Modeling with Truss Elements
Truss elements are elements that can only sustain axial forces. They have displacements as degrees of freedom. Truss elements are sometimes referred to as bars or spars. They live on boundaries in 2D and edges in 3D. Typical uses of truss elements are:
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Trusses
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Cables and wires, given that they are in tension only
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Reinforcement bars
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Two-point springs and dashpots. A special material model called Spring-Damper Material is available for this purpose.
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Thin ‘strain gauges’ attached to for example solid elements
The Truss interface supports the same study types as the Solid Mechanics interface.
Dependent Variables
The degrees of freedom (dependent variables) are the global displacements u, v, and w (3D only) in the global x, y, and z directions, respectively.
Modeling Wires and Cables
You can use the Truss interface for modeling wires and cables, possibly sagging under gravity or other external loads. However, the Wire interface is specifically designed for this purpose, and should be the primary choice.
Below are some suggestions for how to model such structures efficiently using the Truss interface:
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It is generally recommended that you use first order shape functions when modeling wire-like structures. This is the default in the Truss interface.
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If the cable is to be allowed to sag, you must not use the Straight Edge Constraint for those edges. When using first order shape functions, this constraint is disabled as default, so this is an issue only if you use higher order shape functions.
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Most cable problems are geometrically nonlinear. A wire which is not in tension is not numerically stable. Physically, it wrinkles in an unpredictable manner. In order to start the analysis, you either have to add an initial stress or some weak springs.
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If there are no line or volume loads, the wire is straight, as long as it is in tension. In this case, only one element is needed for the whole wire, since you can handle the zero-stiffness in compression in the material data. Enter the modulus of elasticity so that it depends on the axial stress, and has very small value in compression. Such an expression could for example be E0*(1-0.9999*(truss.en<0)).
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In some problems, there are large deformations, but low tensile stresses. This would for example be the case if you model a wire hanging free under self-weight (‘the catenary problem’). Such problems are numerically ill-conditioned, but can still be solved as long as you use linear shape functions for the truss elements, and use tight tolerances for the nonlinear solver.