Inelastic Response Spectra for a Specified Yield Level

This function can obtain these following items for an input accelerogram according to a specified yield level (ay):
1. Inelastic Displacement Spectrum (iSd-T),
2. Inelastic Velocity Spectrum (iSv-T),
3. Inelastic Acceleration Spectrum (iSa-T),
4. Hysteresis of a bilinear elastoplastic system for a requested period (fs-u), and
5. Ductility Spectrum: Ductility distribution over periods (mu-T).
For this purpose, Newmark's method was employed and the Equation of Motions was solved, numerically. This function is completely compatible with the 4th edition of the book "Dynamics of Structures" by A. K. Chopra.