Seismogeodetic Monitoring of Structural Deformation during Shaketable Experiments
Wednesday, 17 December 2014
Accurate low frequency strong motion recordings are important for understanding free field and building response in large engineered structures. We have developed a seismogeodetic monitoring system based on GPS technology for real-time observations of large earthquakes that can also be used for structural monitoring. The data analysis method implements in a tightly-coupled Kalman filter to provide absolute estimates of seismic displacement, velocity, and tilt from GPS and accelerometer observations. Tilt is one of the major error sources that prevents accelerometer data from being integrated correctly to displacements. The technology is currently operational and streaming real-time observations from remote SIO Geodetic Module packages containing MEMS accelerometers at 17 GPS sites in southern California for the purposes of earthquake early warning and rapid response. The instruments were run in real-time on a four-story structure at the UCSD NEES shaketable to test an inertial force-limiting floor anchorage system as an emerging technology for new seismically resistant buildings. Observations were made during a series of earthquake simulations at five points on the roof of the structure, at the base, and at two nearby reference sites off the structure. Two of the points were also observed with observatory-grade Kinemetrics Episensor accelerometers to compare the performance of the MEMS sensors. The unique asymmetric design of the engineered structure deliberately induced large out-of-plane torsion and tilts of the building. This tested the performance of anchorage components to motions in two lateral directions even though the shaketable generated motions in only one component. We performed a seismogeodetic combination of the accelerometer and GPS data in which we simultaneously estimated tilts to take into account the impact of the rotations on vertical tilts of the accelerometers. The seismogeodetic combination reliably recovers drift at the rooftop, demonstrated through the agreement with independent string potentiometer measurements of displacement.