Uplift Wind Loading Values [RE-wrenches]

Wed Mar 19 18:49:29 PST 2003

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

-----Original Message-----
From: keith [mailto:kcronin at lava.net]
Sent: Wednesday, March 19, 2003 9:25 PM
To: RE-wrenches at topica.com
Subject: RE: RE: Uplift Wind Loading Values [RE-wrenches]

Hello Bill

I agree, the number of threads is an issue in conjunction with the wind.
With a sail, things are bound to loosen up!  In Hawaii, 15-30 MPH is an
average day.  I am wondering what is the best method of installation for
a flat roof/pitched roof with metal corrugated designs. To take all of
the pitch angles, wind load based upon exposure, etc., it is difficult
to know the proper mounting methodology.  Is it prudent to consult a
mechanical engineer on this one?  Is it time for a power light style
system, and loose the winter performance?  I know they had to do a lot
of engineering and get all of their stuff stamped to go through all of
the hoops!

Any wrenches want to chime in????

Keith Cronin
Island Energy Solutions
318 A Kuulei Road
Kailua, Hawaii 96734
808-262-3268 Tel
808-263-0338 Fax
www.islandenergy.net

-----Original Message-----
From: Bill Brooks [mailto:billb at endecon.com]
Sent: Wednesday, March 19, 2003 4:02 PM
To: RE-wrenches at topica.com
Subject: RE: RE: Uplift Wind Loading Values [RE-wrenches]

Keith,

The main issue, as I understand it, with fasteners is the number of
threads
in the material. When you get down to sheet metal, a self-tapping screw
only
has part of a tread holding it on. There can be a tendency for vibration
to
cause the screw to loosen. We had some catastrophic damage on some large
arrays at PVUSA where selftapping screws worked loose. If you can find a
way
to lock the screws, it could work. It depends how exposed to the wind it
is,
but you are in Hawaii, so how do you get away from the wind?

Bill.

-----Original Message-----
From: keith [mailto:kcronin at lava.net]
Sent: Wednesday, March 19, 2003 4:57 PM
To: RE-wrenches at topica.com
Subject: RE: RE: Uplift Wind Loading Values [RE-wrenches]

Hey

I have a wind load question....is it possible to use self tapping screws
into a corrugated roof that is supported by z bar purling for pv racking
support?  Or is it advisable to drill the 90 degree angle iron that
makes the z bar purling assembly and have threaded rod through the roof?
I feel this might be better to limit the compression on the corrugation
and the roof.  What is the wind rating or perhaps a mechanical engineer
would know how many self tappers x width of screw or just go for
threaded rod and sleep better!

Thanks in advance!

Keith Cronin
Island Energy Solutions
318 A Kuulei Road
Kailua, Hawaii 96734
808-262-3268 Tel
808-263-0338 Fax
www.islandenergy.net

-----Original Message-----
From: Jeff Oldham [mailto:starpower4u at juno.com]
Sent: Wednesday, March 19, 2003 2:34 PM
To: RE-wrenches at topica.com
Subject: Re:RE: Uplift Wind Loading Values [RE-wrenches]

Let's be sure to all be on the same page -
The original question was "Uplift Wind Loading Values", this is
specificly what I was addressing. The link to the document at UniRac
does not seem to cover this directly and suggests that you contact a
engineering professional. This document and the numbers from Bill here
are only addressing dead and live loading from equipment and wind
combined (the 25-55#/sq.ft.), not UPLIFT. Live and dead loading clacs
are used for verifying the structures ability to support the load.
UPLIFT clacs (13#/sq.f.)are for determining the anchoring system that
holds it DOWN, not the buildings ability to support it. The document
does have pull-outs for various fasteners and this is the data you need
for calculating UPLIFT. A typical 1 kW array will have about 1,200# of
uplift to be anchored down from pull-out.

btw- I've had pretty good success arguing that my array is eliminating
the live load access to a roof area. Live load is usually addressing the
people and materials that might be up there for repair and maintenance.
My argument is that the 17 - 22#/sq.ft. typically allowed/designed for
roofs is taken up by my array as people and materials cannot be piled on
the array, this puts 17#/sq.ft. in my "bank". It can be construed that
wind turns into a live load, but in general most agencies put that into
the dead load. In most cases the array is not increasing roof area so
there in no additional "live"/wind load for the array, it is just a dead
load that is a fraction (typically 1/3)of the allowed live load, that is
no longer available. Give it a try, it works for me more often than not.
This does not mean that the Building Dept. foregoes wet stamped
engineering. Every structural engineer that I've told this to has lit up
and said "Of course! It makes sense to me, where do I sign?"

Jeff Oldham

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