Modal properties of the roebling suspension bridge-FEM modeling and ambient testing

Many of the suspension bridges built in the States in the 19^th century still stand today. These suspension bridges were obviously designed for live loads quite different from automobile traffic they carry today. A good example is the John A. Roebling suspension bridge, completed in 1867, over the Ohio River between Covington, Kentucky and Cincinnati, Ohio. It is necessary to bring these bridges up to current standards of safety. This process requires the combination of the bridge analysis and field testing. This paper will show the first step in the structural evaluation scheme of the Roebling suspension bridge. A three-dimensional finite element model is created in the ANSYS. Starting from the deformed configuration due to dead load, the modal analysis is followed to provide all possible frequencies and mode shapes. The results of the finite element modal analysis have been compared with in situ ambient vibration measurements, and good agreement has been found. This finite element model, after calibrated with experimental measurements, will serve as the baseline for the structural evaluation of the bridge. The outcome will assist in the preservation of the Roebling suspension bridge. The methodology developed could be applied to wide range of old cable-supported bridges.