Why can a stable wind foundation rotate too much in service?

The short answer

Stability means the foundation retains adequate resistance; it does not mean the support remains effectively rigid. Ground compression and foundation flexibility can permit rotation well before ultimate failure. That rotation can change tower alignment, movement and dynamic response, so the foundation needs a serviceability assessment matched to the supported assembly.

Define rotation at the correct interface

Foundation rotation is an angular movement of the support, distinct from bending within the tower itself. Measured tower-top displacement can contain both contributions. Interpreting all displacement as tower bending can conceal a softer support than the structural model assumes.

Specify the reference level, direction and loading condition when discussing rotational stiffness. A value without those boundaries is difficult to use in a coupled tower calculation.

Relate stiffness to the expected movement

For a simplified linear rotational spring, rotation equals applied moment divided by rotational stiffness. This relation is useful for understanding sensitivity, but actual stiffness may vary with load, contact area and soil strain.

Hypothetically, halving the stiffness while retaining the same moment doubles the calculated rotation within that linear model. The statement does not assign a foundation stiffness or permissible angle; contact loss or soil yielding would require a different representation.

Assess the consequence for the whole structure

The tower designer should state the support movement and stiffness assumptions needed for alignment, clearance and dynamic assessment. The foundation designer then checks whether the proposed ground-foundation system provides them under relevant service conditions.

Request rotation estimates separately from ultimate stability factors. Where uncertainty controls the outcome, compare credible stiffness ranges and the resulting tower response. A foundation that passes bearing resistance but misses an accepted rotation criterion may need changed geometry, ground treatment or another support concept. Increasing concrete strength alone does not automatically resolve ground-controlled rotation.