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Virtual Dynamic Shaker Offers Conceptually Simpler, but Equally Reliable, Structural Damping Estimation of Structures

Nina Welding • DATE: May 9, 2018

Categories:  Press Release
Virtual Dynamic Shaker Offers Conceptually Simpler, but Equally Reliable, Structural Damping Estimation of Structures

This images shows how a building vibrates. Photo credit: Enrica Bernardini

How will a tall building or long span bridge respond in the wind? It’s a question that remains unanswered due to the limited full-scale studies of structures over their life cycle. Statistics from the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information confirm that since 1980, the United States has weathered 219 weather and climate-related disasters where the overall damage exceeded $1billion per event. This “cost” encompasses harm to the structural integrity and exterior façade of buildings and bridges, out of service downtime, as well as the impact on humans using the structures.

Numerous building codes and standards specify the minimum requirements for design, construction and maintenance of structures to ensure the health, safety and well-being of occupants. As strong as a building or bridge is, it can be bent or twisted by wind events and seismic loads, making structural damping [like shock absorbers in cars] one of the most important parameters in determining how a building will respond to such loads.

Structural damping, however, is difficult to estimate due to its complex nature and sensitivity to the techniques used to quantify damping. Over the years many experimental campaigns have been carried out to measure damping in multistory buildings and bridges in order to obtain information about such structures. These include work conducted by the NatHaz Modeling and DYNAMO laboratories at Notre Dame involving buildings and bridges in Chicago, Dubai, Seoul and in China.

In the case of wind, one of the most widely used methods of estimating damping is an output-only system identification (SI). A recent study published in the Journal of Engineering Mechanics presents a novel concept for rapid assessments of damping that, while steeped in the basic equations of dynamics, applies a new output-only SI scheme to more accurately and more easily estimate the properties of structures under wind or seismic loads.

The concept, proposed by the paper’s co-authors includes the introduction of a virtual dynamic shaker (VDS), which is attached [virtually] to the top of a structure such as a tall building, long span bridge or deep ocean platform. “Although analogous to the heavy physical shakers that were employed in buildings in the 1950s and ’60s to excite them and estimate their dynamic features, the VDS eliminates the need of physical shakers, attendant cost and the logistics required to haul and operate a gigantic shaker near a building top to set buildings in vibration,” says Ahsan Kareem, the Robert M. Moran Professor of Civil & Environmental Engineering & Earth Sciences and director of the NatHaz Lab. “The VDS estimates a structure’s natural frequencies and also identifies the damping ratios under different external loads, including estimations in the presence of noise contamination in the building response measurements to the loads.”

According to the study, VDS-based schemes can be used as a competitive alternative to existing SI schemes using tuned mass dampers with the added advantage of simplicity and transparency. A user with a basic knowledge of vibration theory could follow the VDS-based information much easier than other SI schemes requiring a more advanced background in signal processing and dynamics.

The study’s authors are Jae-Seung Hwang, professor of architecture at Chonnam National University in South Korea; Dae-Kun Kwon, research assistant professor in the CEEES department at Notre Dame; and Kareem. The work, which was performed at Notre Dame during Professor Hwang’s sabbatical leave from Chonnam National, complements previous studies from the NatHaz Lab involving measurements and modeling of damping and the impact on structural response.

The work was supported by the Basic Science Research Program through the National Research Foundation of Korea and the Ministry of Education, as well as by the U.S. National Science Foundation and a grant from the Technology Advancement Research Program of the Ministry of Land, Infrastructure and Transport Affairs of the Korean government

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