How does FEM-Design handle truss elements in stability analysis?

Question:

Some users have experienced an interesting effect when running a stability analysis. The truss elements can in a specific situation appear to not contribute to stability in a direction where they are expected to. Why is this happening and how can FEM-Design take truss buckling into account in a global stability analysis?

Figure 1. Stability analysis result with a critical parameter near zero despite having truss with tensile stiffness

Answer:

This behavior is linked to how the stability analysis is calculated in the software. The key is how the effective stiffness of truss elements is handled.

Calculation process

1. Load combination analysis
   The chosen load combination is calculated. Uplift and cracked section analysis can be included here.

2. 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.
   
   

3. Geometric stiffness matrix
   Derived from the internal forces of the load combination.

4. Stability solution
   The eigenvalue problem is solved using the effective stiffness matrices and the geometric stiffness matrix.

When are trusses excluded?

If the applied load (for example only vertical loads) does not put the trusses in tension, the truss is excluded from contributing stiffness 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.

Ways FEM-Design gives you control

• Compression limit settings
  Use a non-zero compression limit so that trusses can provide stiffness until the limit is reached.

• Customize 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 load to keep trusses activated, giving a better and realistic stability result.

In summary: if trusses do not appear to be active in the stability analysis, it is usually because they are not in tension in the chosen load combination. FEM-Design ensures realistic stability analysis by letting you control how trusses participate. By adjusting compression limits, load combinations, or imperfections, you can capture their contribution accurately.