Tension/Compression Limits

Form: Frame Assignment - Tension/Compression Limits

An upper limit on the amount of tension and compression force supported by a frame object can be assigned. This is used primarily to model tension-only cables and braces. The behavior modeled is nonlinear but elastic. For example, assume a compression limit of zero has been set. If the object tries to go into axial compression, it will shorten without any stiffness. If the load reverses, it will recover its shortening with no stiffness, then engage with full stiffness when it reaches its original length.

Select the frame object(s) to be assigned axial force limits.
Use the Assign menu > Frame > Tension/Compression Limits command to access the Frame Assignment - Tension/Compression Limits form:

- Tension Limit check box and edit box. Specify a tension limit by checking the Tension Limit check box and entering a value in the edit box that is zero or positive.
- Compression Limit check box and edit box. Specify a compression limit by checking the Compression Limit check box and entering a value in the edit box that is zero or negative.

Note: If zero is specified for both the tension and compression limit, the object will carry no force in a nonlinear analysis.

Click the Apply button. Any previously assigned tension/compression limits will be replaced by this assignment.

When the Apply button is used, the Frame Assignment - Tension/Compression Limits form will remain open until it is closed by clicking the Close button. This allows selection of another frame object(s), to which a different assignments can be made.

If only one assignment is being made to only one set of selected objects, the OK button can be used to both apply the assignment and close the form.

Note:

Tension/Compression limits are nonlinear. Thus, the following apply:

Tension/compression limits will be activated in a nonlinear static or nonlinear direct-integration time-history analysis only. In general, use nonlinear analysis when tension/compression limits are present in the model.
If a linear analysis is performed using the stiffness at zero, the full axial stiffness (AE/L) of the element will be used as if there were no limits.
If a linear analysis is performed using the stiffness at the end of a nonlinear analysis, the axial stiffness will be zero or AE/L, depending on whether the object exceeded its limits at the end of the nonlinear case.

Tip: This feature is useful for modeling braces that can reasonably be represented by a single straight object when the analysis is focused more on the effect of the brace on the structure than on the detailed behavior of the brace itself.

To model the deformation of the brace in detail, break the brace into several sub-objects, and use large-displacements analysis without compression limits. Under compression, the brace will buckle out of the way in a more realistic representation of the true behavior.