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Modeling tension-only members
Tension-only members may be modeled through the following process:
- Select all frame objects which will only carry tension.
- Set the compression limit to zero through Assign > Frame > Tension/Compression Limits.
- Tension/compression limits will only be activated in a nonlinear static or nonlinear direct-integration time-history analysis load case. In general, use a nonlinear analysis technique when tension/compression limits are present in the model.
- When linear analysis is performed using the initial stiffness of a member (stiffness at zero), computation will use the full axial stiffness (AE/L) as though no limits are assigned.
- When linear analysis uses the stiffness at the end of a nonlinear analysis load case, axial frame behavior depends on whether or not the tension/compression limits are exceeded at the end of the case, conditions for which are described as follows:
- Limits exceeded. Axial frame behavior is replaced with a sufficiently soft spring (1e-6*EA/L) which simulates zero stiffness.
- Limits not exceeded. The software uses full axial stiffness (AE/L) as though no limits are assigned.
Tension-only members in moving-load analysis
The only method available to account for the nonlinear properties of members which are assigned tension and compression limits during moving-load analysis is step-by-step time-history moving-load analysis. Additional information is available in the CSI Analysis Reference Manual (Step-By-Step Analysis > Time-History Analysis, page 466).
Unattainable tension and compression limits
When tension and compression limits are specified, the program reduces axial stiffness of the element once the limits are reached as described above. If there is a path for load redistribution, the loads will be redistributed and the limits will be achieved. However, if there is no path for load redistribution (a simple example would be a single member loaded by axial tensile force with zero tension limit specified for the member), the limits may be exceeded. This is however consistent with implementation of this feature in the program.
- Tension-only elements in ETABS article