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RE: Wind Loads on Roof Mounted Solar Panels

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Joe,

The load cases A & B account for the load not being applied concentrically, which could increase the uplift at one end or the other. Same concept as skip loading on a continuous beam. Both cases need to be checked.

As for primary structure vs. C&C, I think you could argue it either way, but I would suggest going with the method that gives the larger load. For calculation as a primary frame, it would be prudent to consider the wind speed increase as it flows over the roof. This would be analogous to the wind at the top of a hill. C&C already has that built in to some extent, thus the higher resulting loads.

Dmitri Wright, PE
Cascade Engineering, Inc.
245 SE 4th Ave, Suite B
Hillsboro, OR  97123-4033
dmitri(--nospam--at)cascade-structural.com
503-846-1131




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





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