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Waiting in: suspense

BRIDGES; Replacing the Forth Road Bridge's ageing suspension hangers is proving more complex than anticipated. Report and photos by David Hayward.

Motorists crossing Scotland's Forth Road Bridge this month cannot help but notice eight large work platforms clamped high up on its main suspension cables.

The curious would find it difficult to guess the exact role of these strange looking pods. And some may be concerned to hear that the platform's occupants are busy replacing all the bridge's 192 suspension hangers.

The crossing's depute (sic) bridgemaster, Barry Colford, is also somewhat perturbed that the platforms are there - but for a different reason. They should have been dismantled weeks ago and the £7.8M replacement contract completed by last month.

But Danish contractor Monberg & Thorsen has so far renewed less than 20% of the 35 year old steel ropes, and the platforms will not be removed until Christmas at the earliest.

'It has proved much more complex than we thought, both designing the temporary works needed and actually replacing the hangars,' is project manager Henning Schultz's explanation for the five month delay. 'What appears straightforward on paper is far from so in practice.'

Central to the contractor's problems is the need for four temporary vertical hangers, positioned around the existing assembly, to take all bridge loads while the original is replaced. Because these temporary cables have to be fixed to the bridge's steel deck sides at a slightly different position to the original hangar, loading on deck beams also changes.

It was the calculation of these additional forces and their overall effect on the bridge - therefore dictating exactly where to fix the temporary supports - that has taxed the contractor's design team. Until the repeatedly refined computer software design analysis came up with a satisfactory answer, little could be done on site.

The need for any contractor presence on the bridge began in 1995 after a routine inspection of the 44mm diameter hangars, each containing 133 wires and ranging in length from less than 1m to over 106m. This revealed several snapped or damaged wires and the crossing's shocked owner, the Forth Road Bridge Joint Board, ordered immediate visual examination of all hangars, plus non destructive tests on a 50 strong sample.

The result showed frayed or broken wires in nine cables, all relatively short mid span hangars and all quickly replaced.

In practice each hangar assembly is four ropes, secured in pairs to a steel casting bolted down to the deck's side stiffening truss. The damaged wires were all within the lower metre of each cable, and the cause is thought to be a combination of fatigue stress and corrosion, aggravated by a lack of protective lubricant around the wire bundles.

Live loading on the deck and its hangars is considerable. During most morning and evening peak hours, the bridge now carries some 10% more traffic than its design capacity of 60,000 vehicles a day. And mid-span cross winds can cause the deck to sway up to 7m sideways.

Replacement of suspension bridge hangars is relatively common, especially in the United States, although the operation at Forth is thought to be the first in Europe involving vertical ropes.

The Bridge Board was taken by surprise by the initial breaks and no plans existed for any widespread hangar replacement.

'Although we found no problems with the longer hangars, non destructive testing techniques are not very effective when you cannot examine every wire in the cable,' says Colford. 'If all the hangars have to be replaced sometime, it was more economic to do the lot at once.'

Loss of section in the worst affected cables had effectively reduced the hangar's 2.5 safety factor by a quarter. But engineers stress this assessment is based on the worst case loading scenario of nose to tail lorries across the bridge - an event that would, in practice, never be allowed to occur. And they maintain that the bridge's overall structural integrity was never at risk.

Monberg & Thorsen's £7.8M winning tender, in April last year, and its own time estimate of 64 weeks to complete the job, also came as a surprise to the client; but this time a welcome one. The contract price was not only the lowest, and over £1M less than the Bridge Board's own target estimate, but the contractor also offered to complete the job some 16 weeks more quickly than most rival tenderers.

The contractor's major constraint was that no traffic disruption was allowed and the entire operation had to assume maximum live loading. Even so, the hangar replacement technique is, at least on paper, relatively straightforward.

Pairs of temporary cables are hung either side of the old hangar, clamped to the main suspension cable and tied at deck level around the top chord of the warren truss style edge beam.

With these cables tensioned to accept full hanger loading, the old assembly is released, removed and replaced by a new set of cables the same size.

This additional support system is regarded contractually as 'temporary works' and so indisputably the responsibility of Monberg & Thorsen to design and erect. Bridge Board consultant, WA Fairhurst and Partners, has some sympathy with the Danish engineer's design problems, but places the delay firmly in the contractor's court.

'It was a very complex design analysis, involving load interaction with the main suspension cables, and there was a danger of overstressing the deck stiffening beam,' reflects Fairhurst partner Alan Simpson. 'But our role was solely to check and approve the contractor's temporary works calculations.'

Finalising the position of the temporary cable supports, exactly 1.36m away from the original hangar, cost the contractor about half the current five month delay, Schultz estimates. But the firm's design work did throw up a real bonus.

It had always been planned to replace the single unit cable socket on the deck with a modern, two section adjustable bolted socket arrangement. Monberg & Thorsen improved on suggested designs by incorporating a screwed thread within the upper part of the socket, so hangers could easily be adjusted or removed totally.

Additionally, it introduced a range of different size bushes to insert into the eye of the socket, allowing 2mm accuracy in adjusting for thermally induced changes in cable length (see box).

The balance of the programme delay is attributed to even more severe weather than the contractor had predicted for the exposed esturial site. Wind speeds greater than 16m/s restrict operations and Schultz reckons the two months downtime already suffered is double what he expected.

Manoeuvring long heavy ropes, and repositioning platforms along suspension cables, has also proved harder in practice than theory. The contractor's original programme cycle of nine days to install a new hangar has turned into an average 12 days - sometimes even double - and acceleration plans are in force just to contain the current delay.

Two extra platforms have been brought in to boost the original six and a month ago a night shift was introduced. Surprisingly, Schultz reports little difficulty changing hangars in the dark.

But there seems little chance of recovering lost time, and - with 25 December the current completion date - Schultz's prime aim is to ensure the team is off site at least a day early.

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