Health monitoring of structures under ambient vibrations using semiactive devices

Structural health monitoring (SHM) is the process of monitoring structural health and identifying damage existence, severity and location. Clear needs for SHM exist for various types of civil structures; for example, approximately 25% of U.S. bridges are rated as deficient and require significant expenditures to rebuild or replace them (FHWA, 2002). However, the dominant method for monitoring the health of civil structures is manual visual inspection - a time- and labor-intensive procedure. Global vibration-based SHM techniques have been studied, but no approach has been well established and accepted due to limitations of ambient excitation sources for most civil structures. One approach that may help alleviate some of the SHM difficulties for civil structures would be to use variable stiffness and damping devices (VSDDs) - controllable passive devices that have received significant study for vibration mitigation - to improve damage estimates. In addition to providing near optimal structural control strategies for vibration mitigation, these low-power and fail-safe devices can also provide parametric changes to increase global vibration measurement sensitivity for SHM. This paper proposes using VSDDs in structures to improve SHM, and demonstrates the benefits in contrast with conventional passive structures. It is shown that using VSDDs in identification gives parameter estimates that have better means and smaller variations than the conventional structure approach. The improvements in the identification process are even more effective when adding higher effective levels of stiffness or damping to a structural system, even though the resulting VSDD forces due to ambient excitation are small.