Evaluation of variation in dynamic response of two instrumented buildings under seismic excitation

Natural frequencies and damping ratios are very important parameters characterizing the dynamic response of buildings. These dynamic characteristics of building structures are observed to vary during different earthquake excitations. To evaluate this variation, two instrumented buildings were studied. One of the buildings is a three storey reinforced concrete (RC) frame building with a shear core, while the other is a three storey RC frame building without shear core. Both the buildings are part of the same large complex but are seismically separated. One of them is instrumented with five and the other with four tri-axial accelerometers. The dynamic properties of the buildings were ascertained using a time domain state-space subspace system identification technique considering 50 recorded earthquake responses. Relationships between identified natural frequencies and damping ratios, and the peak ground acceleration (PGA) at the base level and peak response acceleration (PRA) at the roof level were developed. It was found that response of the buildings strongly depended on the excitation level of the earthquakes. A general trend of decreasing fundamental frequencies and increasing damping ratios was observed with increased level of shaking and response. It is concluded from the investigation that knowledge of variation of dynamic characteristics of buildings is necessary to better understand their response during earthquakes.