# Investigations on Interface Shear Fatigue of Semi-Precast Slabs with Lattice Girders
## 5 Tests on Continuous Semi-Precast Slabs with Lattice Girders  
The design and construction rules for semi-precast slabs with lattice girders for shear and interface shear were derived mainly from tests on single-span beams, representing slab stripes. However, many slabs in building and bridge construction are built as continuous slabs. For continuous slabs and beams, the shear capacity in some member segments is significantly higher than for single-span systems due to the lower shear slenderness, as shown by tests on monolithic structural members [67; 68]. Due to the horizontal deformation restraint at the intermediate supports of continuous members, higher load-bearing capacities of the interface around the intermediate supports can also be expected for semi-precast slabs. The influence of a continuous member on the composite joint and shear bearing capacity of semi-precast slabs has not yet been clarified for both static and cyclic loading.  
The shear capacity of beams and slabs is significantly influenced by the shear slenderness [68; 69; 70; 71; 72; 73; 74; 75; 76]. In the relevant area for the shear design, the shear capacity increases for small shear slenderness and decreases for large shear slenderness. The shear slenderness corresponds to the ratio of the distance between the point of contraflexure and the maximum moment to the effective depth and is generally described by \(\lambda=M/(V\cdot d)\). For statically determinate single-span beams under concentrated loads, the shear slenderness can be described in a simplified way as a function of the load distance to the support \(a\) with \(\lambda=a/d\). This simplified calculation is not possible for statically indeterminate continuous beams. For the same load distance to the support related to the static effective depth \(a/d\), significantly smaller shear slenderness \(\lambda\) results for intermediate supports of continuous beams than for single-span beams and end supports (Figure 21a) and thus also larger shear-bearing capacities.  
Current investigations on slab stripes without shear reinforcement [77; 78] confirm these relationships (Figure 21b). For continuous beams, smaller shear slendernesses and thus larger load capacities result for the same load distances. Investigations on continuous beams with shear reinforcement [79; 80; 81; 82; 83; 84; 85] also confirm the increase in load-carrying capacity,  
Figure 21: Relationship between shear slenderness \(\lambda\) and (**a**) \(a/d\)—ratio and (**b**) shear capacity of single-span and continuous beams (according to [77; 78]).  
which is partially lost with increasing shear reinforcement ratio. The positive influence of shear slenderness is not taken into account according to the current design concepts for semi-precast slabs [41; 42], which usually have small and medium shear reinforcement ratios. For this reason, tests on single-span slabs with a loaded cantilever have been included in the current Series 3 (h = 16 cm) and 4 (h = 34 cm). The single-span slab with a cantilever as a static determined system is representative for a test on a continuous slab and can provide knowledge about the bearing capacities in the area of the intermediate support. Static and cyclic tests are carried out (Figure 22). To extend the data from Series 0, 1, and 2 and to better classify the tests on intermediate supports, static and cyclic tests on single span slabs are also performed for the same combination of parameters as for slabs with cantilevers. In order to characterize the development of shear cracks and cracks along the horizontal interface, the method of digital image correlation is employed [86]. A publication on Test Series 3 and 4 will appear after the experiments have been executed and evaluated.