Tension-only and compression-only effects

While in tension, tension-only members act identically to normal members with axial, flexural, torsional and shear capacity. However, if they go into compression then they are automatically disabled and act as if they have been removed from the model. Similarly, compression-only members act identically to normal members unless they are disabled as a result of going into tension.

Although the solution of tension-only or compression-only members requires an iterative analysis method, SPACE GASS puts it into a slightly different category to the other non-linear effects and makes it available in either a linear or a non-linear static analysis.

Unlike the P-D and P-d effects, tension-only and compression-only effects result in an exact solution provided that convergence can be achieved.

For tension/compression-only effects, convergence is sometimes difficult (if not impossible), especially if the model is highly symmetrical. If convergence is not achieved after three iterations, SPACE GASS relaxes the tension/compression-only criteria slightly in an attempt to improve the chances of reaching convergence.

During the first three iterations SPACE GASS disables tension-only members that have either end in compression. During iterations four and five it disables tension-only members which have the average of their end forces in compression. During the sixth and further iterations it disables tension-only members that have both ends in compression. A similar procedure is followed for compression-only members that have tensile forces at their ends.

If tension/compression-only effects have been activated with "No reversal" then convergence is usually achieved after two or three iterations, even for highly symmetrical structures. This "No reversal" method is not usually recommended though because it sometimes results in members being prematurely disabled and then not being able to be re-enabled in later iterations after the axial forces have been redistributed around the model.

A "Gradually activated" setting is also available for cases where too many members are initially disabled, causing the model to become unstable before it has a chance to adjust to the disabled members.

$image\ebx_-1773499217.gif$ Tension/compression-only effects are ignored by the dynamic frequency analysis module. No tension-only or compression-only members are disabled in a dynamic frequency analysis, regardless of their axial force.

! IMPORTANT NOTE !

Tension-only members should not be used to model cables.