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RE: Steel Modulus and Strength

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In reviewing mill certs for a project recently, our Grade 60 bar yield
strength varied anywhere from about 62 ksi to 71 ksi.  Keep in mind, there
was little if any correlation to bar size, so you would need to evaluate
every location seperately if the difference matters to you.  On the same
job, we have specified 8 ksi concrete in some columns.  28 day cylinder
tests have ranged from about 9.0 ksi to 14.5 ksi.  Again, if the difference
mattered to you, you would need to evaluate every condition seperately, and
you would not be able to do that beforehand, only afterward, since you don't
know how the concrete will turn out.  That assumes that the cylinder
strength corresponds reasonably well to the in-place strength.  I have
rarely seen E vary much from 29,000 ksi, but I have heard of it happening.
Typically, the tighter the control of the materials that you specify, the
more you will pay.  Bar manufacturers and concrete suppliers like to be able
to overshoot the minimums slightly, so that they have a margin for error if
a particular batch of concrete or steel doesn't turn out as well as they
hoped.  If you shoot for 63 ksi and miss by 3 ksi you can still sell your
steel as Grade 60.  If you shoot for 60 ksi and miss by 3 ksi you probably
just toss it back in the furnace and start over.  I believe that putting
fairly severe upper and lower bound limits on an order would be fairly
costly, without necessarily resulting in a better structure.  In 9 cases out
of 10, that extra cost would take strength away from the structure.  

Paul Crocker

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