For tri-directional ground motion, define three amplitudes and assign to X, Y, Z directions. Vertical acceleration (often 2/3 of horizontal) is critical for bridges and long-span roofs.
: Research on energy dissipation systems in linked-column frames, utilizing pushover analysis in Abaqus to recommend optimal beam geometry. IOPscience Soil-Structure Interaction (SSI) abaqus earthquake analysis
Do apply acceleration as a body force to the whole model (this gives erroneous rocking). Instead: For tri-directional ground motion