The Kassel wire bridge over the Fulda River, built in 1870, was retrofitted in 1997 during renovation work with new suspension cables and a new steel superstructure. The round hangers, which connect the main suspension cables to the superstructure, were mostly designed as open spiral cables; however, in the central section of the bridge, they had to be constructed from round steel for structural reasons. During the previous phase of use, fatigue damage occurred precisely at these round steel members.

As part of a master’s thesis in the Department of Structural Engineering, the bridge was modeled using FEM software and analyzed through dynamic calculations. It was found that the bridge’s natural frequencies fall within the range of excitation frequencies generated by pedestrians, and that excitation of the torsional mode in particular leads to twisting in the round hangers (see excerpt from the FEM model). Due to the design of the round hangers, this twisting cannot occur freely, resulting in alternating moment stresses within the round hangers, which were identified as the cause of the fatigue damage.