Wind Exposure Catagory
Wind Exposure Catagory
(OP)
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.
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.






RE: Wind Exposure Catagory
RE: Wind Exposure Catagory
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
RE: Wind Exposure Catagory
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.
RE: Wind Exposure Catagory
Ron
RE: Wind Exposure Catagory
RE: Wind Exposure Catagory
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!
RE: Wind Exposure Catagory
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.