The FlexiArch bridge system – from concept to application
18 July 2017
The strength, stiffness, durability and minimal maintenance attributes of arch bridges are acknowledged by structural engineers throughout the world. In addition, their aesthetic qualities are universally acclaimed, so much so that there are hundreds of thousands of arch bridges in the world (some over 2000 years old).
Thus, in the early 1990s, Gordon Millington (formerly president of Engineers Ireland) asked the first author the question, ‘Why is it that so few arch bridges have been built since the early 1900s?’ Millington’s perceptive comment catalysed research to answer this question (as detailed in Ref 1).
For example, the need for centring and accurate voussoirs for arches meant that they could not compete in terms of speed of construction with prestressed concrete/steel beam and slab systems, which rose to prominence in the 1950s and 1960s and are still widely used. However, many of these beams and slab bridges, even though they have specified design lives of 120 years, have deteriorated after only 20-30 years and indeed a significant number have already had to be replaced.
Where aesthetics were of paramount importance, rigid precast concrete arches, heavily reinforced so that they could be safely lifted into position, were adopted in many instances. However they are difficult to precast, store and transport to site. Also, like beam and slab bridges, they are vulnerable to reinforcement corrosion.
In this context, the UK Highways Agency recommends the use of the arch form where ground conditions permit and also states that ‘consideration should be given to all means of reducing or eliminating the use of corrodible reinforcement’ (Ref 2). Thus, the challenge was to develop an arch system with all the attributes of an unreinforced masonry arch, but with other benefits.
In this brief paper, the concept of the patented ‘FlexiArch’ system (Ref 1), developed to meet this challenge, will be described. In addition, case studies will be presented for three specific applications of this versatile system – chosen as a representative sample from the 60 FlexiArch bridges already in service in the UK and Ireland.
Novel concept and method of manufacture
It was clear at the outset that a radically different approach to the construction of arches would be needed to convince practising structural engineers, but only if the solution was viable, cost effective and sustainable.
The FlexiArch is constructed and transported to site in flat-pack form, using polymeric reinforcement to carry the self-weight of the arch unit during lifting. But once in place, it behaves as a conventional masonry arch. The preferred method of construction of the arch unit is shown in Fig 1 (more details are available in Ref 1).
For the manufacture of each arch unit, the tapered voussoirs are precast individually. Then they are laid contiguously with the top edge touching, in a horizontal line with a layer of polymeric reinforcement placed on top. In-situ screed, approximately 40mm thick, is placed on top and allowed to harden so that the voussoirs are physically interconnected.
The FlexiArch units can be cast in convenient widths to suit the design requirements, site restrictions and available lifting capacity. When lifted at the designated anchorage points, gravity forces cause the wedge-shaped gaps to close, concrete hinges form in the screed and the integrity of the unit is provided by tension in the polymeric reinforcement and the shear resistance of the screed.
The arch-shaped units are then lifted and placed on precast footings at the bridge site and all the self-weight is then transferred from tension in the polymeric reinforcement to compression in the voussoirs, i.e. as a conventional masonry arch.
Experience of using this method of manufacturing (Fig 1) has shown that it has a number of advantages over traditional methods:
- The voussoirs can be produced with the desired taper in relatively simple shuttering;
- High quality concrete can be utilised for the individual precast voussoirs.
The primary function of the polymeric reinforcement is to provide sufficient tensile strength so that the FlexiArch units can be lifted safely. Thus carefully designed tests, which accurately simulated the boundary conditions, were carried out to ascertain the strength of this reinforcement (Ref 1).
Using these results, an appropriate load factor was applied to ensure there was no risk of failure during lifting. Experience has shown that a typical unit can be accurately located on site every 15 minutes and, as a consequence, most bridges can be installed in well under a day, thus affording the FlexiArch enormous benefits relative to a conventionally constructed arch.
Over 60 bridges in the UK and Ireland over the past ten years have benefitted from using the FlexiArch system. In this section, details of three different applications of the system are given, but relevant videos of these bridges as well as photographs of many more applications are available (Ref 4).
- FlexiArch replacement bridge, Sheinton, Shropshire
In 2009, the old bridge in Sheinton, Shropshire, England, was irreparably damaged by flooding and a temporary Bailey Bridge was installed to restore communication across the tributary of the River Severn. Engineers from Shropshire Council decided to replace the three-span bridge with a much longer single-span arch to greatly reduce the risk of flood damage in the future.
As they wished to reduce construction time to a minimum and avoid the use of a bridge with corrodible reinforcement, they selected a 13.7m span x 2.7m rise FlexiArch and ordered eight 1m-wide units from Macrete Ireland Ltd. Each unit (Fig 2a) was placed on the precast sill beams in a matter of 10-15 minutes.
Once all had been located and the precast spandrel walls installed, the bridge was ready for the lean mix concrete backfill (Fig 2(b)). The spandrel walls were then finished in stonework (Fig 2 (c)) to produce an aesthetically pleasing solution. The contractor, DEW Construction, worked closely with the council and Macrete to successfully complete the £450,000 contract in 2010.
