The Behavior of Wind on Building Cladding
- Wind is created by our atmosphere in motion. The faster it moves the more pressure it will exert against a stationary object or surface in its flow path. This type of pressure is generally referred to as a positive pressure.
- As the wind has volume and mass however, when its flow is interrupted by the flat surface of a building, it can “pile up” in the center and become virtually stationary or even “bounce-back.” It will then move laterally. Often such a condition will reduce or eliminate positive pressure in some areas and instead create negative pressures at the building surface.
- The areas around the edges or corners of buildings are usually subjected to the most severe loading. As the mass of air becomes static in the center of our surface, the onrushing air passes out and around the building mass, attaining maximum velocity as it rounds the corner of the building. Wind velocity “v” creates static pressure (Pressure “P” = 0.00256v2); and further, note that a steady wind velocity pressure will sustain a certain head of water (Water Head “h” = 0.00048v2).
- The American National Standard Institute (ANSI), American Architectural Manufacturers Association (AAMA), and all industry test laboratories use the basic form of the above formulas to determine velocity pressure and velocity head at specified wind velocities.
- However, don’t confuse “velocity pressure” a with design wind load. In actual use, codes and industry standards such as ASCE 7 “Minimum Design Loads for Building and Other Structures”, take into account building height, location, orientation, velocity zone, internal loads, etc., to determine how basic velocity pressure will be applied to specific situations. In many cases, Boundary Layer Wind Tunnel Testing is used to quantify the effects of terrain and surrounding buildings on wind pressures.
- ASCE 7 was updated in 2010 to use strength design, while allowable stress design is an alternative methodology. This changed design wind speed maps and pressure calculations. Make absolutely sure any design pressures are appropriately qualified as one or the other, to avoid misapplication.
- Every Wausau quotation includes the following qualification whenever Wausau calculates Design Loads. Determination of loads must be done by the building’s structural engineer of record.
Disclaimer: The Design Loads noted herein were determined by Wausau Window and Wall Systems' Structural Engineering department based on information available. Verification of Design Load(s) is the sole responsibility of the building’s Engineer of Record, and Wausau will not be held responsible for additional costs resulting from differences in Code interpretation or prescriptive requirements not included in Contract Documents in our possession. We strongly recommend that Design Loads (in psf or Pa) specific to all relevant areas of the building be provided by the Architect.