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PPP factor

Bridges

Pigeons, paint and passenger train speeds are the dominant factors during repair of Britain's longest rail bridge. David Hayward reports from Scotland's Tay estuary.

With several years' project managing maintenance work on the Forth Bridge behind him, Railtrack's Hugh McLean hoped he could finally get away from the smell of paint. No such luck - he immediately transferred to Scotland's other historic rail crossing, just one east coast estuary northward.

Here they are also painting the Tay Bridge; not a lot of it, but enough to cause programme challenges and help trigger acceleration of a ú16M (US$25M) repair and strengthening contract now approaching its final winter.

Whether the work involves coating just one of the 86,000 new steel bolts which are being inserted into the superstructure to replace 115 year old corroded rivets, or protecting a 3m long steel plate strengthening a crucial truss chord, each individual paint job demands a climatefriendly working environment.

Epoxy dislikes being applied in temperatures less than 5 0C, and becomes very temperamental in humidity above 85%.

Unfortunately such conditions are not uncommon a few metres above the exposed Tay estuary, even in mid-summer.

'Painting downtime in June was up 50% compared to the same month a year ago' recalls Mowlem's contracts manager Bruce Cameron. 'We were never expecting this to be a painting contract; but these relatively small tasks have become a significant nuisance factor.'

What Cameron was expecting - and has got - is the challenge of both repairing and strengthening one of the rail network's most important river bridges and a vital link in Railtrack's prestigious East Coast Main Line extension to northern Scotland.

Opened in 1887, the wrought iron crossing snakes across the River Tay with few of its spans the same size or profile. The bridge's 'wow' factor may lack the visual impact of its younger, shorter cousin 70km down the line across the Forth estuary.

But just the name Tay summons unparalleled notoriety in engineering archives.

Over 80 reminders of its fatal history are constantly on show, just above high water level, as a row of cut down pier foundations from the original crossing running alongside its successor.

Lesser known reminders of the first bridge's dramatic collapse, are the scores of original slender wrought iron deck trusses reused in the new crossing.

These are supplemented by at least double the number of new, much beefier trusses positioned alongside.

The result is an average 39m long span formed of four, 2.6m deep trusses braced diagonally and supported by twin columned hexagonal wrought iron piers.

These up to 21m tall piers vary in shape but all are hollow with an internal ironwork frame adding rigidity, and a reinforced concrete crossbeam linking the columns just above water level.

The bridge's troughed deck, carrying rail ballast and twin tracks, is an early example of the use of steel in construction. Not surprisingly it was this zone that attracted significant interest from the army of consultants, finite element analysers, abseilers and metalwork tappers that invaded the bridge soon after Railtrack took over custodianship from British Rail in 1994.

This early, dubious quality steelwork is structurally the bridge's weakest link. And, given the complexity of live loadings from wind and train forces on the multiple curved crossing, the conclusion of this ongoing US$770,000 package of surveys is that elements of the structure are understrength.

Simon Short, design engineer for bridge consultant Pell Frischmann, emphasises that, for current train traffic loading, the bridge remains structurally sound, but adds: 'Reserves of strength, especially for freight traffic, do not comply with Railtrack's latest standards.'

For Railtrack itself, Hugh McLean expresses the findings more bluntly. 'We found the bridge in a state of disrepair, with few records and no evidence of detailed inspections, ' he claims.

'Its annual US$770,000 maintenance budget should have been five times higher and we now have a bridge where every member works hard for its living.'

Freight traffic was immediately banned and severe speed restrictions imposed on passenger trains. Current refurbishment is a combination of repair and strengthening, and it is often difficult to determine whether replacement of a cluster of the bridge's total 3M rivets with steel bolts is the former or latter.

Short estimates a roughly 80/20 split in favour of repair; though for Mowlem's Bruce Cameron it is all the same, as gaining access to carry out any of the work remains his prime headache.

Unusually for such major repairs, the railway must remain live throughout the contract, with Mowlem facing hefty train delay penalties. Apart from snatched night time possessions to replace railside walkways, the bridge deck is a no go area.

All repair work, and its supply, must be carried out from 'below decks', and the contractor brought in three sophisticated travelling platforms, each acting as a total work station for a complete span. Slung from lower truss members, and able to slide the length of its span and be lowered to river level, each 10m long, full width platform carries enough equipment to complete all repairs in the area.

With repair work on the crossing's northern section nearly complete, current operations on the south side are becoming increasingly sparse, so gantries are being replaced by several dozen fixed or hanging scaffolds. This change helps - though it does not resolve - the painting problem.

Wind load constraints dictate that no more than 25% of any one span can be sheeted at a time.

Instead gantries or scaffolds must remain in place until all three coats can be completed.

Additional scaffolds, now being built, will ease this logistics challenge and allow more work fronts than were originally planned to open before winter weather sweeps up the estuary.

Wind, rain and falling temperatures affect most repair work, but the downtime decider often lies with wave heights.

An extra 'wrap around' hanging gantry has been added to service repairs on the twin columned piers. This eases the awkward task of entering the columns to examine the internal framework. The frame needs less repair than feared, but one somewhat unrewarding task remains.

Over 1,000t of pigeon guano is the polite description of the bill item Mowlem's men are shovelling into bags. It appears pigeons find this enclosed environment a pleasant home, so leave their not so pleasant deposits piled 1m high on column floors.

The aggressive lung-attacking nature of these deposits, especially if disturbed, means removal teams must be fully suited with pressurised face masks and the columns designated a 'confined space'.

As construction crews leave the bridge next spring - and doubtless the pigeons return - the crossing's loading restrictions will ease significantly.

Commuter trains will cross at up to 100km/h - more than double the current speed - and freight traffic can return.

Infamous history

The 1878 official opening of the first Tay Bridge was marked by a train load of dignitaries crossing at the impressive speed of 40km/h. A century and a quarter later, this is roughly the same pace at which today's trains are allowed to cross the new bridge. The intervening period has seen the slimline old bridge collapse, a new structure built partly from the original's remains, and then this crossing itself fall into 'disrepair', forcing the refurbishment contract.

At around 8pm on storm strewn 28 December 1879, the collapse of the original 18 month old bridge's central navigation section, killing all 73 occupants of a crossing train, marked the birth of much more stringent bridge design standards, especially for live loadings. The new crossing, started four years later, boasts a 2.7kN/m 2wind loading - over five times that of its predecessor - plus reuse of numerous undamaged original trusses.

These slender trusses form the outer two of the current bridge's four truss deck section, spanning twin column piers over twice the width of their predecessors.

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