Challenges in operational modal analysis of suspension bridges

Operational modal analysis is being increasingly used as an experimental technique to identify the dynamic properties of many types of large-scale constructed systems. The writers conducted such an experiment to identify the dynamic properties for the masonry towers of a landmark suspension bridge with the objective of providing data to validate and improve the reliability of a seismic evaluation and retrofit investigation. One of the towers was instrumented by a dense and stationary array of sensors, and the other tower and spans were instrumented with a limited number of reference sensors. The spans had been tested and characterized earlier by different researchers using ambient and forced excitation techniques. The authors did take advantage of those results in designing the instrumentation to characterize the towers. This paper describes some of the challenges encountered in trying to identify the dynamic properties of the towers using this approach, and the techniques the writers used in an attempt to overcome these challenges. Recommendations for future applications with this type of structure are also discussed.