A damage index is a scalar or vector quantity that quantifies the extent of damage in a structure or structural element, typically by comparing a measured or computed response to a reference (undamaged) state. It provides a single measure for assessing severity and supporting decision-making.
A damage index (DI) is a numerical quantity that characterizes the extent of damage in a structure, component, or material. It is typically defined as a scalar (a single number), but can also be a vector or tensor for more detailed damage characterization. The damage index is used to translate complex measurement data into a simple metric that can inform decisions β whether a structure is safe, whether it needs repair, or whether it should be evacuated. There are hundreds of damage indices in the literature, each tailored to specific structural types, damage mechanisms, and applications.
Damage indices are broadly classified into several families. Modal-based indices compare measured or computed modal properties (frequencies, mode shapes, damping) between the damaged and reference states β examples include the modal flexibility method, the mode shape curvature method, and the modal strain energy method. Response-based indices use the measured or simulated response (displacement, drift, acceleration) to assess damage β examples include the maximum inter-story drift ratio, the Park-Ang damage index (combining maximum deformation and hysteretic energy), and the residual displacement index. Stiffness-based indices compare the current stiffness to the initial or expected stiffness β examples include the stiffness degradation ratio and the flexibility-based damage index. Energy-based indices assess the energy dissipated by the structure during loading, often through hysteretic behavior β examples include the hysteretic energy index and the cumulative damage index. Statistical indices use pattern recognition or machine learning to detect and quantify deviations from a healthy baseline β examples include novelty indices and Mahalanobis distance.
Damage indices are essential for structural health monitoring and performance-based assessment because they provide a consistent, quantifiable measure of damage severity. In SHM, they enable automated monitoring systems to flag potential damage and prioritize inspection. In post-earthquake assessment, they support rapid decisions about occupancy, repair, and demolition. In performance-based earthquake engineering, they are used to define damage states (slight, moderate, extensive, complete) for fragility curves and loss estimation. The key challenge in using damage indices is that no single index captures all aspects of damage β different indices are sensitive to different damage mechanisms and loading histories, and the choice of index must be matched to the specific application. Modern practice increasingly uses multiple indices in combination, and increasingly uses probabilistic methods to account for the uncertainty inherent in damage identification. In Iran, damage indices are used in seismic assessment and retrofit design of buildings and infrastructure, following international practice with adaptation to local construction types and materials. The Park-Ang damage index, in particular, is widely used in Iran for reinforced concrete and steel structures.