Displacements

Adding displacements

Displacement objects are used to apply boundary conditions to the model as well as to impose movements directly at selected nodes. Along with Load objects, they form the main actions acting on a structural model during the analysis, to do so, they are combined with Step objects (see next topic). It is necessary for the first Step in the Structure to be associated with the boundary conditions represented by Displacement objects, these are those that do not change throughout the entire analysis. The following example shows how a PinnedDisplacement class is first imported, an object instantiated, and then this object added to the .displacements dictionary of the Structure object. All Displacement objects currently use only the global co-ordinate system with axes='global', this will be improved later with the optional use of the 'local' nodal system. The nodes that the displacement is applied to, should be the string name of the node set, or a list of node numbers.

from compas_fea.structure import PinnedDisplacement

mdl.add(PinnedDisplacement(name='disp_pinned', nodes='nset_base'))  # add a PinnedDisplacement to 'nset_base'

Accessing and editing displacements

To access or edit the data stored on a Displacement object, use its string key and view or edit its attributes as needed from the .displacements dictionary. Like all other objects, printing a Displacement will show a summary to the terminal.

>>> mdl.displacements['disp_pinned'].components  # show the components of Displacement 'disp_pinned'
{'x': 0, 'y': 0, 'z': 0, 'xx': None, 'yy': None, 'zz': None}

>>> mdl.displacements['disp_pinned'].nodes  # show the nodes that 'disp_pinned' applies to
'nset_base'

>>> mdl.displacements['disp_pinned']  # Displacement object name 'disp_pinned'
PinnedDisplacement(disp_pinned)

>>> print(mdl.displacements['disp_pinned'])  # Summary of PinnedDisplacement(disp_pinned)

compas_fea PinnedDisplacement object
------------------------------------
name       : disp_pinned
nodes      : nset_base
components : {'x': 0, 'y': 0, 'z': 0, 'xx': None, 'yy': None, 'zz': None}
axes       : global

The example PinnedDisplacement object above restrains translations in all directions but allows rotations about any axis, as indicated by the automatic assignment of zero for x, y and z for the translational degrees-of-freedom, and None for the rotational degrees-of-freedom xx, yy and zz. Non-zero values for any of the six components could be given to simulate support settlements or to perform a displacement controlled analysis, this is best performed with a GeneralDisplacement object.

Types

Different Displacement object types are available from the classes in compas_fea.structure.displacement, these are detailed below:

from compas_fea.structure import GeneralDisplacement

mdl.add(GeneralDisplacement(name='disp_general', nodes='nset_base', x=0.1, zz=0.05, z=0))  # define x, z, zz

from compas_fea.structure import PinnedDisplacement

mdl.add(PinnedDisplacement(name='disp_pinned', nodes='nset_base'))  # x, y and z are equal to 0 for 'nset_base'

from compas_fea.structure import FixedDisplacement

mdl.add(FixedDisplacement(name='disp_fixed', nodes='nset_base'))  # x, y, z, xx, yy, zz are all 0 for 'nset_base'

from compas_fea.structure import FixedDisplacementXX

mdl.add(FixedDisplacementXX(name='disp_fixedxx', nodes='nset_base'))  # x, y, z and xx are all 0 for 'nset_base'

from compas_fea.structure import RollerDisplacementXY

mdl.add(RollerDisplacementXY(name='disp_roller', nodes='nset_base'))  # only z equals 0 for 'nset_base'