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Re: Ridge beam/diaphragm analysis

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Nels,

This warping, if I follow, is why I'm sure not much diaphragm force can
transfer across the discontinuity in the diaphragm. The poor toenails at the
rafter to ridge board (or beam whatever the case may be) are subjected to
the full in-diaphragm-plane shear as well as the shear load normal to the
diaphragm plane which causes this warping effect (although I'd call this the
effect that occurs 10 sec. before collapse).... Now wood trusses are
continuous across the ridge so in my mind the shear transfer can be
accomplished through the truss. I propose that if the toenails at the ridge
are not adequate to resist the calc'd loads dicussed above, then the roof
diaph should be analyzed considering each plane to be an independent diaph
with it's own chords (two by something cont. running perp to the rafters
placed 12" or so on either side of the ridge. Blocking between rafter
transfers shear from plywd to the 2x chord if the chord is under the rafter
(although you could notch the rafter and let in this chordmember so the
sheathing nails directly to the chord.

Maybe the reason you haven't seen a failure at the ridge (BTW neither have
I) is because the evidence for this kind of failure would be the collapse of
the structure so if you've seen a pile of wood on the ground it may be
because of the ridge failure.

My thoughts anyways,
Rand


----- Original Message ----- 
From: "Nels Roselund, SE" <njineer(--nospam--at)att.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Wednesday, June 30, 2004 4:24 PM
Subject: Ridge beam/diaphragm analysis


> Rand and William,
>
> A gabled roof can act as a diaphragm that transfers across the ridge shear
> forces that result from lateral loads perpendicular to the ridge.  The
thing
> that is difficult to visualize is what happens at the ridge.  Here's my
> explanation:
>
> A diaphragm resists loads in its plane.  The deflection of a sloping plane
> resisting force in its plane is in the plane of the diaphragm.  This means
> that at the ridge, the deflection of the edge of the up-sloping plane has
a
> vertical component upward; the deflection of the down-sloping plane has a
> vertical component downward.  Because a woo-framed diaphragm is relatively
> flexible out-of-plane, the two planes are able to warp at the ridge, and
the
> ridge deflects neither upward nor downward.  There needs to be a nominal
> vertical-load capacity of the rafter connections at the ridge to keep the
> planes from separating, but this is generally part of the system anyway.
> Another effect is a tendency to "roll" the ridge because of the unbalanced
> direction of the displacement.  I've never heard of any observations or
> damage reports that indicated this to be a significant effect.
>
> Hipped roofs are a little more complicated to visualize, but they can be
> thought-through in about the same way.
>
> Despite having satisfied myself that the two planes of a gabled roof can
act
> together as a single diaphragm, nevertheless, for very heavy lateral loads
> [as for a gabled diaphragm bracing a massive unreinforced stone or adobe
> wall in a zone of high seismicity], I generally use a horizontal
> ceiling-level diaphragm if possible, and [again, if possible] locate it
some
> distance below the top of the wall where confinement by superimposed
masonry
> overburden provides for more effective wall anchorage.  If the gabled
> diaphragm and the horizontal diaphragm are interconnected, I think their
> capacities can be considered additive.
>
> William, I've never seen any thing written about this either.
>
> Charley Hamilton: maybe a gabled-diaphragm/hipped-diaphragm study would
make
> a good research project for one of your graduate students.  I'd like to
know
> if I'm right.
>
> Nels
>
> Nels Roselund
> Structural Engineer
> South San Gabriel, CA
> njineer(--nospam--at)att.net
>
> ----- Original Message -----
> From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Wednesday, June 30, 2004 12:58 PM
> Subject: Re: Ridge beam/joist analysis
>
>
> | Bill,
> |
> | I have posted my thoughts on this a few times with little or no
response.
> In
> | short I don't think you can effectively transfer shear across the ridge
> the
> | way most houses are constructed. I will try to find what I have
previously
> | posted. but it may not be today.
>
> | Rand
> |
> |
> | ----- Original Message -----
> | From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
> | To: <seaint(--nospam--at)seaint.org>
> | Sent: Wednesday, June 30, 2004 1:28 PM
> | Subject: RE: Ridge beam/joist analysis
>
>
> | > Unfortunately, I was looking in this book for a good description of
how
> | > lateral wind forces are transferred thru a gabled roof via diaphragm
> | action
> | > - but despite the name of the book, the section on gabled roofs only
> seems
> | > to address vertical load effects. I once convinced myself that the two
> | > plates act effectively as one diaphragm with shear transferred at the
> | ridge
> | > beam, but now I am having trouble reconfirming that conclusion using
> | > free-body diagrams. Is there a reference that explains in detail how
> | forces
> | > are transferred in a gabled roof diaphragm?
> | >
> | >
> | > William C. Sherman, PE
> | > (Bill Sherman)
> | > CDM, Denver, CO
> | > Phone: 303-298-1311
> | > Fax: 303-293-8236
> | > email: shermanwc(--nospam--at)cdm.com
> | >
> | >
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