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A new bridge construction method, combining semi-precast elements and in situ concrete, has been developed at TU Wien, with the aim of decreasing erection time. In the course of construction using this new method, structural conditions arise that render a more detailed investigation necessary. By connecting a precast, thin-walled box girder to a bridge segment located on a pier by means of post-tensioning, a joint is created. By casting in situ concrete on the bottom and top slabs, the joint can be bridged with longitudinal reinforcement; however, the unreinforced vertical joints in the webs remain. This detail is a specific characteristic of the LT-bridge construction method and needs to be further investigated and assessed, as the question arises as to how this circumstance affects the torsional bearing behavior of the bridge superstructure. Torsion tests described in the literature consider ordinary box girders with longitudinal reinforcement or post-tensioned segmental bridges without longitudinal reinforcement at the joints. Therefore, the new reinforcement layout at the joints had to be investigated experimentally. Two large-scale thin-walled box girders—one without joints in the webs and the other with unreinforced joints in the webs—were tested, allowing for a direct comparison of conventionally manufactured bridges and those erected with the new bridge construction method. Furthermore, we investigated whether the results of common calculation methods corresponded to the experimental findings.

Various methods have been developed to produce deck slabs for steel–concrete composite bridges. Usually, the deck slabs are cast with in situ concrete using a formwork carriage, resulting in construction progress of 15 to 25 m of deck slab per week. A new construction method was developed at the Institute of Structural Engineering (TU Wien), which enables the swift erection of the concrete deck slab. This method employs precast deck slab elements with reinforced concrete cross-beams which span in the transverse direction. With this new construction method, producing up to two deck slab sections of 15–25 m per day becomes possible. Further, the performance of novel reinforcement detailing required for the precast deck slab elements is investigated by structural testing. The experiments consist of eight load-bearing tests on four specimens which represent sections of the element during casting and after completion. The investigated parameters in series 1 are the length and spacing of loops, used for protruding longitudinal bars enclosure. In series 2, the enclosure of the shear reinforcement and the height of the cross beams are varied. The results show that the targeted bending capacity could be reached in all tests with no signs of premature failure due to detailing reasons. Based on the experimental results, the feasibility of the new approach is shown and recommendations for detailing are given.

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<p>In the construction of Lafnitz Bridge and Lahnbach Bridge thin-walled precast concrete girders with an U-shaped cross-section were used in order to reduce the weight during the construction process. The usage of light-weight girders was advantageous because the girders were erected in the vertical position and rotated into the final horizontal position during the first application of the balanced lowering method. After filling of the girders with in-situ concrete the webs of the bridges were com- pleted. On top of the webs a deck slab was cast using a formwork carriage. While the application of the balanced lowering method permitted a fast construction of the webs, the production of the deck slab with a formwork carriage was a slow process. Therefore a method for the fast production of the deck slab was developed. In this new method precast slab elements, which are stiffened by cross-beams, are placed on top of the webs. This enables a fast construction of post-tensioned con- crete bridges with a plate girder cross-section.</p>