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AREMA Serviceability Requirements

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Pedro Munoz wrote:

"Can someone familiar with the latest AREMA 2002
Serviceability Requirements for Concrete Structures
comment on the practicality of the Fatigue and
Distribution of Flexural Reinforcement equations. Does
anyone know if AREMA have any kind of guidelines for
the applicability of those two requirements, like some
Examples on how to apply them to real cases ? It looks
like AREAMA and ACI are not up to par with the
Distribution of Flexural Requirements, AREMA still
uses the Z factor. Can anyone, Please comment on this.


Pedro R. Munoz, Ph.D., P.E.
Senior Structural Engineer
DMJM+HARRIS"

Pedro,
I'm not sure exactly what you're asking, but I'll take
a shot at it.  These sections of AREMA are identical
to AASHTO (at least the 16th Edition).  These two
codes typically lag somewhat behind ACI in terms of
incorporating the latest research.  Personally, I
don't feel this is all bad, but that's a much longer
discussion.  The current ACI section 10.6.4 was
apparently derived from the older Z-cracking equation.
 Reading the ACI commentary, it is a somewhat more
direct way to get to what you really want, which is
the required bar spacing.  Given the gross imprecision
of any cracking calculation, I don't think it's
possible to say one way is better than the other on a
technical level.

Both the fatigue and Z-cracking sections require the
engineer to evaluate reinforcing stresses at service
level.  The current 10.6.4 still does, as I read it.

Calculating the reinforcing stresses in a section
which undergoes moment reversal is tricky, because
when the reinforcing is in compression it is part of a
different section modulus, so you get to make some
assumptions about the new tension steel in order to
calculate this.  Fortunately, the calculated
compression is not too sensitive to your assumption. 
If we don't know, we usually assume the steel to be
the same on each face for this calculation.

Whether either equation is "practical" depends, I
suppose, on your point of view.  They've both worked
fairly well for a long time.  They both require a lot
of calculation, which sometimes tends to inspire false
confidence in their accuracy or validity.

If you have a specific question on how to apply it,
respond and I'll take a shot at it too.  I recommend
caffiene be taken beforehand.

If none of this answers your question, I talked to a
co-worker of mine who serves on the AREMA Technical
Committee which maintains Chapter 8.  He suggested you
forward your inquiry to Committee 8 Chairman Don
Ladner @ dladner(--nospam--at)ekmail.com.  Apologies to Don.

HTH,

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota


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