Abstract: A system for damping movement of a structure. The system includes an enclosure located in the structure in a predetermined position therein and having side walls and a floor. A liquid is positioned in the enclosure. The side walls include two first side walls positioned parallel to each other defining a first direction orthogonal to the first side walls and two second side walls positioned parallel to each other defining a second direction orthogonal to the second side walls. The system also includes a number of ribs positioned inside the enclosure and parallel to the second side walls, the ribs defining respective troughs therebetween. The enclosure is configured for imparting a predetermined first sloshing frequency to the liquid moving in the first direction, and for imparting a predetermined second sloshing frequency to the liquid moving in the second direction.