The new Galata Bridge, which is one of the largest bascule type bridges in the world, was built during the period of 1985-1993 to link both sides of the Golden Horn in Istanbul, replacing old pantoon-type bridge built in 1912 at the same location. Soon after it has been taken into service, some very serious cracks and deformations were noticed on the flaps of bridge in 1998. In this study, the results of experimental stress and vibration studies on one of the flaps are given after the damages are repaired. Moreover, the first natural frequency, higher resonance frequencies and corresponding mode shapes have been obtained by using finite element method and results compared with experiment measurements. These studies unveiled that the first natural frequency of the flap structure is in the range of the hydraulic oil circuit natural frequency that causes resonancy during the expanding operation of flaps, especially when flap opening angle is within a certain range. As the second stage of the study, some design variations are applied to flap structure model and frequency responses are obtained for different design trials. Consequently, some design rules are proposed that should be considered during the design stage of bascule type bridges. On the other hand, results show that several natural frequencies of flaps, particularly during the opening period, are within the earthquake frequency ranges. This fact has been proposed as another important problem especially for this bridge as Istanbul is located near a major earthquake fault. (c) 2005 Elsevier Ltd. All rights reserved.