Zero Period Acceleration(ZPA), corresponds to the frequencies on a response spectrum, which cause the structure to trace ground accelerations without amplification of motion.
In the spectrum, the most right hand side of the abscissa (frequency), during which the structure has the same accelerations as the ground without amplification. When the structure's natural frequencies are in this zone, the structure's mechanical behaviour is termed "static equivalent". The evaluation of maximum forces and moments and stresses are obtained by successively applying the maximum ground acceleration (ZPA) without dynamic response amplification to the masses concerned (missing mass) according to the three spatial directions.