Abstract：Damping level of a system is vital in dynamic response analysis. In civil engineering，damping ratio is commonly employed in characterizing the damping level of structure and is of great importance in aseismic design. But so far，for most conventional building aseismic designs，the influence of the soil-foundation interaction on the structures is disregarded. The damping ratio adopted in current aseismic design is based on the experiments carried out on rigid soil-foundation. Since the soil-structure interaction(SSI) effect is universal，for the dynamic responses analysis of an in-situ building，it is in accordance with the actual case to involve SSI effect. For aseismic design of structures involving SSI effect，accurate experimental evaluation of damping ratios of SSI system is a necessary prerequisite. To accurately get damping ratio of the system，many identification methods have been proposed and developed. But so far，most of them cannot achieve success in identifying accurate in-situ damping ratios of buildings，i.e. SSI systems mainly because of the fact that the responses are significantly influenced by noise. In order to accurately evaluate the damping ratio of building involving SSI effect，a combined identification method using ambient excitation technique is proposed and simply introduced. Based on this method，the damping ratios of 82 in-situ buildings located in Xi¢an and their vicinal regions，which include all kind of upper structures in various materials，are investigated. The numerical range of damping ratios of the actual buildings involving SSI effect is shown as well as their distribution pattern. By means of a fitting curve，the variation tendency of damping ratios of in-situ buildings is revealed. And then，the influence factor of the damping ratios of actual buildings is preliminarily analyzed.
