Novel, passive energy management devices that researchers at the University of Illinois developed may effectively mitigate structural damage resulting from large-scale forces and ground motions such as those caused by explosions and earthquakes.
Figure 1: The 9-story structure on the shake table with NES devices installed
The Illinois scientists partnered with the University of Akron in a two-and-a-half-year, $2.1 million project that the Defense Advanced Research Project Agency (DARPA) sponsored.
These vibration mitigation systems employ passive Targeted Energy Transfer (TET) strategies to rapidly direct input energy nearly irreversibly to one or more of the Nonlinear Energy Sink (NES) devices which act to harmlessly absorb and dissipate the energy.
The principal investigators include Profs. Lawrence Bergman, Aerospace Engineering; Alexander Vakakis, Mechanical Science and Engineering; Bill Spencer and Larry Fahnestock, Civil and Environmental Engineering; and Dane Quinn, Mechanical Engineering, University of Akron. Also contributing to the project are Research Prof. Michael McFarland and doctoral students Sean Hubbard, Aerospace Engineering, and Nick Wierschem and Jie Luo, Civil and Environmental Engineering.
According to Bergman, the project began in the fall of 2010, in response to a new DARPA program called “Structural Logic.” In the request for proposals, the agency asked for new materials and/or subsystems that, when placed within a structure, would simultaneously add significant levels of stiffness and energy dissipation over a broad range of frequencies and input amplitudes.
Responding with its TET technology in which strong nonlinearity is intentionally introduced into the structure at the design stage, the Illinois-Akron team was one of six groups invited to participate in the program, and was the only one university-led and composed entirely of academics. The other groups were led by companies – HRL, Inc. (formerly, Hughes Research Laboratories), Lockheed Martin, NextGen Aeronautics, Raytheon, and Teledyne – all of whom teamed with faculty from other universities.
Figure 2: Comparision of 7th floor displacements for Northridge earthquake
The TET concept dates back to two fundamental papers published in 2001 by another collaborator, Prof. Oleg Gendelman, now at Technion, and Vakakis. Working from those early results and with further research and application experience the group had accumulated over a period of nearly 14 years, the Illinois-Akron team embarked upon an intensive program of dynamic analysis, simulation and experiments at various scales. This work culminated in the design and construction of a 9-story, 10-ton steel frame structure incorporating a total of six NES devices, three on each of the eighth and ninth floors, for proof of concept.
The structure recently underwent an extensive series of dynamic tests covering a wide range of broadband ground motions, including impulse-like, swept-sine, and several historic earthquake records. These tests took place on the large-scale shake table housed at the Champaign, Illinois-based US Army Corps of Engineers Construction Engineering Research Laboratory (CERL), with the assistance of CERL engineers Jim Wilcoski and Jonathan Trovillion.
Figure 3: Third floor displacement for blast tests: (a) 30 psi-msec locked and unlocked tests; (b) 90 psi-ms unlocked test
Tests were run comparing the structure with NES devices in both locked and unlocked configurations. In the former, the devices acted merely as integral masses, while in the latter, they were free to perform dynamically as designed. The comparison was dramatic, with the structure undergoing large, potentially damaging motions when the NESs were locked and barely moving after the first few cycles of response when the NESs were unlocked and free to perform.
In early December 2012, the entire structure was partially disassembled and transported to the U.S. Army Corps of Engineers Geotechnical and Structural Laboratory (GSL) Big Black Test Site, near Vicksburg, Mississippi, where it was reassembled and blast-tested with the assistance of GSL engineer Matt Holmer and his team. According to Bergman, the mitigation system performed precisely as predicted. The devices quickly absorbed the blast energy and dispersed it to high frequencies where the inherent damping in the system works most effectively, barely allowing the structure itself to move after the first few cycles of response, even under the largest blast loads.
Figure 4: The nine-story structure undergoing a 90 psi-ms blast test at the Army Corps of Engineers Big Black test site near Vicksburg, MI.
According to Vakakis, the use of intentional strong nonlinearity in structural design remains a contrarian view. Most designers seek to maintain linearity, viewing nonlinearities, particularly strong nonlinearities, as detrimental to their design objectives. However, it’s now been demonstrated that the Illinois-Akron system results in enhanced performance not attainable using conventional passive linear designs, and with no increase in weight.
Having proven the energy management and vibration mitigation concept, the challenge remains to broadly apply the TET technology to produce less expensive, lighter weight structures with equivalent or better performance than traditionally designed structures under extreme loading conditions. The team will shortly shift its focus to a new, Phase II project in order to refine and apply these concepts to other systems of interest to DARPA.
Watch a video about the invention here.