Uplift Wind Loading Values [RE-wrenches]

Sat Mar 22 16:58:02 PST 2003

Jeff,

I didn't feel any stones. In fact, I really thank everyone who contributed to
this very interesting subject. Missionary-position PV gets the job done but
design challenges are much more interesting.

Here's some food for thought. About a year ago, an electronics engineer customer
who lives in New Jersey wrote:

"I finally found the formula in the on-line "Wood Handbook" -
P = 8100 *  G**3/2   *  D**3/4   *  L
G - specific gravity of wood
D - shank diameter of lag
L - length of penetration of thread
For:  G = 0.3 (soft woods)    D = 1/4"  and L = 2"
I get 8100 (0.164) ( 0.353)( 2 ) ~  941 lbs.  !!
I knew these were strong,  but this really amazed me!
Now I feel better!"

I just went to Yahoo and searched for
"on-line" +"Wood Handbook"
and came up with some interesting goodies:
http://search.yahoo.com/bin/search?p=on-line+%2B%22Wood+Handbook%22

Here's to the joy of learning and to peace in our time.

Best regards,
Joel Davidson

"Jeffery Wolfe, Global Resource Options" wrote:

> Joel,
>
> Didn't mean to sound like I was casting stones. I agree that you did your
> homework on that one, and that those systems are quality from what I've
> heard. My point is exactly the point that you make. Without the detailed,
> specific engineering and construction follow-through, one has to over-build.
> For one-offs, which is most of what most of us do, it's cheaper and easier
> to over build than to engineer. For mass-production it cheaper and better to
> engineer and build to a standard.
>
> Thanks for the details.
>
> Jeff
> Selling solar, working for Peace.
>
> -----Original Message-----
> From: Joel Davidson [mailto:joeldavidson at earthlink.net]
> Sent: Saturday, March 22, 2003 7:17 PM
> To: RE-wrenches at topica.com
> Subject: Re: Uplift Wind Loading Values [RE-wrenches]
>
> Jeff,
>
> In 1994, SMUD was the world's biggest PV customer. The challenge was to
> design
> and install the lowest cost grid-tied PV systems in the world on multiple
> sites.
> Siemens Solar and other bidders had little or no construction experience. I
> knew
> that just about anything could be fastened to plywood if enough screws are
> used.
> I discussed my design with an American Plywood Association engineer and then
> I
> paid a registered engineer to review my design. We installed over one
> hundred 4
> kW PV systems on homes and 288 kW on churches totaling over 71,000 square
> feet.
> Installations were limited to gable roofs in good condition with 1/2 inch or
> thicker plywood and composition shingles less than 10 years old. Solar
> panels
> with 7 Solec SQ80 modules (4.33 ft x 13.5 ft) were fastened to roofs with 24
> each 1.5-inch #10 stainless steel wood screws through 0.25-inch thick
> aluminum
> mounting feet and into the plywood. We hired local electricians and trained
> them
> how to make water-tight roof attachments. We paid a construction specialist
> (retired general contractor with over 50 years experience) to fly to
> Sacramento,
> review our design and observe installers at work. To date, there have been
> no
> roof leaks despite the worse rainy season in 100 years in 1994-95, several
> storms since then, winds over 80 mph at some sites and a few minor
> earthquakes.
>
> I used the same screw mounting on my own PV system, but I don't recommend
> fastening PV arrays with screws into plywood or other "out of the box"
> designs
> unless you are willing to do your homework, get stamped drawings and calcs
> from
> licensed engineers and pay a fair price for experienced supervision and
> quality
> work.
>
> Peace,
> Joel Davidson
>
> "Jeffery Wolfe, Global Resource Options" wrote:
>
> > Keith,
> >
> > When I don't get things really engineered, I tend to build a battleship.
> > (Maybe an unfortunate analogy tonight.)
> >
> > I'd consider installing a long piece of angle below the roof purlins,
> > spanning several purlins or beams (depending upon design of the roof
> > structure). I'd then put the same type of angle above the roof, and clamp
> > the two together with bolts. Doesn't need to be real heavy angle, the
> object
> > is simply to create a solid sandwich.
> >
> > Short of that, I've also taken 5/16" all thread rod and bent it into a
> "U".
> > Both ends stick through the roof and hold a foot in place, with the U
> going
> > around a rafter. (I was on a wood roof.) This particular construction
> > survived some pretty good midwestern thunderstorms, but was not subjected
> to
> > any hurricanes.
> >
> > I still haven't got the courage, or the engineering back up, to simply use
> > surface screws for any panel installs. Joel had some stuff he sent around
> a
> > while ago on some screw attached racks he had engineered for SMUD (I
> think).
> > But then you're trusting that for 20 or more years the plywood doesn't
> > degrade, and the screw material doesn't corrode, and that's not a great
> bet
> > with small diameter fasteners. Bad enough with lags into 2x's!
> >
> > My 2 cents.
> >
> > Jeff Wolfe
> > Global Resource Options
> > Selling Solar and Working for Peace
>
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