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Forth Road Bridge Failure Not ‘Foreseeable'

Thursday, March 24, 2016

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An inquiry into the December closure of Scotland’s Forth Road Bridge determined that the fault could not have been foreseen.

After taking evidence from engineers and transport officials, The Scottish Parliament's Infrastructure and Capital Investment Committee recently released its report on the circumstances surrounding the closure, the BBC reported.

ICI Committee members tour repair site
Images: The Scottish Parliament

As part of the inquiry into the circumstances surrounding the closure of the Forth Road Bridge, members of the ICI Committee viewed the repair work on the structure.

The bridge was closed to all traffic late on Dec. 3 after a 2-centimeter (about ¾-inch) crack was found in the structural steel of the bridge’s northeast tower during the last week of November.

At that time, as previously reported, sources stepped forward to point the finger at funding issues, stating that Scottish government should not have abandoned a plan to strengthen steelwork in the tower five years earlier.

In its March 11 ICI Committee report, the group indicated it agreed that, although funding cuts did affect the prioritization of projects and ultimately delayed work on the structure, “the defect which caused the closure in December 2015 could not have been foreseen.”

Collecting Evidence

During the inquiry, the ICI Committee took evidence from engineers, members of the Forth Estuary Transport Authority (FETA) that formerly managed the bridge, and Transport Minister Derek Mackay.

As part of the investigation, Mark Arndt, from Amey, the firm that took over management and maintenance of the bridge in June, told the group a metallurgist had been brought in to inspect the failed member, confirming it had been caused by a fatigue failure rather than a defect in the steel.

“His conclusive recommendation was that there was indeed a fatigue failure because the propagator initiated at the weld interface, which is potentially the weak interface ... then propagated along the weld and progressed into the member itself, which led to a quick failure,” he said in the report.

This was echoed when attention focused on the pin that connects the bottom of the truss end link to the bottom chord of the truss. The pin is in place to allow free movement between the towers of the bridge and the main suspended span under traffic loads, wind loads and changes in temperature, the report noted. However, sources said the pin had seized.

“These movements cannot be stopped so, if the pin is not working correctly, they have to be accommodated by bending in the truss end link instead of by its rotation,” the report stated. “Bending stresses would be set up every time an heavy freight vehicle crossed the bridge and the level of stresses combined with the frequency of their occurrence set up a fatigue induced failure.”

Primary bridge components

Critical components such as the truss end links were inspected every six months, the committee learned. The most recent inspection of that area had been undertaken on May 19, 2015, the report indicated.

Richard Hornby, of engineering consulting firm Arup, explained that it would have begun as very small crack that would have been undetectable in an inspection. “It would have grown gradually at first but then quicker, and would probably have taken only a matter of months to grow from a crack that was visually undetectable to something that had totally failed,” he said.

In terms of inspections that may have turned up any problem in advance, the committee learned the former FETA Bridgemaster and Chief Engineer did not think the standard inspection practice was sufficient for a major structure like the Forth Road Bridge. Instead, they had developed a risk based inspection program through which they increased the frequency of inspections of a member based on its criticality and vulnerability.

Critical components such as the truss end links were inspected every six months, the committee learned. The most recent inspection of that area had been undertaken on May 19, 2015, the report indicated. There had been 23 inspections at that location since 2001, with no defects noted on any of those inspections, it added.

Former bridge engineer Barry Colford testified that, “We carried out our inspections, and the problem was not foreseeable. We spent a lot of time looking at the truss end links, and we had many consulting engineers assisting us in that. We did not foresee the issue with the pin sticking, if that is indeed what the mechanism for failure was.”

Differing Opinions

In reviewing evidence, Members of the Scottish Parliament (MSPs) still expressed concern indicating that FETA was aware that the bridge’s truss end link mechanisms needed attention as far back as 2010-2011, The Courier reported, but could not address them because budget cuts in the 2011 spending review impacted its capital budget.

"The bottom line for me is this work was planned, had this work gone ahead, the truss end links would have been replaced, that's where the problem was caused from,” MSP Alex Rowley said.

"Had that actually happened then the likelihood is the closure, and all the subsequent costs that came from that, could have been avoided," he added.

However, MSP Clare Adamson shared a different view. "While we can say what ifs and what might have been, the fact of the matter is that the scoping of that project had not been done,” she said, “and there is absolutely no guarantee that the replacement of the truss end links would have been completed before this fault emerged on the bridge."

Truss end link failure

A small crack “would have grown gradually at first but then quicker, and would probably have taken only a matter of months to grow from a crack that was visually undetectable to something that had totally failed,” according to an engineering consultant.

Moreover, Transport Minister Derek Mackay expressed that FETA acted appropriately in terms of reprioritizing projects in the face of funding cuts, the BBC reported.

"The defect was not foreseen nor foreseeable. FETA was an independent organisation that maintained and managed the bridge and was independent of Scottish Government,” Mackay said.

"Despite a tough spending review, if emergencies arose Transport Scotland could have intervened to support if asked.”

The majority of MSPs agreed the decision to defer work was appropriate "on the basis of both the prevailing financial circumstances and the engineering advice available," The Courier noted, and felt FETA would have negotiated with Transport Scotland for the necessary funding if public safety was at risk.

Lessons Learned

Despite initial speculation that the bridge might never carry traffic again, and a closure that lasted 19 days, it did reopen to cars later in December and to freight traffic in mid-February.

In that period, Transport Scotland spent £3 million ($4.23 million) to implement immediate repairs. However, the total repair bill for the bridge could reach £19.7 million ($27.81 million), STV News noted.

The Forth Road Bridge is expected to be largely replaced by the end of this year by the new £1.325 billion ($1.87 billion) Queensferry Crossing.

Mac West, chairman of the Institution of Civil Engineers in Scotland, told BBC Radio Scotland that lessons can be learned from the incident.

"This is the first occurrence in the world of this particular failure occurring. It will be of interest to bridge masters across the world," he said. "The Forth Road Bridge already has a very high safety inspection regime in place. It will be even higher now.

"I think the likelihood of a similar occurrence in the future is probably very small," he added.

   

Tagged categories: Bridges; Infrastructure; Inspection; Roads/Highways; Steel; Structural steel; Transportation

Comment from regis doucette, (3/24/2016, 9:23 AM)

Nonvisible contaminants are not unforeseeable. This looks like the same photos from 8 years ago when surface chloride ions were determined to be at fault on this same structure. The issues associated with that small pin that seized and prevented proper movement of the structure critical assemblies, to my thinking, seems preventable. There are chemical solutions to remediate nonvisible contaminants including soluble salts.


Comment from Wayne Senick, (3/25/2016, 4:43 PM)

Focusing the right chemistries that have the field proven right properties to free up the pins, stop further corrosion and let them work properly in this case could have saved the tax payers and the travelling public millions of dollars and a ton of inconvenience plus reduced the risk of a failure. By drilling down to the issue and approaching the problem as a preservationist, situations like this can be avoided. Take it down to basics, bridges need to move how do we insure they do. bridges corrode how do we stop the corrosion causing chemistries for getting to the structure critical components and lastly stop the existing corrosion from progressing using the right field proven chemistries and methods of surface preparation and application. Was a hard lesson leaned in this case, will we learn anything from this near disaster or continue making the same mistakes over and over again of trying to cover up active corrosion cells and hoping they go away without destroying the functionality of the structure.


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