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The Observed Wander of the Natural Frequencies in a Structure

¹ Puerto Rico Strong Motion Program
Department of Civil Engineering and Surveying
University of Puerto Rico at Mayaguez
Mayaguez, Puerto Rico 00681
jclinton{at}uprm.edu
jclinton{at}ecf.caltech.edu
 (J.F.C.)

² Department of Civil Engineering
Division of Engineering and Applied Science
California Institute of Technology
Pasadena, California 91125
case{at}caltech.edu
heaton_t{at}caltech.edu
 (S.C.B, T.H.H.)

³ Lockheed Martin Aeronautics Company
Palmdale, California 93599
javier{at}gps.caltech.edu
 (J.F.)

The Southern California Seismic Network (SCSN) has recently installed seismic stations in two buildings on the Caltech campus (Millikan Library and the Broad Center). Continuous real-time accelerometer data from these structures are now freely available to the community. This dataset provides a new opportunity to observe, and better understand, the variances in the primary dynamic property of a building system, its natural frequencies. Historical data (triggered strong-motion records, ambient and forced vibration tests) from the well-studied Millikan Library show dramatic decreases in natural frequencies, attributed mainly to moderately large local earthquakes. The current forced vibration east–west fundamental frequency is 22% lower than that originally measured in 1968. Analysis of the new continuous data stream allows the examination of other previously unrecognized sources of measurable change in the fundamental frequencies, such as weather (wind, rain, and temperature), as well as nonlinear building vibrations from small local and moderate regional earthquakes. Understanding these nonlinear shifts is one of the long-term goals of real-time building instrumentation and is critical if these systems are to be used as a postearthquake damage assessment tool.

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				J. F. Clinton, G. Cua, V. Huerfano, C. G. von Hillebrandt-Andrade, and J. M. Cruzado The Current State of Seismic Monitoring in Puerto Rico Seismological Research Letters, September 1, 2006; 77(5): 532 - 543. [Full Text] [PDF]