Prestressing force and Ecentricity
Prestressing Force & Eccentricity
Once the actual Zt and Zb have been determined, the next step is to determine what combination of prestress force, P and eccentricity, e, to use at that section. Taking each stress limit in turn:
Tensile Stress at Transfer
This is a linear equation in P1 and e. Therefore a plot of these two quantities will give a region that is acceptable and a region that is not acceptable, according to the inequality.
Compressive Stress at Transfer
Another linear equation which gives the feasible region
Compressive Stress in Service
This equation can again be graphed to show the feasible region. However, this line can have a positive or negative slope. When the slope is negative, P1 must be under the line; when the slope is positive, P1 must be over the line. A simple way to remember this is that the origin is always not feasible.
Tensile Stress in Service