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Second crack found closes Forth Road Bridge

Forth Bridge

Cracks in a second vertical steel member has 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.

Forth Bridge

Forth Bridge

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.

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.

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. 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

 

Readers' comments (5)

  • Assuming the sketch showing the link is correct the member has no structural significance to the bridge at all. I suspect it was put in for aesthetic reasons to appear to show continuity of the truss. The members could be removed entirely without danger to the bridge. The crack was probably induced by secondary stresses from the movement of the joint and corrosion in the pins preventing rotation. If cracks in these members are the only concern it seems an over reaction to close the bridge. Edward Palmer

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  • Edward - a far as I can make out, the sketch does not include the end vertical member of the truss, which sits between the links. I presume this omission is to provide clarity, although that has obviously not been achieved.
    The links provide the vertical supports for the end of the decks on either side of a tower. From there outwards, the deck is supported by the hanger cables. If the links were to fail completely, the deck would not necessarily collapse, but it would drop by 6" or so adjacent to the tower. I think most motorists would not find that acceptable.
    From an educated guess, I daresay the detail of the link plates being welded onto the pin block has given rise to fatigue conditions. It's basically the same as having a square hole in a section, with some nasty stress flows through the welds. Other factors have probably contributed, including lateral wind loads on the link plates and pin friction.
    Let's hope the repairs can be completed quickly, so that the enormous disruption to traffic in the area is relieved as soon as possible.

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  • A. Causes could be one or more of:
    1. Long term stretch of the longest suspenders adjacent to the towers causing extra loads to be distributed to the swing links from traffic loads and wind torsion on the deck.
    2. Friction or siezure of the pins, noting inaccessibility of the movement interfaces, causing bending to the link overstressing the weld area adjacent to the pin. The movement surface is between the pin and truss whereas the pin to swinglink surface is fixed by the keep plate visible on the outside.Pins at the upper end of the links appear to give better access for inspection or lubrication of the movement interfaces.
    3. Incomplete penetration or lack of fusion of the welds noting that access for welding was only possible one side, the rebate on the substantially thick pin plate which formed the backing. At the time of construction reliable ndt methods were not available to check for significant defects.
    4. Brittle fracture, possibly also fatigue related, in the welds or heat affected zone [HAZ] under recent cold weather and high wind conditions.
    SOLUTION
    1. Temporarily a bypass to the cracked area by a welded fabrication is appropriate as has now been published. This is similar to a sketch I sent to the Amey team. Access for welding is a key practical issue bearing in mind intricacy of the pin area and access generally to achieve the repair.

    2. A longer term solution could be to install swinglinks BELOW or underslung so that they are in compression not tension. These need to be of similar length to those existing to be compatible with the expansion joints and other arrangements at the tower.

    Alan Hayward, Consultant to Cass Hayward Consulting Engineers

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  • I am surprised that there has not been any significant discussion 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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  • One may refer to details of the pinned swing link in the ICE Proceedings for November 1965 [Proc. Inst. C.E, paper No. 6890]. Mr. Howarth suggests that the cracking could be due to lateral movement of the truss bottom chord and that the proposed strengthening will actually increase the stresses in the swing link. A rough calculation suggests to me that this is would not be so due to the inherent flexibility of the swing links being 5.75m in length. Although under any assumed movement the bending moment in the swing link would be slightly increased the maximum induced stresses in both the stiffened and unstiffened portions should tend to be less.

    My earlier comment had suggested 4 possible contributory causes of the cracking. Clearly a total siezure of the pin would cause very significant bending stresses in the swing link under longitudinal thermal expansion of the bridge main span, such that the repair might be less effective. Total siezure seems very unlikely to me based on the apparent form of the crack. It would seem important to check that the pins allow rotation and that if practicable lubricant is applied.

    In principle the proposed temporary strengthening system does appear to be appropriate.

    Alan Hayward [F]

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