Seismic Analysis of Structures: Improvements Due to Probabilistic Concepts - Dynamic Behavior of Concrete and Seismic Engineering - page 313

Figure 8.15. Different cycles shapes obtained owing to Wen-Bouc's model
Figure 8.16. Adjustment attempt of a linear oscillator equivalent to a
bilinear strain-hardening elasto-plastic oscillator
Such models give quite good results as far as the prediction of the variances and
PSDs of the response, for linearities that are either weak or with strain-hardening
slope/elastic slope ratios higher than 0.5 or so.