Joel D said: "I would be concerned about the whole roof lifting up in a
strong wind"
Me, too. http://www.youtube.com/watch?v=IHxc8AwG5Ac This is not a
tornado... Not a hurricane... Just wind. And not a particularly strong
wind,
either. Helps explain why TX has built so many wind farms...
I really like the S-5! in the right circumstances. I am concerned that
it's
so "easy" that it can be and has been used in applications where it has
been
vetted about as well as McPain's hunting mate. In nearly all evaluations
I've done of existing S/S roofs, where the structure was actually able to
support the added weight of an array, the weakest link has been the
attachment of the roof skin to the building. Uplift forces at the roof
surface (array location) are transferred to the building structure via 2
attachements, not just 1. The "clip" has a top and a bottom. Both ends
must
be evaluated. I've found more than a few that don't pass a responsible
structural analysis "as-is"... Without solar.
Older documentation from metal bldg/roof mfrs provided a psi or psf rating
for the "top" connection... Where the clip attaches to the metal skin.
"Snap-Click". Be sure to double-check the units... I've seen psi
published.
There are 144 sq inches in a square foot... If you're looking for 35 psf
and
you see 350 psi, you don't have a 10X safety factor! What was less often
published is a rating for the attachment of the clip to the structure.
This
gets really squirrely when the clip is screwed into OSB &/or plywood.
Another point that I have seen engineering calcs fail (but not physical
failures so far), is tear-thru... Where the clip is torn right over the
screw head that holds it to the structure. This is different than
"pull-out", where the screw pulls out of the structural member. Frankly,
I'm
not convinced that the couple of times where the engineers came up with
this
it could possibly happen... I don't think the top of the clip is gonna
hold
on long enough to tear the bottom of the clip this way. But that's what
the
guys with the fancy initials and the little round rubber stamp said...
Hopefully your application will have complete published data on the system
used and conditions found at the site. Don't forget to actually VERIFY the
clip spacing and attachments! Again, metal over OSB/ply requires special
attention. In these cases, I recommend taking some of it off in a few
areas
to investigate if you can't otherwise prove what's in there. One of my
favorites is the wood-truss/osb install where the original documentation
and
engineering is based on (2)ea x 1-1/2 or 2" screws directly into the top
chord of the truss at each clip with 48" spacing up the truss. From the
ground you can see that the seams don't line up with trusses anywhere and
you can clearly see the tip of a single screw come through the OSB by a
whopping 1/4" in the eaves. Sooooo... You want me to believe that you "did
it the right way" where nobody can see it, but not where it's in plain
view?... I've been to Missourri... Try again. Oh, wait... That's not my
favorite part! My favorite part is breaking the news to the owner that his
roof is gonna blow off so he can't have solar on it unless we do
standoffs.
And, yes, I TOTALLY appreciate that he "built it this way so he could have
solar and no penetrations". (Just kidding folks. This is NOT my favorite
part. These situations break my heart and tick me off to no end. Somebody
cheated and now somebody else has to pay for it... One way or the other)
Look out for roof-skins that are already coming loose from the structure.
In
my experience, these situations generally occur where there are long &/or
multiple panels between a "ridge" and an "eave". I haven't personally
observed it in flat-slope apps, but suspect it happens on those, too. Be
VERY cautious on steel buildings with open walls. I've seen plenty where
the
skin is already flopping loose when you get there. The clips have already
released from the skin and/or completely deformed. I've mostly seen these
in
ag applications. Riding arenas, hay-barns, tool sheds, etc. A municipal
dump
transfer station building, too... That one was probably the worst of all
in
this regard. Here was a beautiful, long, wide-open roof that was literally
flapping in the breeze as you walk on it. The pressure differentials on
these buildings are magnified greatly from those of a normal "enclosed"
structure... Positive pressure below the roof + negative above = lotsa
lift.
Probably the single biggest factor that goes into causing these to fail is
that wind is not "smooth and steady". If you are the roof on one of these
buildings, you are being pounded continuously. Stand on or under one of
these buildings when there is a 10 mph or greater wind and just listen to
it... It doesn't just "creak and groan"... It "bangs and slaps".
Another sign of trouble to watch for is horizontal creasing of S/S panels.
In order for a panel to crease, it has to get bent some time... Either
before or after it is installed. When these profiles get bent, the seams
get
distorted. They don't fit like they were designed to. Since the clip
design
is dependent on a perfectly shaped seam, you kinda gotta wonder if the
"new
shape" is gonna hang in there when mother nature starts pulling on it...
That dump transfer station roof had these creases all over it... Don't
know
which came first, the crease or the loose roof, but at this point it
doesn't
really matter, does it?
I'm comfortable with the holding power of an S-5! For purposes of wind
analysis, it's the strongest part of the attachment system. For
applications
where this method is an option, right after making sure the building can
physically support an array, the next thing I look at is the
roof-to-structure attachments. I never kept an actual tally, but I'd
estimate that only about 20% of the potential S-5! projects I looked at
over
the years passed this phase of the evaluation without requiring
modifications of some sort.
For "smaller-ish" arrays, don't waste $ on rails. For "normal" sized,
rectangular, shaped modules, turn 'em sideways (landscape). Figure on
using
an S-5! at every seam or 24", whichever is greater, and run the math that
way. I, personally, want a minimum safety factor of 2+ for these apps. In
this part of the country, it is common for all-steel buildings to have
only
1 clip for every 12-14 sq ft... Horizontal purlins @ 63-66" spacing with
vertical seams every 24 or 32"... You MIGHT get this to calc in zone 1 of
a
barn with low-slope roof, 85 mph basic, cat B exposure, good screws,
high-quality roof profile, and a strong clip... I've seen it done. But it
doesn't get near a 2X safety factor...
Keep in mind the "S-5! WARNING... The S-5! clamp is a handy gadget for a
great many uses, but will not perform miracles." <
http://www.s-5.com/Home/index_457.cfm >
Miracles is what hammocks is made for...
Matt Lafferty
[EMAIL PROTECTED]
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