When you apply static loads in the X direction, the net reaction will be zero in the Y direction. Depending on the type of the structure, the individual reactions may be nonzero, but the sum is not. (For example, consider a single-story open-frame structure with 4 columns arranged in the square layout. These columns are fixed at their base and connected by 4 beams at their top joints, using one beam along each side of the square layout. Applying a horizontal load along the major direction of the structure to a point located on the top of one column will result in a torsion of the entire open frame structure and cause horizontal reactions in the direction orthogonal to the direction of the loading.)

For dynamics, the net reaction is in equilibrium not just with the applied load (as for static), but also with the inertia forces. If the center of stiffness of a structure is not the same as the center of mass, then an X loading may generate Y motion, which in turn generates Y inertia forces. That is why the reaction may exist in a direction different from loading. This situation exists for most non-symmetrical structures.

This behavior is not a fiction of the model, the technique, or the software. This is the nature of dynamic behavior. These are the real forces that should be designed for.

See also

• [Base reactions for response spectrum analysis]