Variation of dynamic characteristics of undamaged structures, and free-field sites identified from strong motions as compared to those identified from weak motions has been the subject matter of numerous studies. Recently, this topic is rekindled with the premise but repeat of the well known conclusion that fundamental period (frequency) varies with amplitude of strong and weak shaking. Some researchers appropriately called this variation of dynamic characteristics as “wandering” of the natural frequencies of a structure. Although due to various sources of excitation and time-varying environmental conditions, variation of the fundamental period (frequency) of even an undamaged structure should not be a surprise to many, it is important to understand why such variation is important for practical purposes. It is important that, if such variation is significant, then, in practice, the dynamic characteristics identified from weak motions should be carefully considered before generalizing these characteristics as representative of strong shaking as well.
In this paper, we investigate the fundamental frequencies of an undamaged case study building for which there are numerous studies of several sets of vibration data, including forced vibration testing, strong shaking due to a distant large earthquake, and low-amplitude shaking due to ambient excitations as well as several small nearby earthquakes. It is shown that the fundamental frequency “wanders” in a consistent way with the level of shaking, and that the significant difference between low-amplitude and strong shaking is attributed to soil-structure interaction during stronger shaking. Furthermore, variation of dynamic characteristics for strong and weak motions are summarized for four additional buildings. The variations are too significant to ignore; thus leading to the conclusion that dynamic characteristics of buildings identified from weak motions cannot be generally used in lieu of those identified from strong shaking.