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Re: OSB Sheathed Truss

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First thought is that there is nothing inherently inferior about staples as
compared with nails for sheathing attachment - both are limited by
applicable design values, both will suffer from the results of over-driving,
and both will corrode eventually in a wet environment.  This is probably a
good application for staples, if all of the load transfer is to be
mechanical, to avoid splitting of the lumber core members.

The analysis will not work quite like you envisioned, as the shear is
minimal (if the loading is balanced) in the center span where you have the
greatest depth of OSB panel, and the shear is maximum where you have the
least OSB.  So it is likely that your analysis will lead to the observation
that the lumber chords are doing nearly all the work in carrying moment and
in delivering the shear to the supports.  If there are no lumber web
members, and the top chords are not capable of clear spanning from the ridge
to the supports, perhaps you can contrive a type of "truss analogy" using
strips of the OSB as pseudo web members.  The likely problem is that the OSB
pseudo webs in compression that are needed to support the physical top chord
will not carry any load due to lack of out-of-plane stability.  If you have
any kind of lumber members in the core, they can serve as the compression
struts in the "truss analogy", so the OSB is needed only to provide tension
capacity as pseudo webs, and there's a good chance that you can make it
work.

Mark D. Anderson  PE
Anchorage


----- Original Message -----
From: <rlewistx(--nospam--at)juno.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, March 05, 2003 6:17 AM
Subject: OSB Sheathed Truss


> I received a call from a homeowner regarding a new home he was having
constructed.  The building inspector would not accept some of the roof
framing without a letter from an engineer.  The carpenter built a gable
shape truss across the garage to support the ceiling and roof.  The truss is
1.5" thick (single ply) made from field fabricated wood (verses
prefabricated pressed metal plates) and covered with 1/2" OSB on both faces.
The OSB is attached with staples.
>
> My gut feeling is that I can get this to work using analogy similar to
plywood box beams.  Although I do not have a rectangular shaped beam
elevation, I was going to calculated shears and moments along the length of
the truss and calculate the force couple in the flange from moment as the
flanges spread apart and shear in OSB.  I don't like staples for structural
connectors so I was going to require nails be added.
>
> Any words of wisdom for analyzing this?
>
> Thanks
>
> Rich
>
>
>
>
>
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