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Re: Precast tension stresses

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Jake,

Al pointed you in good direction. If you want to design it as reinforced then ACI 350 is a good place to look. Keep in mind that the Z factor is meant to keep the cracks very small to either eliminate leaks or minimize them, but the concrete WILL crack using that method.

Another possible route it to design it as unreinforced plain concrete. There is a chapter on this in ACI 318 (chap 22 I believe). Per this method, the intent is to keep the stress low enough that the concrete does not crack. This will result in a VERY conservative design. You can still then include "normally" design reinforcement to guard against catasthropic (sp?) failure.

HTH,

Scott
Ypsilanti, MI

Jake Watson wrote:

From: "Jake Watson" <jake_watson(--nospam--at)comcast.net>
To: "Structural Engineering Association International" <seaint(--nospam--at)seaint.org>
Subject: Precast tension stresses
Date: Fri, 24 Oct 2003 08:34:31 -0600

We are working on a precast concrete project.  One of the pieces is a
planter box.  It will be suspended and vines will grow from it.  Our
contract documents call for prestress and for a design that will not have
any tension in the concrete.  The precaster missed the note and only has
mild reinforcing in his bid.  It would be relatively easy to simply make
them match the contract documents, but we would like to offer them a
performance spec that could potentially be met with mild reinforcing.  Our
goal is to prevent concrete cracking and potentially having a leak problem.

For those of you that design concrete water tanks, how do you insure they
don't leak?  What should we use for a maximum allowable tension stress in
the concrete?  IIRC, according ACI concrete cracks at 5 square roots.  We
need to be sure there won't be problems long term.  Should we spec a
different limit?  Of is there another method than simply specifying a
limiting tension stress?

Thanks in advance for your help,


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