Written by Mungo Stacy for the rail engineer
The bridgescape along the River Tyne is instantly recognisable – a visible, visual celebration of civil engineering.
Less than a mile downstream a tributary, the Ouseburn, commands a further gallery of fine bridges. Compared with the postcard glory-bridges, though, these specimens normally attract little attention.
Yet this collection tells a similar story of engineering development, and the structures vaunt their own distinctive appearance. Modest 18th century Crawford’s Bridge is the lowest and oldest, crossing the burn in a single stone span.
Almost directly above is the red-brick Byker Bridge, opened to road traffic in 1878 with a half-penny toll to traverse its 22 arches clear over the valley.
Efficient twentieth-century precast segmental concrete technology is represented by the elegant curves of the 800 metre long Metro light-rail bridge.
The final group-member is currently in hiding, apparently coy of its dressed-masonry piers, wrought iron arches and delicate spandrel-work.
Behind plastic modesty screening, contractor Carillion is tending to every inch of the Network Rail’s Ouseburn Viaduct in a £10M project to address its condition and load-carrying capacity, to a design by Cass Hayward.
Two hundred years ago, the Ouseburn Valley rang to the sounds of heavy industry. White lead works, lime kilns, pottery, flax spinning, iron foundries – all situated themselves here to exploit the river, both for power and to drain away their noxious wastes.
One hundred years ago, the river beyond the bridges was culverted and covered in up to 30m of landfill. The Ouseburn Tip became notorious for its fetid smells, its tendency to self-combust on hot days and the local scavengers who went ‘scrannin’ on the tip’.
Fast-forward to today, and a plethora of environmental legislation rules the actions of the smallest enterprise. Hence the plastic sheeting which wraps the works and ensures that any contaminants are captured, bagged and removed.
Blasting and painting subcontractor Pyeroy has installed powerful extractor units to recover and process the grit used to strip the bridge back to bare metal.
Sounds of industry and activity once again resound across the valley. Inside the wrap, the angular metalwork acts as a sound-box to echo, amplify and retransmit the smallest noise.
And the work is loud, not quiet – as the big chisel-heads of demolition picks go to work to remove old rivets, the sound-blows feel physical rather than aural.
Up close, the scaffolding resembles a study in persepective by M C Escher. At its highest it has eleven layers, and it took several design iterations to ensure that the right access was available to all areas of the bridge.
Although a haul road was created alongside, large plant was prohibited by the steep-sided valley. In lieu of mechanical telehandlers, scaffolding subcontractor Lyndons manhandled up to 20 tons of scaffolding per day over the six weeks it took to install.
Right first time
The five main 35-metre spans are built of six parallel metal arch ribs per span. The open spandrels are an intricate tracery intended to support the deck from the arch. Masonry piers give the railway a height of 33 metres above the burn. Two 12 metre-span stone arches each end of the bridge complete the full 280 metre length.
However, these details are not completely original. When first opened on 18 June 1839, as part of the Newcastle and North Shields Railway, the viaduct was built of laminated timber.
Compared with stone arches, the only viable alternative at the time, the carpentry saved around 25% on the original construction cost of £24,500.
The designers were father-and-son team John and Benjamin Green, who were also responsible for Newcastle landmarks such as Grey’s Monument and the neo-classical Theatre Royal.
By some accounts, John was the civil engineer, taking part in early chain-suspension bridge experiments with Capt Sir Samuel Brown RN, whilst Benjamin was more of the architect.
That said, it was the son Benjamin who was awarded a prestigious Telford Medal by the Institution of Civil Engineers. His description of the Ouseburn Viaduct in the Institution Proceedings of 1841 described the economies obtained by using timber and the ‘Kyanizing’ process to treat it.
Unfortunately it seems that the preservative was not entirely successful, for the viaduct was rebuilt in metal thirty years later.
Fortunately the original directors of the railway company had insisted on masonry piers which were capable of supporting a heavier superstructure. Care was taken to replicate the form of the timber arrangement, and the engineer on this occasion was T E Harrison.
