System identification of code conforming low-rise rc building in Lalitpur, Nepal

System identification of a code conforming low-rise reinforced concrete frame building in Kathmandu, Nepal is reported in this paper. The fundamental vibration period and damping ratio of the building were determined using ambient vibration records taken in three floors of the building. Ambient vibration measurements were taken using three triaxial accelerometers. Two methods of system identification, Welch spectral method and N4SID method, are used to estimate the modal properties. Fundamental vibration periods of the four-storied building estimated from non-parametric Welch spectral method are 0.292s and 0.289s in the two orthogonal directions. The corresponding results from the parametric state- space N4SID method are 0.266s and 0.264s in the orthogonal directions. The corresponding damping ratios are estimated to be 6.3% and 6.9%. These periods are longer than that of a simplified finite element model of the building assumed to be fixed at the supports. This indi- cates that the flexibility of the soil under the building foundation, plays an important role in the vibration frequencies of the structure.