Structural Damage Detection Through Forced Vibration Testing

Abstract eng:
Existing methods of evaluating buildings after a seismic event often rely on demolition of architectural elements such as cladding, partitions, and ceilings in order to inspect structural members. Evaluations are done by visual inspection with no quantitative data. Current research focuses on the application of Forced Vibration Testing (FVT) for assessment of buildings after a seismic event. This non-destructive method involves temporary instrumentation to record building behavior. This paper describes the use of FVT to detect simulated earthquake damage in an experimental one-story steel structure spanning a forty-eight foot ravine. The structure serves as a dynamic earthquake field laboratory and is outfitted with detachable braces that can be used to simulate damage. It is assumed that an intact structure is represented when all braces are engaged. Damage is simulated by removing one or more braces. FVT, based on a 30 lb linear mass shaker and sensitive accelerometers, has successfully been used to register building dynamic properties such as natural frequency, mode shapes and damping in a wide range of structures ranging from a twostory laboratory structure to an 180,000 square-foot 5-story reinforced concrete library. The paper demonstrates how FVT can be applied to positively identify structural damage in the experimental structure by comparison of experimental mode shapes with theoretical mode shapes from computer models. The modal assurance criterion (MAC) allows for a quantitative comparison of two mode shapes. Experimental mode shapes for a building configuration with a particular brace configuration can be compared to the theoretical mode shapes from a suite of computer models with a variety of different brace configurations. It is shown that the MAC criterion provides a confidence metric that can positively identify the correct theoretical model and thus reveal the location of structural damage.

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