## Invertedpendulum systems

An inverted pendulum is defined as a system with at least 50% of the total mass in the upper third of the height, or with energy dissipation at the base of a single element. Literally, one-storey concrete buildings normally fall in that category. Nonetheless, one-storey frames with the tops of columns connected (through beams) in the two main directions of the building in plan are explicitly excluded from the category, provided that in the seismic design situation the maximum value of the normalized axial load v6 in any column does not exceed 0.3. Such a low value of the axial load, which corresponds to 0.2 for the usual value of 1.5 for the partial factor 7C of concrete, enhances the local ductility at the base of the column. Two-storey frames will not be classified as inverted-pendulum systems, if they have the same mass at the two floors, but will be classified as such if the mass lumped at the roof noticeably exceeds that of the first floor.

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