Adapting the FlexiArch bridge system for widening a bridge in Bristol
01 August 2017
The completed Teewell Hill arch bridge (image: Macrete.com)
This is the second in a two-part series on the FlexiArch system. It is recommended that part one of this series, entitled ‘The FlexiArch bridge system: from concept to application’, is read prior to the article below (1).
The Teewell Hill arch bridge, owned by South Gloucestershire Council in the UK, had been constructed in the 1840s over the Bristol to Bath railway line (now a popular cycle route) in what is now a busy residential area.
The bridge had become an accident blackspot due to the enormous increase in the volume of traffic using the bridge on a daily basis and the absence of pedestrian access. The separate steel pedestrian bridge, adjacent to the Teewell Bridge, also needed to be repaired.
As a result, it was decided to widen the existing bridge by approximately 4m on each side, to provide pedestrian footpaths and improve traffic flow. The bridge carries a major thoroughfare, as well as spanning over a National Cycle Route. As a consequence, demolition to make way for a new structure would not have been acceptable.
In addition, in order to maintain the aesthetics of the existing bridge, it was essential that the arch form of construction was utilised in the widening process. Conventional masonry arch extensions to each side, if feasible, would have been expensive, taken many months to install and the need for centring would have disrupted the National Cycle Route.
On the basis of their experience of using the FlexiArch previously, the consultant WSP | Parsons Brinckerhoff recommended consideration by the Council of this innovative system, provided that suitable bespoke units could be manufactured by Macrete. This would then allow the complex geometry of the existing bridge to be accommodated.
The existing masonry arch bridge had a span of 12.76m and a rise of 3m. To minimise work on site, the FlexiArch units would have to be manufactured in Toomebridge to precisely match the complex geometry of the existing bridge.
Aesthetics was a major design factor for such a high-profile bridge near the centre of the community, as a large number of people passed over and under the bridge on a daily basis. Hence, the end product needed to have comparable aesthetics to the original bridge.
Accommodating the complex geometry
The major challenge to be addressed in this project, however, was that the both facades of the existing bridge were not vertical and were curved in plan as well as in elevation Fig 1(a) and (b).
The ‘concave’ profile of the arches in plan meant that the crowns on each façade were around 500mm inside the vertical plane of the abutments. Thus, the use of standard uniform-width FlexiArch units would have left a large gap at the crowns, which tapered to zero at the abutments of the arch.
Using their experience from developing a skew FlexiArch system, Macrete designed and manufactured bespoke FlexiArch units, with each voussoir individually tailored to match the external profile of the existing bridge. This eliminated the need for special formwork or cast in-situ joints between the old and new elements, thus providing a significant time and cost saving to the Council.
A 3D topographical survey was undertaken at the site, which captured the profile of the arch and the complex geometry of the spandrel walls. This information was then processed and used to create a 3D Revit model which, in turn, was used to develop the bespoke FlexiArch for the required new bridge alignment.
The unsymmetrical nature of the existing masonry bridge meant that its arches on the east and west elevations were slightly different in profile. Using the Revit model, the design team was able to present the client with two different options regarding the development of a bespoke FlexiArch.
The first option consisted of creating two unique FlexiArches (one per side to exactly match), whilst the second option entailed providing one best fit FlexiArch, which would accommodate both sides of the bridge. The latter option, which was selected, brought about significant cost savings in manufacturing as only one formwork was required.
Using the Revit model, the custom FlexiArch arch form was optimised to ensure that the difference in levels between the existing arch and new arch soffits would not exceed 30 mm at any one point, therefore still providing an aesthetically acceptable solution.
Upon completing optimisation of the arch form in elevation, the model was used to accurately determine the plan dimensions of each of the individually tailored voussoirs, for the bespoke FlexiArch units.
As the length of the concrete voussoirs varied from around 750mm to 1500mm, Fig (2) polypropylene fibre-reinforced concrete was used to ensure that each voussoir had not only the required compressive strength, but also additional resistance to possible impact damage during the installation process. The structural performance of the FlexiArches under Eurocode bridge-loading conditions was designed and checked using Archie analysis software.
Construction without causing disruption
As the bridge site was located in the suburbs of Bristol in a busy residential area, all construction activity had the potential to cause significant inconvenience to local residents. Construction of the piled foundations and bridge abutments was carried out without causing major disruption, in preparation for the installation of the pre-cast units.
As site access was very limited, road closures were required to carry out any lifting procedures or deliveries. As a result, it was crucial that the installation of the units could be carried out in as short a time frame as possible.
All precast FlexiArch units arrived flat-pack on the back of lorries and were installed in just a couple of hours, minimising disruption on the cycle path below and the local roads above, which were able to be re-opened before peak travel times on the same day.
Extensive previous experience by Macrete in over 60 FlexArch installations has shown that self-compacting lean mix concrete backfill speeds up construction and results in greater strength, as opposed to the more labour intensive granular backfill, which requires compaction.
In this particular project, unique precast concrete spandrel wall units were designed to follow the road alignment and that of the adjoining embankments, thus eliminating the need for wing walls. The spandrel wall units were also manufactured by Macrete and were supplied with a precast feature corbel, which allowed locally sourced stonework to be built on to the units, ensuring a close match with the existing bridge.
In this case, the client was able to re-create the aesthetics and structural qualities of a masonry arch bridge without requiring costly, time consuming and intrusive centring. The client also benefitted from the speedy installation of the precast units, which reduced the cost for crane hire and minimised road closures. Fig 3 shows the sequence of construction. The overall cost of the Teewell Hill Bridge extension project was £1.5 million (€1.7 million).
The FlexiArch units have no corrodible reinforcement, as they act as arches in service and only compressive stresses are generated. As a result, maintenance of the new bridge elements will be minimal, like the original masonry-arch bridge. Combining both the old masonry arches and new structural elements, which will not suffer from the effects of reinforcement corrosion, has resulted in a sustainable, long-term structural solution.
A blended concrete using GGBS was chosen for the precast concrete units, to reduce the overall carbon footprint of the project. Approximately 250m3 of C6/8 lean-mix concrete was used as backfill material up to the level of the crown of the arches.
Then a reinforced concrete slab 300mm thick was poured on top, using C40/50 concrete to interconnect the existing bridge with the new extensions. Once the concrete backfill had cured, construction traffic was allowed access to the widened bridge.
The owners of the Teewell Hill bridge were vigorously searching for an economic and effective solution to their challenging task of widening the existing 1840s bridge. Positive previous experiences with the FlexiArch prompted the consultants to suggest that Macrete should be approached.
In response, the innovative FlexiArch system was adapted by Macrete to produce bespoke variable width units that allowed the complex bridge-widening project to be successfully completed. By combining digital scanning technology/3D modelling with the versatile FlexiArch system, a cost effective and highly sustainable outcome has been achieved.
Whilst this is the first-ever application of this approach, the authors are of the view that had the FlexiArch system not been available, it would have been exceptionally difficult to find an acceptable alternative solution.
1) Long A.E., Nanukuttan S., Gupta A., (2017), ‘The FlexiArch bridge system – from concept to application’, EngineersJournal.ie
Adrian Long, OBE, FREng, PhD, DSc, FIStructE, FICE, Queen’s University Belfast
Danny McPolin, BEng, MSc, PhD, Queen’s University Belfast
Abhey Gupta, BTech, MTech, MPhil, Macrete Ireland Ltd, Toomebridge, Northern Ireland