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Damping modification factor for elastic floor spectra

Abstract

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 (\( \xi_{\text{C}} \)), whereas recent experiments illustrate that typical NSCs may exhibit \( \xi_{\text{C}} \) values substantially different, mostly lower, than 5%. This study introduces damping modification factors (DMFs) to adjust NSCs elastic seismic design forces for other \( \xi_{\text{C}} \) values. Elastic floor spectra with different \( \xi_{\text{C}} \) 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 \( \xi_{\text{C}} \) evaluated in this study. Assuming a value of \( \xi_{\text{C}} \), 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 \( \xi_{\text{C}} \) 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 proposed to estimate DMFs for elastic floor spectra.

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Acknowledgments

This study was partially supported by a Dissertation Year Fellowship provided by the University of New Hampshire to the first author. This support is greatly appreciated. In 2015–2017, Applied Technology Council sponsored the ATC-120 Project on Seismic Analysis, Design, and Installation of Nonstructural Components and Systems. The authors of the present study were members of the project team. During ATC-120 meetings, discussions ensued on using a viscous damping ratio less than the default 5% for nonstructural components. These discussions, particularly works conducted by Dr. Eduardo Miranda and his coworkers, Dr. Dimitrios Vamvatsikos and Dr. Athanasia Kazantzi, served as a motivation for the present study. These contributions of the members of the ATC-120 project team are gratefully acknowledged.

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Correspondence to Hamidreza Anajafi.

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Anajafi, H., Medina, R.A. Damping modification factor for elastic floor spectra. Bull Earthquake Eng 17, 6079–6108 (2019). https://doi.org/10.1007/s10518-019-00684-3

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Keywords

  • Narrow-band motions
  • Nonstructural component damping ratio
  • Floor response spectrum
  • Damping modification factor
  • Archetype buildings