Apparent changes, over time, of the dynamic behavior of a nine-story reinforced concrete building are examined to determine their plausibility and possible sources. The apparent variations include changes of the resonant frequencies and of the relative contribution of the soil-structure interaction effects during forced vibration tests. Experiments by different researchers on the importance of the soil-structure interaction effects lead to conflicting interpretations of the contribution from the rocking response which differ by factors of 24 and 14 in the transverse and longitudinal directions. These differences are not realistic and may have resulted from errors in the data reduction for the initial experiments. The variations in resonant frequencies are considered to be mainly a result of stiffness degradation of the superstructure as a consequence of the San Fernando earthquake of 1971.

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