## 1 Introduction
Furthermore, it was important to clarify which calculation methods can accurately describe the occurring behavior and, thus, whether it is covered by normative standards. In order to obtain answers to these questions, large-scale experimental torsion tests replicating the conditions described above were carried out in the laboratory of the Institute for Structural Engineering of TU Wien. Figure 1: Concept of the new LT-bridge construction method (drawings based on [12]): (**a**) construction process, showing the resulting horizontal and vertical joints; (**b**) longitudinal section over the pier, highlighting the continuous reinforcement in the deck and bottom slabs due to the in situ layers. It is also conceivable that the hollow box girder itself consists of several segments that are connected to one another by post-tensioning. This would result in further joints without continuous reinforcement in the webs. However, the connection to the pier segment is subjected to vertical compressive stresses resulting from the vertical part of the anchorage forces of the post-tensioning tendons (Figure 1b), as well as from self-weight and torsion at the support (pier). This circumstance was taken into account in the experimental test program, with an investigation of joints without vertical compression of the webs planned in further related test series. Typical torsional reinforcement is composed of longitudinal reinforcement and stirrups with the individual load-bearing distribution described by [PERSON] and [PERSON] [13; 14]. The load transfer is characterized by a strut-and-tie model after crack formation (Eurocode 2 [15]). The distribution of the longitudinal reinforcement along the perimeter of the cross-section can be arranged in different ways. It is therefore possible to either distribute the reinforcement evenly around the perimeter or to concentrate it in the corners of the box girder. This is confirmed by the test series described in detail and performed by [PERSON] et al. [16; 17; 18; 19], where these two reinforcement arrangements led to different crack and deformation behaviors during loading but showed no difference in the ultimate torsional moment [20]. Due to peculiarities of the LT-bridge construction method, a concentrated, continuous torsional reinforcement cannot be located exactly in the corners of the cross-section. The continuous longitudinal reinforcement in the bottom slab--crossing the joint to the pier segment--can only be embedded in the in situ concrete layer. As a result, the concentrated corner reinforcement has a lateral offset. This circumstance, along with the unreinforced vertical joints in the webs, leads to the essential differences between the experimental torsion tests described here and those presented by [PERSON] [20]. Furthermore, a large-scale test with a thin-walled box girder cross-section was carried out during the development of the balanced lift method, but joints in the cross-section were not considered [21]. A comprehensive review of torsion tests found in the literature is presented by [PERSON] [22]. In addition, various torsion tests have been carried out on hollow box girders in recent years, e.g., as presented by [PERSON] and [PERSON] [23], where high-strength concrete was used and the ductility behavior was studied in more detail. Further investigations by [PERSON] et al. [24] and [PERSON] [25] verified the so-called \"Softened Membrane Model Theory\" and assessed the influence of joints on the torsional capacity of externally prestressed box girders, respectively. The latter analyzed girders both with and without unreinforced joints. The unique arrangement of unreinforced joints occurring only in the webs--as is the case within the LT-bridge construction method--therefore requires a new experimental investigation, which is presented from Section 2 onwards.