| Modal flexibility, a measure of structural flexibility obtained through experimental modal methods such as multi-reference impact testing, has been demonstrated through previous bridge research as a potential index for the damage identification and condition assessment of bridges. Research was conducted on three different structures--a structural steel grid located at the University of Cincinnati, a decommissioned steel-stringer bridge (HAM-561-0683) located over Seymour Avenue (State Route 561) in Cincinnati, Ohio, and a commissioned steel-stringer bridge (HAM-42-0992) located along westbound Ronald Reagan Highway over Reading Road in Cincinnati, Ohio--to rigorously demonstrate/verify that modal test methods and modal flexibility are practical, efficient, and legitimate condition assessment/damage identification tools.; Testing of the laboratory grid specimen revealed that modal flexibility, as defined from the data acquired through multi-reference impact test methods, provides a reliable and repeatable measure of existing in-situ structural flexibility, or condition. These same investigations also helped establish preliminary guidelines for conducting modal tests on actual civil structures. When these guidelines were applied to the decommissioned steel-stringer bridge over Seymour Avenue, the bridge was identified as a time-variant structure. Such behavior violated the time-invariance assumption of modal analysis and initiated the development of a unique impact test procedure that would permit acquisition of time-invariant data. The resulting procedure, referred to as "rapid" multi-reference impact testing, was demonstrated as a practical and efficient multi-reference impact test method. In addition, the modal data acquired through this method yielded a reliable and proper measure of existing flexibility (i.e., modal flexibility).; Through the implementation of damage on the decommissioned steel-stringer bridge, the bridge girder condition indicator (BGCI), a condition index derived from modal flexibility, was shown to be an excellent objective index for both global and local condition assessment. Furthermore, to demonstrate that the strategies and methods associated with "rapid" multi-reference impact testing and modal flexibility (BGCI) are applicable to other short-to-medium span steel-stringer bridges, such methods and concepts were applied to a commissioned steel-stringer bridge (HAM-42-0992). Testing at this bridge represented a realistic, or actual, application of these methods, which were used successfully to identify a change in condition from when the bridge opened in the Fall of 1991. |
