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Terry Collins
Sustainability consultant
Asked a question 2 years ago

Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

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Theodore Ray
Planning and development surveyor

Prestressing is now a must to have phenomenon in every mega project whether it includes flyovers or dam or anything.

During prestressing operation at one end, frictional loses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to the deviation of duct alignment from the centerline.

Henry Newman
Water resource engineer

During prestressing work toward one side, frictional losses will happen and the prestressing force diminishes along the length of tendon until arriving at the opposite side. These frictional losses incorporate the rubbing incited because of the change in curvature of tendon duct and furthermore the wobble impact because of deviation of duct arrangement from the centerline. In this way, the prestress force in the mid-span or at the opposite end will be enormously diminished in the event that the frictional loss is high. Subsequently, prestressing, from the two ends for a single span i.e., prestressing one-portion of complete tendons toward one side and the other half at the opposite end is done to empower an even dissemination and to give balance of prestress force along the structure.

Actually, stressing toward one side possibly has the expected favorable position of lower cost when contrasted and stressing from the two finishes. For various spans (for example two spans) with inconsistent range length, jacking is generally used toward the end of the more extended span in order to give a higher prestress power at the area of most extreme positive moment. Despite what might be expected, jacking from the end of the shorter span would be led if the negative moment at the middle support help controls the prestress force. Be that as it may, if the complete span length is adequately long, jacking from the two closures should to be thought of.

Jacking at one end only during prestressing induces a higher amount of frictional losses as compared to jacking at both ends although prestressing forces are the same. So, jacking at both ends (half the number of tendons at each) is generally preferable. Often, cost of prestressing remains the deciding factor.

Jacking at one end is done in the following situations :

  • When the cost of prestressing is more important and a larger loss in prestress is acceptable.
  • When exact parabolic cable profile and stress distribution can be compromised, or when an asymmetric stress distribution is desired.
  • In beams spanning two unequal spans, prestress might be done in such a way that asymmetrical moment distribution is induced to balance the maximum positive and negative moments at the supports.

Jacking at both ends is done when :

  • Symmetric distribution is strictly desired.
  • Cost is not the deciding factor.
  • Frictional losses have to be minimized.

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