Exposure Category D

[drewtheengineer]

Exposure Category D is defined as "Flat, Unobstructed wind flowing over open water..." I have a situation where the site is up on a hill but does have an unobstructed path to over a mile of open water.

I'm tempted to use 'D' however I'm thinking that perhaps the effect caused by the water only occurs on the wind a few feet above the water and that's why it specifies "Flat". Then I should go with Exposure 'C'.

Any takers on this one?

 

[fritzee]

I would agree that you should be using category 'C' for this mountinous site. The contour of the terrain can reduce the wind velocity more than open water. Category D should only be used for shore line conditions.

 

[PMD123]

Please Don't forget to determine your Kzt factor for escarpments, since you are on a hill

 

[drewtheengineer]

Right, of course Kzt. That factor actually added to my confusion though. It states that, among other criteria, if the hill is 15' or over for Exposures C and D, you need to take the Kzt factor into account. But I thought Exposure D was supposed to be FLAT. I guess the question is how flat is flat? If you are sitting on a hill off the water that is say 20', I wouldn't consider that flat, so therefore Exposure D would not apply.

 

[chichuck]

I think that exposure D is the correct one. The hill upon which the structure is located does not change the terrain characteristics upwind. It is the upwind characteristic that determines the exposure catagory.

The fact that your building is situated on a hill is to be accounted for by the factor Kzt.

 

[haynewp]

Try asking the building official what is required in the county.

 

[shemp]

Which version of ASCE 7 are you using? If you're using '98, and your site is in a hurricane prone region, then C is your exposure category.

 

[drewtheengineer]

Yes, '98. It's not in a hurricane prone region. The water is an inland waterway not the Atlantic Ocean.

I guess my confusion lies in the fact that my site is on a hill, off the water. Exposure D specifically defines an applicable site as FLAT. Am I reading FLAT wrong? Can a site still be FLAT with a big ass hill on it? I think I'm going to use "C" with my defense being this site is no where near FLAT.

 

[steve1]

One comment. If you haven't yet done so, please look in the commentary section of ASCE 7-98, there is a very good discussion, with photos, of how each exposure category is defined.

 

[drewtheengineer]

Yes, I did read the commentary. There is little discussion on Exposure D.

 

[chichuck]

drew,

in ASCE 7-98, section 6.5.6.1 it says: "..... For any ginven wind direction, the exposure in which a specific building or other structure is sited shall be assessed as being one of the following categories:"

What the phrase 'exposure in which the buliding is sited' means is, the terrain features upwind of the site, for the distances noted in the definition for each exposure category. In the case of exposure D, this means, open water fo ra distance of atleast 1 mi upwind of the site.

The effects of that water are not limited to just heights near the sea level, rather they extend up to relatively large heights. This is best explained by an example of exposure B. Note that the Code says that it is to be used when the upwind terrain for exposure B (e.g. obstructions the size of single-family dwellings) prevails for at least 1500' upwind of the site. The rationale for this requirement is that wind, blowing over a smoother terrain than expoure B suddenly reaches a place where the (rougher) surface of exposure B prevails, then the wind velocity at the ground level is reduced. As the wind continues farther, over more terrain with the exposure B roughness characteristic, then the wind at the surface keeps slowing down more and more, and at the same time, wind at higher levels continues to be slowed down as well. Eventually, the velocity profile reaches an equilibrium, and does not change anymore. (that is, as long as no other terrain changes occur.) The 1500' upwind min specified was deemed to be enough for this equilibrium condition to be achieved. There is some disagreement on whether this 1500' limit is correct, look at ASCE 7-02, you'll find these have changed a little. But since you have ASCE 7-98 to use, that issue is moot.

For your case, if there is at least 1 mile of open water, then the effects of that water on the wind profile have reached this equilibrium, and have already reached the top of the windspeed profile. So if you use Exposure D, you will be using the code-defined wind profile that corresponds to this condition.

A similar description could be used for wind over water, that suddenly encounters a change to a rougher, Exposure C type terrain. But the 98 code does not indicate a limit in feet, so I tried to use the C-B terrain change as my example.

Is this clear, or have I just confused everyone?

 

[drewtheengineer]

Chichunk,

You were very clear, one might even go as far to say crystal clear.

So if my site was located toward the back of the 1500' distance and had rolling hills in front of it, I could then say it isn't flat and therefore use Exposure C. But what if the site is located say 800' from the water but still had rolling hills in front of it, or 100' with only one hill in front of the hill it's sitting on, can either of these be reduced to Exposure C? How much terrain irregularities do I need to knock it down to "C"?

Let me put it another way, in order to use Exposure B, you need 1500 ft of crap in front of you, slowing down the wind. Similarly, I'd say you'd need 1500 ft of crap in front of you to go from D to C. But by definition, Exposure D only applies if you are within 1500 ft of the water, therefore you'd never have 1500' of crap in front of you while in Exposure D.

True or False:

Exposure D is used when otherwise Exposure C would be used but it is within 1500' to over a mile of water.

Is it just me or is the Exposure Categories extremely subjective?

 

[chichuck]

drew,

Here's what I think:

a) structure 1200-1400' from water, with rolling hills over most of that distance, that's exposure C.
b) structure 800' from water with rolling hills, over most of that distance, I'd be conservative and use exposure D.
c) structure 100' from water with 1 hill upwind, that's exposure D.

For your true-false question, I say it is true.

Lastly, I'd say yes, this is very subjective.

chichuck