Modal identification of the first Bosporus bridge during hanger replacement

Abstract

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