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RE: Wind Loads on Roof Mounted Solar Panels

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Gary,

Initially, I thought “yes” to your suggestion.  I have a bank of panels 15.5’x13.5’ which are (for arguments sake) 20’ from grade.  They will be on a residence 53’x60’+-.  Using the 1.9 factor in that section I get 33.3 psf on a plane normal to the wind direction (vertical plane) as indicated in figure 6-21 where I also get a Cf=1.3.  The panels are at 10 deg. to the horizontal roof, therefore I get an outward pressure from the panels of about 6 psf.  C&C loads for a flat roof will be about 17 psf and using the figure 6-18A the outward pressures are about 15 to 21psf depending on Load case A or B.  Using 6.5.15 seems very low.  I would think they should be somewhere in the same range as C&C loads and those from 6-18A are compare fairly well.  But, If I was totally sure I wouldn’t be asking…..

Joe

 

Joseph R. Grill, PE

Verde Valley Engineering, PLLC

2220 Sky Drive

Clarkdale, AZ 86324

Ph. 928-600-5459

Fax 928-649-3659

email: VVEng(--nospam--at)cableone.net

 

From: Ehrlich, Gary [mailto:gehrlich(--nospam--at)nahb.org]
Sent: Friday, June 04, 2010 11:52 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wind Loads on Roof Mounted Solar Panels

 

Joseph,

 

Actually, I’m not sure Figure 6-18A is the correct one to use. That figure (and the associated provisions of 6.5.13) is intended for large, at-grade structures like picnic pavilions, porte-cocheres and other open buildings, where the wind loads are typically resisted by tranverse moment frames. (Side note: ASCE 7-10 adds language to deal with the longitudinal frames as well.)

 

For a smaller, roof-mounted element such as a solar panel, I think the loading should be obtained from Section 6.5.15 using the force coefficients from Figure 6-21. Equation 6-28 used with the Figure 6-21 coefficients and the vertical projected area of the panel will give you the lateral wind load. Then apply the factor in Section 6.5.15.1 (as adjusted by the ratio of horizontal projected area of panel to horizontal projected area of roof) to generate the uplift wind load.

 

Regards,

Gary

 

GARY J. EHRLICH, P.E.

National Association of Home Builders

D 202 266 8545

gehrlich(--nospam--at)nahb.org

 

From: Joseph R. Grill [mailto:vveng(--nospam--at)cableone.net]
Sent: Friday, June 04, 2010 1:55 PM
To: seaint(--nospam--at)seaint.org
Subject: Wind Loads on Roof Mounted Solar Panels

 

I have been asked to check the existing framing on a roof for the installation of a bank of solar panels.  The roof is flat and is framed with solid sawn light timber.

 

I am trying to put together the wind loads on the panels to be used in conjunction with the gravity loading to check the existing framing.  I am looking at figure 16-18A as the panels are set at an angle and on frames which are above the roof surface.  I think the air flow is “unobstructed”.

 

I can find no definition of the Load cases A&B.  Do these Load Cases have to do with the wind direction?  Figure 6-18B shows one wind direction but both load cases A&B.  Any help with this question?

 

I guess as long as I am asking, I am pretty sure 6-18A is correct but should C&C loading be used?  I am guessing they will be pretty close to the same.

 

Thanks,

Joe

 

Joseph R. Grill, PE

Verde Valley Engineering, PLLC