Node-to-Node anchors as an alternative for Fixed-End anchors

Fixed-End (FE) anchors are single point spring elements that can be used to model anchors, struts and other types of ‘flexible’ supports. PLAXIS 2D and 3D allow for only one FE anchor per geometry point. However, in some situations, it may be required to fix a structure in different directions using spring supports, such as indicated in the figure below.

This would require two or more FE anchors to be applied at the same geometry point, which is currently not possible.

The way to overcome this limitation is to use None-to-Node (N2N) anchors instead of FE anchors. N2N anchors form a spring connection between two geometry points. However, the second point may also be a fixed point at the model (bottom) boundary, which makes that the N2N anchor basically works as an FE anchor.

In the above example case, the vertical FE anchor could be replaced by a vertical N2N anchor with its lowest point connected to the bottom boundary.

Now, there are three issues to consider:

1. The direction of an FE anchor can be defined as an anchor property, whereas the direction of a N2N anchor is just its orientation in the geometry model. The spring support is in this direction. Considering that the bottom boundary is used for the fixed point, the replacement of an FE anchor by an N2N anchor works best for a (primarily) vertical support.
2. The equivalent length of an FE anchor can be defined as an anchor property, whereas the equivalent length of an N2N anchor is the distance between the two geometry points to which the anchor is connected. Since the distance between the point to be supported and the model boundary may be different than the desired equivalent length, a ‘scaling factor’ needs to be applied on the anchor stiffness in the corresponding material data set. For example, if the equivalent length is supposed to be 5 m and the length of the N2N anchor to the bottom of the model is 10 m, then the axial stiffness EA in the material data set must be set twice as high as the actual stiffness. Alternatively, the anchor spacing may be decreased by a factor 2, which has the same scaling effect.
3. Make sure that the boundary to which the N2N anchor is connected is active in all calculation phases in which the anchor is supposed to be active. For the bottom boundary this is generally the case, but note that if for some reason, the part of the boundary where the fixed point is attached is de-activated, the N2N anchor will automatically be de-activated as well.