Uses of Elastic Design Response Spectra

Elastic design response spectra are extremely useful to structural engineers. These spectra are the basis for:

  • Computing design displacements and forces in systems expected to remain elastic
  • Developing design forces and displacement systems that respond inelastically by:
    Modifying elastic spectrum
    - Evaluating response of equivalent elastic structure

These elastic spectra can be obtained by several methods, which are:

  • Processing of site specific ground motion time histories
  • Statistical relationships
  • Empirical relationships
  • Code stipulations

Elastic spectra can be presented in several formats, depending on the needs of the engineer and what information is being presented. Some of the most common formats are:

  • Spectral acceleration vs. period
  • Spectral velocity vs. period
  • Spectral displacement vs. period
  • Spectral acceleration vs. spectral displacement (capacity design spectrum)
  • Tripartite plots (Sa, Sv, and Sd vs. period)

Also, any of the above (except the capacity design spectrum) can be plotted versus frequency rather than period.

Factors which effect elastic spectra include the damping ratio, site conditions, and near fault ground motion effects such as rupture directivity.