Center for Embedded Network Sensing

Ideally both spectral and time domain data could be used to compute the total building response and to make predic- tions of damage patterns based on various input scenarios. The combination of frequency change information coupled with that provided by wavefield properties can pinpoint the time and location of damage more accurately, especially for densely instru- mented structures such as the 17-story UCLA Factor building. The 72-sensor embedded seismic array in the Factor building, recording continuous waveforms at 500 Hz, makes it possible to observe subtle changes in dynamic characteristics between pairs of floors and to relate the measurements to system properties such as changes in stiffness due to a column failure. The high dy- namic range of the 24-bit digitizers allows both strong motions and ambient vibrations to be recorded with reasonable sig- nal-to-noise ratios. Temporary decreases in frequencies of Factor building modes of vibration have been correlated with moder- ate-to-strong shaking, and spectral amplitudes of ambient vibrations have clear daily and weekly patterns that correlate with working hours, wind speeds, and non-seismic vibrations. Waveform data from the Factor array are also being used in comparison with finite element calculations for predictive damage behavior. A three-dimensional model of the Factor building has been developed based on structural drawings. Observed displacements for 20 small and moderate, local and regional earthquakes were used to compute the impulse response functions of the building by deconvolving the subbasement records as a proxy for the free field. It can be shown that small but significant changes in the travel times, mode shapes, and frequencies are observed in the simulation results for strong ground shaking and for modifications to the structural model for hypothesized damage patterns such as broken welds on a particular floor. Wireless untethered devices whose design is guided by data analysis and simulations such as these can significantly increase the spatial resolution of structural response to earthquakes. A Mica-Z mote network controlled by Wisden software that monitors a local area such as a building is being assembled and tested. The software system addresses some of the challenges associated with high sample rates and limited radio bandwidth, yet allows structural data acquisition from a relatively large network of wireless sensors.