# Investigations on Interface Shear Fatigue of Semi-Precast Slabs with Lattice Girders
## 2 Lattice Girders for Use in Cyclically Loaded Structures
### Fatigue Tests from Literature  
The fatigue behavior of semi-precast slabs with lattice girders has been investigated to derive design specifications for technical approvals. In this context, 29 fatigue tests have been conducted and described in literature. For semi-precast slabs with lattice girders KTS [50], EQ [40], and KT100 [51], the specimens were tested with simply supported slabs with two point loads. For element walls with KTW [52], the specimens were tested as simply supported slabs with cantilevers. Table 1 gives an overview of all tests that have been documented in the literature regarding the type of lattice girder, number of tests, and test setups. All fatigue tests were conducted under cyclic loading with a frequency of \(f\) = 2.0-2.5 Hz and a constant load range up to \(N\) = 2.0 million load cycles. The upper load limit for the cyclic loading was determined to generate a tensile stress in the diagonals of 70% of the yield strength, which results in a load exceeding the approved frequent load by 25%. The applied stress range in the diagonals varied between \(\Delta\sigma_{\text{Dia}}\) = 180-230 N/mm\({}^{2}\). After reaching the \(N\) = 2.0 million load cycles, the specimens were loaded monotonically until failure. Comprehensively for all test series, the shear span to effective depth ratio was kept between \(a/d\) = 3.4-3.8, and the concrete compressive strength varied between \(f_{\text{cm,cyl}}\) = 16.1-39.3 N/mm\({}^{2}\) for the precast slab and \(f_{\text{cm,cyl}}\) = 13.9-32.5 N/mm\({}^{2}\) for the in situ concrete. The lattice girders in the slab's cross section were placed in one, two, or three rows to determine the effect of the amount of interface reinforcement. The interface quality was left as cast, with an additional bond breaker or intentionally roughened by raking (according to [29]). Figure 3 shows an exemplary test specimen from the test series conducted for lattice girder KTS [50]. Most tests were stopped before fatigue failure after reaching the reference number of load cycles, and a static test was conducted to assess residual strength. Another group of tests showed fatigue bending failure. Interface shear failure only occurred for four specimens during cyclic loading and for two specimens during st \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Geometry**} & \multicolumn{1}{c}{**Lattice Girder**} & \multicolumn{1}{c}{**Test Setup**} \\ \hline \multicolumn{2}{c}{**Baustahlgewebe**} & \multicolumn{1}{c}{**Filigran**} & \\ \hline \multirow{3}{*}{slabs} & KTS & EQ & \\  & \(n=10\) & \(n=2\) & \\  & KTI00 & & \\  & \(n=14\) & & \\ \hline \multirow{3}{*}{walls} & KTM & & \\  & \(n=3\) & & \\ \hline \hline \end{tabular}  
n: number of tests; test reports: KTS: [50]; EQ: [40]; KT100: [51]; KTM: [52].  
\end{table} 
Table 1: Fatigue tests with lattice girders from literature. Figure 2: Geometries of lattice girder KTS / EQ according to [41; 42]. strength. These specimens had a smooth surface including a bond breaker. Vertical shear failure did not occur at all. To specifically determine the fatigue resistance of the critical section of the lattice girders at the bent and welded node between diagonal and chord, additional small size tests have been conducted with lattice girders KTW and EQ (see Table 1). Therefore, the diagonals were cast in concrete cubes with a concrete strength of about \(f_{\text{cm,cyt}}=20\) N/mm\({}^{2}\) and with an interruption of bond by a cladding tube installed above the welded node (Figure 4a). The cyclic loading was applied to the diagonal with a constant stress range and a frequency of \(f=100\) Hz. If a failure occurred in the small size tests, it was detected in the welded node between diagonal and chord (Figure 4b). In Figure 4c, the applied stress ranges are shown with the corresponding number of load cycles.