# Re: Steel Modulus and Strength

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Steel Modulus and Strength
• From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
• Date: Wed, 26 Jul 2000 08:39:38 -0400
```It doesn't make any difference because *we* don't know the strength of the
concrete in the structure.  The specified strength is the 28-day strength of
laboratory cured concrete cylinders.  Strength of in-place field cured
concrete is only required to be 85 percent of the laboratory cured cylinders.

40 years ago when I was in school, E for steel was 30,000 ksi.  Then it was
changed to 29,000 ksi;  prestressing strand is 28,000 or 27,000, and it
doesn't make any difference.  These values are just *averages* and an
approximation of the modulus of elasticity up to the proportional limit.

BTW, strut theory for concrete has been in used since early in the 20th
century.  Any concept that gives the "right answers" is valid, even it the
concept is "wrong."  (Take the Whitney stress block.)

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Jim Korff wrote:

>>Great...

So why do we bother with specifying concrete strength of  3000 psi.....we
could just say 2900 psi....or 3100 psi....or somewhere in between......if it
doesn't make any difference anyway.....

All I'm after is a default value to use a in a design software
program.....(oops....it's one that originated in Europe.....where they still
seem to think about the structural mechanisms)...so if anyone has a logical
suggestion (29msi vs 30msi)....I would certainly appreciate it.....

jim

A few years ago some "professors" would be upset at the notion of strut and
tie mechanisms......<<

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