The central idea is deceptively simple. In a force-displacement plot, one complete cycle of motion traces a loop through four quadrants: two where displacement and force share the same sign, and two where they oppose each other. The ENSFD is engineered to deliver substantial damping force only in the second and fourth quadrants, the unloading phases of the cycle, while remaining nearly passive during loading. Because the device never pushes back hard against the structure as it moves in the direction of the driving force, it does not increase the maximum damping force transmitted to the building. At the same time, the asymmetric resistance mimics the effect of negative stiffness, a property that, in structural dynamics, effectively lengthens the period of a structure and softens its response to ground motion. The result is a damper that reshapes the hysteresis loop rather than simply enlarging it, concentrating energy absorption where it does the most good and least harm.

To make this concept usable in engineering practice, the researchers needed a way to represent the nonlinear, half-cycle behavior of the damper with parameters that standard analysis tools can handle. They built a nonlinear single-degree-of-freedom model of the ENSFD and applied the method of slowly varying parameters, an analytical technique for deriving the steady-state response of systems whose properties change gradually over a cycle of motion. From this treatment they extracted equivalent linearization parameters, essentially a stiffness and a damping coefficient that capture the averaged effect of the device over a full oscillation. These equivalent parameters were then checked against direct time-history analysis, in which the full nonlinear equations are integrated step by step under recorded ground motions. The comparison confirmed that the simplified parameters reproduce the response accurately enough for design-level seismic analysis, giving engineers a practical bridge between an exotic nonlinear device and the linearized calculation methods embedded in building codes.

Turning the mathematics into hardware required a control system capable of recognizing, in real time, which phase of the motion cycle the structure is in. The team designed a semi-active ENSFD built around an STM32 microprocessor, a widely used embedded controller with enough speed and reliability for structural applications. The complete system has three cooperating subsystems: a signal acquisition system that measures the structural response, a control system that processes those signals and decides the operating state, and a variable friction execution system that adjusts the friction force accordingly. The controller must distinguish among loading, unloading, and the transitional moments when the direction of motion reverses, because the damper’s behavior differs fundamentally in each state. Getting this logic right is the difference between a device that dissipates energy selectively and one that degenerates into an ordinary friction damper.

The experimental program consisted of low-cycle reciprocating tests, the standard laboratory protocol for seismic devices, in which the damper is pushed back and forth through many cycles at controlled displacement amplitudes and frequencies. The measured hysteresis curves matched the theoretical expectations: the device provided substantial damping force during unloading while contributing little resistance during loading, producing the characteristic asymmetric loop that defines equivalent negative stiffness behavior. The three subsystems worked together effectively, with the controller making accurate judgments during loading, unloading, and the transitions between them. This is a notable achievement for a semi-active device, since misclassification of the motion state would corrupt the force profile and undermine the entire design rationale.

Beyond confirming the basic behavior, the tests revealed how the damper’s energy dissipation capacity depends on its operating conditions and internal parameters. The researchers found that energy dissipation is positively correlated with the loading displacement amplitude, which is intuitive, since larger strokes sweep out larger hysteresis loops, and positively correlated with the spring stiffness of the device’s internal components. It is negatively correlated with the loading frequency, meaning the damper absorbs somewhat less energy per cycle when driven faster. When the team ranked the influencing factors by their impact, the ordering was clear: loading amplitude mattered most, spring stiffness came second, and loading frequency had the least effect. For designers, this hierarchy is valuable because it indicates that the device’s performance is governed primarily by the seismic demand itself, with the tunable mechanical properties offering a secondary but meaningful degree of control.

The equivalent negative stiffness damping characteristics of the ENSFD serve two complementary functions. First, by effectively extending the structural period, the device shifts the building’s response away from the frequency content of the earthquake, reducing the accelerations transmitted into the superstructure. This is the same principle that underlies base isolation, but achieved here through a supplementary damping device rather than by modifying the isolation system itself. Second, the concentrated unloading-phase dissipation controls seismic responses without the penalty of increased maximum damping force that accompanies conventional dampers. The authors report that the equivalent parameters derived from their analytical treatment are suitable for engineering analysis, which means the device can be incorporated into standard response-spectrum and time-history workflows without requiring bespoke nonlinear modeling of every installation.

