HR: 0830h
AN: S51E-0102    [PDF]
TI: Is There a Linear Building Transfer Function for Small Excitation?
AU: * Clinton, J F
EM: jclinton@caltech.edu
AF: California Institute of Technology, 104-44 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: Heaton, T H
EM: heatont@institute.caltech.edu
AF: California Institute of Technology, 104-44 1200 E. California Blvd, Pasadena, CA 91125 United States
AB: In the absence of actual building accelerometer data, the linear response of a structure to strong ground motion is estimated by the convolution of the dynamic response of the structure with an input ground motion. The input motion is usually provided by a local `reference' station record. In this study, we look at whether actual recorded ground motion at two instrumented buildings with well studied dynamic properties can be satisfactorily modeled using a local ground station. All stations record continuous 24-bit data streams on the CISN network, so analysis of a variety of weak earthquake motions, as well as ambient noise, is possible. Our buildings are the 9-story reinforced concrete Millikan Library (CISN Station MIK) and the 3-story braced steel frame Broad Center (CBC), both on the Caltech Campus. Motions recorded on their upper floors are compared with motions from ground stations located in the basement of a lightweight wood-frame house (GSA), and in a subsurface vault (CRP). All stations are within $200m$ of each other. Recent work using the new continuous datastream indicates that the natural frequencies of these structures can vary by up to $5%$ during normal ambient conditions, due to such factors as changing building usage, diurnal temperature variation, and wind/rainfall events. These shifts can be sudden, and models of building motions are sensitive to these previously un-documented changes. Further, during stronger motions, such as forced vibration testing, and minor earthquake shaking, natural frequencies are shown to drop by up to $10%$ (2003 M5.4 Big Bear Earthquake, $\Delta = 119km$), with near-instantaneous recovery once the excitation is over. Moderate earthquakes can temporarily reduce frequencies by up to $30%$ with no apparent structural damage (1971 M6.6 San Fernando Earthquake, $\Delta = 31km$). Post-event permanent reductions of about $10%$ have been observed. The ability to monitor these evolving dynamic characteristics makes a re-evaluation of the application of linear transfer functions timely.
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2003 Fall Meeting