Reliability based constant amplitude fatigue limit of shear connection with headed studs for single or multi-axis fatigue loading

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In the construction of many of the new large composite bridges planned for the coming decades, the longitudinal shear transfer between concrete and steel building components is ensured by headed studs. These are subjected to fatigue not only in the main load-bearing direction, as was the case in the past, but also in the transverse direction. In highway bridges, the number of load cycles for the structural members bearing in the transverse direction is many hundreds of millions, even though the stress ranges for highway bridges often are not very high. Thus, a fundamental scientific treatment of the following two issues is imperative:- Theoretical and experimental justification of a hitherto completely unexplored endurance limit for fatigue-stressed shear connectors with headed studs, including the required safety margin for design, - Analysis of the stress conditions under multiaxial fatigue loading for the headed studs and consequences for design. Since few experimental results exist internationally for the fatigue limit under shear loading of headed studs, such fatigue tests with up to 30 million load cycles are mandatory. Because of the high number of load cycles and the resulting test effort, the test program required for statistically valid results can only be reasonably implemented at two universities. In order to avoid the disadvantages of the push-out tests commonly used so far, a novel test rig is designed and tested in parallel, which also allows multiaxial fatigue loading for dowel groups. Furthermore, PAUT-TFM, a new method for non-destructive testing in civil engineering, will be further developed in such a way that the cracks originating from the weld collar of the headed studs can be detected at an early stage and theoretically evaluated with regard to the delimitation of crack initiation and crack growth phases, which will be quite different than previously known for the low stress ranges investigated here. The tests are supplemented by numerical investigations, which, based on previous investigations, focus in particular on the low load level typical of road bridges. Particular attention is paid to the local stresses on the concrete that occur at this load level and their effects on the stress peaks occurring on the outer and inner edge of the weld collar of the studs.Probabilistic fatigue analysis is used to justify the required safety margin in design, i.e. the design value of the fatigue limit, taking into account the distributions of the input parameters. To round off the project, three large bridges of current design will be used to verify by calculation and measurement at what level the global and local fatigue stresses of the headed studs lie in comparison to the justified design value of the fatigue limit.