On identification of Damage in Structures Via Wave Travel Times

The traditional identification methods for detection of damage and for health monitoring of structures are based on detection of changes in the system natural frequencies. These changes tend to be small in the early stages of damage, and therefore may be difficult to quantify, even from accurately processed recorded motions. Other difficulties arise from the nonuniqueness in the model representation. Unless the model accounts for the soil-structure interaction, and it has been carefully validated and calibrated, it is very difficult to identify the true causes and sources of observed nonlinearities in the response. In this paper, it is suggested that the formation of damaged zones in structures could be monitored (identified) via the delays in travel times of seismic waves through these zones. This approach needs further development and testing. A preliminary analysis presented in this paper (of a building damaged by the 1994 Northridge, California, earthquake) shows that this method (1) can lead to detectable changes in the travel times of the waves passing through the areas known to have experienced damage, and (2) in its simplest form does not to require detailed modeling or analysis of soil-structure interaction.