Pre-tensioning I tried before here: https://forum.freecadweb.org/viewtopic. ... 10#p234999. It is mostly used in pre-fabrication of concrete elements that are later transported to and erected on site. Strands are pre-tensioned in a reaction rig, concrete poured and external reaction forces released after the concrete has sufficiently cured. In FreeCAD/Calculix this process can be mimicked by reducing the temperature of the strands, as explained here: https://forum.freecadweb.org/viewtopic. ... 0#p234065 .
In the Post-tensioning process cables are left free to move in sheaths, so they can be tensioned after the concrete is cured. The advantage is that in this case the cables can follow curved paths and thus (like in suspension bridges) directly counter-act external loads. I will try to model this behaviour via contact constraints.
The example I will use is one of a simple bridge deck (30x7.5m), supported by two 1.2x0.2m beams. The loads on the bridge are self-weight (24kN/m^3) and live load (10kPa). The pre-stressing cables/tendons are taken to be curved in both the pre and post tensioned cases, even though it will be clear from the above that this is only realistic for the post-tensioned case.
First the results for the bridge without pre-tensioning:
- Bridge_without_prestress_deflection.jpg (294.36 KiB) Viewed 3047 times
Without pre-stressing, the maximum deflection of the bridge is 176mm (~ L/170), which exceeds typical design guidance (< L/300).
- Bridge_without_prestress_reinforcement_ratio.jpg (306.37 KiB) Viewed 3047 times
The maximum reinforcement required in the beams (see: https://forum.freecadweb.org/viewtopic. ... 21#p234582 ) for this case is also quite high, i.e. 12.5%.
Both the deflection and the required reinforcement indicate that we should be considering a different design concept.
