What is Wobble Coefficient in post tensioning system?

Lets us answer this question from scratch, the word wobble means unsteady jerking movement from one end to other or move unevenly while coefficient is a symbol or number that defines the relation between two or more variables.

While post tensioning we use strand wire that is being joined by twisting around one and other to make a tendon, this tendon is to be inserted within a profile pipe called sheathing or duct after the concrete will achieve a specified compressive strength.

Sometimes the jacking force is applied from one end or both ends. The duct or profile pipe is to be inserted as per the designed curvature and location. The strand wire or tendon can’t be completely set as per the designed curvature and profile due to field restrictions and faulty workmanship.

Due to these reasons the curvature is bit shifted from the designed curvature and thus causes some loss in tensioning force. When jacking force is applied at one end, it decreases as we moves to the farther end due to friction between the tendon and the profile pipe or duct, this friction losses is of two types,

1)      Curvature friction loss

2)      Wobble friction loss

Curvature friction loss is basically the loss in tensioning or jacking force due to design curvature while wobble friction or wobble coefficient is a function of unintended deviation due to faulty workmanship or field restrictions.

Wobble coefficient is usually denoted by K and is measured in units of per length. It is found by multiplying the jacking force at any distance from jacking end with the average of the intended angular deviation from the design profile.

Usually having value of 0.0088.