Vibration Monitoring of a Steel-Plated Stress-Ribbon Footbridge: Uncertainties in the Modal Estimation

Abstract

A low-cost vibration-monitoring system was developed and installed on an urban steel-plated stress-ribbon footbridge. The system continuously measures the acceleration [using 18 triaxial microelectromechanical system (MEMS) accelerometers distributed along the structure), the ambient temperature, and the wind velocity and direction. Automated output-only modal parameter estimation based on the stochastic subspace identification (SSI) was carried out to extract the modal parameters (i.e., the natural frequencies, damping ratios, and modal shapes). Thus, this study analyzed the time evolution of the modal parameters over data monitoring for 1 year. First, for similar environmental/ operational factors, the uncertainties associated with the SSI-based techniques used and to the acceleration records used were studied and quantified. Second, a methodology for tracking the vibration modes was established, because several of them with closely spaced natural frequencies were identified. Third, the modal parameters were correlated against external factors. It has been shown that this stress-ribbon structure is highly sensitive to temperature variations (frequency changes of more than 20%) with strongly seasonal and daily trends. Fairly simple dynamic multiple regression models for the lowest persistent vibration modes were derived, and excellent correlations for some of them were obtained. These correlations enable the influence of these uncertainties on modal estimates to be removed, thus facilitating their use as damage-sensitive features.

Modal identification of structures from input/output data using the expectation-maximization algorithm and uncertainty quantification by mean of the bootstrap

“…The first two ones are torsional modes, and the other two are bending modes. These modes were also identified in Soria et al…”

“…The footbridge has a low‐cost continuous vibration‐monitoring system installed on it to track its long‐term dynamic performance (see Soria et al for more details). This system continuously measures the acceleration using 18 triaxial MEMS accelerometers distributed along the structure, the ambient temperature, and the wind velocity and direction.…”

Further steps towards the tuning of inertial controllers for broadband‐frequency‐varying structures

“…The main drawback of TMDs is that they are highly effective only over a narrow frequency band. Hence, when structures show modal properties changing over time, and/or several vibration modes must be cancelled by the same device, TMDs may detune and experience a significant loss of efficacy. Under these circumstances, the use of semiactive devices may become a competitive alternative .…”

“…Although sensitivity studies of its parameters have been accomplished, as well as optimal tuning proposals, the main purpose of the semiactive device has not considered for its tuning: be effective in a broad frequency‐band spectrum. Cases such as modal properties varying with time due to external factors (temperature, level of occupancy, or aging) and structures with several vibration modes with closely spaced frequencies prone to vibrate are common in practice . Under these circumstances, much greater robustness than that provided by the TMD is required and a design approach that considers both uncertainties in structure modal parameters and an MR damper is desirable.…”

“…Cases such as modal properties varying with time due to external factors (temperature, level of occupancy, or aging) and structures with several vibration modes with closely spaced frequencies prone to vibrate are common in practice. 6 Under these circumstances, much greater robustness than that provided by the TMD is required and a design approach that considers both uncertainties in structure modal parameters and an MR damper is desirable. Thus, the tuning of the semiactive system should take into account its broad spectrum of operation and the significant non-linearities, such as the control law itself, as well as the operation of the MR damper.…”