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Forth Road Bridge remains closed

Forth Bridge inspection landscape

Cracks in a second vertical steel member have triggered the total closure of the Forth Road Bridge until the New Year at the earliest.

The first member, known as the main span truss end link, was found to be fractured on Tuesday. Now, with the crack discovered on another truss end link, bridge operator Amey’s inspection team is carrying out emergency inspections on all 16 such links.

Each truss end link has a welded-on connection to a movement joint where load is transferred from the main truss’s bottom chord. The first failure is thought to have started with a crack in the area immediately above the weld, which would have been affected by the high temperatures during the welding process.

This crack appears to have propagated upwards, eventually leading to a shear failure. The second suspect member was found to have cracks in the same area, although propagation had not yet started.

Forth Bridge

Forth Bridge

Forth Bridge failed member

Concerns that the truss end link next to the one that failed would now be carrying double its normal load was the key factor in the decision to close the crossing completely. If a pair of links were to fail, the corner of the deck above would be likely to subside by 150mm or so.

Deck loads are transferred from the deck truss to the towers via the main suspension cables rather than through the truss end links. These help resist lateral wind forces and keep the truss aligned with the tower, and were not expected to undergo high stresses. So Tuesday’s discovery by Amey inspectors came as an unpleasant surprise.

Each tower has two pairs of truss end links on each face, and inspecting them rigorously is dependent on weather conditions and access logistics. An Amey spokesman said that the next stage was to set up a safe working platform to enable strengthening splice plates to be welded onto the cracked truss links. This operation will take a minimum of three weeks, given favourable weather conditions.

A longer-term solution will depend on the outcome of the inspection programme and the extent of the problems that are likely to be discovered. When corrosion was found in the main suspension cables in 2004 it was ultimately decided it would be uneconomic to replace them, and the Queensferry Crossing was eventually given the go-ahead.

This is due to open next year, after which only coaches and taxis would be allowed on the original crossing. With this in prospect, a major repair programme that would allow the bridge to reopen without further traffic restrictions for less than a year might be hard to justify.

Forth bridge truss drawing

Forth bridge truss drawing

Drawing showing the position of the failure

 

 

 

 

Readers' comments (1)

  • COULD THE PROPOSED REPAIR MAKE THINGS WORSE?

    I am surprised that there has not been any mention of the proposed repair to this member (see BBC - http://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-35060879).

    Obviously, I do not have full details, but simple engineering mechanics suggests that the observed crack could be due to out-of-plane horizontal motion of the bottom chord truss member, leading to bending stresses just above the pin. The proposed solution seems to be at risk of making things worse, by stiffening the affected area. If the displacement remains the same, then the resultant stresses would increase significantly.

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