Explain the loss of prestress due to friction and slip of anchorage and state two remedial measures to avoid them.

Explain the loss of prestress due to friction and slip of anchorage and state two remedial measures to avoid them.
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1 Answer

Answer :

Loss due to Friction: This loss occurs only in post-tensioned members. There always exists a certain amount of friction in the jacking and anchoring system and on the walls of the duct. So, the actual stress in the tendon is less than what is indicated by the pressure gauge. Considerable frictional loss takes place due to friction between the tendon and the material surrounding it, namely the concrete or the sheathing.


Loss due to friction may be classified as below.

(a) loss due to length effect

(b) loss due to curvature effect.

The length effect represents the effect of friction for a straight tendon due to slight imperfections of the duct. In practice, the duct meant for the straight tendon is not absolutely straight. Hence the cable will touch the duct or concrete This loss is also called the loss due to wobbling effect.

In the case of curved ducts, the loss of prestress depends upon the radius of curvature R of the duct and the coefficient of friction between the duct surface and the tendon.

For a straight or moderately curved profile, with curved or straight tendons, the value of prestressing force Po at a distance ‘x’ meters from tensioning end and acting in the direction of the tangent to the curve of the cable, shall be calculated

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 Remedial measures:

1) Over-tensioning the tendons by an amount equal to the maximum

frictional loss, and

2) Jacking the tendons from both ends of the beam, generally adopted when

the tendons are long or when the angles of curvature are large. 


Loss due to slip of Anchorage: In most of post-tensioning systems, when the cable is tensioned and the jack is released to transfer prestress to concrete, the friction wedges, employed to gripthe wires, slip a small distance before the wires can be firmly held between the wedges. The magnitude of slip depends upon the type of wedge and the stress in the wires. This loss of prestress occurs only in post-tensioned members as no anchorages are used in pre-tensioned members.

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Loss due to slip in anchorage is of special importance with short members and the necessary additional elongation may be provided at the time of tensioning to compensate for this loss

Remedial measures: The loss during anchoring, which occurs with wedge-type grip, is normally allowed for on the site by over-extending the tendon in the prestressing operation by the amount of the draw - in before anchoring

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