Figure 8.7: Load Directions Before and Aft er Deflection
Acceleration:
NodalForce:
ElementPressure:
L D Bf
Df D f
Df 8.2.4. Rotations in a Large-Deflection Analysis
Applying an imposed rotation (D command) in a load step is done in a "rotation vector" form, where
the magnitude and rotation direction are given by the values of the ROTX, ROTY, and ROTZ components
on the D command(s). For step loading (KBC,1), the imposed rotation values are applied at the beginning
of the load step. For ramped loading (KBC,0), the values are linearly ramped over the load step.
For compound rotations imposed over multiple load steps, each set of rotations is applied sequentially
to the previous deformed configuration. For example, rotating a body about a nodal x-axis first then
rotating it about a nodal y-axis is done simply as:
D,node,ROTX,value
D,node,ROTY,0.0
D,node,ROTZ,0.0
SOLVE
D,node,ROTY,value
D,node,ROTX,value
D,node,ROTZ,0.0
SOLVE
This simplifies the specification of compound motion such as a robotic arm. See examples presented
in Appendix A (p. 483) to find how different ways of specifying imposed rotations affect the final config-
uration.
The sequence in which the rotations are applied determine how the constrained node rotates from its
initial configuration to its final configuration. The examples presented in Appendix A (p. 483) demonstrate
the effect of the order of imposed rotations on the final configuration.
The rotation components of the output displacements (ROTX, ROTY, and ROTZ) are the sum of all incre-
mental rotations. They generally cannot be interpreted as a pseudovector or rotation vector.
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