Prestressing Force & Eccentricity

Once the actual Z_t and Z_b 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

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