Computation of Structural Flexibility for Bridge Health Monitoring Using Ambient Modal Data

The issues surrounding the use of ambient vibration modes for the location of structural damage via dynamically measured flexibility are examined. Several methods for mass-normalizing the dynamic mode shapes extracted from ambient modal data are implemented and compared. The methods are applied to data from a series of ambient modal tests on an actual highway bridge. The results indicate that the mass-normalization procedures considered all give comparable results. The results also indicate that for the damage case examined, the flexibility from the ambient mode shapes gives a better indication of damage than the flexibility from the forced-vibration mode shapes. This improved performance is attributed to the higher excitation load levels that occur during the ambient test. INTRODUCTION The measured flexibility matrix has been shown to be useful for the indication of structural damage from measured modal data (see, for example, Aktan, et. al. (1994), Pandey and Biswas (1994), Toksoy and Aktan (1994), Doebling (1995), and Robinson, et. al. (1996)). A drawback to using the measured flexibility matrix is the necessity of having mass-normalized mode shapes. For a forced-vibration test, the mass normalization can be implemented from the driving-point inertance measurement. However, for a modal test which uses an ambient excitation source (e.g. automobile traffic), the normalization is not as straightforward. The importance of ambient-excitation modal tests is related to the practical issues surrounding the testing of large civil-engineering structures such as highway bridges. During a bridge test, it is often impractical to eliminate automobile traffic from the bridge, making the controlled conditions required for a forced-vibration test difficult, if not impossible. Also, it can be difficult to provide a significant level of input force for very large bridges. Thus, ambient modal tests are often the only practical source of modal data for large civil-engineering structures. In this paper, three methods for mass normalizing ambient modes are compared, and the resulting flexibility matrices are used to attempt to locate damage imposed in a known location on the structure. The methods are applied to modal data obtained from an actual highway bridge. THEORETICAL BACKGROUND The structural flexibility matrix can be estimated from the measured mass-normalized mode shape matrix and the diagonal matrix of measured structural eigenvalues (where the entries in the eigenvalue matrix are the squares of the circular modal frequencies) as 1. Postdoctoral Research Associate, [email protected], (505) 667-6950 2. Technical Staff Member, Member ASCE, [email protected], (505) 667-4551 G [ ] Φ [ ] Λ [ ]