Composite Multistable Nonlinear Energy Sink for Robust Seismic Vibration Control

Concerns over seismic safety remain central to structural engineering, particularly when it comes to tall, multi-story buildings that cannot escape the unpredictable nature of strong ground motion. Devices such as tuned mass dampers (TMDs) have been the workhorse solution for decades, but their usefulness is tied almost entirely to the assumption that a structure’s frequency will not drift. Anyone familiar with real buildings knows this assumption rarely holds: aging materials, accumulated damage, or even relatively minor renovations can push frequencies away from the design point. Once that happens, a TMD is no longer “tuned” at all, and its value quickly diminishes. This weakness has driven interest in nonlinear energy sinks (NESs). Unlike dampers designed for narrow frequency bands, NESs exploit nonlinear restoring forces to draw energy into higher modes. In doing so, they promise a more flexible and durable means of mitigating seismic vibrations. Recent explorations of multistable and hysteretic nonlinear systems have revealed untapped opportunities to overcome these limitations. Multistable devices, especially those based on snap-through mechanisms, exhibit elastic energy dissipation and flag-shaped hysteresis loops that are both recoverable and repeatable. When negative stiffness elements are stacked in series, multiple snap-through events allow sequential reconfigurations that dissipate energy without residual deformation. Yet earlier designs were often restricted to single-directional loading, and the contribution of their sawtooth nonlinearity to NES performance remained poorly understood. Furthermore, the translation of these properties into scalable seismic control devices had not been fully demonstrated. To this account, new research paper published in Engineering Structures and conducted by Dr. Hongxiang Hu, Dr. Haoran Qin, Professor Zhongwen Zhang and Professor Zhao-Dong Xu from the School of Civil Engineering at Southeast University, the researchers developed CNES that integrates preloaded coned disc springs in series–parallel arrangements and by this produced tunable cubic stiffness alongside multistable sawtooth nonlinearity. The device flexibly adapts to vibration amplitudes, ensuring effective dissipation under diverse seismic intensities. The novel CNES demonstrated superior energy dissipation, robustness to frequency and energy variations, and reduced stroke requirements when compared to conventional TMDs and Type-I NESs.

The researchers characterized coned disc springs which are the building blocks of the CNES device and performed compression tests using a 200 kN universal testing machine to examine single springs as well as groups arranged in parallel or series and their results confirmed the predicted snap-through behavior, with force–displacement curves exhibiting sharp sawtooth patterns. Moreover, parallel stacking increased stiffness and load-bearing capacity but reduced deformation range, while series configurations accumulated multiple snap-through events, extending the overall dissipation capacity. Under repeated cycles, all springs returned to their original shape, demonstrating reliable recovery and stable hysteresis. Afterward, the researchers designed the CNES prototype which consisted of symmetrically arranged sets of series–parallel disc springs attached to a rigid guide bar and auxiliary mass. They introduced adjustable preload by threaded nuts which allowed the device to vary its nonlinear stiffness without altering its geometry. The authors’ laboratory measurements showed that preload directly shaped the hysteresis loop: low preload produced flat initial stiffness suitable for small-amplitude vibrations, while higher preload activated more snap-through units, and yield steeper slopes and quasi-cubic stiffness characteristics.

The team also integrated the CNES into a structural model through a detailed simulation of a six-story steel frame, benchmarked against FEMA P-751 standards. The researchers deliberately placed the CNES at the end mass of the frame, where dynamic input during seismic activity would be most pronounced. This strategic positioning ensured that the device could be assessed under conditions close to what a real building would experience. To challenge the system across a wide spectrum of demands, they drew on twenty-two ground motion records with peak accelerations ranging from subtle tremors of 0.065 m/s² to violent shocks approaching 6 m/s². Such breadth gave credibility to their evaluation, avoiding the pitfall of tuning a device only to a narrow class of earthquakes.

Parameter optimization was not left to intuition. Instead, the team applied genetic algorithms to explore the complex design space, searching for combinations of stiffness and damping that would minimize displacement at the top floor. This computational approach allowed them to test thousands of possibilities and converge on solutions unlikely to be found through trial-and-error. Once optimized, the CNES was evaluated alongside two established benchmarks: the conventional TMD and a Type-I nonlinear energy sink.  The authors reported in the unmodified structure, all devices offered measurable reductions in displacement, however, CNES consistently outperformed the others. In certain seismic scenarios, the reduction reached as high as eighty percent—double the improvement typically associated with TMDs. More importantly, this advantage was not fragile. When the structural stiffness was artificially degraded to mimic damage, the TMD lost much of its effectiveness, sometimes reducing displacements by only a modest margin. CNES, by contrast, showed remarkable indifference to these shifts and reduce the peak displacement by 40% compared with the uncontrolled systems. Its performance remained stable, a clear indication that the combination of sawtooth multistability and tunable cubic nonlinearity confers resilience across a broad frequency spectrum. The Type-I NES fared even worse, delivering erratic results and, under strong motions, occasionally amplifying rather than mitigating displacements. Across 132 simulated scenarios, CNES was unique in recording no outright failures, while the other two devices faltered under specific conditions. The researchers also thought of practical considerations and showed that the stroke requirements for the CNES were just 0.025 m, compared to 0.32 m for a TMD. This difference is not trivial: shorter strokes translate directly into smaller devices, easier installation, and reduced costs. For urban environments where space is constrained and retrofitting large dampers is seldom feasible, compactness becomes as valuable as efficiency. What makes CNES compelling is that it achieves both simultaneously.

The broader implications of the new work extend beyond a single device. By demonstrating that preloaded coned disc springs can act as reliable nonlinear elements, the study points to a path for scalable production. The repeatable snap-through behavior and the durability of the hysteresis response suggest that such devices could serve in long-term installations without significant degradation. Just as importantly, the preload can be adjusted during installation or routine maintenance, offering a degree of flexibility that purely passive devices lack. In this sense, CNES occupies an intermediate position between traditional dampers and complex active systems: it adapts without requiring sensors, controllers, or external power.

In conclusion the findings of Hu, Qin, Zhang, and Xu reinforces the idea that both energy robustness and frequency robustness are necessary metrics for evaluating seismic protection. TMDs demonstrate how sensitivity to frequency shifts undermines otherwise solid performance. Type-I NES devices highlight the risks of dependence on input energy levels. CNES succeeds precisely because it avoids these pitfalls, maintaining effectiveness regardless of how structural characteristics or seismic inputs vary. This dual robustness explains why the device remained reliable even under severe and unpredictable loading conditions. Indeed, the innovation presented in the study offers a compact, durable, and scalable solution for protecting structures against earthquakes. Looking forward, the implications are wide-ranging. While the immediate focus is on building structures, the principles of CNES design may be transferable to bridges, offshore platforms, or even aerospace applications where vibration suppression is equally critical.  The next logical step is large-scale experimental validation, ideally in full-scale test facilities or through deployment in pilot retrofit projects. Integration with monitoring technologies may also provide valuable data on long-term performance and guide adaptive maintenance strategies.