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Re: Why specify 3ksi concrete for grade beams?

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Dennis,

Tom points out what I was going to point out...so I will just supplement
what he offered.  Section 21.2.4.1 has actually had the minimum of 3000
psi for members resisting "earthquake enduced forces" since before 1989.
The oldest ACI 318 that I have is the 1989 edition, which has the same
provision.  And I will note that in the 1989 edition there are no "change
bars" so it was not added in the 1989 edition, so it had to be there prior
to 1989.

The real change that occured in the 2002 edition is there is now a
paragraph in section 1.1.1 that states that "for structural concrete, the
specified compressive strength shall not be less than 2500 psi.  No
maximum specified compressive strength shall apply unless restricted by a
specific code provision."

Regards,

Scott
Adrian, MI


On Sun, 10 Oct 2004 THunt(--nospam--at)absconsulting.com wrote:

> Dennis,
>
> 3000 psi concrete is also required per CBC 2001 section 1921.2.4.1 (and
> even more restrictive in section 1921A.2.4.1) which is the same as section
> 21.2.4.1 of ACI 318 2002
>
> Thomas Hunt, S.E.
> ABS Consulting
>
>
>
>
>
> "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
> 10/10/2004 10:53 AM
> Please respond to
> <seaint(--nospam--at)seaint.org>
>
>
> To
> <seaint(--nospam--at)seaint.org>
> cc
>
> Subject
> Why specify 3ksi concrete for grade beams?
>
>
>
>
>
>
> You guys had me up most of the night and early this morning as I could not
> recall why my specifications on my drawings demand f’c=3000 psi concrete
> for grade beams.
> The best I could do was recall when we first established RGA 1-91 back in
> the late 80’s. At the time, as I recall it, all grade beams used in moment
> frames were required or strongly suggested to be designed using 3,000 psi
> concrete because it forced deputy inspection. As I recall it (and I may be
> wrong but have used this spec since I started doing retrofit work in 1986)
> the city did not have a structural observation requirement and wanted to
> force both the engineer of record and a deputy inspector to make sure the
> concrete exceeded 2,500 psi and that the steel horizontal rebar as well as
> the shear ties (generally in place if required or not) were adequately
> placed.
> It was a measure to force the community to insure that a mix was adequate.
>
> I had one instance where a small frame was to be put in and the contractor
> decided to mix the concrete himself on site. I objected to this after
> finding out what he had done. I demanded cores (it was in the city of
> Beverly Hills) and the stuff came out like soup. The testing lab could not
> get a sufficient strength without the concrete simply falling apart to
> find out what strength it had.
>
> The contractor complained, but we made him remove all of the concrete he
> placed and had him do the job over according to the specifications that I
> provided for special deputy inspection. He paid the price and the work was
> redone to the satisfaction of the testing lab and the inspector.
>
> I think that this practice originated in Los Angeles to force deputy
> inspection and verification of the strength of the concrete when used to
> resist bending in the grade beam so as to reduce the column size.
>
> I’ve been the engineer of record on hundreds of buildings in the Los
> Angeles county area from 1986 until 1993 and all of the grade beams I
> specified were done using f’c=3,000 psi concrete and demanding special
> deputy inspection. It might have been unwritten, I thought it was stated
> in RGA 1-91 or in the 1991 UCBC Appendix Chapter 5 but I can not find it
> so I must assume that it was a recommendation by the professional
> community at the time.
>
> This is not to say that it was required or that every engineer designed to
> this standard, but it was my practice and since it exceeds the minimum
> requirements of the code and none of my clients complained, I believe it
> was the right choice to make.
>
> Dennis
>
> Dennis S. Wish, PE
> California Professional Engineer
> Structural Engineering Consultant
> dennis.wish(--nospam--at)verizon.net
> http://www.structuralist.net
>
>
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