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Wind Exposure Categories

Wind Exposure Categories

Wind Exposure Categories

(OP)
I am a structural engineer for a solar company and we have recently hired a wind tunnel to test our solar panel mounting frame, which is a ballasted system intended to sit on a roof top.  We had them test a few different ballast scenarios, all at exposure category C, to failure in either uplift or sliding.  Once we have a failure wind speed, we can determine the allowable building height based on the site specific basic wind speed shown in ASCE7.  

Since all of our results are given for exposure category C, does anyone know if there is a way to translate my failure wind speeds to category B?  

Since the building in wind tunnel modeled as a 15' tall building, and since wind pressure is a function of the square of the basic wind speed, my first inclination is to square the failure velocity for category C, multiply by the ratio of the Kz values for categories B and C at the 15' elevation, then take the square root of the entire quantity.  This should give me the failure wind speed for the same solar panel array, on the same building in exposure category B.  

The problem is that my results are well beyond what I would have intuitively thought.  For example, in the Basic Wind Speed region of 90 mph, I am getting an allowable building height in exposure category B that is almost 20 times the allowable building height in exposure category C.  Seems too good to be true.
 

RE: Wind Exposure Categories

Use the adjustment factors in Figure 6-2 of ASCE 7-05.

RE: Wind Exposure Categories

(OP)
I have revised my calcs to use Figure 6-2 instead of Table 6-3 and the results are more reasonable.  The allowable building height for category B is now 10 times the allowable building height for category C.   

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