Skew Frame Properties

Beam type spring elements are best defined using three nodes (Figure 1). Nodes 1 and 2 are the two ends of the element and define the local X axis. Node 3 allows the local Y and Z axes to be defined. However, this node does not need to be supplied.

If all three nodes are defined, the local reference frame is calculated by:(1)
x = n 1 n 2 ¯
z = x × n 1 n 3 ¯
y = z × x
If node 3 is not defined, the local skew frame that can be specified for the element is used to define the Z axis. The X and Y axes are defined in the same manner as before.(4)
z = x × y s k e w
If no skew frame and no third node are defined, the global Y axis is used to replace the Y skew axis. If the Y skew axis is collinear with the local X axis, the local Y and Z axes are placed in a totally arbitrary position. The local Y axis is defined at time zero, and is corrected at each cycle, taking into account the mean X axis rotation.

Figure 1. Element Definition