Method for assessing residual use and determination of safe operating time intervals of existing welded steel bridges

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For the assessment of existing welded steel components under cyclic loading, no probabilistically validated verification concept based on fracture mechanics exists so far. However, this is essential in order to be able to make reliable statements on the remaining service life and safe inspection intervals for the large number of historic steel structures. This is the only way to ensure the economic and safe operation of structures with correspondingly critical structural details. The existing verification based on the nominal stress concept is usually not sufficient for this purpose. In the case of bridges, various uncertainties in the input parameters such as the load history or material lead to inaccurate results. In addition, the current condition of the structure is not included in the verification. Therefore, the aim of the planned project is to develop a validated method for the determination of safe operation time intervals and remaining service life by means of crack propagation simulations. In this context, the current condition of the structure and the scatter bands of the input parameters are also to be considered. A distinction is to be made between the required safety level for the determination of the remaining service life and the safety-relevant operating time intervals. The methodology to be developed should be simple and easy to apply without the need for time-consuming crack propagation calculations. It is based on fracture mechanical analyses and is characterized by a probabilistically secured verification concept. Within the scope of the project, it will be developed exemplarily for railroad bridges. However, it is also applicable to a large number of other fatigue-loaded steel structures. Appropriate input parameters with associated statistical distribution functions are to be defined as part of the project. These include appropriate initial crack sizes and crack propagation directions, validated material parameters and the influence of loading including possible residual stresses. In addition, relevant design details have to be transferred into idealized fracture mechanical reference models. On this basis, a general concept for older steel structures will be developed. For this purpose, fracture mechanical simulations are planned, the results of which can subsequently be transferred into correspondingly safe inspection intervals via regression analyses and probabilistic considerations. In order to validate the concept, Monte Carlo simulations will be conducted and intensive scientific work will be carried out on the safety levels and targeted level of safety. The aim of the research project is to develop a reliability-theoretical verification concept based on fracture mechanics, through which further use of existing steel structures can safely be assessed.