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RE: Wind load design for Photovoltaic panel installations

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Jordan,
While I thank you for the information, I think selling this method to a
local residential building plan checker is going to raise more questions
about compliance with what section of the code that is applicable than it is
worth. I think the solution lies with following the component and cladding
design in ASCE 7-05 for an open structure and calculating both the uplift
and the downward pressure on the panel area (remember too that there may be
as many as 60 some panels on the roof depending on the conversion from
Kilowatt usage to the number of panels required, their angle added to the
roof angle and the reactions to the legs of the supports that hold the
panels.
In either case, see my response to Gary Ehrlich as I go into greater detail
about the general use of these panels on a structure that is constructed
using manufactured wood trusses and the potential problem and liability
involved. It may be more trouble than it is worth unless well protected by
E&O insurance. Even still, it probably would not pay the premiums.

Dennis

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint2(--nospam--at)truesdellengineering.com] 
Sent: Wednesday, August 13, 2008 9:58 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wind load design for Photovoltaic panel installations

Depending on how much clearance there is below the panels, and how 
steeply angled they are, you could use elementary aeronautics equations 
for flat plates. The lift is proportional to the angle of attack (angle 
to roof plane, I suppose, here), and the center of pressure is 25% back 
from the leading edge.  The lift is something like 2 x pi x alpha x 
stagnation pressure. That's only good for angles up to about 25-30 
degrees if I remember correctly, then the plate will "stall" and it 
starts to look more like a drag element with Cd approaching 2. A first 
year aero book, like Bertin & Smith (Aerodynamics for Engineers) covers 
the basics if you're willing to wade through - or gloss over - the N-S 
equations to get to the simplified answers for your case.

Jordan




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