Optimal design of TMDs to reduce the longitudinal seismic response of the suspension bridges

Previous earthquakes have been shown that the direction of the bridge relative to the direction of the faulting might have a significant effect on its vulnerability. So, in some cases it is necessary to pay special attention to control the behavior of the bridge at least in one direction. In this study, the reduction of the longitudinal displacement in a suspension bridge was investigated using a tuned mass damper (TMD). At first, the performance of TMD for reducing the longitudinal response of Vincent Thomas suspension bridge against the near- and far-field earthquake records were examined. Then, optimum parameters for TMD were determined while various span lengths for the bridge were considered and different earthquake records were induced to the analytical models. Regression analysis was performed on the results to obtain simple formulas to facilitate the determination of optimum TMD parameters for different bridges with different proportions of lateral spans to mid span lengths. The efficiency of the formulations was confirmed by evaluating the seismic response of three sample bridges. The formulas indicated that the optimum angular frequency and damping ratio of TMD relatively increases as the span lengths ratio is increased. The results also showed that TMD damper is more efficient against the near-field records than the far-field ones so that the longitudinal displacement has decreased by over 40% against near-field records and 27% against far-field records. It was observed that the optimum damping ratio of TMD against the near field earthquakes varies from 0.042 to 0.092.