For nearly fifteen years, the arch strengthening design and build service known as Archtec has been successfully used by many bridge owners to help manage their bridge stock. Now, this technology is being used for railway bridges and is set to introduce similar benefits to those that have been well recognised by highway authorities.
Originally the motivation for developing the Archtec system was the introduction of 40/44 tonne vehicle load rating and to provide a cost effective alternative to traditional assessment and strengthening routes. This driver continues, particularly overseas, but increasingly in response to rail traffic rather than road.
It is now also recognised as offering an alternative, affordable and sustainable solution compared with traditional saddling, or in some cases bridge replacement, by improved use of mostly existing materials.
Archtec has involved a significant research programme over the years including advanced analysis, full scale tests and the monitoring of bridges. A special team was formed, which includes academics, engineers/analysts, project managers and specialist contractors, to deliver this service.
Internationally, around 250 bridges have now been strengthened with many more assessed and found to be adequate. This team is a partnership which brings together several specialists; Cintec International Limited, Rockfield Software Limited and Ramboll UK Limited (formerly Gifford).
Calculating the strength of arch bridges
Despite masonry arches being ancient in form, it remains notoriously difficult to accurately assess their strength. Their behaviour is complex and involves the interaction of individual parts, blocks, bricks, mortar and fill. Several methods for assessing the strength of arch bridges have become well established, for example limit analysis, which is a vital activity where traffic loads increase.
However, their generalised use is limited and their application for designing strengthening difficult. Finite element analysis has also been successfully used although modelling materials to obtain realistic behaviour is challenging.
Instead, the Finite/Discrete Element Method (FDEM), which involves the automatic computation of interacting bodies, is applied in the Archtec strength assessment and strengthening design processes. This is a generalised approach, as is finite element analysis, which means that any geometric form of masonry can be simulated.
As a result, there are no restrictions to the arch bridge arrangements that can be considered, for instance the number of spans, rings and piers that can be assessed. Similarly any type of loading, from highways, railways and even ground movements, can be applied.
The application of FDEM has marked a step change in the level of sophistication which can now be applied to the structural analysis of masonry arch bridges. Not only can it be used to accurately assess strength, but also to determine bridge deformation, including important non-linear effects, making it possible to assess behaviour at both strength and serviceability limit states.
Being a generalised approach, the behaviour of complex bridges can be assessed where, for example, a concrete saddle may exist, a bridge has been propped and, in the case of strengthening, retrofitted reinforcement is introduced.
Strengthening weak bridges
Arches conventionally fail by the development of four hinges leading to a mechanism. The design basis for Archtec strengthening is to locate reinforcement where hinges are predicted to develop so as to improve bending strength. By providing additional strength in this way, the arch barrel is better able to resist live load and peak compressive stresses in the masonry are reduced compared with similar unstrengthened cases.
The same procedure is applied to more complex bridge arrangements, including multi-span arches, although failure mechanisms and reinforcement positioning requires different locations to be considered in design.
The method of strengthening involves the installation of Cintec anchors. These have been developed to allow the retrofitting of stainless steel reinforcement around the circumference of the arch barrel. The reinforcement is then grouted into holes, which have been precisely drilled into the bridge using a diamond coring rig, providing a shear connection with the masonry.
It is this shear connection, and the method of grouting within a fabric sock, that is vital to give the required bond strength. Installation can either be made from the road surface or, in the case of multi-span structures, from below. Once the work is completed there is no evidence of any major intervention to the bridge, a characteristic that is particularly important for historic structures.
Accurate 3D geometric modelling is required both to develop the FDEM model using the true shape of the arch barrel and also for setting out calculations and the accurate positioning of reinforcement. 3D laser surveys are increasingly being used to provide high-density survey measurements (point clouds) and, by using hybrid 3D CAD software, the built environment and proposed works are combined saving time and improving efficiency. Any buried services are also included.
The use of reinforcing systems in arch barrels, and particularly the Archtec system, has attracted considerable interest in recent years as it is becoming recognised that, in many cases, it is a viable alternative to more extreme works such as traditional saddling, lining and replacement. Archtec strengthening provides an effective and economic method of restoring and strengthening masonry arch bridges where the following advantages are often important:
• Through the use of FDEM the first stage in any Archtec project is to accurately assess the existing bridge strength. This allows accurate matching of strengthening to the loading requirements if the bridge is weak, thus minimising the scope of the work. Alternatively, it may be found that strengthening can be avoided;
• Modest scope of works – strengthening requires small scale construction activities such as drilling and no heavy equipment and movement of materials;
• Archtec is un-intrusive compared with more traditional solutions such as saddling, lining or reconstruction, and being embedded into the masonry has no impact on head room;
• A more sustainable solution with little environmental impact, embodied energy and carbon emissions compared with traditional strengthening and replacement;
• Ease of installation – can generally be installed from above or below a bridge with relatively simple access arrangements;
• Speed of installation – can generally be installed in short and often partial bridge possessions;
• Ability to accommodate existing buried services – it is usually possible to work around them thereby avoiding expensive diversions;
• Minimal disruption to users, low cost and reduced programme risk compared with traditional strengthening and replacement;
• Improved Safety as a result of smaller scale of work compared with traditional strengthening and replacement;
• Good whole life performance through the use of long proven materials and components;
• No or minimal effect on the appearance which can be of particular importance in relation to historic and listed bridges;
• A method of installation that is used to verify the way the bridge was originally built – each drilled hole provides a core of information which can be used to confirm construction, for example barrel thickness.
Strengthening road over rail bridges
One of the first road-over-rail bridges to be strengthened using the Archtec system is Keith Haughes Bridge in Scotland. This is a single span brick masonry arch bridge which carries a trunk road across a network railway line. Earlier assessments had indicated that there was insufficient strength to provide the required trunk road live load rating.
Following a further assessment process, including SV196 abnormal loading, Archtec strengthening was selected and together with other remedial work the project was successfully completed in 2010.
Key to the successful installation was minimal disruption to both road and rail traffic. Being a small scale construction activity, with no movement of bulk materials, enabled the work to take place in short possession periods, and by using techniques such as laser scanning and 3D modelling all activities could be accurately planned in advance.