Component and Cladding Positive and Negative Wind Pressures

[cycloneengineer]

Hi,

Regarding Component and Cladding wind pressures as defined in ASCE 7-10: Do the positive and negative pressures on a wall component get combined? For example, I have a girt with a trib. area of ±20sf. The calculated C&C pressures (Zone 4) are: +21.2psf and -25.4psf.

Note 3 on Figure 30.4-1 defines the positive and negative values as pressures that act towards and away from the surface. Where I get confused is Note 4, which says "Each component shall be designed for maximum positive and negative pressures." Do you take this to mean combining the pressures or as two separate cases?

If they are to be combined, would you combine them as:
(a) 21.2psf + (-25.4psf) = -4.2psf or
(b) 21.2psf - (-25.4psf) = 46.6 psf

Attached is a picture of how I'm envisioning the wind pressures to act. Thanks!

 

[driftLimiter]

The pressures are not simultaneous. Would check each case independently. Your girt for example may have a different strength inward vs outward depending on the shape, and bracing etc.

 

[phamENG]

Only time they're simultaneous would be a parapet or similar situation where you have both a windward and leeward face to be loaded. And those are pretty clearly defined in ASCE 7.

 

[cycloneengineer]

Thanks - that is what I thought and what I have done in the past, but as I was running through it this morning I managed to confuse myself and needed a sanity check.