Evaluating a rigid-plastic method to estimate the earthquake ductility demand on structures

In order to evaluate the reliability of a rigid-plastic method in estimating the earthquake displacement ductility demand, the present paper applies the method to hundreds of different elastic-plastic oscillators under more than thirty recorded earthquakes. The mean ratio of the predicted value over the exact value of the displacement ductility demand is computed and plotted as a function of the vibration period of the oscillator for different values of the yield acceleration. The results show that, whatever the oscillator and the earthquake, the rigidplastic method leads to a generally conservative estimate of the inelastic displacement demand. Mean errors less than 15% are found both for comparatively short-period oscillators and for comparatively long-period oscillators. For medium-period oscillators, the relative mean error is generally less than 30%, even for very high levels of ductility demand. Some advantages of the rigid-plastic method with respect to other approximate methods are also discussed in the paper.