Characterization of environmental variability in identified dynamic properties of a soil-foundation-structure system

UnrestrictedFor more than thirty years, researchers have attempted to establish effective local and global methods for monitoring civil, aerospace and mechanical structures. The unpredictable nature of soil and the nonlinear behavior of civil structures, however, make monitoring rather complex. In fact, the combined soil-structure system can be influenced by environmental factors on daily to annual time scales. Identification of structural damage using vibration-based methodologies in the presence of such influences requires some data normalization to reduce uncertainties and variations introduced by environmental factors.; To better understand the correlation between environmental variations and the dynamics of soil-structure interaction, semi-continuous monitoring of a large-scale field test structure has been conducted. The analysis uses data from sensors placed at several locations on the NEES Soil Foundation Structure Interaction (SFSI) Test Structure, which is located on very well-characterized soil in southern California, as well as sensor arrays monitoring the soil response under the foundation. A network of sensors was designed and positioned to record temperature at various locations on the structure; some pertinent soil properties are also monitored.; The observations of measured environmental data and the identified structural system parameters demonstrated strong correlation between variations in the environmental features and in the dynamic properties of the structure; for example daily shifts in the structure’s fundamental frequencies are as much as 10% due to temperature change or seasonal shifts due to ground water table variation under the foundation. An example is given, based on novelty detection, of how “unusual” dynamic behavior can be indicated for the SFSI test structure in the presence of environmental variations.; To explain the effect of the soil saturation on the rocking frequency of SFSI systems, a predictive model based on Wolf's Cone Model approach was developed. The results from parametric study validated the observations from both the SFSI test structure and the scaled model of decreased natural frequency for dry soil. Further, the study shows that the opposite effect — that is, higher natural frequency with soil saturation — occurs for structures with different characteristics relative to the soil. A 1/14 scale model of the SFSI test structure was constructed to validate the observed effect of the water level under the foundation on the rocking frequency of SFSI systems in a controlled laboratory environment. These experiments showed a similar correlation between the identified rocking frequency of the scaled prototype and the rise and fall of the water level under the foundation. This work shows that understanding the environmental variability of an identified model is critical to developing methodologies for cleansing data and reducing uncertainty to allow more robust health monitoring in real civil structures