Modal identification of the first Bosporus bridge during hanger replacement

As the oldest of the three suspension bridges spanning the Bosporus, the (First) Bosporus Bridge was commissioned in 1973. Its main span of 1074 m was originally designed to be suspended by inclined hangers; in 2015, the original inclined hangers were replaced by vertical ones. In-situ ambient vibration measurements were taken at different stages of this operation to gain an understanding of the effects of the hanger cable orientation on the overall dynamic behaviour of the bridge. Measurements were also recorded for various periods spanning several weeks to observe operational variations on the modal frequencies of vibration. Measurements were made on the tower, the deck and the hangers. It was observed that, as a result of the hanger replacement, frequencies of the modes involving predominant deck motion decreased on the order of 6 to 16% while the frequencies of the modes involving predominant tower motion showed almost no change. A finite element model of the Bridge was also developed to further investigate the physical reasons behind the observed changes in modal frequencies and similar trends were observed in the modal frequencies yielded by the finite element models developed for the inclined and the vertical hanger configurations.