Monitoring and Evaluation of the Anomalous Vibration of a Cable in a Stayed Bridge

After an extensive analysis, the Rio Papaloapan Bridge in the state of Veracruz, Mexico, was scheduled for maintenance to replace the upper anchorage element of 20 cables that were identified as structurally deficient. For this rehabilitation, an extensive monitoring was implemented to ensure the integrity of the bridge. As a result, abnormal vibration levels were detected in one cable (cable 9 in semi-harp 1), particularly for winds over 50 km/h. To determine the origin of this behavior, additional vibration measurements were implemented to evaluate the dynamic vibrations of the different elements involved. Comparison of the frequency spectrum of different cables with same characteristics and tensions, it was found that the abnormal cable had high vibration levels within the range of 10 to 20 Hz. At the same time, the frequency spectrum for their corresponding upper anchorage of the cable also showed significant differences for the same range of frequencies and higher levels were detected for the same atypical cable in the semi-harp plane (xy plane). Analysis from the vibration data concluded that the tension of the cable was within specifications and the abnormal behavior was not due to distension. Simulation studies confirmed that reduction in the structural stiffness for the anchorage element induced high vibration levels in the range within 20 Hz and the dynamic coupling with the higher vibration modes of the cable was the most probable cause for the extensive vibration in the cable. Also, simulation analysis showed that a damping system could minimize significantly the vibration levels between 8 and 25 Hz. The foregoing gave us the opportunity to conclude that the cable # 9 o semi-harp 1, is under an abnormal conditions due to a dynamic vibration coupling to its upper anchorage element and the higher vibration in the xy plane in this anchorage element was most probably to stiffness reduction. Based on the previous, monitoring and detailed inspection of the anchorage element was recommended, and at the same time, consideration of a damping system is highly recommended to reduce vibration damage.