How can tower deflection increase the bending moment it must resist?

The short answer

Once a tower deflects laterally, its vertical load can act through an additional lever arm and increase bending. This second-order effect couples axial load with deformation. A first-order calculation based only on the undeformed geometry may understate demand where flexibility and compression make that additional moment significant.

Follow gravity through the displaced geometry

A simple conceptual contribution is axial force multiplied by lateral offset. Hypothetically, a constant 100 kN compressive force acting through an additional 0.05 m offset produces 5 kN m of additional moment at the relevant section.

This example illustrates equilibrium only. A real tower has distributed weight, varying deflection, wind loading and connection behavior; the additional moment is not found by applying one offset universally along its height.

Include imperfections consistently

An initial lean or member curvature creates eccentricity before wind-induced deflection develops. Foundation rotation can add another support movement. If the structural model assumes perfect geometry and a fixed base without justification, it can miss interacting contributions.

Second-order response can also change force distribution in a braced or guyed system. Passing an isolated material stress check does not establish overall stability of that deformed structure.

Ask whether the simplification affects the decision

The tower designer should identify the analysis method and how it accounts for relevant imperfections, stiffness and axial loading. A justified simplified method may be acceptable within its scope, but its assumptions must match the actual tower and support.

If changing support stiffness or installed geometry materially changes demand, resolve that sensitivity before selecting the tower. Do not straighten the structure or alter guy tension based on this explanation. The design outcome should establish the assembled support’s adequacy for the selected turbine, not simply show that nominal first-order stresses remain below steel yield.