Change in instantaneous eigenproperties due to yielding of a structure

This paper presents an insight into the pattern of change in instantaneous eigenproperties of a building as a result of reduction in tangent stiffness due to yielding of different members of the building. Using a perturbation approach, it is found that for a shear building, a reduction in instantaneous stiffness due to a yielding occurring in the bottom story will cause a greater absolute percentage change in eigenvalues (squared frequencies) of a lower mode than that of a higher mode. In contrast, for a reduction in instantaneous stiffness due to a yielding occurring in the top story, this absolute percentage change in the eigenvalues increases with a increase in mode number starting from the fundamental mode. A numerical study with a four-story shear building and a four-story and an eight-story steel moment-resisting frame models representative of existing buildings demonstrates that the trends in change in eigenproperties obtained from the analytical study works well for these buildings. The findings of this study may be utilized to improve the design practices of buildings as well as non-structural components attached to buildings.