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ACI 318-02 reinforcing steel questions

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Clifford Schwinger wrote:

"Section 7.6.5 says that reinforcing steel in slabs
can't be spaced more than 18" on center.
Section 12.11.1 says that at least one third (simple
spans) or one fourth (continuous spans) of the bottom
bars in slabs must extend into the supports.

Here are two questions:

Question 1:  If my slab is reinforced with bars at 18"
on center, can I terminate half the bottom bars short
of the support and only extend bars at 36" on center
into the support, or is section 12.11.1 only
applicable when you have so much bottom steel that
even if you do terminate two thirds or three quarters
of the bars short of the support, the remaining bars
will no further apart than 18" on center?

Question 2:  Section 12.11.1 says that some of the
bottom bars have to extend into the supports, and says
that for beams the bottom bars must go at least 6"
into the supports. This leads me to wonder how far is
"far enough" for slab bottom bars to extend into the
supports. If my slab bottom bars extend 2 microns into
the support, did I meet the requirement of section
12.11.1? 

It might sound like I'm splitting hairs with these
Questions...."


Cliff,
The requirement for slab reinforcing spacing applies everywhere.  By
code, you can't have bars at 36 inches.  More to the point, why would
you want to?  How much money will you save your client?  Enough to
convince them not to extract 100 times that much through the services of
the legal profession when it cracks?  I've never designed a structural
slab that would have worked well with bars at 36 inches on center.

Inadvertently narrow code language ("beams...") aside, I would stick
with the 6 inch extension into the supports for slabs (after all, just a
wide beam).  The point of that rule is that there is a stress riser at
the discontinuity caused by a bar ending.  It's a likely place for a
tension crack, which then propogates (since your bars are 36 inches
apart) and becomes a shear problem.  The 6 inch embedment helps keep
these cracks from starting, helps keep them tight when they do, and
provides some help for the shear strength by way of dowel action (shear
in the actual bar where it crosses a crack).  The embedment does all
this for virtually no cost, since you've already paid for the bar to be
designed, fabricated, shipped, placed, inspected, and entombed in
concrete.  Pretty good bargain, all the way around.

And yeah, it does sound like you're splitting hairs, a little.  These
rules might be the cheapest insurance you'll ever get.  They're safe,
well-reasoned, and cheap.  And nobody's going to sue you for doing
them--only for not.

Mike Hemstad, PE
TKDA
St. Paul, Minnesota

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