ERDC supports tests for devices to prevent structural damage

Published Sept. 10, 2013
The Shaketable testing at ERDC’s Construction Engineering Research Laboratory represented blast effects for the nine-story frame structure.

The Shaketable testing at ERDC’s Construction Engineering Research Laboratory represented blast effects for the nine-story frame structure.

CHAMPAIGN, Ill., and VICKSBURG, Miss. - Last fall, ERDC’s Construction Engineering Research Laboratory (CERL) and Geotechnical and Structures Laboratory (GSL) provided facilities and engineering assistance to the University of Illinois at Urbana-Champaign (UIUC) through access to the ERDC Triaxial Earthquake and Shock Simulator (TESS) and Big Black Test Site.  UIUC was testing a novel approach to mitigating structural damage in buildings from large-scale forces and ground motions such as those caused by earthquakes and explosions.

The approach uses passive targeted energy transfer (TET) strategies to rapidly direct input energy nearly irreversibly to one or more nonlinear energy sink (NES) devices which harmlessly absorb and dissipate the energy. The tests at Champaign and Vicksburg were the culmination of a 2-1/2 year program funded by the Defense Advanced Research Projects Agency (DARPA) and involved a nine-story, 10-ton steel frame structure.

Dr. Larry Bergman, a professor in the Department of Aerospace Engineering, and Dr. Alexander Vakakis, Department of Mechanical Science and Engineering, led the UIUC portion of the project.  “Our team submitted a successful proposal in 2010 in response to a new DARPA program called ‘Structural Logic,’” Bergman said.  “The goal was to develop new materials or systems that would add both significant levels of stiffness and energy dissipation in a structure over a broad range of frequencies and amplitudes.”

With the TET technology, strong nonlinearity is intentionally introduced at the design stage.  This approach is contrary to what most designers strive to achieve, which is to maintain linearity.

“The targeted transfer of energy in a structure is a unique approach to minimizing damage from events such as earthquakes,” said CERL Structural Engineer Jim Wilcoski who, along with Jonathan Trovillion, assisted the UIUC team.  “When the energy from ground motion excites a structure, the building begins to shake at a low frequency due to its fundamental natural frequency, and during the event, the response builds up, which can be very damaging. Transferring the energy to the nonlinear energy sinks excites the building at higher frequencies, allowing the energy to dissipate more quickly.”

In the first segment of testing with the model placed on TESS, it was subjected to an extensive series of dynamic tests covering a wide range of broadband ground motions, including impulse-like, swept-sine, and several historic earthquake profiles.  The structure incorporated a total of six NES devices, three on each of the eighth and ninth floors.  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.

Next, the structure was partly disassembled and transported to the Big Black Test Site located near Vicksburg.  There, it was reassembled and blast-tested with the assistance of GSL engineer Matt Holmer and his team.  “The primary task for us here at GSL was designing and executing the blast experiments,” Holmer explained.  “To do this we had to determine the proper amount and placement of the explosives.  Using software developed by GSL, we were able to predict blast loads and provide UIUC with a charge size and standoff to create the desired loading on the structure.”

Based on UIUC's request, GSL designed blast scenarios to test the NES devices over a range of blast loadings.  The software was very accurate when compared to pressure data collected during the experiments, he added.

According to Bergman, “The mitigation system performed precisely as predicted.” He said the devices quickly absorbed the blast energy and dispersed it to high frequencies where the inherent damping in the system works most effectively.

Following the blast tests, at DARPA’s request, the structure was moved back to CERL. The objective was to do further tests on the structure to verify TESS’s capabilities to produce structural motions similar to those measured in blast testing. 

This story was co-authored by Susan Mumm, editor in the Department of Aerospace Engineering at UIUC.