The study situates itself within a growing body of research on negative stiffness and semi-active seismic protection. Negative stiffness devices, including the well-known device developed by Sarlis, Pasala, Constantinou, Reinhorn, Nagarajaiah, and Taylor, and various quasi-zero-stiffness and magnetic spring concepts, aim to reduce apparent structural stiffness and amplify damping effectiveness. Semi-active resettable devices, explored by Chase, Mulligan, and colleagues and by Bobrow, Jabbari, and Thai, demonstrated that reshaping hysteretic behavior with controllable hardware can outperform passive damping. Earlier work by members of the present team, including half-cycle friction damping devices and double triangular damping devices with equivalent negative stiffness for base-isolated buildings, laid the groundwork for the current ENSFD. The new study advances this lineage by closing the loop: a full analytical model, an equivalent linearization scheme validated by time-history analysis, a working electronic control system, and experimental verification in a single coherent program.

The practical significance of the frequency and amplitude findings deserves emphasis. Real earthquakes are broadband, nonstationary signals, and a damper whose output varies strongly with excitation frequency could behave unpredictably across different ground motions. The ENSFD’s weakest sensitivity being to frequency suggests a degree of robustness that bodes well for real-world deployment, while the strong dependence on displacement amplitude means the device naturally scales its energy absorption with the severity of the response, dissipating more when the structure moves more and less when the motion is mild. That self-scaling behavior is characteristic of well-behaved seismic protective systems and simplifies the task of designing the device for a target performance objective.

Challenges remain before such devices appear in buildings. The laboratory tests were conducted under controlled low-cycle reciprocating loading rather than full-scale shaking table conditions, and long-term reliability of the variable friction mechanism, along with the durability of the sensing and control electronics during years of dormancy punctuated by rare earthquakes, will need further study. Nevertheless, the demonstration that an STM32-based control system can reliably identify loading states in real time, and that the resulting hysteresis behaves as theory predicts, moves the equivalent negative stiffness friction damper from concept toward practice. For a field searching for ways to add damping without adding force, a device that works only half the cycle, and works best when the shaking is worst, is an idea whose time may have come.

Subject of Research: Semi-active negative stiffness friction dampers for seismic response control of structures

Article Title: Experimental study and equivalent parameter analysis of a semi-active negative stiffness friction damper

Article References: Li, W., Peng, L., & Lai, Z. (2026). Experimental study and equivalent parameter analysis of a semi-active negative stiffness friction damper. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02647-x

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02647-x

Keywords: negative stiffness, friction damper, semi-active control, seismic protection, equivalent linearization, hysteresis, STM32 microcontroller, energy dissipation, base isolation, earthquake engineering, structural dynamics, low-cycle loading

Base isolation is one of the most successful strategies in modern earthquake engineering. The basic idea is to decouple a structure from the ground beneath it by inserting flexible bearings between the foundation and the superstructure, so that violent ground motions are filtered out before they reach the floors above. Conventional isolation systems, however, are largely passive: once installed, their stiffness and damping properties are fixed, and they cannot adapt to the particular frequency content or intensity of an incoming earthquake. Active and semi-active control systems can adapt, but they typically require external power supplies, which raises a vexing practical question during exactly the events they are designed for, when electrical grids are among the first pieces of infrastructure to fail.

The Turkish team’s hybrid concept addresses that vulnerability by closing the energy loop inside the isolation system itself. PVDF is a flexible piezoelectric polymer that generates an electrical charge when it is mechanically deformed. In the proposed system, PVDF layers are integrated into the base isolation mechanism, where the relative displacements and strains produced during shaking deform the polymer and generate voltage. Numerical modeling carried out in the finite element software ANSYS confirmed that these layers can convert seismic-induced strains into usable electrical energy, which then powers electromagnets to produce control forces that oppose the motion of the structure. In effect, the earthquake charges the system, and the system spends that charge damping the earthquake.

