Unreinforced masonry buildings are prevalent in historic urban centers and often form clusters of independent structural units built directly next to one another. Conventional seismic performance assessments commonly analyze each building as though it were isolated, overlooking the interaction that can occur between neighboring structures. The research examined this coupling by modeling three distinct unreinforced masonry archetypes and arranging them into seven clusters composed of either two or three units. The configurations made it possible to evaluate the influence of each building’s position and the differences in strength and stiffness between adjacent structures.

To represent the masonry structures, the researchers used a three-dimensional equivalent-frame approach capable of capturing both in-plane and out-of-plane wall behavior. They first developed fragility curves for each archetype when treated as an isolated structural unit. Those curves were derived from nonlinear response-history analyses using a suite of actual ground motions and the multiple stripe analysis methodology. The researchers then constructed fragility curves for buildings located within the two- and three-unit clusters and compared them with the corresponding isolated estimates. This comparison quantified how the modeled seismic interaction changed the estimated likelihood of different damage states across the range of ground-motion inputs represented in the analyses.

The effects depended on the configuration rather than following a single uniform pattern. Units with lower in-plane stiffness and lower strength capacity could benefit from the support of stronger neighbors. That same interaction increased the fragility of the stronger buildings, and the increase became more pronounced as the stiffness disparity between neighboring units grew. Position also influenced the response of individual buildings, but the modeled collapse sequence consistently began with the most vulnerable unit, regardless of whether it occupied an end or interior position. Failure of that unit could trigger progressive collapse that compromised adjacent, otherwise stable structures, showing that a building’s properties and position alone did not determine its behavior during shaking.

The findings are numerical estimates for the selected low-rise masonry archetypes and cluster arrangements, not direct observations of damage following an earthquake. Within the scope of the models, however, the results provide quantitative evidence that assessing adjacent unreinforced masonry buildings in isolation is unconservative. The authors conclude that their seismic fragility cannot be adequately quantified in isolation and that realistic performance assessments for historic urban centers require cluster-level models that explicitly represent configuration, differences in structural properties and mutual interaction. The study thus shifts the assessment from a set of unrelated individual buildings to a system in which the seismic response and possible collapse sequence of one unit can affect its neighbors.

Subject of Research: Modeled seismic fragility of interacting low-rise unreinforced masonry building clusters

Article Title: Impact of pounding interaction on the seismic fragility of low-rise adjacent unreinforced masonry buildings

Article References: Gallardo, J. A., Tomić, I., Palazzi, N. C., de la Llera, J. C., & Beyer, K. (2026). Impact of pounding interaction on the seismic fragility of low-rise adjacent unreinforced masonry buildings. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02722-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02722-3

Keywords: unreinforced masonry, seismic fragility, building clusters, pounding interaction, progressive collapse, earthquake engineering, structural dynamics, nonlinear analysis, masonry walls, ground motions, Impact, pounding

The inerter concept was first introduced in 2002 by Malcolm Smith, who described it as the mechanical dual of a spring. Where a conventional mass has one end permanently anchored to a fixed reference, an inerter connects two points within a structure and generates equal and opposite forces at its terminals in proportion to their relative acceleration. The constant that governs this behaviour, known as the inertance, is measured in kilograms, yet the device itself requires only a fraction of that physical mass. This apparent mass amplification is what makes the inerter so compelling for vibration control: it can deliver substantial inertial effects in applications where space and weight are constrained, from automobile suspensions and railway vehicles to buildings and bridges. In structural engineering, the device has been integrated into tuned mass damper inerter systems, tuned inerter dampers, and a growing family of hybrid configurations that combine the inerter with viscous, hysteretic, or self-centering mechanisms.

Where the inerter is installed matters enormously. The review systematically catalogues the placement strategies reported across the literature, and the findings reveal how strongly configuration shapes performance. A tuned mass damper inerter connected between the top storey and the ground can significantly lower inter-storey drift and response variability, exploiting the large relative acceleration across the device terminals. Tuned inerter dampers placed at the position of maximum modal inter-storey drift yield better vibration suppression than other locations. Inter-storey installations between adjacent or non-adjacent floors improve control of higher vibration modes, while devices coupled between adjacent buildings mitigate pounding and reduce relative displacement. In bridges, inerters installed between deck and pier or integrated with stay cables reduce deck displacement and pier forces. Even clutched configurations, which engage only when dynamic excitation demands it, have been shown to improve control efficiency. The optimal inertance-to-mass ratio, meanwhile, tends to increase with structural period, and studies consistently report that response reduction becomes relatively insensitive beyond an inertance ratio of roughly 0.30 for selected damper configurations, meaning that simply adding more inertance does not guarantee better seismic performance.

Quantifying how well these devices work requires appropriate performance metrics, and the review organizes them into four families. Displacement-based measures, including peak roof displacement, inter-storey drift, residual drift, and base-isolation displacement, remain the most widely used indicators because they directly capture structural deformation, serviceability, and damage potential. Force and acceleration-based indices, such as base shear, floor acceleration, damper force, and root-mean-square acceleration, characterize the dynamic demands imposed on occupants, non-structural components, and the damper devices themselves. Energy-based metrics, including input energy, energy dissipation ratio, and hysteretic energy, reveal how effectively the devices absorb or redistribute the seismic energy that would otherwise flow into the primary structure. Performance-based metrics, the most holistic category, connect structural behaviour to predefined performance objectives through probabilistic frameworks rather than focusing solely on minimizing peak responses.

The reported numbers are striking. Across the reviewed studies, inerter-based damping systems achieved reductions of 20 to 85 percent in peak displacement and inter-storey drift, 18 to 45 percent in floor or roof acceleration, and up to 32 percent in base shear. A grounded tuned mass damper inerter on a base-isolated structure reduced secondary mass displacement and stroke by more than 50 percent compared with a conventional tuned mass damper under far-field ground motion. On a ten-storey shear building, a single-inerter configuration delivered a 45 percent reduction in mean maximum roof acceleration under 25 far-field earthquake records. An inerter-enhanced self-centering damper on a six-storey reinforced concrete frame cut seismic losses by up to 20.46 percent and recovery time by 19.46 percent, while raising the peak ground acceleration associated with a 50 percent probability of failure by as much as 43.86 percent for slight damage states. A low-cost passive inerter system on a two-storey shear frame outperformed traditional tuned mass dampers by 18.5 percent in energy dissipation efficiency.

But the review also exposes a critical caveat that could reshape how these devices are deployed: performance is strongly dependent on the character of the ground motion itself. Near-fault records dominated by early-arriving, low-frequency pulses reduced the effectiveness of grounded inerter systems, and highly impulsive motions produced limited or even adverse responses in some configurations. One study reported a 19.30 percent increase in top-floor acceleration under a near-field excitation condition, despite observing reductions under other ground motions and intensity levels. Relative device efficiency also declined with increasing structural nonlinearity and seismic intensity, and non-clutched inerter configurations were found to amplify ground-storey shear in rocking timber buildings. These contrasting observations demonstrate that improvement in one response measure does not necessarily imply consistent improvement across all structural responses, and that the reported percentages should not be treated as directly comparable across studies with different structural properties, modelling assumptions, and ground motion selection procedures.

Methodologically, the field has undergone a profound transformation. Early performance evaluation relied on frequency-domain analyses, closed-form optimization, and variance-minimization methods applied to simplified single-degree-of-freedom systems excited by stationary white noise or harmonic base excitation. As research progressed, H2 optimization frameworks produced explicit design formulae and robust performance criteria, followed by nonlinear time history analysis, statistical linearization, and experimental shake table testing. The most significant shift, however, has been toward the Performance-Based Earthquake Engineering framework, an integrated probabilistic methodology that connects seismic hazard with structural response and estimates repair costs, downtime, functionality, and associated risk. Rather than asking whether a structure meets code-prescribed strength requirements, PBEE asks what damage is likely to occur and what it will cost to recover, providing a decision-oriented basis for design and retrofit.

