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Interaction diagram

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As Mark said, post-tensioned piles are unusual (but pretensioned piles are common).
The procedure for calculating a post-tensioned pile interaction diagram is:
1. Determine the strains and stresses in the concrete, tendon and rebar before grouting:
    A. Tendon force (and strain) is known from stressing info
    B. Cross section strains are determined from P/EcAt, where P is tendon force and At is transformed area of concrete and rebar (assumes tendon at centroid).
    C. Determine concrete and rebar stresses from cross section strains.
2. Consider Grouting:
    A. Remember net forces and moments of PT, concrete and rebar are zero at this time.
    B. All strain changes from here on affect PT, concrete and rebar
3. Add incremental cross-section strains and calc cross section forces:
    A. From the incremental strains, determine the total strain in each component
    B. From the total strain in each component, calculate its stress and force.
    C. Sum the forces to get total section forces and moments for pile
4. Repeat #3 as necessary to get enough data for a good curve.
If the pile is pretensioned, the calculations in #1 are a little more complicated (need to solve two simultaneous equations), but it is still fairly simple.
Allan Bommer, PE
Structural Concrete Software, Inc.