Measurements of On-Site Dynamic Parameters for Seismic Evaluations

In an effort to develop a low cost and relatively simple method to determine the sensitivity of existing bridges to earthquakes, this study developed a non-contract electromagnetic induction device to measure bridge vibration and damping properties. Three small coils were oriented orthogonally to each other and attached to a bridge to see if a static magnetic field could be brought in proximity, to determine three dimensional motion. This tri-coil sensor was used to test two bridges in New Mexico: Rio Grande Canal Bridge in Fairacres and Big Dry Creek Bridge in Glenwood. The major disadvantage of the device is that the bridge vibrations are qualitative in nature and all data is reduced from relative vibrational magnitudes. Therefore, displacements and stresses cannot be measured. However, this pilot test demonstrated the feasibility of the tricoil sensor and of the relative ease in performing non-destructive forced vibration tests.