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Re: Time Constraints on Stressing Post-Tensioned Concrete Slabs

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At 07:26 29/07/99 -0700, you wrote:
 
-----Original Message-----
From: Gil Brock <gil(--nospam--at)rapt.pcdc.com.au>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Wednesday, July 28, 1999 9:17 PM
Subject: Re: Time Constraints on Stressing Post-Tensioned Concrete Slabs

The 3000psi figure for full stressing of a tendon is based on the capacity of the anchors in bearing/bursting. If you wish to use a lesser strength, you should only do it for anchorages tested and certified to work at the lower strengths. Some unbonded anchorages are rated as low as 2300psi.
By the PTI _expression_, a slight modification of the _expression_ that used to be in ACI 318,  the minimum required strength is calculable and is similar to ACI criteria for bearing stress in that it is a function of the ratio of bearing area to concentric concrete area. In that context "tested and certified" is not germain to individually anchored tendons common to building work. Where it is desired to achieve early stressing such as at 1200-1500 psi most anchor castings can be augmented with steel bearing plates. It is a simple matter to verify the capacity of the system via first principles.
 

I have had a look at the equation you mention. It is very nice except that it does not require/prove  that the bearing plate/casting is designed to carry the stresses induced in it. Steel bearing plates must be thick enough to act as rigid plates for this formula to be valid and stresses in the plate must be limited to proper values and castings must have sufficient strength (depending on grade of casting the strength can vary widely) and stiffness. For lower concrete strengths, the distribution of stresses in the plate will change as will its degree of rigidity. These attributes of a bearing plate/casting must be either determined by test or design and certified by the supplier. 

As a further example, castings for multiple strands used in bonded prestressing often have fins along their length to distribute the forces to the concrete, resulting in much lower dimensions for the front face of the anchor. If the formula in the PTI manual is used for these they will never work. The only way to justify them is by test.
Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd.
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:  gil(--nospam--at)rapt.pcdc.com.au
webpage:        http://www.home.aone.net.au/rapt.pcdc/