Bush Element /Spring Element, what is the difference? 1

[nmruble]

Would someone mind explaining the difference between a bush element and a spring element and their unique applications? The MSC Documentation describes a bush as "generalized spring-damper system", but the spring input allows for a damping value input. When would a bush element want to be used in place of a spring element knowing this information? Thank you!

 

[dmapguru]

The difference between a spring and a bush element in the context of Patran is not particularly well defined because Patran may be used as a pre and post processing tool for several solvers, and the difference between these 2 types of element is defined by the solver. Although there may be similarities between some solvers with respect to these elements, I can only comment on the differences in the MSC Nastran solver, which may be useful.
In MSC Nastran, a spring element is a 0d element, that is, it only requires knowledge of which degrees of freedom (DOF) to connect in the model; it does not require a finite length between the connection points, and the points may be coincident. The direction of the spring is governed by the direction of the DOF at the first connection point, so if the displacement coordinate system (that which Patran calls the analysis coordinate system), not to be confused with the definition coordinate system, of the connection point is a local one, the spring direction will be along one of the local coordinate system directions. If a solver input file is written for MSC Nastran, these spring elements will appear as CELAS1 elements. Although the connection points of spring elements may be separated by a distance, the general advice is to make them coincident and define any offsets to adjacent structure connections with a rigid element. This is because the CELASi type elements define direct entries into the stiffness with no coupling terms, which can cause over-constraint if, in the case of connection points separated by a distance, the DOF direction of the CELASi is not aligned with the vector between the 2 points. I say “can” cause over-constraint because maybe the constraint was intended, although I have yet to see this type of use in an industrial context.
CELASi elements define linear stiffness, so in large displacement analysis, its direction does not change no matter how the structure deforms; in other words it is a small displacement element. CELASi elements do allow damping to be defined, but only structural damping which is a scalar on the stiffness. Viscous damping may not be defined using CELASi (spring) elements, you would need another element type as well in order to do this.
In contrast, BUSH elements are 1d elements, that is, they require the connection points to be separated by a finite distance if the special option to allow coincident points is not taken. The vector between the 2 connection points defines the X direction of the element, it’s Y and Z directions being defined by an additional vector in a manner similar to beam elements. When defined by separated points in large displacement nonlinear analysis, the BUSH element is fully nonlinear and will follow the deformation of the structure in large displacement analysis. When defined by a coordinate system (which is required for the special option of coincident points I mentioned) the element direction remains fixed; it will not be updated in large displacement analysis.
BUSH elements allow structural damping or viscous damping or both to be defined.
When used in frequency response analysis, both CELASi and BUSH elements allow frequency dependent behaviour to be defined, and when used to model the same thing gives the same results. In general use, I prefer using the BUSH element as it is much easier to mess up a model with CELASi elements. CELASi are easy to define, but have hidden pathologies associated with their connection directions, but they do have their uses in somewhat quirky situations, like connection to SPOINTs.

DG