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# RE: reinforced concrete design question

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: reinforced concrete design question
• From: "Jason W. Kilgore" <jkilgore(--nospam--at)leok.com>
• Date: Fri, 20 Aug 2004 10:44:21 -0500

```The Whitney stress block doesn't necessarily describe the concrete stresses
accurately at any level.  It's simply an approximate method used to make
design easier.

At truly low stresses, it is a linear relationship like any other mostly
homogenous material (triangular above and below the neutral axis).

At higher stresses with cracked/reinforced concrete, the compression
stress-strain diagram looks more like a parabola or half a sine curve.  It's
very low (assumed zero) at the top compression surface, increases rapidly to
a max value a short distance below the top surface, then decreases more

Ultimate strength formulas modeling this diagram would be extremely complex,
so Whitney came up with a short cut.

Unfortunately, I don't remember exact numbers from my advanced concrete
class many many years ago.  I'm sure someone out here has more working

---
Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

> -----Original Message-----
> From: Cliff [mailto:clifford234(--nospam--at)yahoo.com]
> Sent: Friday, August 20, 2004 9:31 AM
> To: seaint(--nospam--at)seaint.org
> Subject: reinforced concrete design question
>
> Theres a linear relationship between stress and
> strain in the compression zone of concrete beams when
> stresses are small. When stresses get bigger the
> linear relationship goes away and you use the Whitney
> Stress Block to approximate the compression zone in
> concrete beams.
>
> Does anyone know when the linear relationship between
> concrete stress and strain starts to disappear? Can
>
> I found one book that says the linear relationship is
> valid for stress levels below 0.5 x fc.
>
> TIA,
>
> Cliff Schwinger
>
>
>
>
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