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. A distinction can be made between P-Delta effects on a multi-tiered building, written as P-Δ, and the effects on members deflecting within a tier, written as P-δ (wikipedia)