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Re: Steel Modulus and Strength

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You want a non-flippant answer.  OK.

You specify grade 60 rebar.  Is it 60 ksi or 58 ksi, you ask?

Serious response...

Could be either or neither.  Theoretically, for the most part it SHOULD
NOT be 58 ksi, since if I recall correctly fy=60 ksi, where fy is the
MINIMUM yield strength.  Thus, in theory it should not be less than 60
ksi, but could be 61 ksi, 62.5 ksi, 60.004 ksi, etc.

The point is that (also mentioned by someone else) unless you go out and
purchase the rebar PRIOR to your design and determine the ACTUAL yield
stress for the ACTUAL bars to be used, you will NOT know the EXACT yield
stress to use.

Let's not even get into concrete strengths.

That is why my professor would get annoyed with those of us who would
calculate down to the knats a$$ the values in concrete design.  Based upon
the materials used you are lucky to determine the strength values of
concrete members plus or minus 5 to 10%, whether using US methods OR
European methods.  It has VERY little to do with the method used, but
more to do with the variance in concrete materials.  I doubt that the
Europeans are able to control materials that much more than the US.  I
will certainly agree that the Europeans are much more flexible in
approaching newer technologies.

While I use the codes, I also have at least some reasonable knowledge of
the basis of the mechanisms that the code is based upon.  Would you rather
that I use a more realistic stress-strain relationship for concrete than
the Whitney stress block when designing a concrete beam?  Also, include a
more accurate stress-strain relationship for the steel rebar?  I can do
that (that same professor taught that in a more advanced course), but it
would produce VERY similar answers but take significant more time.  In
your typical, "run of the mill" concrete beam design this would probably
leave me unemployed because I would be inefficient.  However, IF I did
specialized structural design/evaluation (and I don't at this time), a
more exact (or enlightened) mehtod might be warranted.

I apologize and will get off my soap box now.

You wanted a logical suggestion.  I would argue that you got
one...admittedly, the presentation may or may not have been appropriate,
but it was PERFECTLY logical.

I would use 60 ksi, UNLESS you want to do some testing of the actual rebar
to be used.  Testing obviously not feasible if it is new construction that
has not been built yet.  If it is existing construction, then testing
might be feasible.  You are the best judge of that.  Barring any test data
of the actual rebar material to be used, I would say 60 ksi.  The only
thing that would change that would be someone was aware of a study showing
the trend of actual yield stress for bars (similar to the study done that
produced the overstrength factor in the AISC seismic code).

Hope this helps and hope you don't mind my forray onto the soap box.

Scott Maxwell, PE, SE
 



On Tue, 25 Jul 2000, frp 2000 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.....
> 
> flippant answers certainly don't add anything.....
> 
> 
> jim
> 
> A few years ago some "professors" would be upset at the notion of strut and
> tie mechanisms......
> 
> 
> ----- Original Message -----
> From: "Scott E Maxwell" <smaxwell(--nospam--at)engin.umich.edu>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Tuesday, July 25, 2000 8:31 PM
> Subject: Re: Steel Modulus and Strength
> 
> 
> > Does that mean that you don't calculate your reinforced concrete values to
> > the nearest .0000001 k-ft?  ;-)
> >
> > I can just feel my concrete professor getting annoyed with me for even
> > suggesting that in jest!  <grin>
> >
> > Scott Maxwell, PE, SE
> >
> >
> > On Tue, 25 Jul 2000, Roger Turk wrote:
> >
> > > It doesn't make any difference.
> > >
> > > It doesn't make any difference.
> > >
> > > It doesn't make any difference.
> > >
> > > A. Roger Turk, P.E.(Structural)
> > > Tucson, Arizona
> > >
> > > Jim Korff wrote:
> > >
> > > >>Is the correct E Modulus for steel rebar 30,000 ksi  or 29,000 ksi ??
> > >
> > > Is the correct yield strength of  Grade 60 rebar 60 ksi or 58 ksi ??
> > >
> > > Is the correct yield strength of  Grade 40 rebar 40 ksi or 38.7 ksi ??<<
> > >
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