Painting the Tay Bridge
The direct rail route between Edinburgh and Dundee required the construction of the Forth and Tay bridges – respectively the UK’s most iconic and longest.
This was a massive investment and demonstrated the economic importance of rail traffic of the time. However, it didn’t start well. The first Tay Bridge, which opened in 1878, was designed by Thomas Bouch and took six years to build. Eighteen months later, it collapsed in high winds as a train crossed over it, killing 76 people. At the time, Bouch had started to supervise construction of a Forth rail suspension bridge. Not surprisingly this work was stopped.
Following the disaster, the Board of Trade set up a commission to consider wind loading on railway bridges and Parliament specified that the Forth Bridge should “gain the confidence of the public and enjoy a reputation of being not only the biggest and strongest, but also the stiffest in the world”.
Another grim consequence of the bridge’s collapse was a poem about the disaster by William McGonagall, arguably Britain’s worst poet.
The longest bridge
Work on the current Tay Bridge started in 1883 and took five years. The contractor was William Arrol & Co, which also constructed the Forth Bridge. The Board of Trade specified that, for river navigation, the new bridge piers had to be constructed adjacent to those of the original bridge enabling re-use of girders from the original bridge.
Constructing the UK’s longest rail bridge required 25,000 tons of iron and steel, 70,000 tons of concrete, ten million bricks and three million rivets. The bridge is 10,711 feet long and has 85 piers. The south approach from Wormit consists of piers 1 to 28 supporting lattice girders that carry the railway. Piers 28 to 41 are the “high girders” with the railway inside them to give shipping a 77 ft clearance at high water. The north approach from Dundee has piers 41 to 85. From Wormit the gradient falls at 1 in 762 over the south approach, is level through the high girders and then falls at 1 in 113 on the north approach down to Dundee.
The Tay and Forth Bridges are impressive examples of Victorian engineering that have been inherited by Network Rail which now faces the challenge of maintaining these huge structures in a harsh marine environment. With the completion of the £130 million repainting project last December; this is not such an issue for the Forth Bridge. However there is still much to be done on the Tay Bridge. the rail engineer was invited to the middle of the Firth of Tay to learn more.
Painting is the easy bit
Stuart MacDonald is a man who has given a lot of thought to his task of painting Britain’s longest bridge, commenting: “It’s all about logistics, painting is the easy bit”. Stuart has been on the bridge since June this year and is project manager for Taziker Industrial (TI) which has been awarded a two year £15 million contract for phase three of Network Rail’s Tay Bridge refurbishment programme. This comprises piers and spans 12 to 27 (immediately south of the high girders) and 80 to 83 (landspans at the north end of bridge). Network Rail’s construction manager, Ian Simms, has worked on the bridge in various capacities for 20 years. He advises that TI’s contract is part of a long term four phase programme that started in 2006 and is planned for completion in 2017 – the final phase being the high girders.
As well as painting, TI’s contract requires them to repair steelwork defects before applying the surface treatment. Experience to date is that there are typically 240 such defects per span. Most of these are minor repairs requiring small amounts of steel for which TI have a fabrication shop in their compound on the north side of the bridge. To do this work, TI has a workforce of more than 55 employees, including those from Network Scaffolding, a division of TI.
Ten personnel alone are required for loading materials onto the bridge.
The quantities of materials involved illustrate the challenge TI faces. Each span has 1200 square metres of steelwork and two 25 kg bags of grit are required to blast each square metre. The surface coating system is the same as that used on the Forth Bridge. This is a four coat treatment that requires a primer (5 square metres per litre); glass flake coat (1 square metre per litre); an intermediate coat (10 square metres per litre) and a final coat of Tay Bridge Grey (5 square metres per litre). Add to this the scaffolding and other materials required and the logistical challenge of getting these materials onto the bridge 1000 feet from the shore can be appreciated.
One advantage is that, for phase three, there is no requirement for possessions to work on the steelwork below the railway or to access it. Night-time possessions are however required (4.5 hours on weekdays, 6 hours on Saturdays) to transport materials onto the bridge from TI’s storage compound by the sidings at the south end of the bridge. During these possessions, seven hand trolleys are pushed onto the bridge to either spans 15 or 23 on which storage areas have been established. Materials are then passed over the windfence onto a rail level platform from where they are lowered by a hoist. Although such a labour-intensive approach might seem odd, Stuart explains that materials transported are essentially a lot of small items that can be hand carried and that the use of hand trolleys has proved to be more efficient than on-track plant. The 1 in 762 gradient on the south end of the bridge does not present a problem for hand trolleys.
TI’s strategy for spans 12 to 27 is to use four sets of scaffolding to firstly encapsulate spans 12, 16, 20 and 24. As each span is completed, each set of scaffolding then moves to the next span along. Spans are on the critical path and piers, which are independent of the scaffolding, will be sequenced as required. TI is using the HAKI scaffolding system they used on the Royal Albert Bridge (issue 95, September 2012). This uses beaded sheets that slide into slotted tubes to provide encapsulation.
Walking over the Tay
Other than the high girders, a timber catwalk runs the length of the bridge within its trusses along the bottom of the bridge spans. Also running the length of the bridge are electricity and water supplies. TI has also provided a compressed air manifold for the grit blast pots which are located on the storage spans.
This walkway is the only means of access and enables materials to be distributed from the storage spans. It was replaced around ten years ago but, with the bridge being a Grade 1 listed structure, its replacement had to be as original – a pitch pine walkway. Walking a mile or so along this exposed walkway 60 feet above the Tay requires a head for heights. It is also no easy stroll with the need to stoop every 20 feet as the walkway passes under diagonal cross bracing.
Stuart is very conscious of the impact wind has on his project. TI have anemometers on spans 15 and 26 and monitor windspeed in their Wormit compound.
In addition, workgroups have portable anemometers. He explains that windspeed can be very variable and it is not unusual for the weather to be quite calm at Wormit while it is blowing hard in the centre of the Tay.
TI has various rules concerning windspeed. For example, above 40 mph work on the bridge stops and above 27 mph scaffolders are not allowed to lift boards. Applying paint is also weather dependant. Painting is not allowed below 5° C and at certain levels of humidity. Stuart has studied weather records to assess the impact of weather on the contract programme and is confident there is sufficient contingency.
On the bridge, the safety regime is evident. A tagging system is used to record who is on the bridge and where they are, everyone has radios which are frequently tested, the previous day’s Behavioural Safety briefing is a frequent topic of conversation and the standby rescue boat can be anywhere under the bridge in 90 seconds. Stuart mentioned that confined spaces are also a hazard due to the requirement to work inside the piers – twin hexagonal columns joined to form an arch with an internal wrought-iron framework concealed inside the hollow interior.
A battle won by logistics
General Eisenhower once said: “You will not find it difficult to prove that battles have been won or lost primarily because of logistics.” Working in the middle of the Tay is a battle which will be won by planning and preparation, such as the four months that TI have spent putting scaffolding and services on the bridge and delivering materials. This shows just how much is needed to be done before the real work can start. Now that this set-up work is almost complete, TI is able to start their production line refurbishment of spans 12 to 27, about a quarter of the bridge. The weather may not be kind to them but there are contingencies for this and no doubt much new paintwork will be evident by next summer.
With the phase 4 contract to be let after TI complete their work, it will be 2017 before Network Rail can announce that Tay Bridge painting is complete. Then it will once more be the “Beautiful Railway Bridge of the Silv’ry Tay” of William McGonagall’s poem.