- Bridge strengthening, Tameside, Manchester
Tameside’s 78-year-old bridge, which spans National Cycle Route 66 in Manchester, had been weakened by extensive reinforcement corrosion and spalling. Replacement was unacceptable due to the disruption to services and a key transportation corridor. Repair by applying sprayed concrete to the deck soffit had been used in 1974, but it was clearly not a long-term solution, nor aesthetically pleasing.
Wilde Consulting Engineers then suggested using the Macrete FlexiArch. In December 2012, fourteen FlexiArch units (1m wide) were installed (Fig 3(a)) by main contractor AE Yates – the first-ever application for bridge strengthening. The 7.4m-span units were manufactured in Northern Ireland and shipped to site before being individually lifted by crane and placed on lightly greased laterally extended sill beams along each abutment. Then they were pushed horizontally in pairs beneath the bridge (Fig. 3 (b, c)).
When all 14 units had been located, spandrel walls were constructed and the gap between the FlexiArch unit and the original deck soffit was filled with foamed concrete. The £420,000 contract was successfully completed and Tameside Council now have an aesthetically pleasing bridge with a design life of over 120 years (Fig 3(d)).
- Widening a two-span bridge in Beragh, Co Tyrone
This £440,000 contract was completed in 2013 with the existing 100-year-old arch bridge being sympathetically widened to accommodate two 6.5m-wide traffic lanes and two 2m-wide footpaths. Fig 4 shows the finished bridge, where the client was DRD (NI), construction by McCallan Bros Ltd, FlexiArch units and spandrel walls supplied by Macrete Ireland Ltd and design by Doran Consulting.
Future developments and conclusion
The three exemplars give an indication of the versatility of the FlexiArch. However, the authors firmly believe that the system has yet to achieve its full potential. For example:
- The maximum span could be increased to 25-30m for highway loading and, if necessary, the FlexiArch transported to site in half lengths for interconnection prior to installation;
- The system can be adapted for skew-arch bridges over railway lines, where its speed of construction would be invaluable;
- In the construction of multi-span bridges, very slender piers are possible as the lateral forces are minimal. Thus, arch viaducts could be rediscovered and by recognising the ‘breathing bridge’ concept (Ref 5), the need for sophisticated bearings could be avoided (costly to replace periodically for bridges with long lives) and maintenance minimised.
The experience gained from constructing over 60 FlexiArch bridges and from the extensive tests at full and model scale have allowed the following conclusions to be drawn:
- By manufacturing the voussoirs using accurate moulds, interconnecting them via a screed and polymeric reinforcement, arches can be built without centring;
- Lifting the FlexiArch units onto flat-bed lorries, stacking them in their flat-pack form, transportation to and installation on site have proven to be straightforward;
- As a typical FlexiArch unit can be lifted into position in 15 minutes, the speed of installation is comparable with precast concrete/steel beams;
- The FlexiArch, which is made of high quality precast concrete and has no corrodible reinforcement, should have minimal total life-cycle costs;
- After contractors, designers and clients have utilised the FlexiArch they have become much more favourably disposed to this novel system. When combined with the competitive cost, aesthetics, sustainability and durability, its potential is immense.
This is the first in a two-part series on the FlexiArch system. The second article outlines how the FlexiArch system was used to replace Teewell Hill arch bridge, accommodating the existing bridge’s complex geometry while also facilitating footpaths and improved traffic flow. Read more here.
The financial support provided by the ICE R&D Enabling Fund, KTP Scheme, Invest Northern Ireland, DRD Roads Service (NI) and the Leverhulme Trust is gratefully acknowledged. Input by J Kirkpatrick and I Hogg is also acknowledged.
Adrian Long OBE, FREng, FIAE, FIEI, Queen’s University Belfast, Northern Ireland
Sree Nanukuttan BTech, PhD, FHEA, Queen’s University Belfast, Northern Ireland
Abhey Gupta, BTech, MTech, MPhil, Macrete Ireland Ltd., Toomebridge, Northern Ireland
(1) Long A, Kirkpatrick J, Gupta A, Nanukuttan S and McPolin D (2013), ‘Rapid Construction of arch bridges using the innovative FlexiArch.’ Proc. ICE, Bridge Engineering, Vol 166, Issue BE3, Sep, pp 143-153.
(2) HMSO Department of Transport (2004) Design Manual for Roads and Bridges, BD 91/04: Unreinforced masonry arch bridges) [Online] Available at: http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol2/section2/bd9104.pdf (Accessed: April 2017).
(3) Long AE (2004), Concrete arch and method of manufacture. International Patent, Publication 27 May, No. WO 2004/044332A1, Queen’s University Belfast.
(4) Macrete (2013), FlexiArch [Online] Available at http://www.macrete.com/flexiarch-flexiarch-projects/ (Accessed: April 2017)
(5) Long, A.E and Nanukuttan (2017), ‘Archbridges –unlocking their potential’, Proc. ICE, Eng. Comp. Mechanics, March, pp 3-6.