# Investigations on Interface Shear Fatigue of Semi-Precast Slabs with Lattice Girders
## 6 Summary and Conclusions  
To investigate the fatigue behavior of semi-precast slabs with lattice girders, small size tests with lattice girder diagonals cast in concrete cubes were carried out. For all cyclic tests, failure of the lattice girder diagonals occurred in the region of the weld point, with static tests failing in the free diagonal between welding points and load application. For these small-body tests, \(S\)-\(N\) curves in accordance with EC2+NA(D) were derived using the interactive method. The results were already included in the revision of the technical approvals. Subsequently, the fatigue behavior of slab specimens was investigated by 14 fatigue tests with two sub-tests. The test parameters were interface roughness, slab thickness, interface reinforcement ratio, and concrete strength. The following findings can be drawn from the investigations:  
* For the fatigue tests, higher scatter occurred compared to static tests. The scatter concerns the failure mode, interface delamination, strain of lattice girders, and achieved number of load cycles. * The failure of specimens with low degrees of interface reinforcement was generally induced by interface failure after high numbers of load cycles with \(N\) > 2.0 million. Therefore, the bearable fatigue load level generally increases with increasing roughness. Only one specimen with a rough interface had an unexpected premature failure, which needs further investigation. * Specimens with medium interface reinforcement ratios failed predominantly by vertical shear with only small interface delamination. Figure 22: Test setups for each parameter combination in Series 3 and 4. * The failure of specimens with high interface reinforcement ratios was generally introduced by anchorage failure of the lattice girder in the compression zone or by anchorage failure of the longitudinal reinforcement. * The stress ranges in the lattice girders determined by test results were considerably higher compared to the approved stress range according to the technical approvals. So far, a direct comparison between static and fatigue resistance is not possible due to missing tests. Tests currently in progress will close this gap. The fatigue design for lattice girders according to the technical approvals can generally be confirmed by the own tests; however, they are conservative. By applying the derived \(S\)-\(N\) curve, the evaluation leads to a better agreement of design expression and test data for \(N\leq 2.0\) million load cycles. Furthermore, the range of fatigue application for lattice girders could be extended to \(N>2.0\) million load cycles. Since many slabs in building and bridge constructions are built as continuous slabs, for which higher load-bearing capacities of shear and shear interface around the intermediate supports can also be expected, this hypothesis is currently under investigation for semi-precast slabs. Conceptualization, M.H., J.H. and K.W. ; methodology, M.H., K.W., M.C. and J.H. ; validation, M.H., K.W., M.S. and M.C. ; formal analysis, M.H. and K.W. ; investigation, K.W. ; data curation, M.H. and K.W. ; writing--original draft preparation, M.H. and K.W. ; writing--review and editing, M.H., M.S., M.C. and J.H. ; visualization, M.H. and K.W. ; supervision, M.C. and J.H. ; project administration, J.H. ; funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript. The investigations described in this paper are conducted during research programs initiated by the Research Association of the German Concrete and Precast Industry e.V. and funded by the German Federation of Industrial Research Association (AiF, IGF grant numbers 18407 N/1 and 20580 N) and the Federal Ministry for Economic Affairs and Energy based on a resolution of the German Bundestag. The data presented in this study are available on request from the corresponding author. The authors express their thanks to the sponsors and the members of the project committee for the constructive discussions. The authors declare no conflict of interest.