When is a fixed-base assumption inadequate for a wind tower model?

The short answer

A fixed-base assumption is inadequate when foundation or ground movement materially changes the response being assessed. Real supports can translate and rotate; an ideal clamp suppresses both. Whether that simplification is acceptable depends on the relative stiffness of tower and support, the loading condition and the decision the model must support.

Identify what the ideal clamp removes

A tower model fixed at its base attributes all lateral deformation above that point to the tower. In reality, foundation rotation can move the tower top even if the steel remains relatively stiff. Translational ground movement can add another contribution.

For a small base rotation, the resulting rigid-body contribution to top displacement is approximately tower height multiplied by rotation. This geometric relationship does not include bending or establish any allowable displacement.

Compare realistic support alternatives

A practical sensitivity study can compare the ideal clamp with credible translational and rotational support stiffnesses. The same mass, tower geometry and loading assumptions should be retained so the consequence of support flexibility is visible.

Look at natural frequencies, displacements and internal actions rather than frequency alone. A change that appears small in one output may matter at a connection or clearance interface. Nonlinear ground contact may also require more than a constant spring.

Agree the support model across disciplines

The foundation reviewer should explain how stiffness values were derived and where they apply. The tower reviewer should state which stiffness range the structural assessment accepts. These two statements must describe the same foundation geometry and load condition.

If the fixed-base result remains appropriate after justified sensitivity checks, retain that basis. If the choice changes the tower decision, improve the support model or design. A generic assumption that concrete is rigid does not establish that the concrete-ground system is fixed.