Within this probabilistic landscape, fragility analysis has emerged as a key tool for evaluating inerter-equipped structures. Fragility curves express the probability that a structure will exceed a predefined damage state under varying levels of earthquake intensity, typically assuming a lognormal cumulative distribution characterized by a median intensity measure and a logarithmic standard deviation. These curves are developed through incremental dynamic analysis, where nonlinear structural responses under scaled ground motions are evaluated against damage state thresholds drawn from the HAZUS manual, FEMA P-58, and ASCE 41 standards. One of the earliest applications developed a reliability-based design framework for the tuned mass damper inerter, optimizing device parameters to minimize the probability of exceedance. More recent work has extended the approach to hysteretic multi-storey steel buildings, where designs relying only on linear models tend to overestimate device performance once structural yielding occurs, and to structures subjected to pulse-like ground motions, which can cause large displacement demands and significantly alter the effectiveness of vibration control devices.

Despite the promise, the review identifies substantial practical limitations that stand between laboratory success and widespread implementation. Physical inerters deviate from ideal behaviour due to mechanical non-idealities such as friction, bearing losses, and transmission inefficiencies. Large inertial forces impose significant demands on supporting members and connections, while finite device stroke, installation space, and geometric requirements constrain certain configurations. Long-term durability issues, including wear, lubrication requirements, fatigue, and degradation of moving components, remain insufficiently investigated and require further experimental validation. Large-scale or full-scale experimental validation is comparatively limited across the literature, particularly for systematic performance comparisons under real earthquake ground motion records, leaving a gap between analytical models and field-ready devices.

Looking forward, the authors chart an ambitious research agenda. Future work should integrate engineering demand parameters, probabilistic fragility, repair cost, downtime, and resilience measures into comprehensive performance-based seismic design frameworks. Extensive investigations are needed under complex earthquake scenarios, including near-fault pulse-like ground motions and multi-hazard loading conditions. Artificial intelligence and machine learning offer opportunities for optimized parameter selection, probabilistic fragility forecasting, and surrogate models that accelerate seismic decision-making. Life-cycle optimization should incorporate initial construction costs, maintenance demands, anticipated seismic losses, repair expenses, downtime, and environmental impacts within multi-objective frameworks, while large-scale shake-table experiments and full-scale field studies verify analytical models. Digital twin frameworks, creating virtual replicas that continuously reflect physical structural response, could support adaptive parameter tuning, damage detection, and life-cycle performance assessment. For a device born in Formula 1 suspension systems and now poised to protect buildings from catastrophe, the inerter’s journey from mechanical curiosity to seismic guardian is well underway, but the road to reliable, field-proven earthquake protection still demands rigorous validation across the full spectrum of ground motions that nature can deliver.

Subject of Research: Earthquake Engineering

Article Title: Seismic performance assessment of inerter-based damping systems for structural vibration control – a comprehensive review

Article References: Eshapurada, M. B., & Reddy, K. S. K. K. (2026). Seismic performance assessment of inerter-based damping systems for structural vibration control – a comprehensive review. Frontiers in Built Environment, 12, Article 1966528. https://doi.org/10.3389/fbuil.2026.1966528

Image Credits: AI Generated

DOI: 10.3389/fbuil.2026.1966528

Keywords: inerter, seismic damping, structural vibration control, earthquake engineering, performance-based design, fragility analysis, passive dampers, tuned mass damper inerter, Seismic, performance, assessment, inerter-based

The suite, called ParaStereoSync and KineViz-3D, is distributed under the MIT license and runs on Windows, Linux, and macOS using freely available Python libraries including OpenCV, NumPy, SciPy, Matplotlib, and Tkinter. It is organized as five standalone graphical applications linked by CSV files and video streams. Three utility tools, VidTrim, tkCalib2, and TempArUco, prepare the inputs; ParaStereoSync performs the tracking and three-dimensional reconstruction; and KineViz-3D turns the resulting coordinate histories into interactive spatial and kinematic visualizations. No commercial software, hardware dongle, or paid cloud service is required at any stage of the pipeline.

The team’s motivation stems from a well-known mismatch between existing measurement tools and the realities of large-scale testing. Digital image correlation, or DIC, has become the dominant vision-based technique in structural engineering, tracking small patches of a speckle-coated surface between images to recover full displacement fields. Mature packages such as Ncorr, OpenCorr, py2DIC, D²IC, and commercial systems like GOM ARAMIS and VIC-3D have made the method widely accessible. Yet, the authors argue, these tools were all designed for a specific experimental setting: a densely painted surface, uniform controlled illumination, high image resolution, small displacement ranges, and hardware-synchronized cameras. None of those conditions reliably holds on a full-scale ceiling system in a test hall, where speckling an entire bay is impractical, lighting varies, and consumer cameras run on independent internal clocks.

That last problem, camera synchronization, is the suite’s headline technical innovation. When two cameras lack a shared trigger signal, they record on separate clocks and accumulate temporal offsets, so each camera effectively sees a slightly different moment in time. In stereo photogrammetry, the resulting three-dimensional coordinates contain spurious apparent displacements proportional to the structure’s velocity multiplied by the timing error. ParaStereoSync attacks this in software. Its synchronization algorithm tests a range of candidate frame lags between the two videos, interpolating the second camera’s coordinates at each lag, triangulating the matched points, and computing the mean-square reprojection error. The lag producing the lowest error is selected, and a cubic spline fit around the minimum refines the result to sub-frame precision. Crucially, a separate lag is estimated for each tracked point, which absorbs point-specific timing offsets including those caused by the rolling-shutter readout of CMOS sensors, without requiring any explicit sensor timing calibration or special hardware.

Tracking itself relies on the Enhanced Correlation Coefficient criterion under an affine warp model, which is more robust to changes in illumination than simpler sum-of-squared-differences approaches. A multi-step initialization propagates the converged transform from each frame into the next, restricting the search to a localized region and allowing the tracker to follow the large inter-frame displacements that occur during strong-motion excitation. If convergence fails under the affine model, the tracker automatically retries with a simpler Euclidean model, then a pure translation model, with progressively heavier Gaussian smoothing at each retry level. Rather than tracking every pixel the way full-field DIC does, the software follows sparse targets at structurally meaningful locations: printed ArUco fiducial markers, or even natural features such as bolts, connection hardware, and panel corners, eliminating the need for speckle preparation entirely.

Speed comes from camera-level parallelism. Both cameras are dispatched as independent worker processes through Python’s ProcessPoolExecutor, each running the full tracking pipeline on its own video file. The authors deliberately rejected point-level parallelism, noting that video decoding is input-output-bound and many small processes reading the same file would saturate the storage bus. On an Intel Core i7-11800H laptop, the approach achieved speedups of 1.44 to 1.74 times. In their demonstration, tracking 322 ceiling panel points across 2,300 frames took 14.63 minutes sequentially and 8.42 minutes in parallel, with a 100 percent tracking success rate. Tracking 64 T-bar grid intersections over the same frames took 1.95 minutes with parallelism versus 3 minutes without.

The utility tools tackle the setup bottlenecks that historically made high-node-count campaigns tedious. VidTrim wraps FFmpeg in a lightweight graphical interface and trims raw footage losslessly using stream-copy mode, so no frames are re-encoded or degraded; it also offers a motion preview that lets users confirm a target is trackable before committing to a full run. tkCalib2 supports two calibration methods: a standard multi-image checkerboard procedure, and a single-image approach that solves camera pose from known three-dimensional control points, valuable when a specimen is too large for a big calibration board. TempArUco is perhaps the biggest time-saver: it reads the first frame from each camera, enhances contrast with CLAHE preprocessing, detects all ArUco markers, matches them across cameras by their unique binary IDs, and writes template files automatically. In the team’s demonstration, it produced templates for 322 markers in roughly 90 seconds, replacing what would otherwise have been a two-hour interactive clicking session.

The pipeline was put to the test at the Carleton University seismic resilience laboratory on a suspended lay-in ceiling system spanning 5.156 by 5.156 meters, comprising 81 panels on a T-bar grid mounted on a purpose-built shaking table. One-hundred-millimeter ArUco markers sat at every panel corner and 50-millimeter black-and-white markers at every grid intersection, yielding 324 and 64 targets. Two GoPro HERO12 Black cameras recorded at 4K resolution and 60 frames per second from a stereo baseline of 2.08 meters at an average standoff of 3.87 meters. Synchronization analysis found a well-defined minimum at 27 frames, refined to an average of 27.22 frames, with per-node lags ranging from 26.60 to 28.88. Validation against accelerometer-derived displacements in prior large-scale experiments showed a root-mean-square error of at most 2.970 millimeters and a mean absolute error below 1.708 millimeters across all configurations.

