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The bridge doctor

Charles Cocksedge - Charles Cocksedge has worked on major bridge strengthening projects for nearly 30 years. His latest challenge is solving the cable corrosion problems on the Forth Road and Severn bridges. Ruby Kitching caught up with him.

'If you have a problem [with a steel bridge] and no one else can help, and if you can find them, maybe you can hire the B-team [that's Faber Maunsell's bridge team]' -? cue the music and enter the unassuming Charles Cocksedge.

The 50 year old bespectacled technical director of Faber Maunsell's steel bridge team has spent almost his entire career saving bridges from disaster - the Tamar, Humber, Avonmouth and Hammersmith Bridges to name a few. Most recently he has been charged with overseeing the inspection of corroded wires in the suspension cables of the Forth Road and Severn Bridges.

Yet Cocksedge is modest about his achievements. He is the sort of engineer who does not believe he is doing any more than his job description asks of him: will design bridges and will fix them if there is a problem.

Add to this the fact that he has always loved bridges anyway and it quickly becomes clear that Cocksedge is on a bit of a busman's holiday.

'I remember walking across the Forth Road Bridge as a child. I never thought years later I'd be walking along its cables carrying out an inspection. Life's been like that for me - full of coincidences, ' he says.

Cocksedge studied civil engineering at Liverpool University, and started out as an assistant engineer with Freeman Fox, under the watchful eye of bridge pioneer and Messina crossing originator Dr Bill Brown.

'My first job took me to Zambia where I had to investigate whether hydrogen embrittlement was the main cause of some bolts failing on a cable stay bridge. I literally went to Zambia with 200 replacement friction grip bolts in my suitcase - I had a lot of excess baggage that day.' He goes on to explain in careful detail what hydrogen embrittlement actually is: 'It's the reaction between a high tensile steel and water. If there's a metal coating as well, the two metals react and water liberates hydrogen which embrittles the steel and can cause premature failure.' And so began Cocksedge's career in diagnosing and nursing steel bridges back to tip top form.

'Every bridge has its own problems and requirements. The biggest challenge comes from the fact that standard solutions are often too heavy for most bridges so you have to come up with novel ways of getting around the problem.' He adds that coming up with bespoke solutions is fun, but that convincing bridge owners that they will work can be trying.

Take the problems on the Forth Road and Severn bridges where water ingress has caused wires in the main suspension cables to corrode.

When the problems on the Forth hit the press in 2004 (NCE 6 January 2005) Forth bridge owner the Forth Estuary Transport Committee was advised by Cocksedge to install acoustic monitoring to check for wire breaks and retrofit a dehumidification system to dry out wet wires and prevent further corrosion. It was the first time such equipment would be used in the UK and the engineering fraternity was doubtful whether it would work. Further investigation and scoping studies had to be carried out to ensure this was the best solution.

A year on and the same problem, though on a slightly greater scale, has been revealed on the Severn Bridge (News last week). This time bridge owner Severn River Crossing immediately signed up to installing acoustic monitoring and dehumidification equipment, following the example of the Forth. Cables on the Forth were opened up after bridges of similar age in the US were found to have suffered corrosion.

'Three years ago, I'd been watching what was going on in the States, but thought that because the Forth was much younger - being just 40 years old rather than around 70 - and looked after much better, things wouldn't be so bad. It was jaw dropping stuff when we opened up the first panel, ' recalls Cocksedge.

He adds that the factor of safety in the load carrying capacity of the Forth's suspension cables has dropped from 2.6 to 2.3 according to test results from the corroded cables and mumbles cautiously that 2.0 is the lowest acceptable safety factor for the bridge to still be usable until 2014.

The most drastic remediation measure on the Forth will be to replace the cables, costing up to £100M (NCE 25 November 2005).

Cocksedge's work on the Forth and Severn follows the challenging remediation and strengthening work on the Avonmouth and Tamar Bridges.

Avonmouth bridge was widened and strengthened at the cost of £125M between 1995 and 2001. The bridge was originally designed to be a three-lane carriageway in either direction, but with provision for the hard shoulder to be used as an extra lane. But with the advent of heavier 40t lorries, strengthening was required.

'The simple solution was to add steel plate to the structure to strengthen it, but this would have added to the self weight of the bridge which was already quite high. Instead we installed Maccalloy bars across the flanges of the steel box girders to relieve the girders of some of the extra load of the bridge, ' he recalls. On the Tamar bridge also, load tests revealed that the structure would not be able to support new 40t lorries. In fact it looked as if the weight limit for vehicles should be just 17.5t (NCE 19 August 1999).

The solution was to install extra suspension cables to support new lanes of traffic while existing concrete decks were replaced with lightweight steel boxes. 'The cables helped reduce the load in the deck truss and compensated for the fact that existing suspension cables had become overstretched over time'. By this time Cocksedge was working for consultant Hyder. His team defected to Maunsell mid-way through the project.

Other than bridge projects, Cocksedge's other skill is in building the structures that support telescopes.

He describes them as giant theodolites that allow the instrument to rotate about two axes to follow the stars. They are usually sat on a very stiff structure with solid foundations.

Designing these structures has taken Cocksedge to the Canary Islands, Canada and Hawaii.

The project in Hawaii was to design the 40m diameter, 30m high rotating building, weighing 2,000t, to house the 8.3m diameter Subaru Optical telescope, situated on top of the 4.2km high Mount Kea.

'The power of these telescopes means you could use it to read a newspaper in Rome from here.

The ethos was to minimise the effect of heat on the telescope, so we designed lots of vents in the dome.'

Cocksedge's sketches showing how extra cables on the Tamar Bridge helped tighten up the slack cables (left) and strengthen the main spans on Avonmouth Bridge (right)

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