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Re: spacing limits for concrete beam shear reinforcement

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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> 

 

 

<http://usa.autodesk.com/adsk/servlet/mform?id=9131297&siteID=123112&CMP 
=ILC-GN9219645015>  
<http://usa.autodesk.com/adsk/servlet/mform?id=9131297&siteID=123112&CMP 
=ILC-GN9219645015> 

 



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