Force envelope results explained

My force envelope distribution value is larger  than any of the phases' results: How can this be?

In PLAXIS 2D, sometimes, the maximum or minimum values displayed on the force distribution envelope can be larger  than any of the phases’ results.  This can be explained by the fact that these maximum and/or minimum values for the axial force N, the shear force Q or the bending moment M were recorded at an (unknown) intermediate calculation step.

Due to the non-linear behaviour of the soil, each calculation phase is generally divided into a number of calculation steps. In order words, the external loading (Out-of-Balance) is applied and solved in (small) increments.

Each calculation steps is, during the calculation, temporarily saved in the Windows TEMP folder, in a file named data.### (### corresponds to the calculation step number). During a calculation step, the actual forces are compared with the ones from previous calculation steps. The maximum and minimum variables are updated if the current actual force is bigger or lower than the maximum or minimum values recorded in previous calculation steps, respectively.

The force envelope distribution is the visual representation of the maximum and minimum force values recorded during calculation and it doesn’t necessarily mean that these occurred in the last calculation step of any phase. In other words, whenever a force envelope distribution value is larger than in any of the phases' final results, this is an indication that the maximum and minimum force values occurred in an intermediate calculation step.

However, by default, a calculation phase has the Max number of steps stored (in the numerical control parameters settings in the Phases window) set to 1, so only the phase’s last calculation step file will be stored and saved in the project folder. As a consequence, if the maximum and minimum force’s values occurred in an (unknown) intermediate calculation step, the user won’t be able to visualise the calculation results for that specific intermediate calculation step.

Find the calculation step when a force envelope value has been reached and visualize its results

In order to find out in which (intermediate) calculation step the maximum/minimum value occurred, you need to:

1. Store all intermediate calculation steps, during the calculation ( Max number of steps stored = Max steps value on each Phase's numerical control parameters tab)
2. Run your model and save it at the end of the calculation
3. Select to view the results for the plate in Output and then click the Select points for curves button to choose the plate’s post-calc node you wish to retrieve the results for.
4. Create a Step vs post-calc node force's curve, in the Curve manager window. In the generated curve, you can now see the force evolution during calculation and identify in which calculation step the maximum/minimum value occurred.
5. Knowing the calculation step number, you can change the plot of the force distribution to the step in which the maximum or the minimum value was recorded. For that, just make sure that the button in front of the drop-down menu is set to show all saved calculations steps results, as shown in the image below.  From there, you can visualize any calculation steps results.