Added value of regular in-service visual inspection to the fatigue reliability of structural details in steel bridges

In order to design structural details of bridges for fatigue, the current version of the Eurocode for steel structures recommends partial factors for fatigue resistance based on the consequences of failure and on the maintenance method. The safe-life method is used for details where local formation of cracks could rapidly lead to failure or for details not accessible for inspection and has a relatively high partial factor. The damage tolerant method, on the other hand, is used for cases where fatigue crack initiation does not result in immediate failure so inspection and repair can be performed. In the current Eurocode, this comes with a relatively low partial factor. However, since the probability of crack detection of visual inspection by the naked eye is considerably different from more detailed inspection methods, the required partial factor to design a bridge for fatigue should be based on the way and level of inspection planned during the bridge service life. As a common practice, for most bridges, only visual inspections in short time intervals are carried out. In this paper, the added value of periodic visual inspection on the reliability status of a steel railway bridge is studied. The probability of failure after performing visual inspection is investigated by two approaches: 1) A statistical study on the main causes of bridge failure carried out by other researchers to find the relation between the safe-life design method and the design method considering visual inspection; 2) Conducting a survey to collect experts opinions on the matter and using a Bayesian algorithm to assign a probability distribution function to each opinion. A relation between reliability indices for the cases where a bridge is designed with and without considering the in-service visual inspection, is derived