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 research team focused on reinforced concrete hospital buildings resting on friction pendulum isolation systems, devices that sit between the building and its foundation and allow the superstructure to slide gently during strong shaking rather than deform violently. Two technologies were compared: the single friction pendulum (SFP) system, a proven design with one concave sliding surface, and the triple friction pendulum (TFP) system, a more sophisticated bearing whose four spherical surfaces engage sequentially as shaking intensifies. The TFP’s adaptive behavior means it is stiff under small motions, softer under moderate earthquakes, and stiffens again under extreme events, effectively retuning itself to each stage of the ground motion.

What makes the study distinctive is its life-cycle, probabilistic framing. Rather than testing a handful of design-level earthquakes, the authors computed annualized performance rates derived from site-specific seismic hazard curves, integrating the effects of every earthquake expected over the building’s lifetime, from frequent moderate shocks to rare catastrophic ones. Collapse safety was quantified through both the probability of collapse and the mean annual frequency of collapse. Structural performance was tracked through interstory drift, the relative displacement between floors that governs damage to both the frame and its partitions, cladding and mechanical systems. Functional continuity was assessed through peak floor accelerations, used as an acceleration-based proxy for the survival of sensitive equipment such as imaging machines, laboratory analyzers and intensive-care devices.

The modeling effort was substantial. Nonlinear response-history analyses were carried out in ETABS, with detailed hysteretic models capturing the behavior of the isolation bearings and the reinforced concrete superstructure. Concrete material properties were calibrated to Mexican production, and the hysteretic models of the frames drew on well-established formulations for confined concrete and degrading reinforced concrete response. The isolation systems themselves were designed under two regulatory approaches: a performance-based procedure aligned with the seismic isolator standard associated with ASCE 7, and the Mexican seismic provisions of Mexico City’s complementary technical norms, reflecting the study’s setting in one of the world’s most seismically exposed capitals.

The choice of Mexico City as the testbed is significant. The metropolis sits on a deep soft-lakebed basin that dramatically amplifies long-period ground motions, exactly the kind of shaking that resonates with flexible isolated buildings. The 1985 and 2017 earthquakes exposed the vulnerability of medical infrastructure, with hospitals suffering structural and nonstructural damage and, in some cases, losing the ability to treat patients precisely when demand for care peaked. The 2020 Mw 7.5 Oaxaca earthquake, arriving during the COVID-19 pandemic, further underscored how fragile hospital continuity can be. The new framework speaks directly to that experience by treating functionality, not just survival, as a design objective.

The results deliver a clear verdict. Across the configurations investigated, the TFP-based hospitals consistently achieved lower collapse probabilities, reduced interstory drift demands, and significantly lower peak floor accelerations than their SFP counterparts. The triple pendulum’s multi-stage sliding mechanism limited bearing displacements and kept the superstructure response within tighter bounds across the full spectrum of hazard, translating into enhanced structural safety and stronger functional continuity over the life cycle. The SFP configuration, by contrast, showed lower reliability, a higher likelihood of damage, and less effective protection of acceleration-sensitive equipment, meaning that even a hospital whose frame survives an earthquake could lose imaging suites, operating rooms or laboratory capacity to shaking-induced equipment failure.

The acceleration findings deserve particular attention from hospital planners. In conventional fixed-base buildings, floor accelerations can reach levels many times the ground acceleration at upper stories, tearing equipment from its anchors and rupturing piping and ductwork. Isolation dramatically cuts these demands, but the study shows the degree of protection depends strongly on the bearing technology. Because TFP bearings maintain lower transmitted accelerations across a wider range of earthquake intensities, they offer a more robust guarantee that medical equipment remains operational without costly post-event inspection and repair. In resilience terms, that difference can determine whether an emergency department admits patients within hours of a disaster or remains shuttered for weeks.

The framework’s annualized approach also changes how engineers can communicate risk. Instead of reporting performance against a single design earthquake with a fixed return period, the method expresses collapse risk, drift damage and equipment vulnerability as mean annual frequencies, quantities that can be summed over decades of service and compared directly across design alternatives. This gives hospital owners, insurers and regulators a common currency for weighing the higher upfront cost of advanced isolation against the expected lifetime losses avoided, including downtime, repair and the immeasurable social cost of a hospital that fails its community. The authors note that the approach unifies collapse safety, structural performance and functional continuity within a single probabilistic assessment, something conventional code-based design does not attempt.

Broader context supports the urgency of this work. Post-earthquake investigations after the 2010 Chile earthquake and the 2016 Kumamoto earthquake in Japan documented how nonstructural damage and equipment failures, far more than structural collapse, degraded hospital function. Researchers have repeatedly found that hospitals are among the most functionally fragile critical facilities, dependent on a chain of power, water, medical gases and functioning devices that shaking can sever at multiple links. By explicitly modeling acceleration-sensitive contents as part of the performance metric, the Mexican study moves hospital design closer to the resilience objectives that agencies such as PEER have long advocated, in which buildings are expected to remain operational, not merely standing, after design-level events.

The study’s implications extend beyond Mexico. As more countries adopt seismic isolation for critical facilities, and as new regulations for isolation in Mexico City come into force, the choice between simpler single-pendulum systems and adaptive multi-pendulum systems will recur in design offices worldwide. The evidence assembled here suggests that for buildings whose mission demands uninterrupted operation, the extra complexity of the triple friction pendulum buys measurable, quantifiable resilience: fewer collapses, less damage, and a far better chance that the lights, monitors and ventilators stay on when the city outside is in crisis. The datasets, synthetic ground-motion records and numerical routines behind the analysis are available from the corresponding authors on reasonable request, offering a foundation for engineers seeking to apply the same life-cycle logic to their own critical facilities.

Subject of Research: Life-cycle seismic performance and functional continuity of base-isolated hospital buildings using friction pendulum systems

Article Title: Life-cycle performance assessment of resilient base-isolated hospitals: collapse safety, structural performance, and functional continuity

Article References: Equihua, E., Flores, E., & Alamilla, J. L. (2026). Life-cycle performance assessment of resilient base-isolated hospitals: collapse safety, structural performance, and functional continuity. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02694-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02694-4

Keywords: base isolation, friction pendulum systems, hospitals, seismic reliability, functional continuity, collapse safety, interstory drift, floor acceleration, life-cycle performance, triple friction pendulum, Mexico City, earthquake engineering