Wind uplift loads [RE-wrenches]

[Robert Warren]

Mo, 
 Three or four years back, I attended a PV-grid tie class taught by Bill 
Brooks. The material he used from BP Solar showed that the wind load 
figures they used were 24 psf, but that general safe engineering 
practices would take a minimum figure like this and double or even tripe 
it. This class was given in Florida, so a local installer reported that 
Dade County required a safety factor of 2.0. Given that this is 
hurricane country, I was also surprised that this was so low. 
 Anyway, the stock 2kW BP system using 27 modules had a surface area of 
221 sq ft. with 24 mounting brackets,  with only 1/2 inch fastener depth 
(3 or 4 per bracket)into plywood, so they were figuring 24 psf.
 I have seen hurricanes strip off the whole roof, so mounting into 
plywood doesn't do much good. So going by the book can be stupid. I am 
quite sure the general consensus of most all Wrenches on this list are 
that we are all aiming for rafters.
 As I recall, Bill Brooks was recommending 3/8" lag screws 3" long, and 
drilling a pilot hole first so it doesn't split the timber. I am sure we 
will hear from Bill if I mis-quoted him on this.
 The other factor is in an area like where you are in Hawaii, with 
tropical storms, is to keep the top edge of the modules 18 inches below 
the roof peak to reduce direct injection of wind under the top edge. 
Robert Warren  
     


In speaking to one manufacturer of racking systems, the design up-load 
that they use for a PV array is 30 to 50 psf, depending on what wind 
speed criteria is used (80-120mph).

I don¹t know if this helps, but I was certainly ³blown² away by those 
numbers.
I always thought the wind up-load was about half of that. A very 
compelling argument for hitting rafters!

Mo 


What will we do if there is another fuel shortage? 
Did you know that there is a pollution free, completely
renewable fuel which you can use right now in your own car? 
Don't replace the engine, replace the fuel.
Check out my website on making your own fuel: 
http://running_on_alcohol.tripod.com 
robertwarren at mail.com

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