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RE: spacing limits for concrete beam shear reinforcement

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Scott,
Do you know what the basis for ACI allowing beams without shear reinf? In B-regions (vs D-regions), where inclined cracks due to flexure-shear are developed, what elements provide the "tie" in the strut-tie model (particularly for non-deep beams).  It appears, the oversized beam, as Jerry mentioned, will be nothing but a plain concrete section which by nature is governed by flexure, not shear - that means you can not take full advantage of the flexure bars because these are not supposed to yield.
 
Thanks.
 
Suresh Acharya, S.E.
 
 


From: Jerry Coombs [mailto:JCoombs(--nospam--at)carollo.com]
Sent: Tuesday, February 12, 2008 8:48 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: spacing limits for concrete beam shear reinforcement

None are req'd if Vu<.65Vc/2.  I've made a beam bigger just to avoid stirrups.  It's usually cheaper.  I'll still put them at some nominal spacing to hold the reinf. 
 
6" isn't really close at all.  Get's congested for the installer w/ a 12" wide beam. When you get down to 3", look at a larger beam...

>>> On 2/12/2008 at 10:30 AM, "Scott Maxwell" <smaxwell(--nospam--at)umich.edu> wrote:
And I would futher note that the Masonry code (MSJC/ACI 530) has the same
d/2 requirement...for both ASD and Strength design.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Kipp Martin [mailto:KMartin(--nospam--at)carollo.com]
Sent: Tuesday, February 12, 2008 11:07 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: spacing limits for concrete beam shear reinforcement


Christopher,

If I remember correctly from my concrete design classes, the limit on
maximum spacing for shear reinforcing in beams is set so that any possible
shear crack will intersect at least one stirrup.  That is where the d/2
limitation in 11.5.5 comes from.  Remember that shear cracks (diagonal
tension cracks) are at roughly 45 degrees to the horizontal axis of the
beam.  For shallow beams, this does indeed push the stirrup spacing close.
As far as I know, there is no exception or variance to this requirement.  A
6" spacing does not sound like it is all that crowded.  It may not be the
way the residential contractor "has always done it for 27 years" but it is
not a new provision in ACI 318.  The oldest copy that I have is from 1983,
and the d/2 provision is in that edition as well.

--Kipp Martin, S. E., P. E.
  Carollo Engineers
  Portland, Oregon


>>> "Christopher Banbury" <cbanbury(--nospam--at)arkengineering.net> 2/11/2008 2:58
>>> PM >>>
Perhaps someone can help us navigate the ACI 318 provisions and exceptions
related to shear reinforcement spacing limits and area minimums.

When considering 12"Wx16"D c.i.p. floor beams on c.i.p. columns for an
elevated V-Zone residence, ACI 318, 11.5.5 requires stirrups at about 6" oc
if d is taken to be 0.8*h. Based only on strength considerations, the
spacing could be 14" if No.3 stirrups are used. Torsion can be neglected
which helps with Av min per 11.5.6.3 but that doesn't help with the spacing.

Are there any alternatives or exceptions to 6"oc stirrups short of switching
to reinforced masonry which would does not have such restrictive spacing
limits? We are limited on height so increasing d is not an option.

I'm concerned that a stirrup spacing less than 12" oc would be impractical
for the typical residential contractor.

Thanks in advance.



Christopher Banbury, PE

President



Ark Engineering, Inc.

PO Box 10129, Brooksville, FL 34603

22 North Broad ST, Brooksville, FL 34601

Phone: (352) 754-2424

Fax: (352) 754-2412

www.arkengineering.net <http://www.arkengineering.net>





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