Performance of Crossed Long-Gauge Strain Sensors in the Spread Footing Foundation

Estimations of shear stresses are essential design procedures of spread footing foundations and detecting shear strain at critical planes is crucial in structural health monitoring because it provides valuable insights into the structural stability of the foundation. Crossed topology of long-gauge strain sensors can be used to monitor shear strain. By an ideal layout of two strain sensors in a planar case, the effect from the normal force, bending moments, and even temperature gradient can be eliminated. However, some slight position changes, e.g., due to on-site conditions, may affect the performance of crossed sensors. Therefore, it is necessary to study the influence of imperfections in the geometrical position of sensors on the measurements of crossed sensors. This paper presents a study on the performance of long-gauge sensors embedded in a spread footing foundation of a five-floor garage at Princeton University. We investigate the effects of topology on the measurement of average shear strain and the relationship between the evaluated shear and loading from the superstructure. The results show that the imperfections in the positioning of sensors have a significant impact on the measurement of shear strain, which needs to be taken into consideration when interpreting the sensor data. The average shear strain evaluated using the crossed sensors measurements is compared with theoretical values obtained from structural analysis. The results validate the estimated relationship between detected strain and applied forces. The findings of this study have important implications for the use of crossed long-gauge strain sensors beyond spread footing foundations, as the method applied here can be extended to other types of structural elements.