A mechanism evolution index for the seismic assessment of retrofitted non-ductile RC frames

Scalar damage indices for retrofitted reinforced concrete frames are calibrated against one intervention family and record how much damage has occurred rather than which mechanism produced it, so they neither transfer between retrofit types nor warn when a brittle mode is about to govern. The Mechanism Evolution Index (MEI) addresses this gap for non-ductile RC frames retrofitted with bonded carbon fiber-reinforced polymer (CFRP) jackets or steel buckling-restrained braces, within the scope of regular planar systems under single-direction motion. Six independently measurable response constituents combine through one frozen weight set, anchored once at the joint-panel cracking and sliding thresholds of a three-specimen calibration series, into a bounded scalar on the unit interval and a ternary coordinate that separates ductility, shear, and connection demand. The set is fixed once and applied without re-tuning across retrofit type and intensity. The framework is demonstrated on a one-third-scale specimen series, a bare frame with three-ply CFRP and tube-in-tube brace retrofits, reproduced by one calibrated OpenSees model. The bare frame becomes connection-dominated and approaches the sliding threshold at the design intensity, the CFRP jacket suppresses joint engagement then collapses through the same band at the ultimate level, and the brace stays on the ductility axis throughout. Transferability is examined on six external cases outside the calibration, four bare and two retrofitted; the source-reported mechanism sits on the leading ternary axis in every labelled case. The classical scalars track damage severity but cannot name the mechanism, whereas the connection component supplies an indicative retrofit-screening level.