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RE: Exposed Beam Roof Design

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Bill:

You hit the nail on the head, to use an appropriate pun.  <grin>

That is precisely the thing that is not really known too well.  There is
not a whole lot of testing of "big nails" (aka panel spikes) nor "big
screws" (aka panel screws) for use as a diaphragm connection under seismic
loading.  At least one SIP manufacturer has done some testing for wind
application (i.e. monotonic/non-cyclic loading) and as a result does have
a set of diaphragm attachment values in their ICC evaluation report (it is
actually a NER legacy report, if I recall correctly).  I do have a couple
of papers from Frank Woeste that discussion some testing that they have
done with cyclic (psuedo-static) loading, but I just have not had a chance
to sit down and read them yet (Frank was nice enough to send them to me
back around the begining of April).

But, to my knowledge, you assessment of the likely failure mechanism is
correct.  For testing of a single panel screws (and spike) to a "failure"
load in shear, we have found that the failure mechanism is the crushing of
the inner OSB skin (i.e. the OSB skin closest to the supporting structural
member) by the screw or nail shank.  In many ways, however, this is
somewhat similar to the failure mechanism of a typical structural
sheathing diaphragm with standard nails...at leat to my knowledge.  As I
understand the tradition structural sheathing diaphragm (and shear wall
panel too for that matter) failure mechanism, it is the failure of the
perimeter nail connections that governs.  The difference is that I believe
with "traditional" structural wood sheathing diaphragm (and shear wall)
nail connections it is a combination of sheating crushing, nail bending
and nail withdrawl (from prying forces due to the nail bending).  Thus, I
believe that panel screws/spikes will be slightly different and likely not
as ductile as a "traditional" wood diaphragm.  At a minimum, there is not
a whole lot of testing to really say one way or another.

Now, wind is a WHOLE other matter.  Wind does not really require a ductile
response (at least not like seismic loading does) and I am personally
comfortable using known information to come up with SIP diaphragm
attachment panel screw/spike spacing for wind with known information.

The one thing that I would "dicker" with is that in reality it is not a
matter of "lots of small" vs "a few large" fastners, per se.  We can do
panel screw/spike spacing for SIP roof panel attachment of on the order of
3 in on center, which is fairly comparable to able the "tightest" spacing
that one can do with "traditional" nails through sheathing.  Usuually, the
governing thing of nail spacing is the "supporting" material that you are
attaching too and the concern of whether or not your will split the
material.  Since, roof SIPs are usually either supporting by heavy timber
(i.e. either solid, full sawn LARGE timber members [i.e. 5x5 or larger] or
glulams) or SIP (or stick frame) walls which would mean dimensional lumber
with the wide face being nailed/screwed into or steel members with nailers
(again with the wide face as the nailing surface), the concern of having a
crap load of nails going into a very small side face it not the same.

Regards,

Scott
Adrian, MI


On Wed, 21 Jun 2006, Polhemus, Bill wrote:

>
> Are "big nails" (officially known as "spikes") of any great benefit for
> seismic (or wind) resistance?
>
> I would think that tearing of the panel would control, and that "lots of
> small" would be preferable to "a few large" fasteners.
>
> Sorry, it's been a LONG time since I looked at any of this.
>
>
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