FixedLength
The Lagrange multiplier method can be used to define constraints. Extra storage is required for the so-called auxiliary border stiffness matrix. The constraint is satisfied exactly.
The FixedLength2D
and FixedLength3D
constraints behave similar to the Tie
element, the nonlinear constraint is implemented via Lagrange multiplier method.
The position of a given node can be expressed as the summation of coordinate and displacement .
The constraint enforces that
It shall not be used with global damping models in dynamic analysis since it alters the frequency distribution, leading to an unintended damping response. Only damping models that do not depend on system properties can be used.
Syntax
fixedlength2d (1) (2) (3)
fixedlength3d (1) (2) (3)
constraint fixedlength2d (1) (2) (3)
constraint fixedlength3d (1) (2) (3)
# (1) int, unique constraint tag
# (2) int, node tag
# (3) int, node tag
Example

A vertical load is applied to the free end of the top beam. The free ends of the two beams are connected by a FixedLength2D
constraint.
node 1 0 0
node 2 1 0
node 11 .2 0
node 12 .4 0
node 13 .6 0
node 14 .8 0
node 3 1 -.2
node 4 2 -.2
node 21 1.2 -.2
node 22 1.4 -.2
node 23 1.6 -.2
node 24 1.8 -.2
material Elastic1D 1 10
element EB21 1 1 11 12 1 1 1
element EB21 2 11 12 12 1 1 1
element EB21 3 12 13 12 1 1 1
element EB21 4 13 14 12 1 1 1
element EB21 5 14 2 12 1 1 1
element EB21 6 3 21 12 1 1 1
element EB21 7 21 22 12 1 1 1
element EB21 8 22 23 12 1 1 1
element EB21 9 23 24 12 1 1 1
element EB21 10 24 4 12 1 1 1
fix2 1 P 1 4
constraint FixedLength2D 2 2 3
cload 1 0 10 2 2
step static 1 1
set ini_step_size 1E-2
set fixed_step_size true
set symm_mat false
converger RelIncreDisp 1 1E-10 10 1
analyze
exit
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