A nonlinear dynamic assessment procedure that subjects a validated structural model to suites of ground‑motion records incrementally scaled across a range of intensity measures, recording structural response at each scale to produce intensity–demand relationships, estimate collapse thresholds, and derive probabilistic performance or fragility measures.

Principle

Scaling a representative set of ground motions and observing the monotonic increase of nonlinear structural response across intensities yields empirical relations between an intensity measure (e.g., spectral acceleration) and engineering demand parameters, enabling estimation of collapse probability and performance across hazard levels while sampling record‑to‑record variability.

Demonstration

Illustrative scenario: A bridge finite‑element model is run through 20 selected earthquake records scaled to a sequence of spectral acceleration levels; for each scaled record the maximum drift and residual deformation are recorded. Plotting response versus intensity for all records yields IDA curves from which the intensity at which collapse occurs for a chosen percentile is inferred and used to construct fragility functions.

Misapplication

Scaling records beyond physically meaningful limits (changing velocity pulse characteristics), using too few or non‑representative records, treating scaled motions as independent samples of the hazard without accounting for record selection bias, or applying IDA results from a single structural configuration without accounting for modelling uncertainty.

Consequence

Produces quantitative, probabilistic estimates of structural performance and collapse risk that inform performance‑based design and risk assessment; it requires substantial computational resources and careful ground‑motion selection and interpretation.

Reversal

May be unreliable when strong nonlinearity in site response, soil–structure interaction, or scale‑sensitive near‑fault pulses cause scaling to misrepresent realistic motions, or when component-level limit states (e.g., brittle failure) invalidate monotonic scaling assumptions; in such cases alternative record selection, pulse‑correction, or more tailored dynamic analyses are needed.

Boundary

Clearly within: validated nonlinear structural models for which a representative suite of ground motions can be selected and scaled, and where collapse or large‑deformation performance is sought. Boundary case: systems with marked soil–structure coupling requiring concurrent modelling of ground response. Clearly outside: linear analyses, preliminary screening without nonlinear models, or applications that require rapid deterministic checks rather than probabilistic performance measures.

Semantic Tension

High‑fidelity probabilistic dynamic assessment versus computational cost, record selection sensitivity, and practical interpretability for design decision‑making.

Synthesis

IDA converts ensembles of scaled dynamic excitations into empirical intensity–demand relationships and fragility measures, delivering deep probabilistic insight into collapse and performance but demanding rigorous motion selection, modeling validation, and computational investment.