## 1 Introduction  
The use of thin-walled prefabricated concrete elements in bridge engineering has played a major role in the relevant research activities of the Institute of Structural Engineering of TU Wien in recent years [1; 2]. The implementation of such lightweight structural elements allows for the development of new approaches for the construction of bridges, as was impressively demonstrated when the balanced lowering method [3; 4; 5] was used to build two bridges over the Lafnitz and Lahnbach Rivers in Austria. Another application concerns the semi-precast segmental bridge construction method [6; 7; 8], in which thin-walled precast concrete elements are assembled to form a box girder with the aid of steel components. The latest development to emerge from this research area is the new LT-bridge construction method, which combines hollow box girders and special deck slab elements (both components made of thin-walled precast elements), focusing yet again on erecting bridges in a very short construction time [9; 10]. The term \"LT-bridge construction method\" originates from the span directions of the individual elements, with the hollow box girders spanning longitudinally (L) and the deck slab elements spanning transversally (T). Currently, the method has only been developed for single-cell box girders. This paper focuses on the torsional bearing behavior of the resulting bridge superstructure, as it presents peculiarities that require a detailed scientific investigation. The uniqueness of the finished bridge superstructure stems from the fact that it has continuous deck and bottom slabs, with regard to the longitudinal direction of the bridge, while simultaneously working with vertical shear joints without longitudinal reinforcement in the webs. This circumstance results directly from the bridge construction process, which is depicted in Figure 1a. A hollow box girder with a wall thickness of around 100-200 mm is lifted in between two already-placed pier segments. In this construction phase, horizontal and vertical joints are still existent between the girder and pier segments. Within the next construction step, special deck slab elements are placed on top of the previously mounted girder. With the use of in situ concrete layers, the deck and bottom slabs attain their final thicknesses. This subsequently cast concrete allows for the placement of continuous longitudinal reinforcement in the deck and bottom slabs, whereby rebar couplers are used in the latter (Figure 1b). Furthermore, post-tensioning is used to connect the girder to the pier segment. The result is a bridge superstructure with unreinforced joints in the webs at the connection to the pier segment and continuous longitudinally reinforced bottom and deck slabs. The LT-bridge construction method is therefore the motivation for the investigation of box girders with this unique reinforcement arrangement at the joints. The purpose of this paper was to analyze how the unreinforced joints in the webs affect the torsional load-bearing behavior. Compared to a superstructure of a bridge erected with the LT-bridge construction method, simplifications--which are discussed in more detail in Section 2--were made. As the investigation was seen as fundamental, post-tensioning was omitted. This resulted in the shear joints not being under longitudinal compressive stresses, possibly reducing the transmission of shear stresses. In further test series, the influence of post-tensioning will be considered in detail, including long-term effects, e.g., as described by [PERSON] et al. [11]. The main objective of the research presented in this paper was to gain an understanding of how reinforced concrete box girders resist torsion when unreinforced joints are present in the webs. Of particular interest is the comparison to the behavior of regular girders with continuous longitudinal reinforcement throughout the entire cross-section.