Realizing that this subject has been fairly well dealt with, I would
like to add just a few things that I felt got left out or might help a
Exposure D is for lakes or seas as well as oceans, which I
assumed was being referred to by the word "coastal". I've used
Exposure D where buildings were on a lakeshore in 80 mph wind
areas that had more than 1 mile of water extending out from the
building site. If you were building this pole near the Salton Sea,
then you might want to consider the use of the 'D' Exposure.
Also, another way to look at the roughness that Bill pointed out is
to relate the air flow to water flow. If you are looking at flow of
water in a smooth pipe, the roughness coefficient is very low. But,
if you are considering water flowing over a rough rocky river bed,
then the roughness coefficient is much higher. And if you look at
the Mannings roughness for the flow of a river channel in an overland
flooding condition, through houses and trees, the roughness
coefficient is very high. Manning's n for concrete: 0.011; for rough
natural streams: 0.050; for natural range land: 0.13; for Bermuda
grass: 0.41 and for woods: 0.45. This just shows how water is
affected by the varying roughness of the surface over which it
I also seem to remember from hydrology classes, that the
temperature differential between the water and the land affects the
way that the wind flows over the each of the two surfaces, but I
didn't verify that and my memory may not be correct on that point.
Thanks for the chance to spout off. :-)
Lloyd Pack, PE
On 12 Jun 2001, at 14:54, Structuralist wrote:
> This is the first time that I have had this one happen. I called the city of
> Indio California to obtain the local wind load design criteria. I was told
> that to use an 80-mph wind, but when I asked what exposure to use, the
> Building Official responded with - "Your the engineer, you determine that!
> Use a minimum of at least Exposure C."
> There is quite a difference between Exposure C and D which has me worried.
> The sign is a 25'-0" tall pole with a 60-s.f. (6'-0" high x 10'-0" wide)
> sign. The sign occurs in a windy off the freeway, however, the freeway is
> lined with trees intending to create a wind break. This prevents sand from
> blowing over and building up on the freeway.
> How can an engineer determine the specific wind load conditions for a very
> specific site if the building official will not provide the information?
> Any suggestions? The sign is for an Auto Body Repair store located on the
> road running parallel to the freeway. The area is open with industrial
> buildings close by, but the area is open for miles.
> Dennis S. Wish, PE
> Structural Engineering Consultant
> (208) 361-5447 E-Fax
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