This edition of Bridge Engineering is the first of two themed issues devoted to arch bridges. Arch bridges come in many forms, from the conventional stone and brick arch bridge with rubble or granular fill to the more spectacular long-span modern arch bridges constructed with steel and concrete of which there are over 100 worldwide with main spans in excess of 300 m. When the call for abstracts on the theme of arch bridges was published in April 2010, the response was so strong that it was impossible to contain all the papers offered in one issue of the Journal. It was therefore decided to produce two themed issues, one devoted to the more traditional arch bridge form and a second concentrating on the modern development of this fascinating structural system.
This first issue concentrates on the traditional form of construction, the stone and brick arch bridges that have become such an inherent and recognisable part of our road, rail and waterway infrastructure. The second issue, to be published in September 2013, will focus on the more innovative arch bridge forms that have appeared over the last few decades. The papers presented in this issue focus on the various issues associated with the management of masonry arch bridges. The importance of these issues has only come to light in the last few decades in spite of the long track record of these bridges.
Masonry arch bridges are important structures in today's infrastructure not only because of their heritage value, but also for their excellent structural performance and low maintenance cost. In addition, the large number of these bridges on the road and rail network makes them an indispensible component of the national transport infrastructure: in the UK and Ireland, for example, 60% of all bridges are arches. Engineers regard the masonry arch bridge as a very efficient and elegant structural form with a long service life and very low maintenance requirements. Their decline as a preferred solution for short to medium-span bridges stems more from the lack of suitable craftsmen to build these wonderful structures than from any inherent structural weakness. Nevertheless, while these arch bridges have continued to function with little or no maintenance for many generations, it is now recognised that they need careful management if they are to continue to perform as required, particularly given the significant recent increases in volume and intensity of live loading.
The detailed inspection of arch bridges can be used to provide important information on how these structures perform in service. Harvey (2012) has examined a large number of bridges, both in service and during demolition, and has used the visual evidence obtained to determine how these structures carry load. His forensic analysis of the damage found in these bridges presents an interesting way of identifying the typical load paths through bridges with varying geometry, spandrel details and foundation properties, and provides the basis for a new behavioural model.
Wilmers (2012), with his experience of over 30 arch bridges, reports on the typical defects found in Germany and appropriate methods of rehabilitation. Wilmer's paper focuses on methods of repair, particularly those using new materials such as specialised mortar, stabilising chemicals and lightweight concrete. These techniques are used not simply to restore these bridges to their original state, but to make them fit for purpose in relation to modern traffic and extend their useful service life by many decades. It is noteworthy that the problems found by both Wilmers and Harvey stem mainly from a lack of basic routine maintenance, such as protection from scour, clearing of vegetation, effective drainage and re-pointing at appropriate intervals.
Two papers are devoted to the investigation of soil–structure interaction in arch bridges. Callaway et al. (2012) used the results of load tests on small-scale arch models to investigate the contribution that the fill material makes to the strength. Their research focussed on separating the beneficial effects of loads distribution to the arch and passive restraint to the arch barrel. Their conclusion was that this is a more effective and accurate way of modelling arch behaviour and a technique that could be used to improve analysis methods. Foglar and Kristek (2012) examined the soil–structure interaction occurring in buried arch bridges during both construction and operation. They propose a method using centre-line optimisation to make the structural system more effective for the design and construction of buried arches.
The problems of assessment and realistic method of determining the capacity of masonry arch bridges is taken a stage further by Gibbons and Fanning (2012) who carried out a comprehensive review of various analysis methods routinely used for arch bridge assessment. The natural assumption that the level of sophistication of the analysis increases the accuracy of the result is put to the test by examining the results for ten stone arch bridges typical of the bridge stock in Ireland. This analysis indicated that this is not always the case: the simplest assessment algorithms are not always the most conservative. The paper concludes by evaluating the different assessment approach and identifying the circumstances in which the different methods give an appropriate assessment rating. This study is being used to provide the framework for a more realistic and reliable assessment methodology.
Parker et al. (2012) report on the potential effects of ground movements on the masonry bridges supporting London Bridge railway station. Before the construction of the 310 m high Shard tower in London, it was expected that ground vibrations resulting from demolition, excavation, piling and other construction activity might have an adverse effect on the nearby arch bridges. For this reason a comprehensive monitoring system was put in place to measure ground movements, cracking and strain levels. Their paper presents an interesting case study of the effects of new construction on existing structures and demonstrates how monitoring can be used to provide an early warning system of possible problems. It also raises the issue of tolerance to ground movements and discusses why these arch bridges are more tolerant to ground movements than other similar structures, such as those reported by Boscardin and Cording (1987). It was concluded that the use of lime mortar for the arch and foundation construction, and fact that the bridges were founded on flexible alluvial material made these structures more resilient to ground movements.
Ellis and Ellis (2012), in their short article on the replacement of an old 1906 arch bridge in Canada, illustrate how new and old technologies can be combined to retain the historical appearance of the bridge while still carrying out a major rehabilitation project. In this case, cast-in-place concrete was used to retain some of the existing elements of the bridge including balustrades and end-posts. Other modern techniques used were fibre-reinforced-polymer reinforcement in the parapet walls and concrete colourisation and finishing to retain the character of the structure.
This collection of papers represents the many issues relating to the management of this very important component of our bridge stock. Some of these structures are more than 500 years old and are experiencing loads never envisaged by their original designers. Indeed many old arch bridges are quite happily carrying modern 44 tonne road vehicles and even heavier railway locomotives without any apparent signs of distress. This point is emphasised over and over again by many bridge engineers who are amazed by the robustness and resilience of these bridges. The papers presented here reflect just some of the work being carried out to ensure that, with effective management, they continue to perform in the future as well as they have in the past.
