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

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.

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

∴ Loss in pre-stress = 1.75%

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.

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.

Typical values of the total losses of stress that could be encountered under normal conditions of work were recommended by Lin, long term field studies on the loss of prestress in post tensioned concrete bridge girders have been carried out by marks and Keifer, Neville gave the losses in prestress considering various influencing parameters.