KineViz-3D then turns the raw tracking output into engineering insight. Its centerpiece is an animated three-dimensional heatmap of displacement interpolated onto a 50 by 50 grid, with colormap bounds fixed to global extremes so amplitudes remain comparable across frames. A six-panel profile evolution dashboard synchronizes out-of-plane and in-plane displacement profiles, three phase-space orbit plots, and a live bar chart of instantaneous displacement magnitude, and it automatically annotates the frame and node pair with the maximum displacement differential, a direct quantitative indicator of emerging connection failure. Selecting four nodes on a rigid body invokes the Kabsch algorithm to decompose six-degree-of-freedom motion. In the ceiling test, the analysis identified a 4.91-millimeter displacement differential between outermost and innermost grid nodes, and the rigid-body decomposition revealed a 0.3-degree panel rotation about the Y axis, later confirmed by visual inspection of a loosened grid connection.

The implications extend well beyond ceiling research. Any experiment using two or more unsynchronized consumer cameras to observe dynamic phenomena faces the same inter-camera timing problem, and the reprojection-error synchronization approach applies wherever stereo pairs can see identifiable features in motion, from biomechanics to geotechnical centrifuge testing to field bridge monitoring. The modular design lets researchers feed data from commercial systems directly into KineViz-3D, or use ParaStereoSync as a standalone stereo photogrammetry pipeline. The authors identify vectorized, batched triangulation with optional GPU offload as the path to scaling to higher node densities. For now, the message to cash-strapped laboratories is striking: a laptop, two action cameras, and a sheet of printed markers can now capture three-dimensional structural motion across hundreds of points, at a fraction of the cost of equivalent commercial instrumentation.

Subject of Research: Open-source stereo-vision software for tracking 3D structural displacements in earthquake engineering experiments

Article Title: ParaStereoSync and KineViz-3D: A Python suite for parallelized stereo-vision tracking and spatio-temporal visualization of structural displacements

Article References: Seyfu, M. K., Yang, Y.-S., Flude, C. C. W., Lau, D. T., & Erochko, J. (2026). ParaStereoSync and KineViz-3D: A Python suite for parallelized stereo-vision tracking and spatio-temporal visualization of structural displacements. SoftwareX, 36, Article 103139. https://doi.org/10.1016/j.softx.2026.103139

Image Credits: AI Generated

DOI: 10.1016/j.softx.2026.103139

Keywords: stereo vision, structural engineering, earthquake engineering, digital image correlation, camera synchronization, shake-table testing, open-source software, Python, suspended ceilings, displacement measurement, ArUco markers, computer vision

Concrete-filled steel tubular columns, known in the trade as CFST columns, have become a favorite of designers of high-rise and long-span structures. The concept is elegantly synergistic. The steel tube confines the concrete core, boosting its strength and delaying crushing, while the concrete core props up the steel tube from inside, preventing the thin walls from buckling inward. The result is a column with high strength, high stiffness, and remarkable ductility, all in a compact cross-section that frees up floor space. But the column is only as good as its connections, and this is where the technology has historically stumbled.

Conventional CFST column to reinforced concrete beam joints typically require steel bars and plates to pass through the column wall or through the concrete core itself. That means drilling, cutting, and welding inside a confined space, interrupting the continuous tube and sometimes reducing the column’s load-carrying capacity precisely where the forces are greatest. The detailing is complex, quality control on site is difficult, and construction crews lose time on operations that cannot easily be standardized. For a technology whose selling points are speed and efficiency, the connection detail has been an awkward contradiction.

The solution proposed by Zha and Zhang moves the problem outside the column. Their externally stiffened ring connection wraps a stiffening ring around the outside of the steel tube, giving the beam’s forces a continuous external path to travel through rather than forcing them to penetrate the tube wall. The researchers tested two configurations: a welded version, in which the connection components are welded together into a continuous load path, and a lapped version, in which elements overlap and rely on bearing and bond. Eight specimens, all built at 1:5 scale, were fabricated and subjected to low-cycle reversed loading, the laboratory equivalent of the back-and-forth swaying a structure experiences during a strong earthquake.

The test matrix was designed to isolate two key design variables. The first was the axial compression ratio, set at either 0.33 or 0.50, which represents how heavily the column is squeezed by the gravity loads above it. Higher axial compression generally changes how a joint fails and how much ductility it retains. The second variable was the length of the corbel, the short steel bracket that projects from the column to support the beam, tested at 120 millimeters and 240 millimeters. Corbel length matters because it determines how far the beam’s forces are carried before they are transferred into the joint region, influencing whether the concrete cracks early or the steel yields in a controlled, energy-absorbing way.

The headline result is reassuring for designers: every one of the eight specimens achieved a ductility coefficient greater than 5, satisfying seismic design requirements. Ductility, the ability to deform well beyond first yield without losing strength, is the property that lets a building absorb earthquake energy and warn occupants before collapse. That all configurations cleared this threshold suggests the externally stiffened ring concept is fundamentally sound. But the tests also revealed a clear hierarchy among the variants, and the differences came down to how smoothly forces could flow through the joint.

Welded connections emerged as the standout performers. Their hysteretic loops, the force-displacement curves traced over repeated loading cycles, were full and spindle-shaped, the signature of a joint that dissipates energy steadily cycle after cycle without pinching or degradation. Stiffness degradation, the gradual softening that accumulates as damage spreads, remained stable and gradual. The researchers attribute this superior behavior to continuous force transmission through the welds, which keeps the load path uninterrupted and lets the steel yield in a distributed, controlled manner. In seismic terms, the welded joints behaved like well-tuned shock absorbers.

The lapped connections told a more cautionary tale. Those built with short 120-millimeter corbels suffered premature concrete splitting and used their materials inefficiently, because the shortened bearing length concentrated forces and cracked the concrete before the steel could do its share of the work. Lengthening the corbel to 240 millimeters improved ductility, giving the forces more room to spread and delaying local failure, but the researchers judged the longer version less economically attractive, since the added steel and dimensions eroded the cost advantages that motivated the design in the first place. The lesson is that in connection design, geometry is destiny: too short invites brittle cracking, too long wastes material.

Alongside the physical tests, the team built nonlinear finite element models in ABAQUS, the widely used simulation platform, to validate the experiments and peer inside the joints where instruments cannot reach. The models employed the concrete damaged plasticity model, a constitutive framework that tracks cracking and crushing of concrete under cyclic loading, together with a layered simulation approach that captures the distinct behaviors of steel and concrete. The simulations accurately reproduced the observed failure modes and the backbone curves, the envelopes of peak response traced across loading cycles, with errors within 10 percent. That level of agreement means engineers can now use these validated models to explore design variations without fabricating and breaking a new specimen each time, dramatically accelerating future development.

The practical upshot is a clear recommendation: welded externally stiffened ring connections are the configuration the researchers propose for real CFST frames, offering a balance of seismic performance, constructability, and design simplicity. Because the stiffening ring sits outside the column, the tube remains continuous, the concrete core is undisturbed, and the awkward through-tube detailing that has plagued conventional joints disappears. For cities in seismic zones racing to build taller and span farther with structures that must also survive the big one, a connection detail that is simultaneously stronger, simpler, and faster to build is not a small refinement. It is the kind of unglamorous innovation that quietly determines whether buildings stand or fall when the ground starts shaking.

