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RE: Tension in Concrete

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Jim,

Mabuhay to you also. I would like to thank you for your comments and for the
formula
you provided .
My Thanks also to the others who contributed.

My figure of 10% of Fc' is for the ultimate tensile stress and yes, 178 psi,
the allowable flexural tensile stress is lower.








> -----Original Message-----
> From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
> Sent: Saturday, January 23, 1999 1:07 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Tension in Concrete
>
>
> Alex :
>
> Mabuhay!
>
> Like anything else, the use of unreinforced concrete should be
> evaluated by you for the
> actual conditions of the project. If it is a minor structure,
> subject to forces which are
> relatively predictable (I would not consider earthquakes or
> typhoons very predictable),
> redundant structure, shrinkage cracking not affecting the
> strength of the structure, good
> sound foundation soil, etc. then it might be OK. Additionally,
> you might consider the
> quality of the construction, the risk to the occupier of the
> structure as well as the risk
> to your professional status and license if there is a failure.
>
> ACI 318-95 Chapter 22 (or old ACI ?) recommends an allowable
> flexural tensile stress not
> to exceed  5 x 0.65 x sqrt (f'c). This equals about 178 psi for
> 3000 psi concrete. This is
> a lower stress level than you are proposing. Also the types of
> aggregates that you are
> using for this project can dramatically affect the tensile
> strength of the concrete
> (large, sharp, angular crushed hard aggregate creates stronger
> concrete than small,
> rounded soft aggregate).
>
> For example, home builders, in my region, typically use
> unreinforced column footings under
> posts in basements. The footings are small, subject only to light
> gravity loads and
> typically have more than enough unreinforced capacity for
> flexural. Basement walls and
> wall footings are also built to act as unreinforced elements
> also. Steel is added in the
> long direction to control shrinkage cracking. If the shrinkage
> cracks do occur, they don't
> affect the strength, since they occur parallel to the span of the element.
>
> Just my opinion......I hope this helps.
>
> Jim Kestner, P.E.
> Green Bay, Wi.
>
>