A quick way of estimating the bending moment due to prestress in a beam/slab is to use the product of the tendon force and eccentricity: P x e. This is a simplistic elastic method that is accurate only if the following holds true:
- The eccentricities of the tendons are small in comparison with the span length.
- The frictional loss of prestress is negligible.
- The same tendons run through the whole length of the span.
For the beams and slabs designed with Captain, one or more of the above requirments are usually not met. Captain uses a rational approach of load balancing instead -- the effect of prestress is accounted for by considering the equivalent vertical forces and moments (balancing loads) that it exerts on the structure.
For detailed information on how the program uses load balancing, see the "Load Balancing" topic in the program Help.