Subject of Research: Seismic performance of externally stiffened ring connections between concrete-filled steel tubular columns and reinforced concrete beams

Article Title: Seismic performance of externally stiffened ring connections between concrete-filled steel tubular columns and reinforced concrete beams

Article References: Zha, B., & Zhang, S. (2026). Seismic performance of externally stiffened ring connections between concrete-filled steel tubular columns and reinforced concrete beams. Scientific Reports. https://doi.org/10.1038/s41598-026-75173-0

Image Credits: AI Generated

DOI: 10.1038/s41598-026-75173-0

Keywords: concrete-filled steel tube, beam-column connection, externally stiffened ring, seismic performance, quasi-static test, finite element analysis, ductility, hysteretic behavior, energy dissipation, earthquake engineering, structural engineering, reinforced concrete

The distinction between ductile and limited-ductility reinforced concrete frames lies at the heart of why this line of research matters for earthquake engineering practice. Ductile frames are detailed with close transverse reinforcement spacing, robust confinement of column cores, and capacity-design principles that steer inelastic action into flexural plastic hinges where energy can be dissipated stably through many cycles of deformation. Limited-ductility frames, by contrast, are typically designed and constructed with minimal seismic detailing, often in regions of moderate seismicity or in older building stocks designed under earlier code generations such as the Uniform Building Code provisions referenced in the study. Because their members receive less transverse confinement, these structures are prone to shear-critical behavior, in which diagonal cracking and degradation of the concrete compression zone can precipitate brittle failure before the substantial deformation capacity associated with flexural yielding is ever mobilized. Capturing this behavior in analysis therefore requires models that couple flexural and shear response rather than treating them independently.

The flexure–shear interaction modeling approach adopted in the OpenSees numerical models reflects a significant evolution in nonlinear simulation capability. Earlier generations of frame models relied on lumped or distributed plastic hinges governed purely by moment–rotation relationships, which implicitly assumed that shear strength remained intact throughout the analysis. For shear-critical members, that assumption can be dangerously unconservative, because shear capacity degrades as displacement ductility demand increases, and axial load carrying capacity can follow. Modeling frameworks that track the interaction between flexural yielding and shear degradation, drawing on the deformation capacity formulations developed for members with continuous or lap-spliced reinforcement, allow the analyst to reproduce the gradual loss of lateral strength and the eventual axial failure modes observed in laboratory tests of poorly detailed columns. For limited-ductility frames, this modeling fidelity is not a refinement but a necessity, since the governing failure mechanism is precisely the one such models are designed to capture.

Masonry infill walls introduce a further layer of complexity that has been studied extensively for frames of intermediate and high ductility but far less thoroughly for limited-ductility systems. Infills interact with their surrounding frames through a diagonal compression strut mechanism once the wall-frame interface separates and cracking localizes the contact region. At low levels of ground shaking, infills stiffen the structure substantially, shortening the fundamental period and raising the lateral strength of the building. This stiffening effect can be beneficial, reducing displacement demands, but it also redistributes internal forces, potentially inducing shear failures in columns trapped between stiff walls, a phenomenon widely documented in past earthquake reconnaissance. Equivalent strut macromodels, which represent the infill as one or more diagonal compression elements with nonlinear cyclic behavior, have become the standard practical approach for capturing these effects in frame analysis, and their state of development has been reviewed extensively in the literature on infilled frame modeling.

The finding that infills raise seismic capacity by up to fifty percent at lower performance levels but contribute progressively less near collapse is consistent with the known mechanics of infill behavior. At minor and moderate damage states, the walls act as effective lateral load-resisting elements, sharing demand with the frame and limiting drift. As shaking intensifies, however, the infill panels crack, lose their diagonal compression capacity, and may fail in out-of-plane modes or slide along bed joints, effectively removing themselves from the lateral system. What remains is the bare frame, now potentially compromised by the earlier interaction effects, such as local column shear damage caused by the strut action of the walls. Near collapse, therefore, the structure behaves increasingly like a bare frame with accumulated damage, which explains why the beneficial contribution of the infills diminishes precisely where it would matter most for life safety.

Soil–structure interaction adds the third coupled phenomenon examined in the study. When a structure is founded on flexible soil rather than a rigid base, the foundation itself deforms, introducing rocking and translational flexibility that lengthens the effective period of the combined soil-foundation-structure system and adds radiation damping as well as hysteretic energy dissipation in the soil. The classical expectation, rooted in period lengthening, has often been that SSI is beneficial, reducing the spectral accelerations experienced by flexible structures. However, the roughly twenty percent reduction in seismic capacity reported here, together with amplified displacement demands under both mainshock and aftershock excitation, illustrates the counterintuitive side of SSI for this class of buildings. Period lengthening can move a structure closer to a site period where spectral amplification is greater, and the additional flexibility concentrates displacement demand at the foundation level, increasing drifts in the superstructure and potentially aggravating P-delta effects. Nonlinear constitutive models for the soil, including the widely used material formulations for shallow foundation response developed through the Pacific Earthquake Engineering Research Center, allow these effects to be captured rather than idealized away through a fixed-base assumption.

The treatment of mainshock–aftershock sequences addresses a dimension of seismic hazard that conventional design practice largely ignores. Design codes typically assess structures against a single design-level event, yet real earthquake sequences frequently deliver multiple strong shocks, and damaged structures must withstand the aftershocks in a degraded state. Research on inelastic displacement ratios under repeated earthquakes has shown that residual deformations, accumulated damage, and the reduced stiffness of a structure after a mainshock can all conspire to produce aftershock demands that exceed what an intact structure would experience under the same level of shaking. For limited-ductility frames, whose members degrade with each cycle of deformation, the vulnerability to aftershocks is particularly acute because the shear-critical failure modes are progressive in nature: cracking and strength loss from the mainshock lower the threshold at which brittle failure can be triggered by subsequent shaking.

The quantitative relationships reported between mainshock and aftershock demands offer a useful empirical picture of this degraded-state response. On average, roof displacement demands during aftershocks reach approximately half of the corresponding mainshock values, while inter-story drift demands reach sixty-five to seventy-five percent, indicating that the structure does not simply re-experience its mainshock response. The asymmetry between these two ratios is itself informative: roof displacement reflects the global, first-mode-dominated response of the damaged system, whereas inter-story drift is more sensitive to local stiffness concentrations and damage patterns. The observation that several ground-motion sequences produced aftershock drift demands exceeding the corresponding mainshock demands by up to one hundred seven percent is a striking reminder that aftershocks are not merely weaker copies of the mainshock. Depending on the frequency content, duration, and phasing of the aftershock relative to the damaged configuration of the structure, the second shock can be more damaging than the first.

The fragility methodology underlying the study connects these nonlinear response results to probabilistic statements about damage. Incremental Dynamic Analysis scales each of the twenty selected mainshock–aftershock ground-motion sequences across a range of intensities, producing a population of response trajectories from which the probability of exceeding defined damage states can be estimated. Mapping structural response quantities onto the HAZUS damage state definitions provides a standardized vocabulary for damage, aligned with the loss-estimation framework widely used in earthquake consequence modeling. Fragility curves constructed in this way express, for any given ground-motion intensity, the likelihood that a building will reach or exceed each damage state, from minor cracking through complete structural damage. This probabilistic framing, which builds on the performance assessment philosophy articulated in foundational work on seismic performance evaluation, is what allows the relative influence of infills, SSI, and sequence effects to be compared on a common scale.

The choice of two-, and four-story archetype buildings designed under the Uniform Building Code gives the results a concrete anchor in a recognizable class of existing construction. Low- and mid-rise frames of this type are ubiquitous in seismically active regions worldwide, and many were designed before modern capacity-design and ductility detailing requirements took hold. The two-story configuration represents the short-period end of the building population, where infill stiffening effects are proportionally largest and where period lengthening from SSI can shift the response into more demanding spectral regions. The four-story configuration introduces greater flexibility and higher drift demands, making it more sensitive to P-delta amplification and to the concentration of drift in lower stories where infill interaction and foundation flexibility effects converge. Studying both heights therefore brackets a meaningful range of behavior for the limited-ductility building stock.

The implications for performance-based assessment and retrofit are considerable. If fragility estimates for infilled, flexibly based, limited-ductility frames are made using fixed-base bare-frame models, the analysis may simultaneously overestimate capacity by crediting a rigid base that does not exist and underestimate the stiffening benefit of infills at lower damage states, while missing the near-collapse degradation of both. Retrofit strategies that add strength without addressing shear-critical member behavior, or that stiffen the structure without accounting for foundation flexibility, could fail to deliver the intended improvement in reliability. The study’s demonstration that the coupled effects of infills, nonlinear SSI, and aftershock sequences significantly alter the computed vulnerability profile argues for assessment workflows that represent all three phenomena explicitly, particularly for the large inventory of limited-ductility RC buildings whose seismic safety remains governed by brittle failure modes.

