ABSTRACT
This paper presents the replacement works of cable stays of the 33 years old Tjörn Bridge located in Sweden. The bridge has a main span 366 m and a total length of 664 m. The bridge has a locked coil cable system of 64 cables, where some cables had been subjected to corrosion and wire failures due to exposure to water and ice. The paper describes the full story from detecting the corrosion problem to restoring the safety of the bridge by replacing the cables in a safe way.
In the coming years, the replacement is likely to become a major challenge to many owners of older cable stay bridges. Corrosion of cables is an inherent risk to many cable systems as water may penetrate into the low anchorages. This problem may accelerate, if the cables are subjected to vibrations or unintended stress concentrations.
For the Tjörn Bridge, the history of inspection and identification of the corrosion problem is described and the challenges and solutions of the replacement of cables are presented and discussed. As several cables typically had been exposed to the corrosion risk, the replacements carried out, required an in depth safety analyses to consider the strength of the other possible degraded cables. This was accomplished based on inspection results and a finite element model of the degraded cable system. The model was calibrated to measured cable forces and tensile tests carried out in the laboratory.
The cable forces were determined from a 3D point cloud of the cables by use of laser scanning and also frequency testing. The laser scanning technique has shown up to be a very time and cost efficient technique when cable forces of many cables have to be measured. Tjörn Bridge. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig119_1.tif"/>
New cables were procured by Trafikverket and a tender was prepared. As basis for the tender, the replacement of cables also required an accurate analysis of the process of removing the old cable and installing the new cables. This process included consideration of the risks to traffic, safety of personnel, and risks of damages to the bridge and other cables. In total, the rehabilitation also included necessary mitigating measures for protecting against future corrosion of the new cables and for stopping the corrosion process in the existing cables. These mitigating measures were carried out 2 years before the cable replacements to stop the degradation problem. They comprised an improved drainage system, enhanced water tightness and better access for regular inspection and maintenance.
After successful replacement of two cable stays and other mitigation actions, the safety of the bridge was updated based on supplementary inspections, new measurements of cable forces and tensile testing carried out in laboratory of the replaced cables. With the improved safety of the bridge, the owner Trafikverket now has obtained a time window to carry out preparation of a long-term mitigation and maintenance program.
