The Park-Ang damage index is a linear combination of the contribution from the deformation and the hysteretic energy. Based on the material properties and the force-deformation relation obtained from the analysis, you need to define the ultimate deformation, the max deformation at each time step, the yield strength, the accumulated energy at each step. You also need to define a proper parameter beta that governs the damage due to cyclic loading.

Details of the model can be found in the paper:

http://ascelibrary.org/doi/10.1061/%28ASCE%290733-9445%281985%29111%3A4%28722%29