I have modeled a simple cable structure with prestress where the support forces are smaller than the prestressing force. To check it, I have analyzed two different spatial systems.
Example 1 is defined with a straight cable while the cable of example 2 has a rise.
Where does the difference come from?
The program treats a 'prestress' load as an external load like a member load. Moreover, the 'prestress' load affects the entire model.
Let's say it 'flows' off into the structural system and accordingly creates deformations and support reactions.
The prestress is converted into a length change (strain).
The axial force that remains after the calculation depends on the member's restraint of deformation.
If the stiffness of the connected system is soft, nothing or only very little of the prestress remains but a deformation exists.
In the end, the prestress is to put on a same level with a temperature change.
Once it is expressed with a force, once it is expressed with a temperature.
In example 2, the prestressing force is more converted into a deformation, which leads to lower axial forces, and thus to lower support forces.