Need a book? Engineering books recommendations...

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

RE: Shear

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Jim,

It would seem that the reason that the difference for wood would be due to
how wood if typically "configured".  Keep in mind that the shear failure
in wood members is typically due to horizontal shear failure (i.e.
parallel to the wood grain).  I did find this one statement that explains
why in my Beyer text book:

"The concept of omitting loads within d from the support is based on an
assumption that the loads are applied to one side of the beam (usually the
top) and the member is supported by bearing on the opposite side (usually
the bottom).  In this way the omitted loads are transmitted to the
supports by diagonal compression.  A similar type of adjustment for shear
is used in reinforced-concrete design."

It would seem to me is that the difference is that a wood member could
handle a significant concentrated load in that range since the shear is a
horizontal shear failure so the diagonal compression due to a significant
concentrated load could still be handled by the wood.  The concrete on the
other hand probably cannot handle substantial concentrated loads in that
zone since the shear failure is due to diagonal tension in most cases.

I realize that there is not too much "meat" to my explanation, but it
makes intuitive sense to me.

HTH,

Scott
Ypsilanti, MI


On Tue, 30 Jul 2002, Jim Kestner wrote:

> Scott:
> 
> I thought your description of the shear mechanism in concrete is described
> well and I am in agreement with this. I believe ACI has this correct and am
> therefore not concerned about their response. I am more concerned on how NDS
> views the situation with a concentrated load within a distance "d".
> 
> Jim K.
> 
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Tuesday, July 30, 2002 8:42 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Shear
> 
> 
> Jim,
> 
> I believe that this is largely due to the way that the shear failure
> mechanism will form in concrete.  Under a uniform loading condition, where
> the uniform load between the face of the support and d from the support,
> the failure mechanism is typically that a crack (typically due to diagonal
> tension) will form at the face of the support in the tension zone and
> spread at roughly a 45 deg angle away from the support toward the
> compression zone.  The crack will typically not "penetrate" the
> compression zone due to the compression force present conteracting the
> diagonal tension.  Thus, the crack will "hit" the compression zone at a
> distance of about d from the face of the support.  The result is that a
> (small) amount of load can be carried to the support in a more arching
> action type method rather than shear...this would be the uniform load
> between the face of the support and d from the support.
> 
> When you now place a (significant) concentrated load between the face of
> the support and d from the face of the support, the arching mechanism is
> no longer sufficient to carry this type of load, so it wil be necessary to
> carry this load by a shear mechanism as well.  Thus, the reason why you
> must now take Vu at the FACE of the support.
> 
> I must admit that this is my own take on the situation, so it could be
> wrong.  If you want a more "definitive" answer than I can talk to someone
> that I know who is on the 318 committee.
> 
> HTH,
> 
> Scott
> Ypsilanti, MI
> 
> 
> On Tue, 30 Jul 2002, Jim Kestner wrote:
> 
> > Curt:
> >
> > I am only speaking here of the exception for concentrated loads (located
> > closer to the support than "d") that appears in section 11.1.3 b.  It is
> > illustrated in Figure R11.1.3.1 d. You must include these concentrated
> loads
> > in your shear calculation for Vu. I believe only uniform loads within a
> > distance "d" can be ignored. The commentary elaborates more on this
> > requirement.
> >
> > Jim K.
> >
> >
> > -----Original Message-----
> > From: LaCount, Curt [mailto:Curt.LaCount(--nospam--at)jacobs.com]
> > Sent: Monday, July 29, 2002 4:27 PM
> > To: Seaint (E-mail)
> > Subject: FW: Shear
> >
> >
> > Jim,
> >
> > I believe that the ACI section you mention does permit the loads closer
> than
> > "d" to the support to be ignored (or in otherwords Vu shall be as that
> > computed at a distance "d" from the face of support).  There are
> exceptions
> > that are discussed in the commentary.
> >
> > HTH,
> >
> > Curt La Count
> > Jacobs Engineering
> > Portland, OR
> >
> >
> > -----Original Message-----
> > From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
> > Sent: Monday, July 29, 2002 1:42 PM
> > To: 'SEAINT'
> > Subject: Shear
> >
> >
> > Does anyone know why for Concrete design per ACI 318-95  11.1.3.1 you are
> > not allowed to ignore concentrated loads within a distance "d" from the
> > support for shear yet for Wood design, you can ignore all loads within a
> > distance "d" from the support per NDS '97 3.4.3a (Figure 3D). Shouldn't
> > proceedures for wood and concrete be the same? Is NDS ignoring some effect
> > that they should not be?
> >
> > Jim K.
> >
> >
> >
> >
> >
> ============================================================================
> > ==========
> > NOTICE - This communication may contain confidential and privileged
> > information that is for the sole use of the intended recipient. Any
> viewing,
> > copying or distribution of, or reliance on this message by unintended
> > recipients is strictly prohibited. If you have received this message in
> > error, please notify us immediately by replying to the message and
> deleting
> > it from your computer.
> >
> >
> >
> ============================================================================
> > ==
> >
> >
> > ******* ****** ******* ******** ******* ******* ******* ***
> > *   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 
******* ****** ****** ****** ******* ****** ****** ********