When static loading is applied in the X direction, net reaction in the Y direction will be zero, though individual reactions may be nonzero, depending on the type of structure. For example, when a single-story frame system with four columns, arranged in a square plan with interconnecting beams at the roof level, is subjected to a horizontal point load in the major direction at the top of one column, torsion will cause horizontal support reactions in the direction orthogonal to loading.
Given a dynamic loading condition, net reactions are in equilibrium for both applied loading and inertial forces. When the structural center of stiffness differs from the center of mass, loading in the X direction may generate motion in the Y direction. This motion will correlate with a set of inertial forces and support reactions orthogonal to the direction of loading.
Most non-symmetric structures exhibit orthogonal reactions and response quantities when subjected to either static or dynamic loading. This behavior naturally occurs as a structure establishes equilibrium. Design must account for these orthogonal forces, along with all others which result from load application.