More broadly, the work sits within a growing body of research that treats seismic vulnerability as an emergent property of interacting subsystems rather than a property of the frame alone. Recent studies of infilled frames under mainshock–aftershock sequences, of SSI effects on the damage states of infilled structures considering both in-plane and out-of-plane wall behavior, and of modeling uncertainties in residual drift assessment under sequences all point toward the same conclusion: the most consequential aspects of seismic performance arise at the interfaces between structural, non-structural, and geotechnical components. For limited-ductility construction, where the margin between serviceable response and brittle failure is inherently narrow, resolving those interfaces in analysis is not an academic exercise but a prerequisite for credible risk assessment and for prioritizing the retrofit investments that follow from it.

Subject of Research: Probabilistic seismic fragility assessment of limited-ductility infilled RC frames with nonlinear soil–structure interaction under mainshock–aftershock sequences

Article Title: Probabilistic seismic fragility assessment of flexibly based infilled limited-ductility RC frames under earthquake sequences

Article References: Al-Mamoori, T. M. H., Zafarani, M. M., Ali, A. M., & Birzhandi, M. S. (2026). Probabilistic seismic fragility assessment of flexibly based infilled limited-ductility RC frames under earthquake sequences. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02716-1

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02716-1

Keywords: seismic fragility, limited-ductility RC frames, masonry infill walls, soil–structure interaction, mainshock–aftershock sequences, incremental dynamic analysis, OpenSees, HAZUS damage states, flexure–shear interaction, performance-based seismic assessment, earthquake engineering, retrofit

The physical risk component of the framework is built on a Python-based computational tool that couples disaggregated exposure data on a 30-arc-second grid with site-specific probabilistic hazard outputs. In practical terms, this means the model divides the country into cells of roughly one kilometre at the equator, each carrying information about the building stock within it, and then overlays probabilistic seismic hazard curves that describe the intensity of ground shaking expected at each location over given return periods. Exposure modelling at this scale requires careful taxonomy: buildings must be classified by material, structural system, height and age, because a reinforced concrete frame with soft storeys responds to shaking very differently from a load-bearing masonry house. The researchers map building taxonomies to fragility functions, which are probabilistic descriptions of how likely a structure class is to reach defined damage states under given shaking intensity.

A key technical feature of the methodology is its logic tree approach to fragility assignment. Rather than committing to a single vulnerability model for each building class, the framework draws on both regional fragility functions developed specifically for Turkish building stocks and global models developed for broader seismic risk analyses, weighting them in a logic tree that propagates model uncertainty into the final results. This is a deliberate response to one of the most stubborn problems in catastrophe modelling: different fragility curves derived for nominally similar buildings can produce materially different loss estimates, and pretending that a single curve is the truth understates the true uncertainty. The logic tree generates a suite of risk metrics for every province, including economic losses, casualties and the number of displaced households, each expressed as a distribution across the model branches rather than a single deterministic number.

The socioeconomic side of the framework departs from the common practice of appending a social vulnerability index as a supplementary layer. The researchers constructed a socioeconomic vulnerability index from thirty indicators spanning economic, health and educational dimensions, drawing on national statistical data. Indicators such as income, employment, access to health services and educational attainment are known from decades of disaster research to influence both the immediate toll of an earthquake and the speed of recovery, but combining them raises a methodological trap: how much weight should each indicator receive? Subjective expert weighting introduces bias, while naive equal weighting ignores the fact that many socioeconomic indicators measure overlapping aspects of the same underlying condition. To resolve this, the team employed an objective weighting technique designed to manage interdependencies between indicators, using the statistical structure of the data itself to determine weights and eliminate potential biases in the aggregation.

Perhaps the most consequential design decision is how the two halves of the analysis are combined. Instead of treating socioeconomic vulnerability as a separate variable to be added to the physical risk score, the methodology integrates it as a multiplier that scales the structural risk outputs. The logic is multiplicative rather than additive: two provinces with identical building fragility and hazard exposure will not experience equivalent disasters if one has substantially weaker coping capacity, and the multiplier structure captures that interaction directly. A province with moderate physical risk but severe socioeconomic vulnerability can therefore rise in the integrated ranking above a province with high physical risk but strong coping capacity, a reordering that conventional engineering-only rankings cannot produce. The final output is an integrated seismic risk index for each province, intended to identify hotspot areas where long-term mitigation strategies are most urgently needed.

The results demonstrate that the integrated ranking differs in meaningful ways from the traditional physical risk ranking. Provinces whose populations face compounding economic, health and educational disadvantages move up the priority list when their diminished coping capacity is factored in, while some areas that score highly on structural risk alone slip when social resilience is accounted for. For decision-makers charged with allocating scarce retrofitting funds, this difference is not academic. Türkiye has enacted legislation on the transformation of areas under disaster risk, and national reconstruction assessments after the 2023 earthquakes have grappled with the scale of rebuilding required. A screening tool that surfaces where combined physical and social risk is concentrated gives policymakers a defensible, data-driven basis for deciding where detailed, finer-resolution assessment and investment appraisal should be directed first, before expensive building-by-building surveys are commissioned.

The framework also situates itself within a broader methodological lineage. Integrated risk assessment, in which social vulnerability is combined with physical loss modelling, has been developed for other countries and regions, including applications within the OpenQuake computational platform and national studies for countries such as Portugal. The Turkish application extends this line of work to one of the most seismically active countries on Earth, where the North Anatolian Fault and the convergence of the Anatolian plate with surrounding plates have produced a long historical record of destructive earthquakes. The exposure and hazard inputs draw on the European seismic risk and hazard modelling infrastructure, including regional exposure models and site response characterisations, adapted to the national scale of a country with tens of millions of residential buildings.

Technically, the framework’s dependence on 30-arc-second gridded exposure data and probabilistic hazard outputs reflects a deliberate trade-off between coverage and resolution. National-scale prioritisation cannot wait for building-by-building surveys of the entire stock, which would take years and rapidly go out of date. Grid-based exposure modelling, in which the spatial distribution and characteristics of buildings are inferred from statistical and census data disaggregated to cells, allows the entire country to be assessed consistently within a single computational run. The cost is that local effects, such as soil amplification at specific sites or the particular deficiencies of an individual structure, are only represented in aggregated form. The authors are explicit that the framework is a screening and prioritisation instrument, not a substitute for detailed structural evaluation, and that its proper role is to direct finer-grained studies toward the places where integrated risk is highest.

The 2023 Kahramanmaraş earthquake sequence provided a stark real-world validation of the framework’s premise. Post-disaster assessments by international institutions documented catastrophic damage and loss across the affected region, and empirical studies of the damaged building stock have since generated new fragility evidence for Turkish reinforced concrete and masonry construction. The researchers’ insistence that coping capacity is inseparable from structural vulnerability echoes what reconnaissance teams observed on the ground: recovery trajectories, shelter needs and long-term displacement were shaped as much by the socioeconomic condition of affected communities as by the shaking itself. By encoding that insight into a quantitative, reproducible methodology, the study offers a template that other earthquake-prone countries can adapt.

The larger significance of the work lies in what it changes about the conversation around seismic mitigation. For decades, retrofitting priorities have been argued largely in engineering terms, measured in expected monetary loss, collapse probabilities and casualties. Those metrics matter, but they capture only part of the disaster equation. A national-scale index that makes social vulnerability an explicit multiplier of physical risk reframes the question from ‘where are the most dangerous buildings?’ to ‘where will earthquakes cause the greatest and least recoverable harm?’, and the answer to the second question is frequently a different map. For Türkiye, a country where earthquake preparedness is a matter of intense public debate and where large parts of the building stock predate modern seismic codes, that reframing could influence how limited public investment is targeted in the years ahead, and for the international risk-modelling community, it provides a concrete demonstration that integrated, data-driven prioritisation frameworks are now computationally feasible at full national scale.

