System identification of a residential building in Kathmandu using aftershocks of 2015 Gorkha earthquake and triggered noise data

Abstract

System identification is conducted to estimate the fundamental vibration period and damping ratio of a residential building in Kathmandu. Ground motion and structural response due to aftershocks of the 2015 Gorkha Earthquake, as well as noise data triggered by ambient vibration is used to identify the dynamic properties of the structure. In total, motion due to 3 aftershocks and 362 ambient vibration is used. The identification is based on estimating the frequency response function of the structure. When using the aftershock data, this function is estimated from power spectral density functions of motion recorded at the ground floor and roof of the structure. In case of triggered noise, it is assumed that the input motion is a white noise. Fundamental vibration period is estimated from the first dominant peak of the transfer function, and damping ratio is estimated by using the half-power bandwidth. The building being studied is a 4-storey reinforced concrete frame with masonry infill walls. The fundamental period of the building estimated from aftershock data and triggered noise data was found to be similar in the range of 0.24–0.4 s. Empirical relations available on the literature predict a fundamental period of 0.25 s for the building being studied. It can thus be concluded that the fundamental period of the building can be estimated with confidence using both aftershock and ambient vibration data. The damping ratio, however, showed greater variation. This variation is, in part, due to the inherent uncertainty in spectral estimation which requires smoothing operations that directly affect the bandwidth of the dominant peak of frequency response function.