Node definition - eesd-epfl/OpenSees GitHub Wiki
The three nodes that the macroelement connects are standard three-dimensional nodes equipped with 6 degrees of freedom each. Therefore, the model dimensions must be set before building the model as:
# Create Model builder with 3 dimensions (-ndm) and 6 dof/node (-ndf)
model -ndm 3 -ndf 6
Afterwards nodes can be normally defined through their coordinates:
# tag X Y Z
node 1 0.00 0.00 0.00
The user defines the position of the central node as a normal node, but should not apply moments nor imposed rotations to it, as it is used to contain degrees of freedom that do not correspond to the standard set of displacements and rotations.
The coordinates of the central node will define the barycentre of the macroelement. The other two nodes can be placed at the two extremities of the macroelement. If placed elsewhere, a rigid offset is automatically defined at the moment of creation of the element, without need of user input. This allows creating equivalent frames with rigid nodes directly, without the need of defining further rigid links.