Subject of Research: Integrated seismic risk estimation and retrofitting prioritisation for Türkiye combining physical loss modelling with socioeconomic vulnerability

Article Title: An integrated seismic risk estimation framework for Türkiye

Article References: Aydin, M. F., Mucedero, G., & Monteiro, R. (2026). An integrated seismic risk estimation framework for Türkiye. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02718-z

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02718-z

Keywords: seismic risk, Türkiye, earthquake engineering, socioeconomic vulnerability, fragility functions, probabilistic hazard, building retrofitting, risk prioritisation, loss estimation, Kahramanmaraş earthquakes, exposure modelling, integrated risk index

The research team focused on three configurations of a five-storey reinforced concrete building. The first was a regular building standing on flat land, serving as the baseline for comparison. The second was a step-back building, in which floors are progressively shortened on one side as the structure climbs the slope, creating a vertically irregular profile. The third was a step-back setback building, which adds a further geometric complication: not only do floors shorten toward the hill, but columns at the downhill side are of vastly different heights, a configuration common in towns built along Himalayan and other mountain slopes. These three systems allowed the researchers to isolate how each type of geometric irregularity alters seismic demand.

At the heart of the study lies a demanding computational campaign. The authors subjected their building models to nonlinear time-history analyses, meaning the structures were simulated through the full recorded shaking of real earthquakes, with the model free to crack, yield, and redistribute forces just as a real reinforced concrete frame would. Twenty far-field ground motions drawn from the publicly available PEER NGA database supplied the excitation. Each record was applied first in a single horizontal direction and then in both horizontal directions simultaneously, reflecting the reality that earthquakes almost never shake the ground along a single clean axis. The team also varied the record scale factor, systematically scaling the intensity of each motion to probe how demand grows as shaking strengthens.

The headline metric was the maximum inter-storey drift ratio, essentially how far one floor displaces sideways relative to the floor beneath it. Drift is a trusted proxy for structural damage: excessive drift means cracked columns, yielded reinforcement, and, in the worst cases, collapse of individual storeys. When the buildings were shaken in one direction only, the flat-land building produced maximum drift ratios ranging from 0.39 to 1.35 percent across the scaled records. The step-back setback building ranged from 0.48 to 1.67 percent, and the step-back building, the most geometrically compromised of the three, ranged from 0.74 to 2.23 percent, already signaling its heightened vulnerability.

The striking result emerged when shaking was applied in two directions at once. Under bidirectional excitation, the drift demands climbed sharply: the flat-land building’s range rose to 0.70 to 2.13 percent, the step-back setback building to 0.86 to 2.33 percent, and the step-back building to 0.93 to 2.79 percent. In other words, simply accounting for the second horizontal component of ground motion, something many simplified analyses still ignore, pushed every configuration toward substantially larger deformations. The physical explanation lies in bidirectional interaction: as columns yield in one direction, their capacity to resist forces in the perpendicular direction degrades, so the two components of shaking effectively erode each other’s resistance and demand accumulates in ways a unidirectional analysis cannot capture.

But the study did not stop there. The direction in which a ground-motion record is oriented relative to the building, known as the incidence angle, was varied systematically. This matters because earthquake records contain two horizontal components, and rotating the record changes which component aligns with the building’s strong and weak axes. For regular flat-land buildings, this rotational sensitivity exists but is comparatively modest. For the hillside configurations, the effect was dramatic. At critical incidence angles, the normalized drift demand of the hillside buildings reached roughly 2.0 to 2.8 times the response of the flat-land building. The step-back building proved especially direction-sensitive, its worst-case angles exploiting the mismatch between the short, stiff columns on the uphill side and the tall, flexible columns at the bottom of the slope.

To convert these deterministic results into a probabilistic picture of risk, the team turned to incremental dynamic analysis, a method in which each ground motion is scaled through a series of increasing intensities until the structure approaches its limits. From this family of analyses they constructed fragility curves, which express the probability that a building will exceed a given damage state as a function of ground-motion intensity, here measured by peak ground acceleration. The damage thresholds were drawn from the drift limits specified in FEMA 356:2000, the widely used American prestandard for seismic rehabilitation, allowing the curves to distinguish between levels of damage from minor cracking through to more severe structural compromise.

The fragility analysis delivered a nuanced verdict. Across the board, the step-back configuration generally emerged as the most seismically vulnerable of the three, consistent with its larger drift demands under both unidirectional and bidirectional shaking. Yet the researchers found something more unsettling: the relative ranking among the flat-land, step-back, and step-back setback buildings is not fixed. It shifts with the seismic incidence angle. A building that appears safer than its neighbor when the ground motion strikes from one orientation can appear more dangerous when the same record is rotated. For code writers and practitioners, this undermines any single-scenario assessment and suggests that hillside structures should be evaluated across a spectrum of incidence angles rather than at one assumed orientation.

For a rapidly urbanizing mountain world, the implications are hard to ignore. Cities such as those sprawling across the Indian Himalayas, the Andes, the Mediterranean hillsides of Turkey and Italy, and countless other sloping terrains host millions of reinforced concrete frames whose geometry resembles the step-back and step-back setback prototypes studied here. Previous research has already documented elevated collapse risk for hillside buildings in the Himalayan belt, and recent studies have proposed new descriptors of the coupled irregularities these structures exhibit. The present study adds a critical layer: even a thorough fragility assessment may significantly understate risk if it considers only one direction of shaking or one angle of incidence. The authors’ demonstration that record-to-record variability, illustrated by plotting mean responses together with mean plus and minus one standard deviation for selected cases, further complicates the picture, reminding engineers that individual earthquakes can deviate substantially from average predictions. The practical message is sobering but actionable: as hillside construction accelerates, seismic evaluation of buildings on slopes should routinely incorporate bidirectional excitation and sweep across incidence angles, because on sloping ground, the direction an earthquake comes from may matter almost as much as its strength.

Subject of Research: Seismic response and fragility of reinforced concrete hillside buildings under unidirectional and bidirectional ground motions

Article Title: Seismic response of hillside buildings under unidirectional and bidirectional ground motions with varying incidence angles

Article References: Ansari, M. G., Dutta, S. C., & Kumar, S. (2026). Seismic response of hillside buildings under unidirectional and bidirectional ground motions with varying incidence angles. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02720-5

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02720-5

Keywords: hillside buildings, seismic response, bidirectional ground motion, incidence angle, inter-storey drift, fragility analysis, incremental dynamic analysis, nonlinear time-history analysis, vertical irregularity, step-back setback, reinforced concrete frames, earthquake engineering

The research, led by Dongyeon Lee of Hanyang University together with colleagues from Korea Hydro and Nuclear Power and the Korea Advanced Institute of Science and Technology, tackles a long-standing gap in earthquake engineering practice. Spatial coherency describes how the similarity between ground motions at two locations decays as the separation between them grows and as the frequency of the shaking increases. Empirical models built from dense arrays such as those at Lotung in Taiwan or Chiba in Japan have long served as the reference standard, but they are inherently site-specific. Extending them to a new site means installing instruments and waiting for earthquakes to happen, which is neither practical nor economical for most engineering projects. The Korean team’s alternative is to predict coherency from first principles, using the measured spatial statistics of the soil itself as the input.

The physical basis of the new framework is the recognition that incoherency at a single site arises largely from the heterogeneity of the subsurface. Shear waves traveling upward through soil encounter pockets and layers of varying stiffness, and each encounter scatters and delays the wavefield slightly differently at every location. If the spatial distribution of shear wave velocity, denoted Vs, is known in statistical terms, the resulting distortion of the wavefield can, in principle, be computed. The researchers therefore set out to measure Vs variability directly and feed those statistics into a two-dimensional finite-element model of the soil deposit, generating Gaussian random fields whose coefficient of variation and correlation lengths match the field measurements.

Validating such a framework requires ground truth, and this is where the centrifuge enters the story. Geotechnical centrifuges allow engineers to test scaled physical models of soil deposits under properly scaled gravitational stress, reproducing the stress-dependent stiffness and damping behavior that governs wave propagation at full scale. The team constructed two soil models in the centrifuge, one a single-layered deposit and the other a double-layered deposit, and subjected them to recorded earthquake motions using an on-board earthquake simulator. Crucially, they characterized the spatial variability of the models themselves using bender elements and cone-penetration tests at multiple locations, obtaining the coefficient of variation and correlation lengths needed to build representative random fields. This pairing of physical testing and statistical characterization is what gives the study its unusual rigor.

