Need a book? Engineering books recommendations...

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

Re: Ridge beam/diaphragm analysis

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Nels,

"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..."

The sheathing usually hits the shoulder of the  typical 2x ridge board which
doesn't make for adequate nailing--one nail on each side of the ridge at an
angle to the face grain of the ridge board at say 4" o.c. wouldn't have full
capacity for shear transfer. However maybe its customary in other areas to
provide 4x ridge boards with beveled tops to facilitate adequate nailing...
And then of course there's ridge vents.

 I'm imagining that the load path is, that the shear in the sheathing is
transferred to the nails in the panel edge, then into the rafter adjacent to
the ridge, then the toenails from the rafter into the ridge, the ridge to
the rafter on the other side (through toenails), and then into the sheathing
through the sheathing nails into the rafter. This is assuming the sheathing
cannot be significantly nailed to the ridge board.

"-- this is true even if trusses interconnect the two planes."

the way I see it the truss acts as a shear splice assuming the connection
plate can hold the two top chords together given the diaph load. You will
agree that the shear load wants to tear the diaph in two orthogonal
directions thus the 45 deg split. one direction is parallel to the roof
truss one perp to it. The truss and its connection transfer the parallel
force (either as comp or tens) the perp loadalso  is transferred via the
truss. If it were the rafters-connected-to-the-ridge-board type setup it is
conceivable that the toenails could fail in shear and separate from the
ridge. In a truss system it is because the plate connection is so strong
that if you pull on the truss perp to it's main axis it will pull the whole
truss over up to some critical load which I'm sure is much stronger than the
toenails in the rafter to ridge board connection. Ideally though the ridge
gets blocked to restrain the ridge joint an strengthen the plate connection.

My two cents,

Rand



----- Original Message ----- 
From: "Nels Roselund, SE" <njineer(--nospam--at)att.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, July 02, 2004 11:37 AM
Subject: Re: Ridge beam/diaphragm analysis


> 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
> | > | >
> | > | >
> | > | > ******* ****** ******* ******** ******* ******* ******* ***
> | > | > *   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
> | > | > ******* ****** ****** ****** ******* ****** ****** ********
> | > | >
> | > |
> | > |
> | > |
> | > | ******* ****** ******* ******** ******* ******* ******* ***
> | > | *   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
> | > | ******* ****** ****** ****** ******* ****** ****** ********
> | >
> | >
> | >
> | > ******* ****** ******* ******** ******* ******* ******* ***
> | > *   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
> | > ******* ****** ****** ****** ******* ****** ****** ********
> | >
> |
> |
> |
> | ******* ****** ******* ******** ******* ******* ******* ***
> | *   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
> | ******* ****** ****** ****** ******* ****** ****** ********
>
>
>
> ******* ****** ******* ******** ******* ******* ******* ***
> *   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
> ******* ****** ****** ****** ******* ****** ****** ********
>



******* ****** ******* ******** ******* ******* ******* ***
*   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 
******* ****** ****** ****** ******* ****** ****** ********