IBC Wind Exposure C definition - Florida Building Code

[lbeci]

The 2006 ammendments to the Florida Building Code 2004 have recently changed the definition of wind exposure C to match the definition as listed in the IBC code. I have had many interpretations of this definition and have not had one agree with the other. I am hoping to get some feedback from other engineers who have been working with this definition in the IBC.
The definition reads: "Exposure C. Open terrain with scattered obstructions, including ... ... extending more than 1500 feet from the building site in any quadrant."
The definition then goes on to say "This exposure shall also apply to any building located within exposure B type terrain where the building is directly adjacent to open areas of exposure C type terrain in any quadrant for a distance of more than 600 feet." --This is the part that is getting me. -

Does this mean that an exposure C wind design (which is 1500 feet or wider) must be extended 600 feet into an adjacent exposure B design?

or

Does this mean that the open area (C terrain) only needs to be 600 feet wide next to a building in B terrain to design the building as exposure C? - this seems to contradict the first definition of 1500 feet for exposure C

Any incite into the definition would be greatly appreciated.

 

[swivel63]

use RWDI, LOL

 

[haynewp]

If you look on page 280 of the wind commentary of ASCE 7-02, it shows a picture of exactly what they intend by this provision.

 

[Ron]

lbeci...your second assumption is the correct one as haynewp pointed out in the ASCE commentary. An otherwise Cat B exposure becomes a Cat C exposure with an open area of 600 feet or greater upwind.

 

[lbeci]

ok..so you guys are saying to use the ASCE7 commentary to expalain the definition in the IBC

so if my second assumption is correct than does that mean a building in an area of C type terrain which is say 1400 feet wide (<1500 feet) would be designed per exposure B and a building located within B type terrain which is adjacent to an area of 1400 feet wide C type terrain should be designed as exposure C?

this does not make sense to me..what am I missing?

 

[haynewp]

Look at the figures on pages 301, 302, and 303 of the ASCE wind commentary. I think it may have something to do with this "open patch" effect but it does seem a little weird. I will compare FBC and ASCE again tomorrow, but the language is pretty straight in the FBC 06' amendments that this is how they expect it to be treated.

 

[FSS]

Your building in the 1400' wide area would also be a C because you would have exceeded the 600' patch threshold.

 

[lbeci]

FSS - what about the original definition of exposure C? An area of 1500 feet or greater?

 

[FSS]

I may have misunderstood options. I reread and say the 1400' wide area is a C because it is in a C area. Patch language does not let you go down to a B from a C, because your total area is a C. The patch language is intended to account for localized areas within B where winds are more like a C.

 

[bcca]

Sorry that I am jumping in a bit late. Obviously Florida is affected greatly because it is very flat. I heard a different interpretation from a plan reviewer in Fla.

1. Exposure C is only defined to land or a site as 1500 ft of open terrain in both directions. It then takes 600 (600 x 600 field) ft of obstruction (30 ft or greater) to transition land as B when it is adjacent (any quadrant) to it.

Building within 600 feet of exposure C (1500 x 1500 flat terain) would still clasify you as C. Since developments are never perfectly squared, this add'l definition clarifies how to handle odd shapes.

This is intended to handle exposures to land adjacent to large bodies of water rather than just addressing the Coastal (CCCL) line.

 

[ash060]

Ibeci
I also work in Florida and the code change has finally caught up with things. I use my judgement for B and C and if it is really is borderline than I call the Building Offical and they will tell you what exposure you have.

 

[bcca]

The 600ft part does seem to be the most misunderstood part.

I believe the "open patches" is more of an opening (600'+ x 600'+) of the 1500'+ exposure C terrain that actually would be directly adjacent to the building (in any quadrant). It could have been worded better reversed that it takes 600' of transition.

If it was interpreted as 600' deep flat field next to a house constitutes a C, then I believe we would have too much work to do...

 

[structuralaggie]

My wind guru professor in grad school said he would always use exposure C as a minimum. Easy to say when you are not worried about a client firing you.