On the computational side, each physical model was represented by a two-dimensional finite-element mesh, and the researchers ran a substantial campaign of simulations: ten bedrock motions recorded during the centrifuge tests were combined with thirty random-field realizations for each model, producing three hundred time-history analyses per deposit. Each realization represents one plausible arrangement of the measured soil heterogeneity, and the ensemble captures the range of responses that variability can produce. From the simulated surface motions at multiple locations, the team computed coherency functions in the same manner used for array recordings, binning results by separation distance so that they could be compared directly with the coherency derived from the centrifuge sensors.

The agreement reported is striking. The numerical models closely reproduced the measured response spectra and the shear stress-strain behavior observed in the centrifuge, confirming that the wave propagation physics was being captured correctly. More importantly for the study’s central claim, the coherency models computed from the simulations showed strong agreement with the centrifuge-derived coherency across all separation-distance bins. In other words, once the spatial variability statistics of the soil are known, the simulation does not merely approximate the general character of incoherency; it tracks the measured decay of coherency with distance and frequency in detail.

The implications extend well beyond the laboratory. Sites hosting critical infrastructure, particularly nuclear facilities, increasingly demand site-specific coherency models for soil-structure interaction analysis, and regulators in several countries have pressed for models tailored to local conditions. In Korea, for example, recent work has focused on generating coherency models suitable for nuclear power plant sites where no dense arrays exist. A validated simulation-based framework means that a project team could characterize a site with conventional geotechnical investigation tools, bender element tests and cone-penetration soundings are routine in practice, and then compute a coherency model numerically rather than waiting years for array data to accumulate. The study’s authors present this as a practical alternative to empirical models whenever site-specific spatial variability parameters are available.

The work also connects to a broader trend in geotechnical engineering: the treatment of soil spatial variability as a quantifiable random field rather than a nuisance to be averaged away. Probabilistic approaches using random-field theory have transformed reliability analysis of slopes, foundations, and liquefaction assessments over the past two decades. What this study adds is a bridge between that statistical tradition and the wave-propagation community concerned with spatially varying earthquake ground motion. By demonstrating that measured Vs statistics suffice to predict coherency, the researchers effectively show that the two communities’ toolkits can be combined, with site investigation data flowing directly into ground-motion incoherency predictions.

There are, of course, boundaries to what the framework has so far demonstrated. The validation was performed on idealized laboratory deposits in a centrifuge, where the soil variability could be measured exhaustively and the input motions were fully known. Real field sites present additional complications, including three-dimensional subsurface topography, deeper and more variable layering, and uncertainty in bedrock input motions. The authors note that their findings establish the feasibility of simulation-based coherency estimation, and extending the approach to field conditions is the natural next step. Even so, the core result stands: the physics linking soil heterogeneity to ground-motion incoherency is now demonstrably reproducible in a validated numerical setting.

For engineers designing the next generation of long-span bridges, lifeline networks, and nuclear structures, the message is that coherency analysis need no longer be the exclusive privilege of instrumented sites. A well-characterized soil profile, a finite-element model, and a modest computational campaign of random-field realizations can deliver a defensible, site-specific coherency model, one that has now been checked against physical reality at centrifuge scale. As seismic design codes continue to move toward performance-based criteria that explicitly account for the spatial character of shaking, tools of this kind are likely to move from research papers into standard practice, changing how the profession thinks about what lies beneath the foundations of everything it builds.

Subject of Research: Centrifuge-validated numerical simulation of spatial seismic ground-motion coherency in heterogeneous soil deposits

Article Title: Centrifuge-validated numerical framework for estimating seismic coherency

Article References: Lee, D., Lee, Y., Choi, D.-H., Kwon, T.-H., & Park, D. (2026). Centrifuge-validated numerical framework for estimating seismic coherency. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02714-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02714-3

Keywords: seismic coherency, spatial variability, shear wave velocity, centrifuge testing, finite element analysis, Gaussian random fields, site response, soil-structure interaction, earthquake engineering, geotechnical engineering, dense seismic arrays, Bulletin of Earthquake Engineering

The research, carried out by Onur Onat of the Department of Civil Engineering at Inonu University in Malatya, rests on a deceptively simple idea. Every structure, from a guitar string to a skyscraper, has a set of natural frequencies at which it prefers to oscillate. These frequencies depend on the structure’s mass and stiffness, so when an earthquake cracks masonry, opens bed joints, or otherwise softens a building, the natural frequencies drop. By measuring the minarets’ vibration signatures before and after the Kahramanmaraş sequence, engineers can effectively ask the towers themselves how much damage they sustained, even in places the eye cannot see.

Ambient vibration measurement, the technique at the heart of the study, requires no artificial shaking. Wind, distant traffic, and micro-tremors in the ground provide a continuous, gentle excitation that is more than enough to coax a tall minaret into swaying at its natural frequencies. Sensitive seismometers recorded these barely perceptible motions at both towers, and signal processing identified the natural frequencies, damping ratios, and mode shapes, the characteristic patterns of deformation associated with each frequency, for the first four vibration modes of each minaret. The pre-earthquake and post-earthquake modal vectors were then compared using the Modal Assurance Criterion, a standard statistical measure of how similar two vibration patterns are.

The results tell a consistent story of measurable but not catastrophic degradation. The first four identified frequencies decreased by an average of 5.46 percent for the Yusuf Ziya Pasha minaret and 5.98 percent for the Sunullah Melik minaret. A frequency drop of roughly six percent may sound modest, but in structural terms it corresponds to a meaningful loss of lateral stiffness, the property that allows a slender tower to resist horizontal earthquake forces. Intriguingly, the damping ratios, which describe how quickly vibrations die away, showed no uniform trend, a reminder that energy dissipation in cracked masonry is a far messier phenomenon than stiffness loss.

Field inspections corroborated what the vibration data implied. At the Yusuf Ziya Pasha minaret, observers documented bending-induced tensile cracking in the lower body of the tower, exactly where the flexural demands of earthquake shaking would be expected to concentrate in a cantilever-like masonry shaft. At the Sunullah Melik minaret, the damage took a different form: sliding and separation along the bed joints, the horizontal mortar lines between stone courses, around the pulpit region. These distinct crack patterns illustrate a crucial point about masonry behavior, namely that the same earthquake can produce qualitatively different failure mechanisms in structurally similar towers depending on their geometry, construction details, and local ground conditions.

To translate these observations into a quantitative assessment of seismic performance, the study employed finite element models, numerical representations of each minaret built from thousands of interconnected elements. Rather than assuming material properties from textbooks, the researcher independently calibrated separate pre-earthquake and post-earthquake models using the measured fundamental frequencies of each tower. The models were then verified against the higher-mode frequencies and the modal vectors, ensuring that the numerical replicas matched reality not just in their simplest vibration pattern but across the full measured spectrum. This two-stage approach, calibration followed by independent verification, guards against the common pitfall of a model that fits one number by accident while being wrong everywhere else.

With trustworthy models in hand, the study moved from diagnosis to prognosis. Nonlinear time-domain analyses subjected both the pre-earthquake and post-earthquake models to five unscaled ground-motion records, letting the computer simulate how the towers would respond to real earthquake shaking in their intact and damaged states. Nonlinear analysis is essential here because masonry does not behave elastically under strong shaking; cracks open and close, joints slide, and stiffness degrades progressively as the motion intensifies. The models incorporated material behavior consistent with established plasticity formulations for masonry, allowing the simulations to capture the initiation and propagation of the very cracking patterns observed in the field.

The comparison between intact and damaged simulations produced one of the study’s most consequential findings. The post-earthquake stiffness reductions were reflected far more clearly in the deformation demands than in the global force responses, particularly under the stronger ground motions. In other words, a damaged minaret may carry roughly similar peak forces as an intact one, yet it must bend, sway, and distort substantially more to do so. Since deformation, not force, is what ultimately tears masonry apart, this result carries a sobering implication for heritage managers: towers that appear to have weathered an earthquake without collapse may nonetheless have crossed into a regime where the next strong event produces disproportionately larger displacements and more severe cracking.

