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Re: Tension in Concrete

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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.

Alex C. Nacionales wrote:

> I would like your opinion regarding using the tensile strength of
> concrete(10% of Fc') in structural computations.
> This would be economical in thick shallow footings subjected to large
> moments where top reinforcement would be needed if the tensile strength of
> concrete is neglected.
>
> Alex C. Nacionales
> Iloilo City, Philippines
>
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