How can I assign a member to be tension-only?

Go to the Members tab of the spreadsheet input, and go to the hinges sheet. The first field is for "Axial rigidity". Set this to tension only by selecting that option from the drop down list, or use the Ribbon menu tool "Tension only" once you have only the desired members selected.

Why is my tension-only brace failing in bending?

There are three typical reasons why a tension-only member will induce bending.

1. If the member ends are fixed, a tension-only member can still develop an end moment. Tension-only members are typically hinged at both ends in both M3 and M2 axis, if not also torsion.
2. If self-weight loads are applied this will cause the member to bend or sag under its own self-weight. To alleviate this issue we suggest using a weightless, zero density material for tension only member. An "A36 weightless" material is provided out of the box for this purpose.
3. If the tension-only member intersects other members and the Finite Element analysis option to "Add intermediate nodes at member intersections" is checked, then there will be intermediate points of connection (without hinges) leading to force transfer and potentially bending. 

We suggest not using this option for models with tension-only braces. If there are other places where you really want joints at member intersections, manually segment those portions using Process - Segment selection.

Why are my combined forces not equal to the sum of the load case forces?

When you run a P-Delta analysis, or a tension-only analysis, or compression-only analysis, each combination is solved directly. We do not use addition, or superposition, to calculate combined effects since a member that might not be in compression due to one load may be in compression due to that same load combined with another.

Under Process - Analysis - Load Conditions tab the user can select which cases or combinations to include.

See Also

RAM Instability In Finite Element Analysis

RAM Frame - Tension Only [FAQ]

Ram Elements Load Combos [FAQ]

Product TechNotes and FAQs

Structural Product TechNotes And FAQs