Lorry braking suddenly could have ‘unzipped’ Menai bridge causing …

A lorry performing an emergency stop could have brought down one of the two bridges linking Anglesey ^[1] to the mainland, engineers have said. Improvements were carried out on Menai Suspension Bridge more than 30 years ago but calculations may have underestimated its load capacity.

The bridge was shut with five minutes’ notice in October 2022. While it reopened in February, with a reduced weight limit, full upgrades are not due to be completed until August 2025.

Its closure was ordered following concerns about the safety of 242 steel hangers that hold up the road deck. Most of these dated back to 1938 when two suspended wooden decks were replaced by a single steel deck.

As part of planned maintenance, all 242 hangers were to be removed for painting off-site. But when engineers began checking old documents – merely to “inform the specification” of this work – to their alarm they discovered “significant concerns” about the bridge’s structural integrity.

Their analysis revealed a ”credible risk of an unzipping failure of the bridge deck”. The engineering team wrote: “Such an event might lead to a catastrophic loss of life and this was considered an unacceptably high-consequence event.” Their conclusions were reported in the magazine of the Institution of Structural Engineers ^[4], whose first ever president was the bridge's builder, Thomas Telford.

Between 1988-91, load assessments on the bridge were carried out prior to the introduction of 44-tonne vehicles on the UK road network. As no test data existed for the bridge’s 208 spiral strand hangers - the rest were solid steel - four were removed for "testing to destruction".

In all four, sockets holding the spiral strands were found to be brittle. As a result, 25 more hangers were tested, “almost all of which failed by brittle fracture of the socket”.

Engineering concerns focused on the brittle nature of sockets clamping the bridge's suspension wires (Image: Rhodri Williams)

Based on this, 11 of the most highly loaded hangers were replaced in 1990. Including those tested, it meant a total of 40 hangers were renewed between 1988-91. “This was deemed sufficient to assess the bridge as capable of carrying full HA traffic loads (normal design loading for roads in Great Britain),” wrote engineers and managers at consultants COWI, UK Highways A55 ^[5] and its contractor, Spencer Bridge Engineering.

Rolling inspections of the hangers were carried out over subsequent years. These did not recommend replacing any more hangers, the Welsh Government ^[6] has said. It took on responsibility for the bridge in 1999. Maintenance has been carried out by UK Highways A55 since 1998.

North Wales Live has launched a WhatsApp community group where you can get the latest stories delivered straight to your phone ^[7]

Given modern design standards, brittle steel is rarely encountered by today’s engineers. Although guidance is available for historic bridges, this is for structures with rolled or milled steel, not cast steel like that use for the Menai bridge.

Armed with the historic data, COWI recommended all hangers on the suspension bridge be replaced “at the earliest opportunity”. It also launched a review to assess what would happen if a single hanger failed.

Were this to occur, most of the hanger’s load would then “jump” to the two adjacent hangers, they found. In all models, the adjacent hangers would then fail. The process would then be rapidly repeated, leading to “global unzipping failure of the entire bridge deck”.

When opened in 1826, Thomas Telford's suspension bridge over the Menai Strait had the world's longest span at 176m (Image: Rhodri Williams)

The team made one other crucial discovery. In the late 1980s, hanger capacity was correctly assessed with an adjustment for “low-temperature ductility”. However the calculation was based only on a “slow loading event”, such as a vehicle driving onto the bridge.

“It did not anticipate rapid loading events such as an HGV performing an emergency stop,” said the engineers. “Furthermore, in the event of the sudden brittle fracture of a socket, the rate of loading of adjacent hangers would be almost instantaneous. Thus the calculated hanger capacity may overestimate the capacity of the hanger in this situation.”

There were no quick fixes. Getting replacement hangers typically takes 12-18 months. Moreover, no off-the-shelf sockets were compatible with the Menai bridge, so they all had to be custom-made. These will be subtly different from the ones they replace, incorporating modern design codes while being in keeping with the originals.

The Grade I-listed bridge - the world's second oldest vehicular suspension bridge still in use - is due to celebrate its 200th anniversary in 2026 (Image: Peter Braddock)

Prototype sockets cast in India were assembled into hangers and subjected to radiography, ultrasonic and magnetic particle testing in Switzerland. After the prototypes were tried out in situ this summer, they went into full production.

Work to install and paint the hangers began in September ^[9] and is due to be completed by March 2025. Lane closures will be lifted over the two Christmas periods ^[10].

In the meantime, COWI and the bridge contractor quickly developed bespoke “secondary fail-safes” (SFS) for all 242 hanger locations. These were designed to take the strain if a hanger failed – and to carry loads when the old hangers were replaced.

Having devised solutions, the engineers said what happened at Menai Suspension Bridge (Pont y Borth) highlights “the importance of high-quality record-keeping and documentation”. Tests carried out 34 years ago helped the team establish current threats based on modern standards.

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References

^{^} Anglesey (www.dailypost.co.uk)
^{^} Armed police make arrest after reports of man 'brandishing firearm' (www.dailypost.co.uk)
^{^} Anger as rare horse and unborn foal killed by the kindness of ‘ignorant’ stranger (www.dailypost.co.uk)
^{^} Institution of Structural Engineers (www.istructe.org)
^{^} A55 (www.dailypost.co.uk)
^{^} Welsh Government (www.dailypost.co.uk)
^{^} WhatsApp community group where you can get the latest stories delivered straight to your phone (www.dailypost.co.uk)
^{^} Sign up for the North Wales Live newsletter (www.dailypost.co.uk)
^{^} began in September (www.dailypost.co.uk)
^{^} lifted over the two Christmas periods (www.dailypost.co.uk)