In structural engineering, the P-Δ or P-Delta effect refers to the abrupt changes in ground shear, overturning moment, and/or the axial force distribution at the base of a sufficiently tall structure or structural component when it is subject to a critical lateral displacement.
The P-Delta effect is a destabilizing moment equal to the force of gravity multiplied by the horizontal displacement a structure undergoes as a result of a lateral displacement.
To illustrate the effect, take the example of a typical statics case: in a perfectly rigid body subject only to small displacements, the effect of a gravitational or concentrated vertical load at the top of the structure is usually neglected in the computation of ground reactions. However, structures in real life are flexible and can exhibit large lateral displacements in unusual circumstances. The lateral displacements can be caused by wind or seismically induced inertial forces. Given the side displacement, the vertical loads present in the structure can adversely perturb the ground reactions. This is known as the P-Δ effect.
In some sense, the P-Delta effect is similar to the buckling load of an elastic, small-scale solid column given the boundary conditions of a free end on top and a completely restrained end at the bottom, with the exception that there may exist an invariant vertical load at the top of the column. A rod planted firmly into the ground, given a constant cross-section, can only extend so far up before it buckles under its own weight; in this case the lateral displacement for the solid is an infinitesimal quantity governed by Euler buckling.