Problem description

When having multiple lines on the same location or partly overlapping lines in PLAXIS 2D AE’s Structures mode, the program will intersect and merge these lines when going to one of these green modes: Mesh mode, Water conditions mode and Staged construction mode. When conflicting items are defined, PLAXIS 2D AE takes the settings from the first defined line: this can include:

• different line load values on these overlapping lines;
• or when both lines have been defined as a plate and the assigned plate material is not the same.

Usually this is not a big issue since it can be changed easily in the phases: e.g. change the load values in the appropriate phase or change a material assignment to a plate.

However, one item that cannot be changed in the green modes, is the definition of the local axis direction. For any local axis dependent parameter, this can become a problem then. Some examples when this can go wrong:

• Interface definition. A positive interface is defined on the side of the surface in which the positive direction of local axis 2 (perpendicular to the line) points. If there is a conflict between two overlapping lines, it could mean that the positive interface and/or negative interface can flip sides. The direction of the local axis 2 is by default defined by the drawing direction of the line. So having drawn multiple lines could already lead to a conflicting direction of this local axis 2. Users are advised to check the interface sign (+ or -) at the sides in the staged construction mode to avoid confusion later on.
  
• Force results in structural elements: the sign convention of shear forces and bending moments in structural elements is based on the local axis. If for part of a line the local axis flips, this will also mean that the sign convention for the entire structural line flips. The sign for normal forces is positive when the structure is in tension, while compression is negative. However, the side on which the positive or negative forces are drawn in the results plot are based on the local axis, so visually it could appear that the normal force flips sign: this however is not the case, so check the tables for the correct values. As for the local axis of plates and geogrids, these can only be seen in the final results when viewing the force results. Make sure to activate the local axis from the view menu.
  

Solution

This issue is now solved in PLAXIS 2D AE.02: when (multiple) overlapping lines are found, the line with interface will take preference for the determination of the local axis in the green modes.

To avoid this in PLAXIS 2D AE.00 and 2D AE.01:

• Try not to use multiple lines on the same location, but add several features (line load, plates, interfaces etc) to the same line. This can easily be done by just right clicking the line and add the appropriate feature from the pop-up menu.
• To merge multiple overlapping lines, please use the merge equivalent objects tool by selecting these lines and then right click the selection and choose to Merge equivalent geometric objects. This only works when the lines fully overlap, i.e. they have the same begin- and end point.