Prestress Loss due to Friction in Post-Tensioned Beams

Explanation:
Prestressing is a technique used to improve the performance of concrete structures under various loading conditions. The technique involves applying a compressive force to the concrete before it is subjected to any external load. This compressive force is known as the prestressing force and is applied using either pre-tensioning or post-tensioning techniques.

One of the major challenges in prestressing is to maintain the prestressing force over the life of the structure. This is because the prestressing force is gradually lost due to various factors such as creep, shrinkage, and friction.

Friction is one of the major factors that cause prestress loss in post-tensioned beams. When the prestressing force is applied to the tendons, the tendons tend to move through the ducts or anchorages. This movement causes friction between the tendons and the surrounding materials such as the ducts or anchorages. The frictional force opposes the applied prestressing force and causes a reduction in the prestressing force.

The amount of prestress loss due to friction depends on various factors such as the surface condition of the tendons, the geometrical arrangement of the tendons, the stiffness of the ducts, and the magnitude of the applied prestressing force.

In post-tensioned beams, the frictional force is significant due to the long length of the tendons. Therefore, the prestress loss due to friction is more pronounced in post-tensioned beams than in pre-tensioned beams.

Conclusion:
Prestress loss due to friction is one of the major factors that cause prestress loss in post-tensioned beams. This loss can be minimized by using appropriate lubricants, reducing the length of the tendons, and using low-friction materials for the ducts and anchorages.