Fatigue Capacity of Concrete Structures : Assessment of Railway Bridges

Present codes are mostly written for the design of new structures. When assessing existing structures it is possible to ascertain actual properties and to use them instead of using very conservative estimates. Possible reinforcement fatigue damage can e.g. be assessed with partial damage methods in the same way as is done for steel structures and with similar failure stresses.- The definition of a load cycle depends on the structure and what part of it that is studied. For ballasted bridges often two bogies for adjacent wagons can be identified as one load cycle. For the highest stress ranges often a whole train can be looked upon as one load cycle. The influence of earlier traffic can be checked with a damage hypothesis.- Material properties. The concrete capacity is often underestimated, especially its capacity to carry shear forces in slabs without stirrups. For stirrups usually no reduction normally needs to be considered of the stress range capacity due to bending of the bars. This is true as long as the cracks in concrete crossing the bars are not situated in the corners of the cross sections- Dynamic factors can often be reduced from the ones obtained from standard code values after an evaluations and/or measurement on the structure in question.- The need for closed stirrups and reductions of capacity due to splicing of reinforcement bars can be reduced if the reinforcement is fully bonded as e.g. when it is situated in compressed concrete.- More research is needed to calibrate design and assessment methods to real full scale tests on bridges. Here new measurement technology makes it possible to check real strain and stress ranges, which may be considerably smaller than the ones obtained from conservative design models.- Recommendations for assessment procedures are given in Appendix A. Examples of assessments of two concrete trough bridges are presented in Appendix B (Övre Bredån) and Appendix C (Kallkällan).Godkänd; 2015; 20150215 (elfgren)