Answer:
The equivalent loads are determined according to the multi-modal response spectrum method. To analyze a system spatially, it is necessary to activate the masses in all directions so natural frequencies and mode shapes can be determined on the spatial system. Equivalent loads are then determined on the basis of the mode shapes.
Which mode shapes are taken into account should be decided on the basis of the effective modal mass factors. According to EN 1998‑1, 90% of the structural mass must be activated to achieve accurate results.
