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Re: Wind load design for Photovoltaic panel installations

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David,
 
        Canadian codes specify a minimum live load to provide for workers constructing or maintaining the roof system.  I expect your codes would have a similar requirement.  Just a thought that may be helpful.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Wednesday, August 20, 2008 9:21 AM
Subject: Re: Wind load design for Photovoltaic panel installations

Dennis,
 
I can understand the plan checker saying the panels are dead load, but why would the live load not be reducible?  Or even zero as others have pointed out?  It becomes physically challenging to "load" the roof where a solar panel occurs... 
 
I'm curious to follow this thread as I am going to a site later today regarding a solar panel installation.

Thanks.
 
On Wed, Aug 20, 2008 at 12:58 AM, Dennis Wish <dennis.wish(--nospam--at)verizon.net> wrote:

I had a chance to speak with the plan checker on the project (it seems that it is back on if the client is willing to work with me). The facts are as follows:

1. The weight of the panels and rack system that they rest in are considered a dead load which is most critical considering that even at 16-psf of dead load in the roof a 3-psf additional load represents nearly 20% of the full dead load to be added.

2. The live load is not allowed to be reduced according to the 2007 CBC.

3. I had a chance to review the calculations from the last engineer who designed the panels for a trussed roof. The problem was that he used a FEM program to design the trusses but checked only the chords and struts for stress. The problem as I see it is that the connection where the press plate is located becomes the weak link and is also proprietary based on the plates used, and the pressure of the machinery the yard uses to make the connection. This proprietary information can only be found from the original truss package which in most cases does not exist.

4. The plan checker was also concerned about the uplift and pull-out values on the wood screw connection. Assuming it is a pre-drilled connection, then the NDS or AITC manual provides sufficient information to verify the size and embedment of the lag screws needed to hold the panel rack in place.

 

I see the bigger problem as the point load reaction at the connection of the rack to each truss acting against the roof based on the additional 3-psf dead load that needs to be added on some fifty or sixty panels plus the wind that acts normal to the roof at the top of the panels which becomes a concentrated rather than uniform load against the trusses. Again, my concern is addressing the metal side plate pressed connection rather than the ability of the chords and struts to resist the additional force.

 

I think that we have forgotten that the small dead load of the panels can have a tremendous affect upon the trusses when you consider that only 3-psf can represent between 15 and 20% over the design values for a manufactured truss roof. On a conventional stick roof I would have less fear since I can run the numbers on my own and anticipate how the joists/rafters will perform.

 

Any ideas?

 

Dennis

 

From: Tarek Mokhtar [mailto:Tarekmokhtar(--nospam--at)earthlink.net]
Sent: Friday, August 15, 2008 3:47 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wind load design for Photovoltaic panel installations

 

Yeah, I guess all these roofs that were designed for 16 PSF from the start of time

are gonna fail now, what a concept >:-o

 

Tarek Mokhtar, SE

 

 

 

 

Yeah, no shit. :-)

-gm

On Fri, Aug 15, 2008 at 12:46 PM, Bill Allen <T.W.Allen(--nospam--at)cox.net> wrote:

16 PSF on a 4:12 slope?

 

Not dialed into the 2007 CBC / ASCE 7-05 I see.

 

:o)

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS

Consulting Structural Engineers
 
V (949) 248-8588 * F(949) 209-2509

-----Original Message-----
From:
sscholl2(--nospam--at)juno.com [mailto:sscholl2(--nospam--at)juno.com]
Sent: Friday, August 15, 2008 11:33 AM
To:
seaint(--nospam--at)seaint.org

Subject: RE: Wind load design for Photovoltaic panel installations

 

I have entered this late but wish to learn what the problem is.

It seems to me that photovoltaic panels (which I believe weigh about 3 lbs/sq. ft.) will prohibit anyone walking on the roof (for which we design for 16psf on a 4/12 slope) and thus the load is less not more than typical design load.  Is this what is of concern?

Stan Scholl, P.E.

(someone with a very low  PE number)



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David Topete, SE