Giving new life to fatigue life-expired critical details

Structures with theoretical fatigue life-expired details in critical locations may pose a significant risk to the ongoing safety of the transport network. This paper examines a case study in which a structure with such critical details with theoretically expired fatigue lives was successfully managed and remediated without extensive traffic closures. Gade Valley Viaduct is a multispan composite box girder viaduct completed in 1986 located in Hertfordshire, UK, near the town of Kings Langley. It carries the M25 London orbital motorway with dual 4 lane carriageways together with tapering on and off-slip roads carried on separate structures. Initial observations of movement of the slender unstiffened bottom flange panels were identified early in the life of the structure and confirmed by strain gauge studies of sample panels. A detailed assessment using the as-built geometry confirmed the extent of the fatigue shortfall and the sensitivity to small cracks. The fatigue shortfall locations were confirmed by monitoring-based assessment and full remediation has been implemented at these locations. A combination of welded stiffeners, bolted doubler plates and weld treatment by plasma dressing was deployed to give a full 120-year fatigue life to previously life-expired safety-critical details. The development of the plasma dressing procedure and associated quality controls used in the existing live structure are described. The extensive use of off-site testing followed by on-site testing in a mock-up girder before application on the live structure were key features of this project. This case study illustrates how heavily used structures with fatigue shortfalls can be managed to ensure safety and rehabilitated without costly and disruptive traffic closures.