# Investigations on Interface Shear Fatigue of Semi-Precast Slabs with Lattice Girders
## 1 Introduction  
Lattice girders are prefabricated reinforcement systems to be installed in semi-precast concrete elements, e.g., in semi-precast slabs (also known as precast half slabs, filigree slabs, or precast Omnia floor slabs) or in semi-precast walls (also known as double layered reinforced concrete slabs or precast concrete double/twin walls). On site, semi-precast elements are combined with in situ concrete. Due to the high economic advantages and flexibility, around 70% of all floor slabs in building and industrial engineering in Germany are made of semi-precast slabs with lattice girders [1]. Prefabrication reduces construction time in the building process and minimizes the risk of flaws in the construction while assembling. Precast elements can be used as permanent turnoverk, and the lattice girders combined with in situ concrete ensure a quasi-monolithic structural behavior. In the ultimate limit state, lattice girders serve as interface reinforcement and can also fulfill the function of shear reinforcement to ensure adequate resistance against vertical shear depending on type and disposition [1]. In addition, they play an important role during the construction stage (e.g., during transportation, lifting, and assembling). In buildings, semi-precast slabs are generally subjected to monotonic loading. However, in industrial constructions due to, e.g., fork-lift trucks or oscillating machinery, as well as in bridgeconstruction, semi-precast slabs may be subjected to cyclic loading conditions due to traffic loads. After shear under cyclic loading was studied for a long time almost exclusively on structural elements such as reinforced and prestressed concrete girders, e.g., [2; 3; 4; 5; 6; 7; 8; 9; 10], fatigue research is increasingly focusing on slabs as well [11; 12; 13; 14].  
Because of their advantages, in the state of construction, semi-precast slabs with lattice girders are used in many countries [15; 16; 17; 18; 19; 20]. However, the advantages in the ultimate limit state are almost not applied at all, even though international research has been performed [21; 22; 23; 24]. Much of the research for the ultimate limit state was conducted in Germany (e.g., [25; 26; 27; 1]), where the design of semi-precast slabs with lattice girders follows technical approvals, which are based on Eurocode 2 [28] in combination with the National Annex of Germany [29]. In the ultimate limit state, lattice girders are used as shear, interface shear, and punching shear reinforcement. Since the first semi-precast slabs with lattice girders were built in the 1960s, this construction method has been continuously developed and researched. In recent years, punching and fatigue design have been the main areas of research for the use of lattice girders [30; 31; 32; 33; 34; 35; 36; 37]. The \(S\)-\(N\) curves (\(S\): stress, \(N\): number of load cycles) defined in [29] for profiled bars, which are either bent or welded, are not applicable for the fatigue design of lattice girders. Due to the geometric condition with a combination of bent and point-welded bars with a smooth surface, the design in the technical approvals was limited to a simplified fatigue verification, e.g., [38; 39]. The design regulations in the general technical approvals were derived based on fatigue tests in literature, e.g., [40]. To improve and expand the limits of application, theoretical and experimental investigations are conducted at the Institute of Structural Concrete (IMB), RWTH Aachen University. Some of the results have already been incorporated into the latest technical approvals [41; 42], e.g., an \(S\)-\(N\) curve for lattice girders for a more refined fatigue design.  
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 numbers 18407 N/1 and 20580 N). A detailed description of the results so far can be found in [43].