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Re: Steel Modulus and Strength

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Based on my experience with failure analyses as well as design
work....you're (all) right in the fact concrete is a crap shoot in terms
of precision.  There are just too many variables that you can't control
without putting a QC tech out there on site, basically 24/7 during
concrete mixing and pouring, and repeatedly taking data.  Go with the
standards for design purposes, make sure you have good contractors (who
assume proper liability) for the concrete, and pray you don't need pull
exact data down the road in response to a failure issue.

HOWEVER, metals tend to be a bit more quantified.  Most graded materials
made in the US tend to meet their minimum strength requirements pretty
reliably.  Again, a lot of the problem that do arise come from vendors
and contractors, only in this case its substitutions being the biggest
concern.  If you are planning to use Grade 60, then use 60ksi for your
yield.  I would be more likely to adjust safety factors upwards than to
arbitrarily downgrade material properties if reliability is a concern.

 If its a critical application, such as designing close to your minimum
SF, get a rebar pulled from the stockpile every so often and given a
pull test (almost any metallurgist lab can knock that out fairly easily)
as a spot check on that batch/purchase of rebar.  The concern would not
be whether there was a bad run of rebar as much as substitution or
counterfeiting.  I haven't heard much of counterfeit rebar, but there
was a SERIOUS problem with imported counterfeit fasteners some years
ago, ie, regular fasters marked as being high-strength fasteners.  Since
a lot of work is "over engineered", the cheaper stuff doesn't usually
become apparent unless those "what if" situations we are required to
design for (hurricanes, earthquakes, etc.) occur.  Then again, if the
Titanic had been made with the specified plate, it wouldn't have sunk.

Hope this helps.  

Bart Kemper, P.E.

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