3.3 Wind Suction Effect on Building Walls and Roofs

When wind interacts with a building, both positive and negative (i.e., suction) pressures occur simultaneously. Internal pressure changes occur because of the porosity of the building envelope. Porosity is caused by openings around doors and window frames, and by air infiltration through walls that are not absolutely airtight. A door or window left in the open position also contributes to porosity.

Walls: Wind striking an exterior wall exerts a positive pressure on the wall, which forces air through openings and into the interior of the building (this is analogous to blowing up a balloon). At the same time the windward wall is receiving positive pressure, the side and rear walls are receiving negative (suction) pressure; therefore, air within the building is being pulled out at openings in these other walls. As a result, if the porosity of the windward wall is greater than the combined porosity of the side and rear walls, the interior of the building is pressurized. But if the porosity of the windward wall is less than the combined porosity of the side and rear walls, the interior of the building is depressurized (this is analogous to letting air out of a balloon).

Roof: When a building is pressurized, the internal pressure pushes up on the roof. This push from below the roof is combined with the suction above the roof, resulting in an increased wind load on the roof. The internal pressure also pushes on the side and rear walls. This outward push is combined with the suction on the exterior side of these walls. Therefore, a pressurized building increases the wind load on the side and rear walls as well as on the roof.

Local Suction: Local suctions are most serious for wind at an angle (usually about 45 degrees) to the side of the building. Local pressure coefficients of -2.0 are not unusual, and in certain model studies values down to -5.0 have been measured over very small areas near corners projecting into the wind; this indicates the importance of proper fastening of roof coverings at such points.

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