Perhaps the most methodologically valuable conclusion concerns what vibration monitoring can and cannot detect. The combined measurement and modeling framework demonstrated that mode-dependent dynamic changes, the frequency shifts distributed across the first four modes, can identify earthquake-related stiffness loss even when the principal global mode shapes remain broadly preserved. This is good news for the growing field of structural health monitoring of cultural heritage, because mode shapes are notoriously difficult to measure accurately in the field, while frequencies can be tracked reliably with modest instrumentation. The study suggests that a monitoring program need not capture perfectly resolved shape changes to flag damage; a careful reading of frequency trends across multiple modes, interpreted through a calibrated model, suffices.

The implications extend well beyond two minarets in Malatya. Türkiye holds an extraordinary inventory of historical masonry minarets, and the 2023 Kahramanmaraş sequence damaged or destroyed many mosques and their towers across the affected region, prompting widespread post-earthquake assessments. The approach demonstrated here, pairing ambient vibration surveys with independently calibrated nonlinear finite element models, offers a repeatable template for triaging that inventory: identifying which towers have silently lost stiffness, quantifying how much seismic reserve remains, and prioritizing retrofit resources accordingly. As climate extremes and seismic activity continue to threaten unreinforced masonry heritage worldwide, the quiet hum of a minaret vibrating in the wind may prove to be one of the most eloquent damage reports a historic structure can give.

Subject of Research: Structural health monitoring and seismic performance assessment of historical masonry minarets damaged by the 2023 Kahramanmaraş earthquake sequence

Article Title: Structural health monitoring and seismic performance assessment of two historical masonry minarets before and after the 2023 kahramanmaraş earthquake sequence

Article References: Onat, O. (2026). Structural health monitoring and seismic performance assessment of two historical masonry minarets before and after the 2023 kahramanmaraş earthquake sequence. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02721-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02721-4

Keywords: structural health monitoring, masonry minarets, Kahramanmaraş earthquakes, ambient vibration, modal analysis, finite element model updating, nonlinear seismic analysis, natural frequencies, damage assessment, cultural heritage, earthquake engineering, Türkiye

The research, published in the Bulletin of Earthquake Engineering, was carried out by Mustafa Al-Mashaykhi and Pathmanathan Rajeev of Swinburne University of Technology in Melbourne, together with Kushan Wijesundara and Sahan Bandara of the University of Peradeniya in Sri Lanka. Their focus was a workhorse of steel construction: the concentrically braced frame, or CBF, a structural system in which diagonal steel braces carry lateral forces through axial tension and compression. CBFs are popular because they are stiff, economical and efficient at controlling drift, yet their braces are notoriously vulnerable to the kind of repeated inelastic cycling that long-duration earthquakes deliver.

The problem with drift-based assessment, the authors explain, is that it captures the peak deformation a frame experiences but says nothing about how that deformation was reached or how many times the structure yielded along the way. When a brace buckles in compression and stretches back in tension, cycle after cycle, the steel accumulates low-cycle fatigue damage even if the maximum drift never changes. Two ground motions can produce identical peak drifts while leaving the structure in radically different states — one largely intact, the other with fractured braces and a frame on the verge of collapse. Classic post-earthquake surveys, including studies of the 1994 Northridge and 1995 Hyogo-ken Nanbu earthquakes, documented exactly this kind of cumulative damage in steel buildings, motivating engineers to look beyond single-deformation measures.

To capture that cumulative effect, the team turned to an energy-based damage index, building on earlier work by Rajeev and Wijesundara that applied the continuous wavelet transform to steel braced frames. The wavelet approach, rooted in mathematical techniques first formalised by Dennis Gabor in 1946 and developed extensively since, allows the seismic energy absorbed by the structure to be computed as a function of time. Instead of treating the ground motion as a single peak event, the method decomposes the response record into localised wave components, making it possible to track how hysteretic energy — the energy dissipated through plastic deformation — accumulates over the full duration of shaking. The resulting damage index blends the maximum deformation demand with the total dissipated energy, echoing the influential Park and Ang damage model of 1985 and later energy-based formulations by researchers such as Bojorquez, Teran-Gilmore and Fajfar, who explicitly linked ductility demands to low-cycle fatigue.

The mechanics of the study were ambitious in scale. The team designed eight different CBF steel structures, varying both building height and bracing configuration — the geometric arrangements of diagonals, such as diagonal and chevron patterns, that determine how forces flow through the frame. Each frame was then subjected to thirty different ground motion records spanning a wide range of seismic intensities and hazard levels, drawn from European strong-motion databases. Using the open-source simulation platform OpenSees, with brace models capable of capturing cyclic inelastic buckling as characterised by Uriz, Filippou and Mahin, the researchers ran Incremental Dynamic Analysis, the method introduced by Vamvatsikos and Cornell in 2002, in which each record is scaled to progressively higher intensities until the structure reaches defined limit states.

From this mountain of nonlinear simulations, the team built probabilistic seismic demand models — statistical relationships between ground-motion intensity and structural demand — for both the conventional drift measure and the new energy-based damage index. These models then fed into fragility curves, the standard tool of seismic risk assessment, which express the probability that a structure will exceed a given damage state at a given level of shaking. Fragility curves are what engineers and insurers ultimately rely on to estimate expected losses and to decide where retrofit money should be spent, so their statistical quality matters enormously.

The headline finding is that the energy-based approach produces fragility estimates with lower statistical dispersion. In practical terms, the record-to-record variability — the scatter that arises when different ground motions, even at the same intensity, push a structure to very different levels of damage — is reduced when damage is measured by accumulated hysteretic energy rather than peak drift. Lower dispersion means tighter, more confident fragility curves, and tighter curves mean more reliable estimates of collapse risk. For a profession that must make safety decisions under deep uncertainty, reducing this scatter is not a cosmetic improvement; it directly changes the width of the confidence intervals within which real buildings are judged safe or unsafe.

Equally significant is where the two approaches agree and where they diverge. At lower damage levels, corresponding to the Immediate Occupancy performance state — the condition in which a building remains safe to enter and use immediately after a quake — the drift-based and energy-based methods yield broadly comparable fragility predictions. This makes sense: at light damage, the structure has barely yielded, little hysteretic energy has been dissipated, and peak deformation is a reasonable proxy for the state of the frame. But at the high damage end, at the Collapse Prevention limit state, where the structure is barely standing and braces have undergone many large inelastic cycles, the two methods diverge substantially. The energy-based index, sensitive to those cumulative inelastic hysteretic cycles, paints a different — and, the authors argue, more physically faithful — picture of how close a CBF really is to collapse under severe, long-duration shaking.

The implications reach beyond braced steel frames. Energy-based damage indices have been gaining traction across earthquake engineering, with recent work applying energy concepts to mainshock-damaged buildings, to integrated displacement-and-energy design procedures, and to the plastic design of steel moment frames that explicitly account for cumulative damage. The new study adds an important probabilistic leg to that edifice: it demonstrates, quantitatively and across a family of real frame designs, that an energy-based damage measure delivers a more stable estimate of seismic performance precisely where it matters most — near collapse. For code writers, that suggests energy demands deserve a place alongside drift limits in performance-based design frameworks, particularly for structural systems whose components, like steel braces, degrade through repeated buckling and yielding.

There are, of course, practical hurdles before energy-based fragility becomes routine practice. Computing hysteretic energy requires nonlinear time-history analysis and the wavelet-based signal processing that underpins the damage index, tools that are more demanding than the elastic drift checks familiar from everyday design offices. The authors note that all data, models and code supporting the study are available from the corresponding author on reasonable request, which should help other researchers replicate and extend the method to other structural systems, longer-duration motions and site classes not covered here. But the direction of travel is clear. As earthquake engineers push toward more reliable, performance-based assessment of the buildings people depend on, measuring the total energy a quake pumps into a structure — not just how far it sways — may become the difference between a fragility curve that looks reassuring and one that tells the truth about collapse risk.

Subject of Research: Energy-based seismic fragility assessment of concentrically braced steel frames

Article Title: Energy-based seismic fragility for concentrically braced steel frames

Article References: Al-Mashaykhi, M., Rajeev, P., Wijesundara, K., & Bandara, S. (2026). Energy-based seismic fragility for concentrically braced steel frames. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02717-0

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02717-0

Keywords: seismic fragility, concentrically braced frames, energy-based damage index, inter-storey drift, incremental dynamic analysis, wavelet transform, hysteretic energy, collapse prevention, probabilistic seismic demand, steel structures, low-cycle fatigue, earthquake engineering