Incremental Dynamic Analysis (IDA) is a nonlinear response history analysis method in which a structure is subjected to a suite of ground motions, each scaled to multiple intensity levels, to develop a comprehensive picture of its performance across the full range of seismic demands — from elastic response to collapse.
Incremental Dynamic Analysis (IDA) is a nonlinear response history analysis procedure used to evaluate the seismic performance of structures over the full range of ground motion intensities. In an IDA, a single ground motion is scaled to multiple intensity levels (typically using a scalar intensity measure such as spectral acceleration at the first-mode period, Sa(T₁)), and the structure is analyzed at each level using nonlinear response history analysis. The results are plotted as an IDA curve — a relationship between the intensity measure (IM) and an engineering demand parameter (EDP), typically maximum inter-story drift. The process is repeated for a suite of ground motions (usually 10 to 30), producing a family of IDA curves that characterizes the structure's response across the intensity range. The procedure was formalized by Vamvatsikos and Cornell in 2002 and has become a standard tool in performance-based earthquake engineering.
IDA provides more complete information than single-intensity analysis. By computing response across a range of intensities, IDA captures the evolution of structural behavior — initial elastic response, onset of yielding, progressive damage, and eventual collapse. The IDA curves show how different ground motions produce different responses at the same intensity level, providing a direct measure of record-to-record variability. The median IDA curve and its dispersion provide a statistical characterization of the structure's behavior, and can be used to develop fragility curves for specified damage states. IDA is also used to identify collapse capacity — the intensity level at which a structure becomes dynamically unstable — and to evaluate the effect of different design alternatives on structural performance. The full IDA methodology requires 10 to 30 ground motions scaled to 10 to 20 intensity levels each, resulting in hundreds of nonlinear analyses — a computational challenge that has driven the development of efficient solution algorithms and parallel computing approaches.
IDA has important limitations and is often compared to other methods. The scaling of ground motions to high intensity levels can produce unrealistic records (excessive inelastic content) that may not represent real earthquake behavior; this has led to alternative approaches such as multiple stripe analysis (MSA), where each intensity level uses different (unscaled) ground motions. The choice of intensity measure affects the results: Sa(T₁) is standard for regular structures, but other IMs may be more appropriate for tall buildings (where higher-mode effects matter), near-fault sites (where pulse-like motions dominate), or structures with significant period elongation (where Sa(T₁) loses correlation with response). Modern practice often uses vector-valued IMs — combinations of intensity measures — to capture multiple aspects of ground motion. Despite these complexities, IDA remains one of the most powerful tools for understanding structural behavior and is widely used in performance-based design, particularly for tall buildings, base-isolated structures, and other structures where collapse capacity is a critical concern. In Iran, IDA is increasingly used in research and in performance-based design of important structures, with applications in the assessment of existing buildings, the design of new high-rise buildings, and the evaluation of retrofit strategies.