On the use of plastic energy to establish strength requirements in ductile structures

Although there is a consensus that low cycle fatigue should be avoided, there are different opinions regarding how to account for it during seismic design. Different approaches have been used to formulate structural damage indices; in such a way that it is important to clarify if the number and amplitude of plastic incursions have a significant influence during the assessment of structural damage. Under the assumption of proportionality between the demand and supply of plastic incursions as a function of their amplitude, low cycle fatigue can be assessed exclusively through the normalized plastic energy demand. The response of simple systems to motions recorded in different soil conditions suggest that plastic energy tends to be dissipated in a well defined manner in earthquake-resistant structures. Particularly, energy dissipation follows similar tendencies as those exhibited by typical cycle capacity curves, in such way that the plastic energy dissipated during the design ground motion provides reliable information for assessment of low cycle fatigue. In the case of soft soil, the use of the plastic energy demand to characterize plastic cycling may result in an important underestimation of the level of structural damage.