<p>The main goal of this research is to propose methods for calculating the structural damage index under seismic loads based on the dissipated plastic strain energy in the structure. Three structures with 3, 6, and 9 stories were designed and nonlinearly modeled using the Perform-3D software. The energy capacity (E_c) of the structures was calculated using three methods based on the area under the pushover curve (E_p), design energy (E_t), and energy of instability level (E_instability) and damage indices were then defined based on the ratio of dissipated plastic strain energy in the structure to E_c, calculated using these three methods. Incremental dynamic analysis curves based on plastic strain energy were extracted, and damage indices were calculated for various seismic loads and intensities. Subsequently, the corresponding damage indices for low, moderate, high, and collapse damage levels were determined for each proposed method using the Park-Ang index. Fragility curves were then calculated for different levels and verified utilizing the Park-Ang method. The calculations were performed for two scenarios including earthquake-only and earthquake followed by aftershock. The exceedance probability obtained from all three proposed methods for the 6-story structure at all peak ground accelerations demonstrated close agreement with the fragility curves derived from the Park-Ang damage index. For the 9-story structure, the fragility curve from the design input energy method illustrated some errors, while the other two methods provided sufficient accuracy. For the 3-story structure, the fragility curves from the proposed methods were sufficiently accurate at peak ground accelerations below 1&#xa0;g.</p>