Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: OSB Sheathed Truss

[Subject Prev][Subject Next][Thread Prev][Thread Next]
> -----Original Message-----
>
> 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.
>

It's not clear to me whether this truss is supporting only a typical 24"
tributary width, or whether it is supporting a larger area.

All things being equal (which of course they never are!) it's unlikely that
a single ply truss constructed of the visually-graded lumber typically used
on site (compared to machine-graded lumber often used for shop-fabricated
trusses) can be upgraded to carry substantially more than a 24" width.

I've checked site-fabricated trusses using nailed ½" plywood plates, and
they can be made to work, although the number of nails required can be
difficult to fit in with the restrictions on edge distances, spacing, etc.

The critical connection is at the heel. Once you know the reaction you can
calculate the horizontal component of the compression force in the rafter -
and this is both the tension force in the bottom chord and the force that
has to be transferred through the heel connection.

Some possible problems:

Is the bottom chord spliced?
Are nail shear capacities available for OSB?
Is there backing/blocking behind the OSB joints?

Peter James



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********