Advanced Assessment of Welded Bridges for Fatigue and Brittle Fracture

Rapport, 2018

Around 50% of steel and steel—concrete composite bridges owned by Trafikverket
have been built before 1970, when there was limited knowledge about fatigue and brittle
fracture of structural steel products. This was the case in particular for the usage of these
products in construction of welded bridges. At the same time, fatigue deterioration in
these structures is undergoing due to the escalation of freight volume as well as the
increased axle loads of modern trains and lorries. Realistic assessment of residual
service life of these structures is crucial in more efficient planning of repairs and
avoiding unnecessary replacement costs. Therefore, more accurate condition
assessment methods are important from both economic and sustainability aspects.
While the improved assessment methods for existing structures have been subject of
extensive research since 1990's, their use in regular engineering practice is still limited.
This study is an attempt to review the recent advances in structural assessment of
welded steel bridges and to present them in a structured manner for the use of practicing
engineer. In this report, a multi-step assessment methodology for fatigue and brittle
fracture of welded bridges is presented, mainly based on the past research in Europe.
The assessment steps are comprised of preliminary assessment (phase I), detailed
investigation (phase II), and expert investigation (part III). Detailed information is
given in the report for the corresponding input data (resistance-side and action-side)
and modelling options for each of these assessment steps. As expected, the more
advanced method is used, the more extensive set of input data and analytical effort is
required. When applicable, the assessment method has been adapted to the relevant
technical requirements stipulated by Trafikverket.
Finally, the application of the presented multi-step assessment is examined on a case
study bridge (Göta river bridge). Phase II assessment of the bridge based on historical
loading data shows insufficient fatigue life for a critical detail that has been replicated
in many locations over the bridge. The results of phase III assessment suggest that
intensified inspections of vulnerable details in regular time intervals are sufficient for
ensuring the safe service of the structure until the end of its planned service life.