Damping modification factor for elastic floor spectra
Nonstructural components (NSCs) housed in building structures are subject to semi-narrow band excitations generated via filtering of the ground motion by the supporting building. NSC design forces are generally provided based on adopting a 5% NSC viscous damping ratio (ζC), whereas recent experiments illustrate that typical NSCs may exhibit ζC values substantially different, mostly lower, than 5%. This study introduces damping modification factors (DMFs) to adjust NSCs elastic seismic design forces for other ζC values. Elastic floor spectra with different ζC values are developed for floor motions obtained from several code-based designed buildings subjected to ground motion sets with different intensities. Numerical analyses illustrate that due to the narrow-band characteristic of building floor motions, applying conventional DMFs proposed for adjusting ground spectral ordinates may lead to the underprediction of floor spectral ordinates up to 55% for the range of ζC evaluated in this study. Assuming a value of ζC, the amplitude of the DMF for a building floor spectrum is primarily a function of the NSC tuning ratio. For example, for a given floor motion and a ζC value of 2%, the DMF can vary from 1.0 (for a non-tuning condition) to 1.6 (for a tuning condition). To a lesser extent, the DMF is also influenced by supporting building characteristics (e.g., the level of inelastic behavior, fundamental period, height, and lateral-force resisting system), the vertical location of the NSC in the building, and ground excitation characteristics. Practical expressions, which are developed through simple modifications of the prescribed ground spectra DMFs by ASCE/SEI 41-13, are pro-posed to estimate DMFs for elastic floor spectra.
Bulletin of Earthquake Engineering