Rosensteinsteg II—A cable footbridge with deck elements of CFRP‐reinforced concrete

Rosensteinsteg II is a lightweight cable girder footbridge with a span of 29 m in the city of Stuttgart, Germany (Figure 1). Together with Rosensteinsteg I, a suspension bridge with a total length of 78 m, it bridges the tram line U14 and the federal road B14 and connects Rosensteinpark with the eastern city district and the mineral bath Leuze. The bridge family was built on the occasion of the federal garden exhibition in 1977 and was designed by a group of renowned German engineers, involving Jörg Schlaich, Rudolf Bergermann and Günther Mayr. It can be seen as a typical example of the Stuttgart school of lightweight design. In 2014, the superstructure and one foundation of Rosensteinsteg II had to be disassembled, to allow for the construction of a new road tunnel as part of B14. Also, the bridge had already been in poor condition for a long time due to heavy corrosion of the concrete deck elements (Figure 2). As per design standards in 1977, the extraordinarily slender concrete elements with a total thickness of only 10 cm had been designed with a concrete cover of merely 30 mm on the top side and 20 mm on the bottom side. Corrosion of the steel reinforcement led to concrete spalling in large areas on the lower surface of the concrete elements, thus all

Rosensteinsteg II is a lightweight cable girder footbridge with a span of 29 m in the city of Stuttgart, Germany ( Figure 1). Together with Rosensteinsteg I, a suspension bridge with a total length of 78 m, it bridges the tram line U14 and the federal road B14 and connects Rosensteinpark with the eastern city district and the mineral bath Leuze. The bridge family was built on the occasion of the federal garden exhibition in 1977 and was designed by a group of renowned German engineers, involving Jörg Schlaich, Rudolf Bergermann and Günther Mayr. It can be seen as a typical example of the Stuttgart school of lightweight design.
In 2014, the superstructure and one foundation of Rosensteinsteg II had to be disassembled, to allow for the construction of a new road tunnel as part of B14. Also, the bridge had already been in poor condition for a long time due to heavy corrosion of the concrete deck elements ( Figure 2). As per design standards in 1977, the extraordinarily slender concrete elements with a total thickness of only 10 cm had been designed with a concrete cover of merely 30 mm on the top side and 20 mm on the bottom side. Corrosion of the steel reinforcement led to concrete spalling in large areas on the lower surface of the concrete elements, thus all F I G U R E 1 Rosensteinsteg II after reconstruction in 2019. concrete elements were due for replacement. After completion of the road tunnel, the demolished foundation and the bridge superstructure were to be reconstructed.
For the reconstruction of the footbridge, the design team from schlaich bergermann partner aimed to achieve the same slenderness of concrete deck elements as in the original structure, while at the same time minimizing the risk of corrosion and thus ensuring a much longer service life for the reconstructed structure. Together with the client, they decided to utilize the relatively novel reinforcement material CFRP (carbon fiber reinforced polymer) in order to not only reduce the risk of corrosion, but to use an entirely noncorrosive reinforcement material.
Concrete elements were reinforced with meshes of CFRP reinforcement manufactured by the German firm solidian in Albstadt. By recommendation of the manufacturer, a concrete cover of 15 mm was maintained for the CFRP meshes. Casting of the 3.3 m wide and ca. 1.1 m long concrete elements ( Figure 3) was carried out by the precast concrete firm informbeton in Schwepnitz, with a concrete C50/60 with a maximum aggregate size of 8 mm.
As for CFRP reinforcement no valid design codes were available at the time of planning of the project, a special building permission (Zustimmung im Einzelfall) had to be obtained for the execution of the project. For this, full scale destructive testing had to be undertaken on the concrete elements to experimentally verify the assumptions of moment capacity, shear capacity and the capacity of connections of the railing to the concrete elements. Testing was performed at the Chair of Conceptual and Structural Design at Technische Universität Berlin, who have more than 15 years of experience with the design and testing of concrete elements reinforced and prestressed with CFRP.
For the bending design of the CFRP-reinforced concrete elements, designer, testing institution and the checking engineer agreed to maintain an additional factor of 1.5 between the design bending capacity and the design bending moment for additional safety in the case of brittle failure by reinforcement rupture, similar as suggested by the current Canadian Code for FRP-reinforced concrete structures. In the design calculation, failure by reinforcement rupture was predicted. For the testing program, the following load scenarios were considered: • Maximum bending moment at midspan of the concrete elements, which span between the two main cables. Maximum bending results from a point load of 10 kN as per Eurocode. In the tests, this moment was applied in the layout of a typical four-point bending test (Figure 4) in order to achieve a larger area of maximum bending. • Maximum shear, which is induced by a point load of 10 kN with a distance of 1.5 times the concrete element thickness from the support. • Maximum moment at the railing connection, which is induced by a horizontal load of 1 kN/m on the railing handle (height of 1 m assumed in the original calculation).
For the case of unforeseen errors in the test execution, the client had provided a fourth concrete element to the testing facility. After successful completion of the three required tests, this element was used to determine the behavior of the concrete element under repeated loading (15 loading cycles) until the design load for maximum moment at midspan, before being loaded until failure in bending. As in the cable girder the concrete elements are fixed on only 4 points to the main cables, also in the tests the elements were supported on 4 points with steel plates on roller bearings. In all four tests, the experimentally determined load capacity of the CFRP-reinforced concrete elements exceeded the assumptions from the design. Bending behavior of the elements was semiductile, with large deformation (102 mm deflection at a span of 2.30 m) and clearly visible crack development (Figure 4). Contrary to predictions from the design, one of the two elements tested in bending failed not by CFRP rupture but by crushing of the compression zone. This can be explained by the fact that for reinforcement of the edges of the element, additional U-shaped reinforcement meshes had been added, which were not considered in the bending design but would shift the stress distribution toward concrete crushing. The second element tested in bending failed by rupture of the CFRP mesh, as expected. The fact that both failure modes occurred, shows that the elements were very close to the "balanced section," which reaches the ultimate design strain simultaneously at the tensile and compression side. Repeated loading in 15 cycles led to a very modest increase in midspan deflection from 3 to 4 mm and cracks of ca. 0.1 mm crack width at the design load. In the serviceability limit state, very small deformations (2 mm, only 22% of the permissible L/250) and crack widths smaller than 0.1 mm were observed, which promises excellent behavior of the elements in service conditions.
After completion of the B14 tunnel, the previously removed foundation of Rosensteinsteg II was newly constructed and the second, remaining foundation was repaired. The superstructure consisting of the cable girder and the deck elements was then assembled in the course of one weekend, while operation of the tram line U14 was suspended. With the reopening of Rosensteinsteg II in December 2019, the city of Stuttgart reobtained one of its characteristic lightweight structures ( Figure 5). The use of CFRP-reinforced concrete elements will hopefully contribute to making a long service life of this beautiful structure possible.

ACKNOWLEDGMENT
Open access funding enabled and organized by Projekt DEAL.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.