In this paper, the energy absorption capacity of a single-degree-of-freedom system with an arbitrary load-deformation relationship is defined as the maximum input energy for which the system starts to show plastic deformations lower than a perfect elasto-plastic system. Based on the damage distribution law proposed by authors, a formula is derived to evaluate the energy absorption capacity of a system with tri-linear load-deformation relationship, consisting of elastic, hardening, and degrading parts. In this formula, the energy absorption capacity is expressed as a function of the three parameters, namely the hardening rigidity after yielding, plastic deformation to maximum strength and decay rigidity after maximum strength. This formula is found to represent well dynamic responses of structures with various hysteretic behaviors.