The self-powered character of the design is what makes the results striking. In the experimental campaign, the PVDF layers generated voltage outputs of up to 102 volts, demonstrating the feasibility of operating the electromagnetic control stage without any external power source. The authors are careful to note the caveats: the measured voltage outputs may vary depending on the structural irregularities of the building and on the frequency content of each earthquake, and the repeatability of the results is subject to the experimental and scaling limitations inherent in laboratory testing. Even so, a hundred-volt-class output harvested from a scaled shaking table test represents a meaningful proof of concept for energy-autonomous structural control.

The experimental program was unusually thorough for a study of this kind. The researchers built eight steel frame models at 1/10 scale and tested them on a shaking table under eleven representative earthquake records from Türkiye, a country that sits on some of the most active fault systems in the world and suffered devastating earthquakes in Kahramanmaraş in 2023. Crucially, the eight models were not all identical. The team included both regular configurations and irregular ones, specifically frames with torsional irregularity, soft-story conditions, and vertical discontinuity. These irregularity classes are among the most dangerous features in real buildings, because they concentrate seismic demand in unpredictable locations and have been implicated in many collapse events.

Testing across such a diverse set of structural configurations matters because base isolation does not perform uniformly on all buildings. Irregular structures respond to ground motion in complicated ways: torsionally irregular frames twist as well as translate, soft stories concentrate deformation in a single vulnerable level, and vertical discontinuities create abrupt changes in stiffness and mass that can amplify local response. A control system that works well on a regular, symmetric laboratory model may behave very differently on a real building with a tall open ground floor or a setback upper story. By subjecting all eight configurations to the same battery of eleven ground motions, the study provides a more realistic picture of how the hybrid system performs across the messy variety of the actual building stock.

The underlying physics of the system draws on two well-established research streams that have rarely been merged at the base of a building. Piezoelectric energy harvesting has been explored for decades in contexts ranging from instrumented knee implants to pavement-embedded transducers that generate power from footsteps, and researchers have long recognized that structural vibrations could power wireless sensors and small devices. Electromagnetic damping and electromagnetic isolation, meanwhile, have been studied as means of providing tunable, controllable restoring and dissipative forces without mechanical contact. The innovation in the new work lies in coupling the two: the piezoelectric layer acts as both a sensor of structural strain and a generator of the electricity that the electromagnetic actuator consumes, eliminating the need for a separate power and sensing infrastructure.

The study builds on a doctoral thesis completed at Atatürk University and was supported by the university’s Scientific Research Projects Coordination Unit under project numbers FKP-2023-12377 and FBG-2022-11620. The research sits within a broader international effort, documented in recent review literature, to develop smart and hybrid isolation systems, some of which envision integration with earthquake early warning networks. What distinguishes the PVDF-based approach is its sustainability argument: rather than dissipating seismic energy purely as heat in viscous or frictional elements, the system captures a portion of that energy, converts it to electricity, and redeploys it as active control force, improving structural resilience while providing what the authors describe as sustainable seismic energy dissipation.

The implications for both existing and newly designed buildings could be significant. Retrofitting vulnerable structures is one of the great unsolved challenges of earthquake engineering, particularly in seismically active, economically constrained regions, and any technology that reduces dependence on external power and complex control hardware lowers the barrier to adoption. Because the hybrid system operates at the base of the structure, it could in principle be incorporated into isolation layers added beneath existing buildings or designed into new foundations from the start. The authors report that the overall findings confirm the hybrid system improves structural resilience while offering a promising pathway for both categories of construction.

Considerable engineering work remains before such a system could be specified for a real building. The experiments were conducted on 1/10-scale models, and scaling effects mean that voltage outputs, force magnitudes, and control performance at full scale cannot be read directly from these results. The sensitivity of the harvested voltage to earthquake frequency content means that the system’s self-sufficiency would need to be verified across the full spectrum of possible ground motions, including near-fault pulses with long-period velocity components that pose particular challenges for isolated structures. Power conditioning, energy storage, and the reliability of electromagnetic actuators under sustained seismic loading would all require dedicated development. Yet the core demonstration, that a building’s foundation can harvest the energy of an earthquake and immediately spend it protecting the structure above, stands as a compelling glimpse of where structural control may be heading: toward buildings that do not merely endure earthquakes, but feed on them.

