Need a book? Engineering books recommendations...

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

Ridge beam/diaphragm analysis

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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 
******* ****** ****** ****** ******* ****** ****** ********