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Re: blocking at eaves..Roof diaphragm

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 Andrew wrote:
5. Do you see any problems with a discontinuous roof diaphragm at the roof
peak because it was cut for ridge vent placement?

I think I remember a discussion on this list about ridge vent cuts not
affecting the diaphragm, but our APA buddies can probably chirp in about
that. One argument is at least in shear in the diaphragm parallel to the
ridge, the theoretical shear in a rectangular roof will be zero at the
middle (ridge). Perpendicular the truss top chords act as drag struts to
carry the shear past the ridge. Just some thoughts...

Andrew,

Ridge vent or not you have a continuous discontinuity at a place where the
shearing forces are trying to pull the diaphragm apart. Same (even worse) at
hips unless you have a wide hip rafter that's beveled to meet the plane of
the underside of the roof sheathing and well nailed at sheathing to hip
rafter. Now if your using roof trusses then the truss members will transfer
that shearing stress across the discont. (but that's hard to quantify), with
stick framing... other than collar ties you don't have anything to provide
the transfer (no collar ties at hips). The model we use to analyze diaphragm
behavior is based on a flat roof acting similar to an "I-beam". By analogy
would an I beam function the same if the web was contorted in the shape of a
gable roof and with a continuous slit right down the middle (with some
nominal tack welds to hold the two pieces together)?

Everyone I've talked to on this subject can only say that they've never seen
this type of failure. I think with this type of failure all you'll see is a
pile of 2X4's after the failure. As far as I know APA has not tested a full
scale diaphragms, I think wood truss guys have to some extent but like I
said wood trusses have built-in resistance to this type of failure.

Rand





----- Original Message ----- 
From: "Andrew Kester" <akester(--nospam--at)bbma.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Thursday, May 06, 2004 8:57 AM
Subject: blocking at eaves


Martin:

1. Do you require eave blocking?  2. If the forces at the roof diaphragm
were greater or less than about 200 plf would your answer to #1 change?

I would not cave into arguments for eliminating eave blocking such as " lots
of older houses without blocking that have withstood the test of time and of
course  and "in the lower 48 nobody blocks at the eave". To me it is a case
by case basis, and it depends  a lot on heel height of the truss or depth of
the rafter at the bearing wall, size of the building, and obviously the
shear in your diaphragm. Some engineers think that the force "just gets
there" from the diaphragm to the wall. But ask a truss manufacturer if they
designed their truss with a 2ft heel to take 400lb out of plane, and they
will either hang up the phone or say "uhhhmmmm, NO".

Most of my projects are in wind-only country, but we get some very high
shears in single story diaphragms, although I don't do residential. I would
think even if solid blocking was required by design or code, that you could
allow holes drilled at certain spots in the blocking (for ventilation) and
you would still get enough shear out of the member. I am not sure how to
quantify that.....

Some options I think work well other then solid blocking:

-Diagonal tension straps (Simpson) to take the shear down to the base of the
adjacent truss. But that truss connection has to be sized appropriately for
that shear and combined with uplift.

-Mini-shear walls at every third or fourth truss, with independent tie downs
for OT. Some may argue that the plywood, laid lengthwise across the trusses,
will act as the diaphragm shear strut and carry it to that shear wall (which
is the way a diaphragm transfers shear, to my understanding). I prefer a
little 2x4 blocking and a flat strap at the diaphragm boundry element that
directly ties the roof diaphragm (plywood) into the shear walls.

-Solid blocking at every third or fourth truss space (depends on the shear),
tied together as mentioned above. The blocking has its own connection to
take the shear into the bond beam or top plate. If the shear is not too
high, the truss connection itself may handle the shear.

-Not a contractor favorite, or my favorite, good ol' 2x4 diagonal bracing.
This is labor intense and a pain to install. Also getting a good mitre cut
and a snug fit from top chord to bottom is pretty difficult. You still have
to make sure the truss connection takes the shear and uplift. These are
compression only, so you need one at each truss space, unless you combineit
with a strap for tension.

I'd love to hear others ideas on this issue, or if they think any of the
above are crappy ideas.

5. Do you see any problems with a discontinuous roof diaphragm at the roof
peak because it was cut for ridge vent placement?

I think I remember a discussion on this list about ridge vent cuts not
affecting the diaphragm, but our APA buddies can probably chirp in about
that. One argument is at least in shear in the diaphragm parallel to the
ridge, the theoretical shear in a rectangular roof will be zero at the
middle (ridge). Perpendicular the truss top chords act as drag struts to
carry the shear past the ridge. Just some thoughts...

Andrew Kester, PE

Longwood, FL




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