The research, conducted by Resat Oyguc and Evrim Oyguc of the Institute of Disaster Management, Earthquake Engineering Department at Istanbul Technical University, focuses on a class of buildings that keeps structural engineers awake at night: non-ductile reinforced concrete frames. These are structures designed before modern seismic codes demanded the careful reinforcement detailing that allows a building to bend and deform without collapsing. Many such buildings across seismically active regions, from Türkiye to California, remain in service today, and retrofitting them has become a global engineering priority.

Two of the most common retrofit strategies anchor the study. The first involves wrapping columns with bonded carbon fiber-reinforced polymer, or CFRP, jackets—thin, immensely strong composite sheets that confine the concrete and delay brittle failure. The second deploys steel buckling-restrained braces, ingenious devices in which a steel core designed to yield in both tension and compression is housed within a tube that prevents it from buckling, allowing the brace to dissipate seismic energy stably through hundreds of loading cycles. Both approaches can dramatically improve a frame’s performance, but they do so in fundamentally different ways, and this difference has long frustrated attempts at unified assessment.

The problem, as the authors explain, lies in the very design of existing damage indices. Scalar indices calibrated against one intervention family record how much damage has occurred rather than which mechanism produced it. An index tuned to the behavior of CFRP-retrofitted frames transfers poorly to braced frames, and neither can sound an alarm when a brittle mode—such as joint panel shear failure or sliding—is about to take control of the response. In a severe earthquake, that distinction can mean the difference between a repairable structure and a catastrophic collapse.

The researchers’ answer is the Mechanism Evolution Index, or MEI, a framework that condenses six independently measurable response quantities into a bounded scalar on the unit interval, while simultaneously decomposing the structural state into a ternary coordinate that separates demand into three interpretable channels: ductility, shear, and connection. The six constituents include quantities familiar to any experimentalist: the normalized hysteretic energy dissipated during a run, the ratio of peak base shear to design-level capacity, the peak interstorey drift ratio, the residual drift left after shaking stops, the dynamic amplification between the shaking table and the structure above it, and the period softening ratio that captures how much the structure’s fundamental frequency has degraded from one excitation to the next.

What makes the framework remarkable is its economy of calibration. The six constituents are combined through a single frozen weight set, anchored once at the joint-panel cracking and sliding thresholds of a three-specimen calibration series. Each constituent passes through a sigmoid normalization function whose half-saturation value is fixed to the measured response of the bare specimen at design intensity. Once these anchors are set, the weights are frozen and applied without any re-tuning across retrofit type and shaking intensity. This is a deliberate departure from the prevailing practice, in which every new structural configuration demands its own bespoke index calibration, rendering comparisons across interventions nearly impossible.

The ternary coordinate is where the index earns its diagnostic power. The three components—ductility, shear, and connection—sum to one, placing every response state as a point within a triangular coordinate space whose corners represent pure ductile, pure shear, and pure connection demand. As a frame is shaken progressively harder, its point traces a trajectory through this space, and the leading axis at any moment names the mechanism that governs. The framework also defines a dominance margin, the difference between the two largest coordinates, which quantifies how decisively one mechanism leads over the others—a measure of confidence in the diagnosis itself.

To build and test the framework, the researchers drew on a one-third-scale specimen series: a bare non-ductile frame, the same frame retrofitted with a three-ply CFRP jacket, and a version strengthened with an innovative tube-in-tube buckling-restrained brace. All three configurations were reproduced by a single calibrated OpenSees finite element model, whose joint hysteresis was tuned to match measured displacement traces, base shear histories, hysteretic energy, and the fundamental periods identified from white-noise excitation before and after each strong-motion run. The calibration objective weighted all four error measures simultaneously, ensuring the simulation platform captured not just peak responses but the degradation pathways that unfold between them.

The findings tell a story that should concern anyone responsible for retrofit decisions. Under design-level intensity, the bare frame became connection-dominated, its ternary point migrating toward the joint sliding threshold exactly as its unreinforced beam-column panels began to fail. The CFRP jacket, perhaps counterintuitively, suppressed joint engagement through the design band but then collapsed through the very same connection channel when pushed to the ultimate level—the strengthening shifted the brittle mode upward in intensity rather than eliminating it. Only the braced frame remained on the ductility axis throughout, converting the failure mode into a stable, energy-dissipating one. The connection component of the index thus serves as an indicative retrofit-screening signal, flagging interventions that leave a residual brittle pathway even when headline performance metrics look satisfactory.

Transferability—the acid test for any index claiming generality—was examined on six external cases lying outside the calibration set, four bare and two retrofitted. In every labelled case, the mechanism reported by the experimental or analytical source landed on the leading ternary axis of the index. The classical scalars, by contrast, tracked damage severity faithfully but could not name the mechanism in any of them. This separation between knowing how much and knowing what kind is, in the authors’ framing, the essential contribution of the work.

The practical implications extend well beyond the laboratory. Post-earthquake reconnaissance following recent sequences in Türkiye has repeatedly documented the lethal consequences of brittle joint failures in non-ductile frames, and retrofit campaigns guided only by severity-based indices may inadvertently favor interventions that mask rather than remove the underlying vulnerability. A screening tool that can be applied identically across bare, jacketed, and braced configurations, using response quantities measurable from instrumentation or extracted from a calibrated simulation, offers engineers a common currency for comparing retrofit options on the dimension that matters most in a collapsing building: which mechanism will win.

The authors are careful to delineate the scope of their framework. It applies to regular planar systems under single-direction motion, and the frozen weight set is anchored to joint-panel thresholds specific to the calibration series used. Extending the approach to three-dimensional response, torsionally irregular buildings, and bidirectional ground motion remains open. The data supporting the study are available from the corresponding author upon reasonable request, and the authors report no external funding or competing interests. Even within these bounds, however, the Mechanism Evolution Index represents a meaningful conceptual advance: a damage measure that evolves alongside the structure, tracing not just the accumulation of harm but the shifting identity of the mechanism inflicting it.

As cities with vast stocks of pre-modern-code concrete buildings confront escalating seismic risk, the difference between a retrofit that redirects failure into a ductile pathway and one that merely postpones a brittle one is not an academic distinction—it is a matter of life and death. By giving engineers a single bounded number paired with an interpretable mechanism diagnosis, the Istanbul Technical University team has provided a tool that could make that distinction visible before the next earthquake makes it tragically apparent.

Subject of Research: A mechanism evolution index for the seismic assessment of retrofitted non-ductile reinforced concrete frames

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

Article References: Oyguc, R., & Oyguc, E. (2026). A mechanism evolution index for the seismic assessment of retrofitted non-ductile RC frames. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02679-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02679-3

Keywords: seismic retrofitting, reinforced concrete, damage index, ductility, CFRP, buckling-restrained brace, OpenSees, non-ductile frames, beam-column joints, shake table testing, earthquake engineering, structural assessment