Abstract
The apparent natural period (the time required for one cycle vibration) of steel frames accompanied with plastic deformations under strong ground motion is longer than the natural period calculated from the initial elastic stiffness. Because earthquake input energy by ground motion depends primarily on the apparent natural period, the plastic deformation of steel frames increases, the apparent natural period becomes longer, and earthquake input energy becomes higher. The present paper proposes the expression to approximate the mean of the apparent natural period in the whole vibration and demonstrates that the variation (increase) of the earthquake input energy of steel frames accompanied with plastic deformations can be predicted by using this expression.
