How Engineers Can Avoid “Improper” Filling of Masonry Arch Bridges
A leading academic has called for more education around masonry arch bridges, as well as supporting new guidelines on how to treat these structures.
Speaking at NCE’s Future of Bridge conference, Matthew Gilbert, Director of the Integrated Civil and Infrastructure Research Center at the University of Sheffield, pointed out that National Highways’ approach to filling in bridges was “inappropriate”.
He told the conference that bridges are currently being reinforced unnecessarily and that there are alternatives to infilling.
“Anecdotally, you hear about [bridges being strengthened unnecessarily] very regularly,” he said. “Filling a population of redundant bridges has been in the news. I think these interventions were inappropriate – huge amount of material and the fill is not stiff enough to provide support for the bridge.
“It will stop catastrophic collapse if it happens, but in general it’s a very crude engineering solution and I don’t think it would be high on my list of preferred options.”
Last week, local planning officials urged Eden District Council to refuse the National Highways’ request to retain the controversial Great Musgrave Bridge infill.
With a formal ruling expected this week, the council’s deputy director of development has recommended it be denied as it causes ‘considerable detriment’ to the appearance of the structure and ‘does not complement or enhance the area’ . If the board votes against the request, National Highways would be required to remove the infill.
The infill of the 8.4m single-span masonry arch bridge was carried out by Amco Giffen on behalf of National Highways between May and June last year. It has since sparked controversy, with local campaign groups calling for the work to be canceled and engineers expressing ‘shame’ at their profession after images of the filling appeared in national media.
Last month, a report by masonry bridge specialists Bill Harvey Associates concluded that the bridge was not weak at the time of filling, posed no threat to public safety and suggesting that it was at risk of collapse was ” absurd”.
Regarding identifying a more appropriate course of action for masonry arch bridges, Gilbert said he “wasn’t familiar with the assessment process” for bridges that have been filled in. by national highways, so he did not know “exactly what had been undertaken”, but he had heard that “some crude assumptions were made”. He said National Highways “potentially wanted to change the status of a large number of passives to non-passives.”
“It’s kind of an accounting exercise rather than an engineering problem,” he said.
During the session, Gilbert presented the new and improved guidelines for the assessment of masonry arch bridges contained in CIRIA C800. Almost half of all bridge spans in the UK are masonry construction, but inaccurate valuation has led to the unnecessary demolition and replacement of many bridges.
Gilbert said engineers cannot be expected to ‘miraculously’ ‘gain an understanding of the full complexity of railway structures that they may not have dealt with at university and in their offices. “. As such, he said the advice is a way to bring them up to a standard, although there will always be cases where consultation with other professionals is needed.
He also feels there is an argument for the subject to be brought back into university courses.
“We’re talking in the coming years about building less and making the most of what we have and masonry arch bridges currently make up almost half of all spans in the UK,” Gilbert said. “There is a strong argument that this should be a central topic again. For new construction where required, masonry arch bridges are a very attractive option, particularly if you can source the stone locally. because you have a very low carbon solution compared to concrete.
“Working in an academic institution, I know the programs are very crowded, so it’s tricky. But looking at the environment we face, we have to spend more time on the existing structures – whether it’s buildings or bridges – and less to new construction because that’s the balance the industry faces.”
Despite much criticism, National Highways has always stood by its decision to fill in the Great Musgrave Bridge. An internal review of the embankments carried out at the end of last year deemed the work “necessary”. Last month, Hélène Rossiter, Historical Railways Estate program manager for National Highways, said “the infill was crucial to public safety and made the future use of the structure viable”.
If National Highways were to remove the infill, it has compiled a list of five potential reinforcement options and, following the fallout from Great Musgrave, National Highways has also developed a new way to assess abandoned railway bridges and tunnels under its control. .
The new way of working will see decisions on major works planned for the historic railway estate reviewed in conjunction with experts from the heritage, environment and active travel sector who have been selected to form a stakeholder advisory forum.
The forum includes the Department for Transport (DfT), Sustrans, Railway Paths Ltd, Railway Heritage Trust, The HRE Group, Heritage Railway Association, Natural England, Historic England (also representing Cadw), Historic Scotland and ADEPT.
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