Revelations from ambient shaking data of a recently instrumented unique building at MIT campus

A state-of-the-art seismic monitoring system comprising 36 accelerometers and a data-logger with real-time capability was recently installed at Building 54 on the campus of the Massachusetts Institute of Technology [MIT], Cambridge, Massachusetts. The system is designed to record translational, torsional and rocking motions, and to facilitate computation of drift between select pairs of stories. The cast-in-place, reinforced concrete building is rectangular in plan but has vertical irregularities. Heavy equipment is installed asymmetrically on the roof. Spectral analyses and system identification performed on one set of low-amplitude ambient data reveal distinct fundamental translational frequencies in the structural NS and EW directions [0.75 and 0.67Hz, respectively], a torsional frequency of 1.49 Hz, a rocking frequency of 0.75 Hz, and very low damping. Such results from low-amplitude data serve as baseline against which to compare the behavior and performance of the building during stronger shaking caused by future earthquakes in the region.