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Re: Ridge beam/diaphragm analysis

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Rand,

It is the ability of the diaphragm to warp out of plane that keeps the
diaphragm edges together at the ridge and prevents tearing damage.  If
tearing were to occur, the capacity to transfer shear from one plane to the
other would be reduced or lost.  If the ridge shear connections are
adequate, shear can be transferred across the ridge.  The key here is that
the sheathing edges of both planes need to be nailed to a ridge member or
blocking to transfer the shear across the ridge -- this is true even if
trusses interconnect the two planes.

The ridge is in the plane of each slope.  Thus, shear stresses, which act in
the plane of a diaphragm, can be transferred across the ridge from one plane
to the other -- provided each edge is connected at the ridge for in-plane
shear.

I would distinguish between the transfer of the in-plane shear forces across
the ridge, discussed above, and the transfer of lateral loads into the
diaphragm planes each side of the ridge.  In order for the two planes of a
gabled roof to act as a single diaphragm, each plane must be receive its
share of the lateral load in its own plane -- as would be the case with your
example of a system of trusses on which both roof planes are built.  The
trusses act to distribute the load to both planes.  However, they are not
effective in transferring the shear stresses across the ridge.

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: Thursday, July 01, 2004 7:59 AM
Subject: Re: Ridge beam/diaphragm analysis


| 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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