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RE: Wind load design for Photovoltaic panel installations

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Dennis,

 

I don’t recall any clear guidance in ASCE 7-05 for the particular issue of solar panels on a roof. We did have some recent discussions in the ASCE 7 Wind Subcommittee about signs (or similar elements) attached to building walls that may be of use. The concern was that people were using the (very complex) solid freestanding sign/wall provisions for a sign board attached to a building (think your typical supermarket or big box store). The conclusion was that as long as the sign was not located in an edge zone, and was not offset more than three feet from the solid wall behind it, that the sign could be designed using the same positive and negative C&C pressures as would be used for any component that was part of the wall itself. In other words, as long as the sign was out of the highest-turbulence, largest wind flow-separation zones and fairly close to the roof, any wind pressure developing on the bottom surface of the sign would not be enough to cause concern and the sign would behave like part of the wall. It makes sense to me that the same principles would apply to an element on the roof.

 

As for dead loads for solar panels on truss roofs, there was a brief discussion in SBC magazine on the subject (http://www.sbcmag.info/Archive/2007/dec/0712_tqa.pdf)  which gives some guidance.

 

Regards,
Gary

Gary J. Ehrlich, PE
Program Manager, Structural Codes & Standards
National Association of Home Builders (NAHB)
1201 15th Street, NW, Washington, DC 20005
ph: 202-266-8545  or 800-368-5242 x8545
fax: 202-266-8369
gehrlich(--nospam--at)nahb.com

 


From: Dennis Wish [mailto:dennis.wish(--nospam--at)verizon.net]
Sent: Wednesday, August 13, 2008 4:04 AM
To: seaint(--nospam--at)seaint.org
Subject: Wind load design for Photovoltaic panel installations

 

Can anyone guide me to the section of the ASCE 7-05 that I would need to use to design the wind load conditions for contractors and distributers who are retrofitting solar electrical panels onto existing roofs? I would consider the component to be open from the bottom and would need to find the design section in the current IBC 2006 and ASCE 7-05 for wind design acting upward on the panels as well as the resultant wind force acting downward on the panels based on the slope of the roof and the geographic location or the building departments requirements for local wind speed and wind category.

 

Can anyone get me started? I think the problem is going to narrow down to checking the existing roof capacity for the additional downward and upward force of wind plus the weight of the panels and their means of connecting to the roof. In areas where the roof is designed using metal plate wood trusses there is going to be a potential problem with proving the retrofit weight of the panels on systems that are traditionally designed only for the weight of the materials original to the design of the home. At least two local contractors have contacted me about this problem since the local building departments are requiring a design by a local engineer to assure the capability of the roof to carry the additional load and the potential for uplift that could dislodge the panels from the roof – tearing the roof away with the panel mountings.

 

Has anyone worked on this type of problem since the adoption of the new code in January? If possible, please reply to me privately (unless others are interested in this type of problem for their area) as I need to decide whether or not to take on this type of work assignment considering the potential liability of the wind effect under the new code on an existing roof.

 

TIA

Dennis

 

Dennis S. Wish, PE

 

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

La Quinta, CA 92253

760.564.0884 (Phone, Fax and Answering Machine)

dennis.wish(--nospam--at)verizon.net

http://structuralist.wordpress.com

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