Potential major traffic disruption averted through NAG's software and SETRA's expertise

The Tancarville bridge in Normandy, France, was built in 1959 and is still one of Europe's longest suspension bridges. When one strand in a suspension cable failed, it was of paramount importance to carry out the necessary repairs effectively and rapidly, given the pivotal role of the bridge in Normandy's traffic system in supporting 16,000 daily users. On the one hand, the evaluation of the corrosion damage to the suspension mechanism was a difficult task, while on the other hand, suspension cables and rods needed to be replaced quickly. Replacing the Tancarville bridge's suspension system without interrupting the traffic flow required both sophisticated numerical simulations and advanced technical logistics.

The decision was taken to install and tension a new suspension system before starting work on the old weakened system. This was clearly a potentially hazardous task. During one stage of the process, when new and old suspension systems were both in place, the bridge was subject to unusual forces. These caused a maximum vertical displacement of the centre of the bridge deck of nearly 2.5 metres, and an inward displacement of the top of the towers of around 0.3 metres. When the work was completed, and the old suspension system was removed altogether, the bridge returned to its design configuration and range of displacements. It was imperative to compute the maximum forces and deformations that the bridge could withstand safely, and to time precisely the work on both new and old suspension systems and the transfer between the two, according to accurate simulations.

The large displacement calculations were carried out by Groupe GTM (now VINCI), a Company dedicated to construction and associated services. The calculations were then tested by SETRA, a French Government owned civil engineering company, which is involved in all aspects of road transport technology and in the planning and maintenance of the road transport infrastructures in France (including conception, construction, exploitation, security and protection of the environment). Initially, SETRA employed a code named "PCP", developed in-house, on Compaq Alpha Unix workstations. It was soon apparent to SETRA that the computing times required were too high for this particular project and, in particular, they would need faster sparse solvers to carry out the simulation work within the very tight time schedule.

SETRA then chose to take advantage of the functionality of the modules from the Sparse Linear Algebra Chapter of the NAG Fortran Library. The decision was taken on the grounds of robustness, accuracy and performance of NAG algorithms, as confirmed by previous work carried out by the SETRA team using NAG software.

"The reputation for reliability of NAG software gave us confidence that we would get the right results. This was critical given that we wanted to replace the suspension cables without interrupting the traffic. We could not tolerate any mistake that would endanger the lives of drivers, and thanks to this NAG software, the computing times were reduced by a factor of 2-3," says Jean Gual, head of the software development team at SETRA.

A major technological feat was carried out successfully, and a potential major traffic disruption averted, through the combination of NAG's software and SETRA’s expertise. Despite the large, but thoroughly safe, displacements undergone, the Tancarville Bridge was repaired successfully without causing any disruption to the traffic flow.

Jean Gual, head of the software development team, SETRA