Mitigation of Higher Mode Effects in Base-Rocking Systems by Using Multiple Rocking Sections

Base-rocking systems have been proposed as a way to limit the seismic forces experienced by a structure without accepting structural damage. However, structural forces can increase significantly, even when the base moment is limited, because of higher mode effects. This article suggests that these effects may be substantially reduced by designing to allow rocking to occur at multiple locations over the height of a base-rocking system. This is confirmed by a statistical study of the response of 24 systems of varying height and joint configuration to two suites of 20 earthquakes, as well as by a case study of the response of five 12-story systems designed using a displacement-based procedure. The bending moment envelope above the base of the wall is shown to be greatly reduced by providing multiple rocking sections, while the peak displacements do not increase in magnitude or in variability.