Environmental effects on measured structural frequencies-model prediction of short-term shift during heavy rainfall and comparison with full-scale observations

The sensitivity of measurements of the frequencies of vibration of structures to environmental factors, such as weather, has been of interest for structural identification and health monitoring, as changes in these frequencies are often used as indicators of degradation of the structure. This paper presents a simple 2D soil-structure interaction model that can predict shifts in the apparent frequency of a building due to water saturation of the soil during heavy rainfall, and recovery as the degree of saturation decreases. The soil is modeled as a porous half-space, dry or fully saturated with viscous fluid (water in this case), the motion of which is described by Biot's theory of wave propagation in fluid-saturated porous media. This model is a generalization of a previous model considered by the authors to more realistic soils-with finite permeability and saturated with viscous fluids. Results of model predictions are shown for model parameters that correspond approximately to the NS response of Millikan library in Pasadena, CA, and are compared with the observed trends at the same site under ambient noise excitation (up to about 3.5% increase in the first frequency during heavy rainfall, and recovery within about a week following the rainfall). Results are shown for a range of values of soil permeability as well as for undrained soil (elastic solid with Poisson ratio close to 0.5). The model predictions agree qualitatively with the observed effects at Millikan library, but the analysis does not exclude other possible causes for the observed effect. Copyright (c) 2008 John Wiley & Sons, Ltd.