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Re: Another ASCE 7-05 Wind Load Question

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Conrad Harrison wrote:

Interesting, using letters for exposure classification. I guess you cannot interpolate between classifications and multipliers, or start with roughness length (z0) and side of a hill or escarpment. So AS1170.2 is fine for wind flowing off the Adelaide plains up the Adelaide Hills, but not realistic within the hills themselves.

How does ASCE7-05 treat topography?

ASCE 7-05 defines a roughness length parameter, z0, in the Commentary to Chapter 6 (Wind loads) on page 287. The typical values and ranges for each exposure category is given in Table C6-8 on page 319. The formula for z0 is:

z0 = 0.5*Hob*Sob/Aob where

   Hob = average height of the roughness in the upwind terrain
   Sob = average vertical frontal area per obstruction
   A0b = average area of ground occupied by each obstruction

I find it easier to look at the photos in the Commentary that are representative for the exposures or use C if in doubt and I know I'm not near water.

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