Increasing traffic led to the viaduct being widened in 1887 to add a further two tracks on a similar but slightly narrower new structure. In the 1950s, the tracks were ballasted and additional vertical frames were added to transfer the extra dead load without overloading the spandrels.
In the worst tradition of architectural fancies, the ornate spandrels appear to put form before function. Their pretty appearance has contributed to the Grade II listing of the structure. However, the narrow sections are prone to corrosion, have a tendency to develop cracks and create an indeterminate load-path.
Concerns over the capacity of the bridge led to a speed restriction of 20mph being imposed. Refurbishment was essential as this affected the East Coast Main Line.
Network Rail commissioned Gifford to assess the bridge and design a strengthening scheme to Form A, Approval in Principle stage. The scheme will provide full RA10 capacity for 60mph freight and RA8 capacity at 100mph for passenger traffic.
The Gifford assessment chose to treat the spandrels as non-structural, accounting for their dead weight but ignoring their stiffness in the computer model of the bridge. This left a well-defined load path, taking vertical loads from the deck into longitudinal stringer beams just below deck level and thence into the 1950s vertical frames down to the arch and to the piers. Strengthening measures were required and detailed for all these elements.
Under this scheme the spandrels are not required to carry load, although some residual stresses will remain in them. Loads from the railway will be carried by the existing vertical frames behind the fascia, which can be seen by careful examination of an elevation of the bridge.
The tendered scheme was won by Carillion with designers Cass Hayward to develop the detailed design. Cass Hayward exploited their knowledge of the bridge, gained during the Category III check of the Gifford assessment, to identify opportunities to optimise the strengthening.
James Parsons, partner at Cass Hayward, explains:
“Originally, doubler plates were shown for the full perimeter of the arch ribs. During detailed design, we were able to look more closely at the areas needing extra material which allowed us to remove around two thirds of the main rib strengthening.”
The newer 1870s section of widened bridge only carries a single line plus the overhead line equipment bases, and around 90% of the strengthening to the ribs was designed-out for this section.
Construction has been carefully sequenced whilst still allowing the contractor to work on several spans at once. Clearly, strengthening had to be in place before introducing additional load into existing members. Thus, the works to the arch web splices are first, followed by the doubler plates to the arch flanges.
After this, the lower connections for the vertical frames are to be strengthened. Then new stringer-beams will be added to support the deck off the vertical frames. Additional infill panels will be fitted to the spandrels at the centre of the arch where new stringers cannot be installed.
Extensive discussions were held with English Heritage and Newcastle City Council’s conservation officer about the works to the bridge. Darryl White, Network Rail’s scheme project manager, commented: “They were most concerned about the appearance of the outer elevations”.
He added that, following eight to nine meetings over a couple of years, a suite of details was agreed and consent obtained.
A key consideration has been retaining the original structure as far as possible. The reduction in arch rib strengthening has assisted with this as it allows more of the original material to be visible rather than overplated.
Other detailed-design changes have also helped. The longitudinal stringer beams were intended to be standard universal beam sections.
These were changed to T-sections trussed together by channels, on first sight a cumbersome arrangement, but actually assisting with installing the smaller pieces in a congested area and reducing the changes needed to the existing vertical frames to make a connection.
Recent policy on painting for historic bridges has been to recreate the original colour where possible. Specialist conservators Crick Smith were engaged to analyse the original paint.
Although it seemed that the bridge had been taken back to bare metal in the past, a few traces were found. Trial panels in these colours were previewed by the interested parties to make a colour selection. In engineering terms, the M20 RT98 system is expected to have a 25-year life.
When the bridge re-emerges next summer after a year of work, the most visible change may be the fresh paint.
It is a tribute to the attention paid to the heritage that the extensive works, which will fundamentally change the way the structure carries loads, have such a minimal impact on the appearance of the bridge.
With the scaffolding removed, and with the capacity to meet the needs of twenty-first century rail traffic, the Ouseburn Viaduct will regain its standing amongst the other bridges as belle of the valley.