Uplift Wind Loading Values [RE-wrenches]

ASAP POWER! asap at podnine.com
Mon Mar 24 22:06:35 PST 2003


Sorry to back up a bit and I agree with both of you but now I'm confused.
Bill, you agree with Jeff that live loads can be shown to be redundant or
irrelevant.  That's a nifty argument too Jeff, but Bill, I don't see why the
live load could be called eliminated as a reason to excuse the extra dead
weight.  Maybe some good definitions are in order...complete with all
equivalent phrases and terms, units.  And why and when can you consider
downward wind pressure a dead load (vs. live load)?  I'm under the
assumption that uplift = downward design wind pressure and that it doesn't
matter for tilt since we're already assuming worst case, with wind always
coming orthogonal to the plane of the array.  Also, I'm thinking uplift wind
load value is always a live load and a live load is always wind loading.

Eliminating potential live loads--if in the form of people or equipment
blocked by an array surface area coverage--is easily understood.  That is,
if live loads are temporary loads caused by the need to service a dead load,
but in which case, there are still people & equipment on the roof doing the
install, etc, walking all around the array, albeit temporarily...isn't this
considered in the designed roof's dead loading anyway?

The main issues as below seems to be what the inspectors are concerned
about.  They want "roof dead and uplift loading values" the latter of which
I believe they are construing as the definition presented in Bill's main #2.
But the question remains.  How do you calculate uplift?  Get a calculator.
Like Tim H at Uni-rac or understand the formula.

Then also, the Uni-Rac code compliant document says "Based on the
characteristics of your roof truss lumber and the lag screws, consult Table
4 to determine the Lag Pull-Out.  Lag Pull-Out Value must be the greater
than the Foot Design Live Load multiplied by a safety factor appropriate to
your installation."  First of all, is Uplift Wind Loading = to Foot Design
LIVE Loading?

For the safety factor and how it's determined:  that depends on who is doing
the job and what you're working on.  A footbridge with people on it would
have a higher safety factor than my small residential rooftop PV install for
instance even though they may have different design wind pressure
requirements, so typical residential PV on a sound roof is 1, PV array on a
hospital or public building, it might be 8 though.  So this is just
typically 2-10 multiplier of final values.

Beyond these attempted basic definitions, what happens with tilt vs. flush?
Regardless of what I said earlier, ProSolar says they will give wet-stamped
engineered designs for any array for a fee, but do have some figures ready
to go (faxed with orders) for "one angle and flush."  What I got was 1/4 in.
deflection report at 125mph wind loading equivalent test for the Sharp
modules in the array.  Done with sandbags resting on the panel at 50#/sq.ft
for 30 minutes for and simulated for download and uplift conditions (both
sides of the panel).  From what I can tell, that's all that was done but the
engineer states "This document certifies the Tile-Trac mount AND (my caps)
accompanying tilt up support, used with Sharp Solar Modules, withstands a
50#/square foot static pressure load, this loading is equivalent to a wind
speed of approximately 125 mph as defined in the 1997 Uniform Building
Code."  I was half-expecting to see different numbers for a tilted Sharp
module vs. a flush mounted one.

Obviously, wind can get behind a tilted array much more effectively, and
there's the lateral loading and flexure of tilt legs/system as a whole
(folding) to consider too.  But isn't wind design pressure for a
flush-mounted system (and any tilted array for that matter) assuming that
design wind speed is hitting the back of flush mounted panels straight on,
at right angles as mentioned?  But can tilted array wind loading values be
more considering of lateral forces?  Especially if area is prone to
gusty/burst winds?  Perhaps the safety factor can be invoked for these
concerns.  I guess it's really the foot design that will determine how well
a tilted array resists lateral folding, and of course, how and what it's all
lagged into.

Simplest to me seems to just calculate the surface area under (of) the
array, figure out at 75mph (where this inspector wanted it) what kind of
wind pressure potential that is presenting (as if the wind is hitting it
dead on) using the formula, and then with a figure for the whole array,
divide by number of fasteners/feet!  Hey, that's 2. again!  Now should that
resultant be what is 13#/sf or less?  Now it sounds like I just need to
calculate that uplift.  Back where I started?  Which page am I on?  Trying
to tie it all together (and down)...

PD


-----Original Message-----
From: Bill Brooks [mailto:billb at endecon.com]
Sent: Wednesday, March 19, 2003 5:31 PM
To: RE-wrenches at topica.com
Subject: RE: RE: Uplift Wind Loading Values [RE-wrenches]


Jeff,

Back to the same page. I am absolutely talking about UPLIFT. Live loads are
irrelevant with PV systems and many inspectors will allow you to use the
elimination of live loads as a reason to add weight to a roof. My
calculations are specifically related to withdraw strength of fasteners in
wood. The uplift on a 1kW PV system can be much more than 1200# depending on
the conditions. Dead load is only relevant for the added weight to the
structure. The dead load increase of the wind is the same with or without
the PV system.

Two main issues:

1. Can the structure handle the additional dead load of the array?

2. Can the fasteners handle the expected withdraw loads imposed by the
uplift of the wind? (total uplift divided by number of fasteners)

Point loading is another issue, but a good design has enough attachment
points to keep this from becoming a problem.

Bill.

-----Original Message-----
From: Jeff Oldham [mailto:starpower4u at juno.com]
Sent: Wednesday, March 19, 2003 4:34 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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