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How do I model lift off supports?

Whilst lift-off/contact supports can be easily modelled in either a linear or nonlinear analysis, simply by choosing the "Lift-off" option for a particular freedom in a defined support attribute, there are cases where simple lift-off/contact between mesh is required or more control over the behaviour is desired/required.  Lift-off (compression only) contact or supports can also be modelled using joint elements.  Key considerations include the geometry of the model, selection of appropriate mesh and material attributes, and calculation of appropriate joint stiffnesses.

Joints defined for this purpose will have different properties according to loading conditions; this requires repeated updating of the stiffness matrix and a nonlinear solution is therefore required. 

Superposition of load is not applicable in nonlinear analyses. It is therefore important to ascertain if the use of lift-off supports is appropriate. Does the total loading on the structure lead to lift off? Certain individual loadcases may appear to cause lift-off, but when combined with dead loading and other permanent or coexistent downward or opposing loads, lift-off may not be possible.  If it is necessary to adopt a nonlinear lift-off approach, a full nonlinear loading procedure will need to be adopted since superposition of loads is not valid. 

Use the links below to obtain advice on generating lift off supports:

Lift off joints at (or between) point features or at line ends

Lift off joints between edges of surface features or along length of line features

Lift off joints over faces of surface features

Joints: Mesh | Geometric attributes | Material attributes | Spring stiffnesses | Supports | Loadcase properties

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