In 2016.00, the automatic centerline might not detect the structural forces correctly for an internal tunnel lining connected to concrete elements at the end points.
For example, the axial forces and the bending moments might be incorrect, as shown Figure 1 and Figure 2.
Figure 1. Incorrect bending moments calculated with automatic centerline (left) and correct values with manual centerline (right)
Figure 2. Incorrect normal force values calculated with automatic centerline (left) and correct values with manual centerline (right)
In Figure 1 the bending moment line generated from the automatic centerline shows no bending moment at the top and bottom of the side drift, which is incorrect as all connections are rigid in this tunnel case. The correct behaviour should display a bending moment at the top and bottom of the line.
Similarly, in Figure 2 the axial force should be the lowest at the top and gradually increase downwards due to the self weight of the concrete of the side drift. This is not the behaviour displayed in PLAXIS 2D 2016.00.
This issue with the automatic centerline detection is now solved in PLAXIS 2D 2016.01.
If you are working with an older version, please update to the latest version.