Geographical Values

Exposure Category

The Exposure Category refers to what types of structures or obstructions are around the installation site.  Details of how to accurately determine the Exposure Category are covered in the appropriate version of ASCE 7 §26.7.  For quoting and preliminary design purposes, the simplified descriptions below can be used.

Exposure B – Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of singe family dwellings or larger.

Exposure C – Open terrain with scattered obstructions – open country and grasslands

Exposure D – Open costal terrain w/ no obstructions

 

Topographic Factor, K_zt

The Topographic Factor corrects for wind speeding up at elevation changes.

K_zt = 1.0 for most site locations including:

          Flat or gently sloping ground

          Gently rolling hills with less than 2 miles between crests

          Sites less than halfway up an isolated hill or ridge

          Sites on slopes of escarpments but not near the crest

For installation sites in valleys, more than halfway up an isolated hill or ridge, or near the crest of an escarpment, consult the appropriate version of ASCE 7 §26.8 for the method to calculate K_zt.

 

Elevation Factor, K_e

The higher the elevation of an installation site, the lower the wind force is for a given wind speed due to reduced air density.  ASCE 7-10 does not use this factor so in that case, the value of K_e is set to 1.0.

The table below shows the K_e values for various elevations.  Round the site elevation down to the nearest value on the table below, or K_e can be calculated directly using the formula at the bottom of the table.

As an example, for the same wind speed, the wind forces in Denver, CO, (Elevation 5,280 ft) would be around 17% lower than the wind forces in Houston, TX. (Elevation 80 ft)

If the site elevation is unknown, K_e can be conservatively set to 1.0.

per Table 26.9-1 K_e = e^{-0.0000362Z_e}

^{e = 2.71828  (Euler’s Number)}

Some localities may not allow the use of K_e < 1.0.  For example, at the time of this writing, the building code for Washoe County, Nevada states “Air density corrections to design wind pressures are prohibited”

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