With the help of prestressed FRP elements, the connections are relieved and the service life of the bridge is extended. A similar project had already been implemented in 2013, but at that time it was about strengthening the cross girders. In this situation, it was possible to rely on the proven S&P FRP laminates in combination with end anchorages. 

In the present case of the Aabach Bridge, it quickly became apparent that FRP laminates were not suitable due to the structural characteristics of the bridge. A more space-saving solution was required that could be installed close to the connection of the longitudinal and transverse girders in order to absorb the loads in the best possible way. 

Through the rivet hole

The project team therefore developed a new prestressing system with a round carbon rod. The rod is guided through one of the rivet holes where the rivet was previously removed. However, this also required the engineers to develop a new end anchorage with pretensioning device for the carbon rod. To do this, the developers relied on the proven principle of a cylinder with clamping wedge. The challenge here was to design the clamping wedge in such a way that it presses on the carbon rod as evenly as possible under load and without punctual pressure. Otherwise, the carbon fibers could break, which would weaken the rod and its hold in the anchorage.

In addition, a device was developed to pretension the wedge in the cylinder together with the carbon rod. This allows the required prestressing force to be built up much better and with less slip during the actual prestressing process on the bridge. The clamping devices for fastening to the steel girders also had to be specially calculated, planned and manufactured.

Clear advantages with FRP

The high degree of individual calculations, planning and production processes is typical for the refurbishment of such steel bridges. This is because they often differ greatly in terms of their design, the material used and their current condition. Nevertheless, strengthening with FRP systems has decisive advantages over the previously known options:

• The bridge does not have to be closed, there is no interference with train traffic
• Cost-effective preparation: the existing corrosion protection does not have to be removed
• No welding work is required, which could weaken the steel
• The components of the prestressing system are corrosion resistant
• The system can be easily inspected, maintained and readjusted

Service life massively extended

The project executed in 2013 with FRP laminates as well as the described reinforcement of the Aabach bridge are permanently monitored by means of electronic sensors as part of the project work. The data from the monitoring and from the laboratory tests as well as the calculations show that the service life of steel bridges can be extended by up to 50 years with the S&P FRP system.

Broad-based project

The system was developed and implemented in close cooperation with the Swiss Federal Institute of Technology Lausanne (EPFL), the Swiss Federal Laboratories for Materials Testing and Research (Empa), the Swiss Federal Railways (SBB) and dsp Ingenieure + Planer AG. In addition, the Swiss Agency for Innovation Promotion (Innosuisse) supported the project.