Damping estimation of a double-layer grid by output-only modal identi cation

Abstract

Modal parameters of large civil engineering structures such as Modal Damping Ratios (MDRs) are determined mainly through output-only modal identi cation. In this paper, MDRs of a double-layer grid were obtained using output-only modal identi cation. For this purpose, a double-layer grid constructed of a ball-joint system was tested. Through some random tapping on the structure, the acceleration response in multiple locations was measured. The acquired data were processed using output-only modal identi cation to arrive at MDRs. The MDRs corresponding to the rst eight modes of the grid were extracted by ve output-only modal identi cation techniques, namely Enhanced Frequency Domain Decomposition (EFDD), Curve- t Frequency Domain Decomposition (CFDD), and three di erent methods of data-driven stochastic subspace identi cation. To determine the appropriate model order used in SSI methods, sensitivity analysis was carried out and the resulting number of orders was 200. The proper frequency resolution of 1600 was determined to estimate the MDRs of the grid by EFDD and CFDD. The results showed that the MDRs of the grid obtained from di erent methods were in good agreement with each other. The grid has very low MDRs because the MDRs of the modes measured using di erent methods varied from 0.06% to 0.11%.