Subject of Research: A PVDF-based hybrid base isolation system that harvests seismic energy to power electromagnetic vibration control

Article Title: Seismic energy dissipation and vibration mitigation through a PVDF-based hybrid base isolation system with electromagnetic control

Article References: Çelebi, O., & Aydın, A. C. (2026). Seismic energy dissipation and vibration mitigation through a PVDF-based hybrid base isolation system with electromagnetic control. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02671-x

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02671-x

Keywords: base isolation, PVDF, piezoelectric energy harvesting, electromagnetic damping, seismic energy dissipation, shaking table tests, structural control, structural irregularities, earthquake engineering, vibration mitigation, self-powered systems, steel frame models

The appeal of friction dampers lies in their physics. Conventional steel frames absorb earthquake energy by yielding, which means the structure itself accumulates damage with every loading cycle, a phenomenon engineers call cyclic deterioration or pinching. An RFD behaves differently. Governed by Coulomb’s law of dry friction, it produces stable, rectangular hysteretic loops whose enclosed area, and therefore the energy dissipated per cycle, remains constant no matter how many times the ground shakes. The device consists of a central rotating plate sandwiched between side plates with friction discs, all clamped by a pre-tensioned bolt. When inter-story drift forces the plates to rotate against one another, friction resists the motion with a constant moment, protecting the primary frame. Experimental work dating back to Mualla and Belev in 2002 showed reductions of up to 89 percent in lateral displacement and 41 percent in base shear under optimized configurations.

Simulating such devices accurately, however, is notoriously difficult. The abrupt transition between sticking and slipping is mathematically discontinuous, and the two common workarounds both introduce error. Regularization smooths the discontinuity with an artificial curve, adding fake compliance exactly where the response should be sharp, while equivalent-damping approaches replace the device with a viscous dashpot that cannot reproduce the rate-independent, rectangular hysteretic loop. Both require case-specific calibration, which would be hopeless at the scale of hundreds of thousands of simulations. The authors instead built on the Non-Smooth Dynamics method, reformulating Coulomb friction as a Linear Complementarity Problem solved at each timestep with a Projected Successive Over-Relaxation algorithm. This formulation reproduces the exact friction law with no tuning parameters, and because each step typically converges in fewer than five iterations, sweeping thousands of structural configurations becomes computationally feasible. Crucially, it also means the training labels fed to the machine learning models contain pure physics rather than artifacts of a smoothing choice.

The dataset behind the study is staggering in scope. The team generated 1,000 shear-type frames ranging from 3 to 20 stories, with randomized Latin Hypercube sampling covering fundamental periods from 0.3 to 3.0 seconds, floor masses from 150 to 350 tonnes, and four different mass and stiffness distributions. Each frame was fitted with roughly 50 different damper configurations, varying the number of devices, their vertical placement, and friction moments between 25 and 500 kilonewton-meters. Eight real seismic records, including El Centro 1940, Kobe 1995, and Northridge 1994, were applied at their original historical intensities. In total, 433,984 nonlinear simulations were run, each quantified with a composite index called CR2 that combines peak roof displacement, peak roof acceleration, and peak base shear, each normalized by the uncontrolled response, so that lower values mean better performance.

Six regression models were then trained to predict seismic performance from the design variables, and the comparison itself delivered a scientific finding. Linear models achieved a cross-validated coefficient of determination of only 0.64 for the typical scenario, while XGBoost reached 0.907 and Random Forest 0.904, cutting prediction error nearly in half. The gap proves that the relationship between damper design and seismic response is fundamentally nonlinear, with threshold effects and interactions that simple equations cannot capture. The nonlinearity deepens under severe ground motion: for the conservative 90th-percentile scenario, linear models managed only 0.31, while Random Forest reached 0.786. To prevent data leakage, the team used structure-grouped cross-validation, ensuring models were always evaluated on buildings they had never seen, and verified accuracy on 200 entirely held-out structures with calibrated 90 percent prediction intervals.

