Test: Loss Due to Anchorage Slip - Civil Engineering (CE) MCQ

Losses in Prestress members:

• In prestressed concrete, the most important parameter is the prestressing force.
• If the prestressing force reduces with time, prestresses also get reduced. 
• Various reductions of the prestressing force are termed as the losses in prestressing.
• Total loss of prestress consists of those losses which are instantaneous at the transfer stage as well as those which are time-dependent. 
• Loss due to friction and anchorage slip occurs only in the post-tensioned beam.

Important Points
Losses of prestress are as follows:

Anchorage slip is the distance moved by the friction wedges (in post tensioned members) after releasing the jacks at the ends of the member and before the wires get fixed perfectly in wedges, the loss during anchoring which occurs with wedge type grips 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.

The anchorage slip is observed in post tensioned members at the time of transfer of prestress to the concrete and the friction wedges employed to grip the wires, slip over a small distance before the wires are firmly housed between the wedges.

Calculation:
Given,
Anchorage slip = 3 mm; Beam length = 30 m; Modulus of Elasticity of steel = 2.1 × 10⁵ N/mm^2, initial pre-stress = 1200 N/mm²

∴ Loss in pre-stress = 1.75%

The anchorage slip involves the definite total amount shortening of concrete and hence it is low in members with small spans compared to the members with large spans while prestressing a short member, due care should be taken to allow for the loss of stresses due to anchorage slip, which forms a major portion of total loss.

The amount of anchorage slip generally depends upon the type of wedge used at the ends and the magnitude of stress in the wires, since the loss of stress is caused by a definite total amount of shortening the percentage loss is higher for short members than for long ones.

The systems in which tendons are lopped around concrete anchorage blocks, as in the case of Leonhardt-Baur system, loss of stress may take place, the Baur Leonhardt system is included in the third type of anchorages which work under the principle of looping tendon wires at the ends of concrete member, in this method double tendons are wrapped around the end block.

In case of long line pre tensioning system, anchorage slip is much less in comparison with the length of the tensioned wires and hence it can be ignored for calculation of stresses, slip of anchorages, length of cable, cross-sectional area of the cable, modulus of elasticity, prestressing force in cable are considered.

Loss of stress due to anchorage slip = (E_sΔ/l),
For a 30m long beam, loss of stress = (210×10³×5)/(30×1000) = 35n/mm²,
Loss of stress = 35/1000×100 = 3.5%.

The method used for slip in anchorage by extending tendon is satisfactory provided by overstress does not exceed the prescribed limits of 80-85percent of the ultimate tensile strength of the wire and the magnitude of the loss of stress due to slip in anchorage is computed as
Anchorage slip Δ = PL/AE_s, But moment Loss of stress, (Δf)_a = P/A,
By considering both equations (Δf)_a = E_sΔ/l.