Structural assessment and fatigue evaluation of a 60-year-old linkspan bridge in Northeast of England

Abstract

This thesis aims to investigate the condition of a linkspan bridge by performing structural inspection and structural assessment concluded with the fatigue evaluation based on the collected inspection data. To that end, a linkspan bridge has been selected which is constructed in 1965 and operational in an international port that is located in Northeast of England. The original linkspan structure was suspended from a fixed gantry and subsequently extended two times which is approximately 32m and is supported by four hinged bearings two on bankseat and two on the pontoon end. Periodical inspections and evaluation of any changes in the structure over time is a noteworthy concern for the port operators as well in terms of asset management. Accordingly, the condition of the structure can be determined based on overall and member-based structural status, thus the residual life. The structural inspection is conducted in two steps, consisting of visual inspection and thickness readings by using ultrasonic testing device to determine the overall condition of the linkspan bridge and have further idea about the member thicknesses. Considering the continual corrosion resulting with the loss of member sections, a comparison has been made to determine the structural capacity of the link-span bridge based on the existing condition. For that purpose, a member-based computational models of the linkspan bridge have been produced in SCIA Engineer Analysis software based on the historic information and existing drawings of the bridge. Original member thicknesses are compared with the reduced member thicknesses to represent the allowance for corrosion. The results have been defined and evaluated for each member type; accordingly, the minimum allowable section thickness was identified. In addition to the inspections, fatigue analysis is an important method to have an idea of the residual life of the structure based on the actual data. Whilst it is possible to undertake a retrospective analysis of fatigue, which would need to include information on yearly traffic (numbers of vehicles in certain weight bands for example). Therefore, the actual ferry timetables are used for reference and yearly traffic estimation has been made as an exemplar study. As structures age (and particularly if their condition and thus capacity deteriorates), the accumulating effects of fatigue can be a significant factor in the eventual failure of a structure, as well as one-off overloading, the chances of which increase under a given load as the structure corrodes and thus its capacity gradually reduces.