Flexibility by Multireference Impact Testing for Bridge Diagnostics

Modal testing by impact is a practical tool for structural identification, integrity monitoring, and diagnostics of bridges. Typically, nonlinearities in bridge response as well as sensitivity to ambient conditions are observed during modal tests. Therefore, impact tests should be planned and executed so that the effects of nonlinearities are incorporated in estimating modal parameters of a test structure. Furthermore, indexes more reliable than the modal parameters are needed to diagnose local and or obscure damage. These and other shortcomings in current modal test-based diagnostic studies are reviewed. A multireference impact testing of a bridge, in which frequency-response functions (FRF) are measured and a large number of modal parameters are reliably identified, is presented. The mode shape coefficients obtained through processing measured FRF are directly transformed into flexibility of the test bridge without assuming mass. Analytical studies of a calibrated analytical model are presented to demonstrate that flexibility coefficients are more sensitive to local damage than either frequencies or mode shapes.