A comparison of modal properties derived from forced and output-only measurements for a reinforced concrete highway bridge

Development of effective structural health measuring systems for the existing bridge inventory is an active research area among state and federal transportation agencies. One Level IV method under development by the authors uses periodically collected global modal data to trend the health of structural systems. These data are currently extracted via incremental Single-lnput-Multiple-Output (SIMO) test methods. SIMO methods require an active measured input to the structure. Output-only methods, which do not require measured inputs, could significantly decrease the difficulty of collecting point-in-time data from remote bridge sites by eliminating the need for on-site excitation devices. Several output-only methods in the literature rely on unmeasured excitation from ambient environmental sources and passing traffic to generate the modal response data used to extract modal properties. This paper presents results of an investigation comparing extracted modal properties derived several output-only methods to results of a SIMO test using a common time-based modal dataset. The data were collected from a new reinforced concrete two-span highway bridge. This comparison of methods includes a discussion of the similarity of natural frequencies and mode shapes. The relative computational effort required to generate and extract the modal properties of the structure is also discussed.