Significant attention has been recently paid to the determination of floor acceleration demands used for the design of acceleration-sensitive non-structural components in buildings. This study makes a contribution through the use of experimental shake table data from one 6-storey and two 3-storey reinforced concrete buildings tested at the E-Defense testing facility. All buildings were exposed to multiple excitations, and floor acceleration demands were studied through peak floor accelerations and floor response (acceleration) spectra. The obtained ratios of peak floor to peak ground accelerations confirmed findings from previous studies. The ratio distributions along the height provided in Eurocode 8, ASCE 7-16 and NZS 1170.5 were found to be conservative. Besides the known properties of floor response spectra, the results for two of the buildings revealed that in some cases, spectra can have two prominent peaks corresponding to the fundamental mode period of different damage states or degree of nonlinearity during a single shaking event. A prerequisite for this occurrence, which in this paper is termed the “Elongated Fundamental Mode Effect”, is that due to the characteristics of the input motion, a significant part of the energy is involved both before and after the change of structural response nature, from (mostly) linear elastic to nonlinear. Among others, such change is perceptible through an extension of the fundamental period. The Elongated Fundamental Mode Effect was investigated by using scalograms generated through the Continuous Wavelet Transform, which was found to be an efficient tool for the visualization of energy localization in the time–frequency domain.
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