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RE: Shear

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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.
> 
> 
> 
> 
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