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Forth Bridge defective member previously deemed 'not highly stressed'

Forth Bridge inspection landscape

Failure of a structural member in the Forth Road Bridge’s main steel southbound deck truss discovered on Tuesday came as a shock to the bridge maintenance team as it had previously been assessed as not highly stressed.

Traffic restrictions remained in place today while a team of engineers assessed the safety of the 53 year old structure and attempted to come up with both short term and long term solutions.

The vertical member that failed is located at the end of the main truss, hard up against the connection to the north tower, and is said to be difficult to access safely. Darkness, high winds and driving rain made inspections impossible overnight, with the team beginning the work this morning.

There have been concerns about the long term load carrying capacity of the deck trusses for a number of years, and strengthening plates were bolted on to highly stressed sections around 10 years ago. There have been several truss strengthening operations since then.

Bridge operator Amey discovered the failure during routine inspection. An Amey spokesman said that the failed member had never been identified as undergoing abnormal loading, although it was in a location where some neighbouring members of the truss were under high stress.

Strengthening work was already taking place in these areas.The failure occurred “just above a pin joint”, leading to speculation that the joint may have jammed.

In the short term it is likely that a temporary splice plate will be welded on, which would allow the crossing to reopen to normal traffic flows. A longer term solution will also be sought.

A first report on the results of the investigation is expected later today.

Amey Forth Bridges unit operating company representative Mark Arndt said “A key structural component beneath the southbound carriageway has failed so it is important that we carry out all the necessary calculations before relaxing safety restrictions.

“Following the detailed inspection carried out this morning we now have the information required to develop both short and long term solutions. Our team will be working through the night to design and check our plans and we aim to have these confirmed during the day tomorrow.”

The Scottish Government’s Forth Bridges Unit took over responsibility for the operation, management and maintenance of the Forth Road Bridge on 1 June after the dissolution of the Forth Estuary Transport Authority. Amey has held the Forth Bridge Operating Contract since November 2014.

 

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