The following is a description of BI-SPEC, a nonlinear spectral analysis program that performs bi-directional dynamic time-history analysis of a simple pendulum system, or a series of dynamic analyses resulting in a "Bidirectional Nonlinear Spectrum", or a set of such spectra.

The program offers great flexibility for changing the dynamic properties of the dynamic system, and the nonlinear properties of the nonlinear element. The program currently supports several element models: a linear model, two nonlinear models: Bilinear, and Bilinear with Degrading Stiffness, and a plasticity-based bilinear element with bidirectional interaction.

Linear spectral analysis can be performed by specifying multiple frequencies. Ranges of strengths could also be specified in nonlinear analysis resulting in "Constant Strength Spectra". The program can also perform target searches for ductility resulting in "Constant Ductility Spectra".