Engineers at University College Dublin are helping to assess the weight-carrying capacities of some of the State's oldest masonry bridges, reports Dick Ahlstrom.
The Republic's highways and byways are dotted with hundreds of graceful old masonry bridges. Many were built in the 1700s and, while they look attractive, road engineers would like to know whether these old structures can handle today's heavy traffic loads.
Masonry bridges have been built and used for thousands of years but, surprisingly, engineers have difficulty deciding exactly how much these bridges can carry, explains Dr Paul Fanning, of the Department of Civil Engineering at University College Dublin. Fanning heads a research project to make masonry bridge performance more predictable.
"With steel and concrete bridges you can easily go out and see what is there," says Fanning. "With masonry bridges you can't do that."
Funding for the work comes from the National Roads Authority, Dublin Corporation and the Institution of Civil Engineers. "The reason the National Roads Authority is interested is it is developing a new bridge management system," he explains.
The system will include a database of all of the State's road bridges, including old masonry and modern concrete and steel bridges. It will catalogue regular bridge safety inspections but also provide information about size and weight-carrying capacity.
The database will allow a transport company to find a safe route for an overweight load, avoiding any bridge crossings incapable of handling it.
Many important single-arch masonry bridges were built more than 200 years ago. Their constructors could never have imagined the 32-tonne juggernauts these bridges have to handle today.
In the late 1700s, the State's longest masonry arch bridge, one with just a single arch, was the Lismore Bridge in Lismore, built in 1775 and measuring just over 30 metres. Dublin engineers wanted to claim this record and in 1792 built the Sarah Bridge at Islandbridge, featuring a 31.8-metre arch. The record was broken again with the Lucan Bridge over the Liffey, built in about 1814 and measuring 33.5 metres. It remains the State's longest single arch masonry bridge.
Fanning is developing a model that can calculate how these bridges will handle loads driven over them and has tested the model against actual bridge measurements. "A lot of people believe that the response of masonry arch bridges is unpredictable - but it is predictable," says Fanning.
The distortion to the bridge arch when under load is measured using a device known as a linear variable differential transformer. A series of these is mounted on scaffolding under the bridge and a four-axle truck carrying a measured 30-tonne load is driven across.
The units measure, in fractions of a single millimetre, the movements of the bridge arch as it distorts under the stress. "We can map the displacement pattern" of the arch, says Fanning, with the amount of movement varying at different places as the truck passes.
These actual measurements are compared to results from the mathematical model that calculates loads on the basis of "finite element" analysis. This involves taking the bridge apart in a mathematical sense, piece by piece, and calculating how individual components will perform. These individual elements are then added together to give a prediction of what the bridge can carry.
"We wanted to see if we could use the finite element method as a general tool," Fanning says. In fact, good agreement was found between actual measurements taken at bridges and modelled predictions.
Fanning has done detailed load-bearing and distortion measurements on seven masonry arch bridges. The Sarah Bridge was studied along with two others from the 1790s, the Killeen Bridge (which crosses the Grand Canal along the Kylemore Road in west Dublin) and Griffith Bridge.
Griffith Bridge crosses the Grand Canal near the junction of Dolphin and Davitt roads as the canal passes under Suir Road near Kilmainham.
The bridge used to give access to a steelworks and was built to withstand very heavy loads, but it is now the entrance to an apartment complex.
More tests would be needed to confirm the model, particularly on spans of under nine or 10 metres, Fanning says. "An awful lot of masonry bridges here, about 40 per cent of them, are less than five metres long." The longest ones have been done; now he needs to do a few small bridges.
The best way to fine-tune the model would be to test a bridge to destruction, something the NRA might not be too happy about. Fanning might yet get his chance, however.
Two masonry arch bridges near Ballincollig, Co Cork are due for demolition to make way for the Ballincollig bypass. Fanning is negotiating with Cork County Council and hopes to push these past the point of collapse, something that in the long run will contribute to the safety of similar masonry bridges.