The oldest bridge in the world is thought to be an arch bridge built in Turkey around 850 BC.
Although most bridges are not expected to last so long, an increasing number are still in use well beyond their intended design life. At some time in their existence most bridges will require repair and strengthening, not only to correct the inevitable age-related structural defects, but also to enable them to support the live loading required by modern design standards. This themed issue includes eight papers, which describe the various technical and practical challenges involved in the repair and strengthening of a range of structures.
In the first paper, Banerji et al. (2014) set out a procedure for determining the structural response of a strategically important prestressed concrete box girder bridge with longitudinal web cracking. The precursor to any repair and strengthening process is the understanding of the structure's response. The paper sets out the monitoring undertaken, the assembly of data and its incorporation in the simulation modelling necessary to inform both current structural capacity and future remediation strategy.
Colford et al. (2014) describe a significant project to replace the bearings for the Forth Road Bridge approach viaducts in Scotland. ‘Lock up’ of the roller bearings and local concrete deterioration led to the decision to replace the bearings. Since the original design made no provisions for bearing replacement, the bridge piers first had to be strengthened and modified, while maintaining the traffic flow. This paper outlines the innovative approach taken together with a detailed description of the temporary works needed to maintain the bridge's articulation during jacking.
Silla–Sanchez and Noonan (2014) make a welcome contribution to the themed issue from Australia with their paper describing the management of fatigue cracking on the West Gate Bridge in Melbourne. Following a recent major strengthening project, ongoing management of existing and potential fatigue cracking in the steel spans was necessary. The paper describes the analysis undertaken, the management strategy and the range of treatment details developed to address existing cracked components and those identified as being vulnerable to crack initiation.
The paper by Rammer and Espion (2014) starts by offering an insight into the early development of Belgian post-tensioning before focusing on one of the last applications of the Blaton–Magnel ‘sandwich’ system. The prestressing cables, being external to the concrete and only protected by cement grout, became so severely corroded as to jeopardise the safety of the structure. Repair was effected by the replacement of all existing cables with new tendons placed in high-density polyethylene ducts. The assessment and execution are described, the latter not without problems, as is frequently the case in incorporating new components into old structures. The authors draw a number of useful lessons from the experience and present these along with their conclusions.
Stacy et al. (2014) describe the process and challenges of incorporating existing bridges into the Manchester Metrolink phase 3 project. The paper highlights the particular issues encountered when existing structures require refurbishment to accommodate a modern transport system, all at an affordable cost. The paper gives examples of the repair and strengthening techniques adopted for a variety of structural forms. It also discusses the refurbishment strategy and the approach taken to risk mitigation.
In the sixth paper, McCarron and McAdam (2014) describe the innovative approach to the design and construction of the repairs to several iconic nineteenth-century railway underbridges on Ireland's national railway system. The paper discusses in detail the combination of advanced analysis techniques, laboratory research, testing and construction methods used to develop and build the preferred solutions. Although the techniques described were bespoke, they are repeatable and therefore applicable in other similar situations. The paper perfectly illustrates how existing infrastructure can be preserved for future service, rather than the costly and disruptive alternative of complete replacement.
Lowe et al. (2014) describe the strengthening of the oldest reinforced concrete bridges in Shropshire (and, indeed, a very early national example). The listed shallow arch bridge, having already given 100 years of service, has been strengthened by means of the comparatively recent material, carbon fibre reinforced polymer (CFRP). The authors describe the ground and concrete investigation, the consideration of feasible options, design and construction. Furthermore, in common with many others of this vintage, this bridge forms an important and longstanding link in the local network, and community engagement throughout is vital in allaying commercial and public concerns. The authors outline their consultations along with the opportunity taken to inspire possible future engineers in a local primary school.
In the last paper, Redpath et al. (2014) describe the design and construction of structures designed to protect the piers supporting Tay Road Bridge from collision by errant marine vessels. The singular function of the three stand-alone structures is to completely absorb the energy resulting from a collision, arresting a vessel that would otherwise severely damage the bridge. Cost, form, geology, bathymetry, geometry and execution are weighed against a background of functional, environmental and aesthetic considerations. The authors set out the contractual, design and execution methods deployed in providing protection to this important estuarial crossing.
