Earthquake Floor Spectra for Unrestrained Building Components

This paper introduces the use of elastic displacement floor spectra to model the seismic performance behavior of unrestrained components. The displacement spectrum has the attribute of tracking period-shifts associated with "rocking" behavior. Analyses of the displacement floor spectra in high-rise buildings demonstrate the significance of contributions by the higher modes of vibration. Significantly, such higher mode contributions could not always be captured by established code procedures wherein the seismic demand on a building floor is obtained by linear interpolation between the seismic demand at the roof and ground level. The significant influence of the higher modes in high-rise buildings also precludes the use of simple static models to predict seismic demand. Results are also presented herein to demonstrate the sensitivity of the displacement floor spectrum to a multitude of modeling uncertainties.