This paper presents results of vibration tests conducted on a recently reconstructed suspension bridge in Ostrožac, Bosnia and Herzegovina. The Ostrožac bridge spans 200 m over the Jablanica Lake and represents a case of exceptional engineering. However, it is also a typical example of ageing infrastructure characterized by a lack of adequate maintenance and severe degradation by corrosion. The main girders are a complex steel portable Bailey-type trusses suspended by Ø30 mm steel hangers and 6Ø52 mm primary load-bearing cables. The curved cables support the road pass over 18 m high reinforced concrete pylons. Six 44 m long straight inclined back-stay cables transfer the tensile force using threaded rods to the reinforced concrete block anchorages at both ends. The dynamic parameters were obtained by on-site measurements using Digitex xWave triaxial accelerometers and the output-only operational modal analysis which does not require knowledge of the input excitation. The vibrations were induced by wind and by the passage of cars. The structure vibrates spatially even though the nature of excitation is dominantly vertical and the adjacent structural segments of the Bailey truss oscillate almost independently. Power spectrum, damping and Scruton number were determined from acceleration and displacement measurements of the inclined cables. Natural frequency was employed for estimation of the cable axial force. Even though the inclined cables have solid dynamic parameters, notable wind-induced vibrations were visually detected implying the need for structural strengthening intervention.

suspension bridge; Bailey truss; ambient vibration testing