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# Re: Concrete Column Design -Plastic Centroid vs. Centroid

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Concrete Column Design -Plastic Centroid vs. Centroid
• From: Tantzu Daiwan <daiwandow(--nospam--at)yahoo.com>
• Date: Wed, 31 Aug 2005 11:22:24 -0700 (PDT)

Wontae,

As I understand, the plastic centroid of a section is the centroid of the resistance to the load computed from the assumptions that the concrete is stressed uniformly to 0.85 f?c and the steel is stressed uniformly to fy.

For symmetrical sections like round or rectangular columns, the plastic centroid is at the middepth of the section. Most of the time we have the symmetrical section, so the centroid and the plastic center are always the same point, that is the center of the section.

T. Daiwan

Louisville, KY

Wontae Kim <Wontae.Kim(--nospam--at)unistresscorp.com> wrote:
Hi, everybody!
{After posting this question, I have never received any response.}

According to "Reinforced Concrete - Mechanics and Design, 2nd Ed." by J. G. MacGregor,
"The moment capacity M_n for the assumed strain distribution is found
by summing the moments of all internal forces about the centroid of the column.
...In the 1950s and 1960s moments were sometimes calculated about the plastic centroid,
...[P. 410]

Is this true?
All books are using P.C. instead of centroid
to calculate moment capacity of columns.
Why did Dr. MacGregor say that?

Does the latest edition of his book contain the same statement?

****Wontae Kim****
Unistress Corp.
Pittsfield, MA 01201
(Tel) 413-629-2031
(Fax) 413-499-0824

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