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1960s 'design oversight' cause of Forth Road Bridge truss end link failure

Truss end links

The failure of the pin that caused the truss-end link fracture on the Forth Road Bridge may have been due to an original 1960s design oversight.

In December 2015, a truss end link – a critical structural element that supports the suspension bridge’s main trusses which in turn supports the road deck – in the north-east side of the main span of the Forth Road Bridge was found to have fractured. The bridge was immediately closed to traffic while contractor Amey carried out emergency repair works to allow it to reopen.

The works took three weeks to complete and included installing a support splint around the fractured steel member and the installation of a 9.5m high steel frame above the deck to re-support the truss.

In an article in December’s edition of New Civil Engineer, experts say it has now emerged that originally, there was a pin-ended link member at the ends of the longitudinal trusses below the deck and near the main towers. This vertical link had a top pin and a bottom pin – the top pin was permanently lubricated via a hose feed, but the bottom one was not.  

It was this bottom pin on the north east side of the main span which seized up, causing the link fracture in December 2016 and prompting seven other similar connections to be replaced, said Amey major bridges director Ewan Angus.

“If the bottom pin had been lubricated, it wouldn’t have got stuck,” he said. “The root cause of the fracture was that the pin was stuck.”

An improved version of this detail, which was simpler to inspect and replace, was originally suggested for the truss-end repair, but, as it was so similar to the failed link, it was considered too contentious. 

The built solution removes the need for pins altogether by using a bearing and bracket detail instead to support the bottom corner of a new truss-end post. The bearing surface allows 1m of movement. New staircases will make them easier to inspect.

This issue of pin lubrication has been known about since before the fracture was first discovered and discussed in January 2016 at an infrastructure and capital investment committee meeting of the Scottish Parliament. 

At the time, then Forth Estuary Transport Authority former chief engineer and bridgemaster Barry Colford said: “Unfortunately, on the Forth Road bridge, the pin is enclosed and you cannot get into it either to lubricate it effectively or to inspect it. It is not a great detail: if you were doing it again, you would not design it in that way.”

With the focus being on remedying the problem, thereafter, its implications were not more widely debated.

Read the oral evidence given by representatives of the former Forth Estuary Transport Authority and representatives of Transport Scotland, Amey, Arup and Fairhurst here www.parliament.scot/parliamentarybusiness/report.aspx?r=10341&mode=pdfx§

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Readers' comments (1)

  • It is stated that the lower pins were not served by a grease injection system of any kind whereas the upper ones are. It is also stated that these pinned joints were recognised as a critical feature in a risk assessment of the structure. Therefore, it would be natural to conclude that if the lower pins could not be assured to remain a rotating hinge (not siezed) there should be an assessment of the consequences of seizure and/or a specific check for locally induced bending and cyclic bending stresses (fatigue). This is surely obvious and yet this was not offered up for discussion at this meeting - why not?

    Also, the official enquiry’s conclusion, after emergency bridge closure and repair was that the large crack was discovered by chance by a diligent inspector and that the failure mechanism had been unforeseable. This may work for the layman but surely not for the ICE or the Institution of Structural Engineers. Where was our honest critical voice?

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