47.5 MB - The Whole Building Design Guide

Internal pressure (building pressurization/depressurization): Openingsthrough the building envelope, in combination with wind interactingwith a building, can cause either an increase in thepressure within the building (i.e., positive internal pressure),or it can cause a decrease in the pressure (i.e., negative internalpressure). Building envelope openings occur around doors andwindow frames, and by air infiltration through walls that are notabsolutely airtight. A door or window left open, or glazing that isbroken during a storm, can greatly influence the magnitude ofthe internal pressure.Wind striking an exterior wall exerts a positive pressure on thewall, which forces air through openings and into the interior ofthe building (this is analogous to blowing up a balloon). At thesame time that the windward wall is receiving positive pressure,the side and rear walls are experiencing negative (suction) pressurefrom winds going around the building. As this occurs, airwithin the building is pulled out at openings in these walls. Asa result, if the porosity of the windward wall is greater than thecombined porosity of the side and rear walls, the interior of thebuilding is pressurized. But if the porosity of the windward wall islower than the combined porosity of the side and rear walls, theinterior of the building is depressurized (this is analogous to lettingair out of a balloon).When a building is pressurized, the internal pressure pushes upon the roof. This push from below the roof is combined with suctionon the roof above, resulting in an increased upward windpressure on the roof. The internal pressure also pushes on theside and rear walls. This outward push is combined with the suctionon the exterior side of these walls (see Figure 3-6). When abuilding becomes fully pressurized (e.g., due to window breakageor soffit failure), the loads applied to the exterior walls and roofare significantly increased. The rapid build-up of internal pressurecan also blow down interior partitions and blow suspendedceiling boards out of their support grid. The breaching of a smallwindow can be sufficient to cause full pressurization of the facility’sinterior.MAKING CRITICAL FACILITIES SAFE FROM High Wind3-11