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RE: "z" limit requirement

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One thing I might add also is that the calculation of "z" should only be
based on a maximum cover of 2".  (This is covered in ACI 350, I believe.)
For example, if you are designing a grade beam with a cover of 3" at the
bottom, you would only use a maximum of 2" in the calculation.  (Using 3" of
cover really affects your "z" calculation.)  Another thing to remember is
that, as stated in the comentary for ACI 318, the equation for "z" is based
on a beta value of 1.2.  (Beta is the ratio of the distance to the neutral
axis from the extreme tension fiber and from the centroid of the main
reinforcement.)  The 145 and 175 values in ACI 318 are non-conservative for
slabs since the actual beta value for a typical slab is greater than 1.2,
closer to 1.35.  The limiting "z" values for slabs will be closer to 130 and
155.  (The ACI commentary points out that the z values of 145 and 175 in the
code correspond to crack widths of 0.013 and 0.016 inches respectively, with
a beta value of 1.2.)  Note that the opposite holds true if you have a
deeper beam.  Deeper beams will give you a beta closer to 1.1.  The
allowable "z" values could be adjusted for these deeper beams to 155 and 190
and you would essentially be checking for the same 0.013 and 0.016  inch
crack width.  

Jim Hagensen, SE 

> -----Original Message-----
> From:	pcm23(--nospam--at) [SMTP:pcm23(--nospam--at)]
> Sent:	Monday, December 07, 1998 7:44 PM
> To:	seaint(--nospam--at)
> Subject:	"z" limit requirement
> Hello Everybody,
> For any concrete beam (floor, roof or grade beams), are we REQUIRED by
> 1910.6.4 to ALWAYS check the "z" (distribution of flexural
> reinforcement)?
> Most concrete beam design softwares and beam design examples from books
> does
> not check this. When I design a concrete beam (especially a grade beam),
> most of the time I need a lot more rebar due to the "z" requirement than
> required by  flexure. Should we ALWAYS check them? Or only on certain
> conditions? 
> Thanks in advance for the input.
> A.M.
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