In the event of a high wind event, it is the concern of building codes ASCE-7 and ASTM specifications that windows and doors don’t fail and develop an opening breach in the building. An opening would allow more wind to enter the structure and pressurize the inside of the building, leading to other doors and windows blowing out, the roof lifting, etc. This is also known as developing a ‘partially enclosed’ building condition in ASCE-7.
In order for an opening to remain classified as ‘non-porous’, the opening must not allow an excessive wind amount to enter the building. This includes even after the window glass breaks or door swings open and the opening is protected by an impact protection system (shutter, fabric, etc.).
Per ASTM 1996, section 3.2.11 (2014a), an opening is defined as porous when the open area of an assembly exceeds 10% of its projected surface area.
ASTM 1996 goes on to say that this porosity factor is defined as the design wind force (per ASCE-7) divided by the horizontally projected area of the entire assembly.
When storm panels are applied to a structure, they frequently develop openings that can create a porous condition. Examples include when corrugated storm panels are built out from a structure for anchorage or separation from glass purposes, turn a corner, etc. For this reason, closure panels are typically designed to restore the opening down to below 10% porous. Closure angle examples are illustrated here which comes from our digitally assisted product approval FL28390 which assists in better calculating anchorage and separation from glass requirements.