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RE: Wind load design for Photovoltaic panel installations

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From: "Jordan Truesdell, PE" <seaint2(--nospam--at)truesdellengineering.com>

To: seaint(--nospam--at)seaint.org

Subject: Re: Wind load design for Photovoltaic panel installations

Good point. We generally don't have plan checks of calculations here in

the mid-atlantic, so I tend to be more at leisure to use a rational

engineering process. Abandoning the obvious calculation method for a

shoe-horned procedure that may not really applicable - but will not

confuse a poorly educated code checker - must be very frustrating.

Jordan

 

 

Dennis Wish wrote:

> 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. <snip>

 

Dennis (and others) - From my experience (limited, but some), the issue with roof-mounted photovoltaics is wind uplift normal to the surface.  The panels are fairly light, and, yes, you can say that the panels eliminate roof live load where they occur.  Use the same formula for roof uplift that you would use on the roof surface for the roof member design. As you are probably aware (but some others not), the roof panels are connected to a rail system which has stanchion connections to the roof at varying intervals - I've seen them spaced at 6 to 16 feet spacings in each direction.  The fewer the connections, the fewer potential water infiltration areas you will have (not normally an engineer's concern, but something that needs to be considered from a whole-building design perspective).  So you will have concentrated roof uplift loads on particular roof members where the stanchions occur.  Here in San Diego, I have seen those uplift forces in the 15 to 20psf range using ASCE methods of calculation. If your panels do not cover the entire roof surface, you could apply an appropriate percentage of this value as acting on the stanchion connection.  The stanchion connections can also be adjusted to reduce the uplift on any one member. 

Karen E. Roberts, SE

San Diego, CA