Missed Stressing Operation

Q: What happens if you forgot to stress an unbonded post-tensioning tendon at the construction joint and have already poured the subsequent pour?

A: The most preferred option is to stress the tendon in the subsequent pour. However, the following items should be factored in:

a) The 7% Elongation Range must be revised.

Since you have a longer pull, the elongation calculation needs to be revised also. This should be noted in the elongation records from the PTI-Certified Inspector. A thumb-rule is to just add Min1+Min2 and Max1+Max2 together. For example,

  • First pull of 50′ would be Min1 = 3.72″ and Max1 = 4.28″
  • Second pull of 100′ would be Min1 = 7.44″ and Max1 = 8.56″
  • New elongation range for 150′ pull would be Min1 = 3.72″ + 7.44″ = 11.16″ and Max1 = 12.84″.
  • The new range is an close approximation. A more accurate range can be calculated with PT software. This is especially important for very long pulls.

b) The force in the tendon will likely be less.

Since the stressed tendon length is now longer, your angular and wobble friction have likely increased somewhat due to the increased number of spans. In other words, the final average force will drop in both pours for that particular tendon.

  • Previously, the 50′ pull may provide 27.5 kips of force in Pour #1.
  • Previously, the 100′ pull may provide 27.3 kips of force in Pour #2.
  • Now, the 150′ pull may provide only 26.9 kips of force in Pours #1 and #2.

c) The intermediate anchor has been abandoned.

The force within the 150′ tendon will be transmitted uniformly through the construction joint. Conversely, the 50′ tendon transmits force only within Pour #1 and the 100′ tendon transmits force only within Pour #2 (i.e. the construction joint “locks-off” the force within each pour). This is important should be there be any repair/renovation conducted in the future.

– Neel Khosa, Vice President, AMSYSCO

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Copyright © 2011 by AMSYSCO, Inc. All rights reserved.

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