” structure. This ultimately describes that will generate some level of internal pressurization by allowing wind to flow into or through it, but still has enough open space on the rest of its walls & roof to allow that wind to safely flow out.
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Partially Open Structures with large openings on multiple sides will allow wind to flow through, but still trap some air inside, resulting in internal pressures |
Creative structures with some openings and some walls will likely result in Partially Open classification. |
For structures with considerable porosity or unique layouts, consult with an engineer or use one of our online calculators |
Determining enclosure classification is an important step when calculating internal pressures that will affect building components such as windows, doors, and storm shutters. Many times, structures that are to remain habitable during a storm require their main wind resisting systems to be designed considering the most extreme “partially enclosed” internal pressures (in case of a window blow out or damage due to windborne debris). Even with these more stringent designs, hurricane-resistant windows and shutters may only require moderate “enclosed” pressures be considered in the design process. Consult with your designing engineer and local building departments to confirm all of your enclosure classification and calculation requirements.
**Here’s where it gets tricky. We have C&C (& Garage) wind charts, as well as an online calculator that all calculate pressures for structures. These tools all assume an enclosed design (same calcs for Partially Open) and disclaim this in the notes / report page (report page may only be visible after purchase – need to check this). Additionally, the code has sections for sunrooms / pergolas that detail that they can be designed as partially enclosed or enclosed at the engineer’s discretion. However, most buildings need to be permitted as partially enclosed, meaning all the components & elements in the design are rated to resist partially enclosed internal pressures.
Now, after this article detailing the difference between all of these classifications, how can we provide just “enclosed” wind charts? Further, the “partially enclosed” wind charts and calcs will be triple what is shown in the enclosed charts (making them nearly worthless??).
Related to all this, we have an enclosure calculator (which surely needs work). Is this a tool that would be helpful to people? Can we market it? What sort of seal would go on this? Could we even certify reports without actual plans? Tricky stuff.
Last Update: July 9, 2024
