Wind Exposure Catagory

[flgulfcoasteng]

I would appreciate any opinions on determining the most correct exposure category for a residential (<30 ft mean roof height) structure located in a hurricane prone region (gulf coast of Florida).

This will follow the Florid Building Code Residential rules (a bit revised from ASCE 7) that I have attached for your convenience. I have also attached a crude sketch of the problem.

Scenario: low rise residential structure to be built 800’+ inland from a bay within surface roughness B. The bay has a 1000’ fetch (surface C) toward the coast. Additional 4500’ surface roughness B between the bay and the Gulf of Mexico.

Argument for Exposure B: per R302.2.1.4.3 (attached) under Exposure B the surface roughness B may prevail upwind over a distance of 1500 ft. (more than half of the 1500’ is roughness B (800’)). Is this a loose interpretation of prevail?

Argument for Exposure C: Upwind for 1500 ft roughness B is only 53%+ which is not considered prevailing.

I consider it Exposure C but would like some outside opinions before I force the issue. Thanks.

 

[flgulfcoasteng]

and the sketch...

 

[Ron]

FGCE...it isn't that the Exposure B "may" prevail for the stated distance (either 2600' for =/>30'h or 1500'<30'h), it is that it MUST prevail for that distance. In your case, Exposure B does not prevail for at least 1500 feet in the direction you noted; therefore Exposure C must be used.

Just had a similar argument in a trial this week. Engineer used Exposure B for an airport hangar....3000 feet of open area (Exposure B and C definitions in the 2004 FBC were changed by a declaratory statement in 2006 to read similar to your version)

In my opinion, exposure categories are poorly defined in each of the codes.

Ron

 

[flgulfcoasteng]

Thanks Ron. I am staying with exposure C.

I agree with your opinion that the exposure catagories are poorly defined. i.e. the text under Surface "Roughness" C (not Exposure) describing the case for 100' from 600'Lx150'W Exp C terrain in Exp B has provided a basis for several heated discussions. I'll leave that alone today.. Thanks again Ron.

 

[Ron]

FGCE...read the commentary in ASCE 7-05...it provides a little insight. Further, there's a book written on application of wind loads under ASCE 7 that was fairly good, but I believe the last version only covered ASCE 7-02, but it does cover this issue. I'll pull it out when I get back to my office and give you the title.

Ron

 

[FSS]

Without seeing a more detailed map, I would say Exposure B is OK. You have more than 1500' from the gulf, so you have completely transitioned into a B exposure category (another 3000' until you hit the lake). Your lake is less than 1500' across, so it is treated like an "open patch" and only the areas on the lake get the higher Exposure C. Since you are 800' in from the lake in a presumably Exposure B terrain, you can stay with B.

 

[flgulfcoasteng]

FSS, I see your point and that is one of the issues we (locally) have discussed before. If the Exposure B terrain extends 5000'+ toward shore and has an open patch >600'x150' that is more than 100' away, does it not meet Exp B requirements by the letter... and the next day I read the code again and convince myself of the opposite. That brings me to Exp C as the conservative option I normally follow in these grey areas.

I have been practicing locally for 10+ years and have seen as many different applications as there are engineers. In fact when comparing wind calc continuing cducations classes I have attended with those other engineers have taken it is apparent the we, as a profession in Florida, are all over the board on this. I'm certain the problem is within my understanding, or lack thereof, and unfortunatley I am in the majority on this

Do we have any ASCE 7 committee members in our ranks? Set us straight!

 

[ron9876]

Looks to me like the lake is not a C zone since it isn't 1500' wide therefore it is a patch and it is greater than 100' from the project therefore B.

If the lake was 1500' wide then it would be C. Thats what I think it says.

But the definition for B says at least 1500' so, in my opinion, this is a direct conflict. Whenever there is a doubt I feel you have to use the greater value in our world that is ruled by lawyers.

And ASCE7 certainly isn't clear enough in many areas.