The most striking practical insight concerns coverage density, the fraction of floors equipped with dampers. Feature importance analysis identified this single variable as the dominant predictor of performance, and the response surface revealed clear diminishing returns. By formulating the threshold as an explicit optimization over the trained surrogate, the researchers found that the median coverage at which 80 percent of the maximum achievable gain is reached sits at about 0.83, or 0.86 when read from the empirical envelope across the whole population. Resolved by building height, the picture inverts common intuition: low-rise frames of 3 to 5 stories require full coverage, mid-rise frames of 6 to 12 stories need at least 86 percent, and high-rise frames of 13 to 20 stories need only about 73 percent, with none requiring full instrumentation. The mechanical explanation is elegant, since tall buildings concentrate drift demand in a subset of stories that can be covered selectively, whereas the flat drift profile of a low-rise frame leaves no inexpensive stories to omit.

The analysis also uncovered a counterintuitive failure mode. A small number of oversized dampers, high friction moments paired with low coverage, can leave a frame worse off than no dampers at all. In this lock-up regime, the devices never reach their slip threshold under the imposed demand, act as rigid links, and locally stiffen the structure, amplifying floor accelerations and base shear. These configurations account for just 1.4 percent of the dataset but nearly 10 percent of the total prediction error, and more than 80 percent of them occur in frames of five stories or fewer. The practical lesson is a lower bound on coverage for any given device capacity: coverage cannot be traded freely against friction moment. The study identifies an optimal friction moment window of roughly 300 to 460 kilonewton-meters for mid- and high-rise frames, with a lower optimum near 250 kilonewton-meters for short buildings.

Robustness across seismic scenarios proved to be another strength of the framework. When the researchers compared the optimal damper count recommended under typical conditions with that recommended under the conservative 90th-percentile scenario, 67.9 percent of the 1,000 structures received identical recommendations, and in the remaining cases the median discrepancy was just one damper. The conservative scenario never suggested fewer devices, only occasionally one or two more, meaning designers can optimize for typical demand and then verify against severe conditions with minimal adjustment. This multi-record approach, summarizing performance with 10th-percentile, median, and 90th-percentile statistics consistent with the FEMA P-58 framework, addresses a long-standing gap in earlier studies that optimized dampers against a single ground motion.

The authors are candid about the limits of their results. The structural models are elastic shear frames, and a substantial fraction of the simulated demand levels actually lie at or beyond the yield limit of real steel frames, so the coverage recommendation should be read as an upper bound for quasi-elastic serviceability design rather than a validated rule for ductile response. The design space is a factorial sweep rather than a representative building stock, and the eight-record suite carries sampling uncertainty of about 0.10 in CR2 units, which exceeds the surrogate’s own prediction error of 0.070 by roughly half. That last point may be the study’s most consequential methodological message: enlarging the ground motion database, not refining the machine learning model, is what would most sharpen future design guidance. Future extensions include soil-structure interaction, three-dimensional frames with torsional coupling, nonlinear material behavior, floor-specific friction moments, and cost-based optimization.

Even with those caveats, the work marks a genuine shift in how earthquake protection devices might be designed. Instead of running expensive simulations case by case, engineers can now query a trained surrogate that exposes the entire response surface across damper placement, capacity, and building geometry in milliseconds, complete with calibrated uncertainty bounds. The finding that taller buildings need proportionally fewer instrumented floors, that a coverage threshold near 86 percent captures most of the achievable benefit for mid-rise construction, and that a handful of oversized devices can actively harm a structure, together give practitioners an evidence-based starting point that previously required months of computation. As machine learning surrogates spread through earthquake engineering, this study offers both a template, exact physics in the labels, grouped validation, honest uncertainty, and a reminder that the biggest gains sometimes come from questioning design intuition.

Subject of Research: Machine learning surrogate modeling of nonlinear seismic response of steel moment frames with rotational friction dampers

Article Title: A machine learning-based surrogate model for analyzing the nonlinear seismic response of steel moment frames with rotational friction dampers

Article References: Ponce-López, E., & Yáñez-Godoy, H. (2026). A machine learning-based surrogate model for analyzing the nonlinear seismic response of steel moment frames with rotational friction dampers. Machine Learning with Applications, 26, Article 101030. https://doi.org/10.1016/j.mlwa.2026.101030

Image Credits: AI Generated

DOI: 10.1016/j.mlwa.2026.101030

Keywords: rotational friction dampers, machine learning, seismic response, steel moment frames, XGBoost, non-smooth dynamics, earthquake engineering, surrogate model, coverage density, structural optimization, energy dissipation, time-history analysis