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Re: Difference between Initial Prestressing and Stressing Force
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- Subject: Re: Difference between Initial Prestressing and Stressing Force
- From: GSKWY(--nospam--at)aol.com
- Date: Thu, 13 Mar 2003 21:29:51 EST
The terms "jacking force" and "stressing force" are used interchangably. In post-tensioning, this is the force that the jack applies to the strand. The actual force in the strand will decrease along its length, due to friction. For a half-in strand, the stressing force is usually 33 kips (0.153 sq. in. * 270 ksi * 0.8 Aci 318 Safety Factor).
"Initial Prestressing Force" is not defined anywhere in the Code. However, the term Ppi is used in the calculation of Prestress losses according to the equations developed by Zia et al. in the 1979 classic, Estimating Prestress Losses.
"Ppi - prestressing force in tendons at critical location on span after reduction for losses due to friction and seating loss at anchorages but before reduction for elastic shortening, creep, shrinkage and relaxation."
Ppi will vary along the length of the tendon, due to friction losses. fpi is defined as Ppi/A. An average value for fpi is used when selecting a "C" coefficient for calculating relaxtion. Typically, this is assumed to be 0.7.
The term "Initial Stresses" is also used as a generic term to identify the stage before live load is applied - in a heavily post-tensioned member, you may end up with cracking at the top of the member at midspan at this stage and thus should add some nonprestressed reinforcement. In calculating initial stresses, you make an estimate of what the stress in the strand is (before long term losses), what your concrete strength is, and what your loading is. Then you look at the potential for cracking.
At lock-off (immediately after anchoring the strand), it is typically assumed that you have about 28.9 kips average force in the strand. This being the average of the force along the length of the strand. After long term losses, it is typically assumed you have about 27 kips in the strand. This is referred to as the "final effective force". The post-tensioning supplier determines the number of tendons required for a beam by looking at the final effective force required on the structural drawings and then dividing by 27 or whatever they are using as the final effective force for a tendon.
When determinin the ultimate moment strength of the member, you can calculate the stress in the straing using approximate equations given in ACI 318, as long as the final effective stress in the strand (stress after prestress losses) is not less than half of its ultimate strength. Unless the stressing force was less than the typical 80% of the ultimate tensile strength, the final effective stress will always be more than half of the ultimate strength. If not, you have to calculate the stress in the strand using strain compability.
Gail Kelley
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