Question:
When running a stability analysis, the truss elements are sometimes not contributing to stability in one direction, even though they should be able to take tension. Why is this happening?
Figure 1. Stability analysis result with a critical parameter near zero despite having truss with tensile stiffness
Answer:
This behavior is linked to how stability analysis is calculated in the software. The key is how the effective stiffness of truss elements is handled.
Calculation process
- Load combination analysis
The chosen load combination is calculated. Uplift and cracked section analysis can be included here. Effective stiffness assignment
Based on the load combination results, the program saves the stiffness matrices based on the possible uplift or cracked section properties.
For example, if a truss switches off in this combination (due to zero compression limit), its stiffness becomes zero.Geometric stiffness matrix
Derived from the internal forces of the load combination.Stability solution
The eigenvalue problem is solved using the effective stiffness matrices and the geometric stiffness matrix.
Why do trusses “switch off”?
If the applied load (for example only vertical loads) does not put the trusses in tension, their stiffness becomes zero in step 2.
As a result, the structure appears much weaker in certain directions.
This leads to a very low critical load parameter in the stability analysis, even if in reality the trusses could stabilize the system when loaded differently.
How to resolve it
Check compression limit settings
Use a non-zero compression limit so that trusses can provide stiffness until the limit is reached.Adjust load combinations for stability
If uplift is not relevant for the stability case, disable uplift in that combination.Introduce small imperfections
Apply a small horizontal deviation load to prevent all trusses from switching off at once, giving a more realistic stability result.
In summary: if trusses do not show up as active in the stability analysis, it is usually because they are not in tension in the chosen load combination. Adjusting the analysis settings or introducing a horizontal load will ensure they contribute as expected.