Building a precast masonry-arch bridge that lasts centuries
23 May 2017
Newtownabbey Pathway Project. Image: Queen’s University, Belfast
Masonry arch bridges are some of the most beautiful bridges in the world. This type of construction goes back to Roman days and, in fact, some of the surviving bridges are over 2,000 years old.
Although they are strong, long-lasting, and aesthetically pleasing to the eye, traditional arch bridges are expensive and time-consuming to build. As a result, most arch bridges built since the 1970s have been constructed from steel-reinforced concrete arches and slabs, which are faster and cheaper to install.
The downside, however, is that steel-reinforced concrete corrodes – so much so that a masonry arch bridge that should last several hundred years starts to crumble after forty or fifty years if it is built with steel-reinforced concrete. Because of this rapid deterioration, engineers are looking more toward steel-free structures.
To improve on this situation, Adrian Long, a civil engineer at Queen’s University in Belfast, challenged himself to develop a modern arch bridge system with all the attributes of an unreinforced masonry arch bridge, but that also can:
- Be installed as quickly as other types of bridges;
- Eliminate the need for centring;
- Use existing methods of design and analysis;
- Be constructed off site using precast concrete methods; and
- Be cost-competitive with other types of bridges.
After ten years of research, Long perfected the FlexiArch bridge system in 2007. Because the main forces are compressive, no reinforcing steel is required. No centring is needed and the bridge can be assembled in a day.
Because there is no corrosion, a FlexiArch bridge is expected to last at least 300 years with minimal long-term maintenance, which only adds to its cost-competitiveness in comparison with other types of bridges.
How the FlexiArch works
“The FlexiArch units can be cast in convenient widths to suit the design requirements, site restrictions and available lifting capacity,” according to Long. “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.”
Long notes that the degree of taper of the voussoirs controls the geometry of the arch: flatter arches require less taper, for example. The arch-shaped units are then lifted and are placed on precast footings at the bridge site, “with all the self-weight then transferred from tension in the polymeric reinforcement to compression in the voussoirs, acting the same way that a conventional masonry arch does”, Long added.
The polymeric reinforcement provides the tensile strength that is needed to lift the FlexiArch units safely. Laboratory tests that simulated the bridge-site conditions were undertaken to test the strength of the polymeric reinforcement.
“Using these results and taking into account creep effects, an appropriate load factor was applied to ensure that there was no risk of failure during lifting,” added Long. “A typical unit can be accurately located on-site every 15 minutes, therefore, most bridges can be installed in well under a day.”
A wide range of static loading tests were also carried out to validate the performance of the FlexiArch system. These have included model tests in the laboratory (at fifth, quarter and third scale) with granular or concrete backfill where it was possible to achieve the ultimate capacity.
Full-scale tests under maximum loads (equivalent wheel load of 320 kN, or lane loading of over 1,000 kN) also showed that the bridge system more than satisfied the stringent requirements for highway bridges.
To date, more than 50 of these bridges have been built in the UK and Ireland, and discussions are in progress with several companies in the United States. Span lengths can reach 30 metres.
“By interconnecting the accurately precast voussoirs via a screed and polymeric reinforcement, arches can be produced to the precision required by designers without the need for centering,” said Long.
“The speed of installation is comparable with precast concrete/steel beams. As such, it can be used for road bridges over railway lines where construction windows are restrictive. As there is no corrodible reinforcement, total life cycle costs are therefore minimal.”
This article is reproduced with permission from the American Society of Mechanical Engineers.http://www.engineersjournal.ie/2017/05/23/flexiarch-precast-masonry-arch-bridge-lasts-centuries/http://www.engineersjournal.ie/wp-content/uploads/2017/05/FlexiArch-Newtownabbey.jpghttp://www.engineersjournal.ie/wp-content/uploads/2017/05/FlexiArch-Newtownabbey-300x300.jpgCivilbridges,Queen's University Belfast,structures and construction