Why can foundation rocking make tower response nonlinear?

The short answer

Foundation rocking can change the area of ground in compression as load rises and reverses. Because ordinary soil contact does not sustain tension like a bonded solid, part of the base may separate and later recontact. The resulting moment-rotation relationship can vary with amplitude and loading history, rather than remain a constant linear spring.

Identify the changing contact mechanism

At low eccentricity, the full base may remain in contact. Increasing overturning can unload one edge and concentrate compression elsewhere. The resisting lever arm, local pressure and rotational stiffness then change together.

This is different from simply assigning a lower soil modulus to an otherwise unchanged full-contact model. The contact geometry itself has become part of the response.

Distinguish recoverable motion from accumulated movement

Opening and closing contact can be partly recoverable, but soil yielding or repeated loading can also produce permanent settlement or rotation. The relevant behavior depends on stress level, ground properties and drainage.

A measured return toward the original position after one event does not establish that unlimited repeated rocking is acceptable. Conversely, a nonlinear response does not automatically mean collapse; its consequences require the applicable stability and serviceability assessment.

Check the model at the decision’s load level

Ask whether the adopted rotational spring represents small-amplitude operation, an extreme event or another specified condition. If contact loss is credible, evaluate whether the analysis method captures the associated pressure redistribution and movement.

Any proposed use of rocking as a design mechanism requires an explicit engineered basis. Do not infer that allowing movement is a safe substitute for foundation resistance. The support decision should address peak bearing, accumulated tilt and tower dynamics together. No universal rocking allowance, soil-contact threshold or MARSTON operating limit is established by this explanation.