|Location Map ( geo)|
|M1 • A631|
The Tinsley Viaduct is a major crossing in Sheffield, where the M1 and A631 both cross the River Don and the local railway network on separate tiers. The viaduct is also next to the city's key retail area of Meadowhall.
The crossing of the Lower Don Valley was always going to be challenging. Not only was the crossing to carry a six lane motorway and a 4 lane trunk road, the crossing had to be snaked between industry, cooling towers and survive a high level of condensation and toxic atmosphere.
The need for a crossing at this location came about due to a decision in the late 1950s to provide a route from the London - Yorkshire Motorway (now the M1 & M18) that passed closer to the heartland of industrial Yorkshire, this route was to be designed by West Riding County Council extending from Aston to Leeds via Sheffield.
The route was to become the Section II Scheme and as part of this scheme, a three quarter mile long two level viaduct was proposed, carrying the motorway route (M1) on the top level, a major a class road (A631) on the lower deck that would link the two main roads through the Lower Don Valley (the A6109 and A6178) thus avoiding the need to build a separate junction on each side of the valley.
The proposed viaduct caused significant unrest from the local authorities, British Rail and public utilities as to how the construction would impact on their services. These organisations had the power to force a public enquiry had their objections not been withdrawn. The design had to be such that it had minimal impact on the surrounding infrastructure and in particular power supplies to the steel industry.
The detail of how to construct such a massive structure in a highly polluted area was always going to be key to the viaduct and indeed, haunted the viaduct for decades afterwards.
The original design approved by the Ministry of Transport was a 1000m (3,400ft) long prestressed concrete, with some structural steel members that had a high resistance to corrosion. The concern about corrosion arose from the nearby industry, in particular the (then) seven cooling towers and close proximity of the River Don and the Sheffield-Keadby Canal, which gave the area an humidity above the 80% corrosion threshold. This humidity was made worse by the sulphur dioxide from the adjacent power station and a nearby sulphuric acid plant and was regard as the most corrosive atmosphere in Britain at the time.
Other problems centered on ground conditions, the site of the viaduct is crossed by the Don geological fault system, had numerous ancient and uncharted mines and subsidence had been recorded at 15 inches in the previous 30 years.
The viaduct was placed out to contract in February 1964 and five tenders were opened, with one from Cleaveland Bridge offering an alternative lump sum quote for a steel box girder design that was £1 million cheaper than the next lowest tender of £6 million and was accompanied by 8 pages of welding and painting specification and 5 outline drawings prepared by Freeman, Fox and Partners (see side note). This compared to the 4,500 sheets of calculations, computer analysis, 176 drawings and 239 pages of specification and bills of quantities of the design put out to tender.
A report recommended acceptance of the lowest tender and rejection of the steel alternative, and that the alternative would in the long term increase, rather than decrease costs - A prediction that would prove to be true in later years with the strengthening through the 1970's, and later strengthening in 2004-2005 that cost £82 million - The Ministry invited re-tenders on a steel design that was 5ft narrower and with slightly easier access arrangements due to land purchases since the original tender. Although a lower tender for £4,344,000 was supplied for a concrete design, the tender was awarded for a more costly (£4,616,000) steel design by Cleaveland Bridge and Freeman, Fox and Partners were appointed to supervise the contract.
Its difficult to imagine, but 12,500 tons of mainly high tensile steel and 80,000 tons of concrete were used in the construction of the viaduct. With gas, electricity and water supplies carried in the lower deck.
The deck of each level is a composite of a reinforced concrete slab, on two main longitudinal box girders with cross girders spanning onto 17 pairs of steel columns spaced across the valley. Each deck is anchored at each the northern end with about 600mm (2ft) of expansion provided for at the southern end. Each support along the length have rocker type bearings, allowing for independent thermal movement and the main box girders on roller bearings at the southern end. This design was strengthened later with the diagonal braces that can be seen today. The upper deck is longer than the lower deck, necessitating four separate abutments constructed from reinforced concrete on spread footings.
Construction was arranged such that disruption below was at a minimum, so as to avoid closing railways and the operations of the industry below. A 35 ton derrick was used to lift the box sections of the longitudinal box girder, with each span completed in about 11 days. The 215mm (8.5in) deck was begun before the steel erection was completed and featured heating cables built into the road surface of the top deck to reduce the affect of ice formation as a result of condensation from the nearby cooling towers.
Partly as a result of the collapse of the West Gate Bridge in Australia in 1970, a design check revealed the need for strengthening to the structure including the main box girders and the introduction of the diagonal steel box struts that are clearly visible today. This first round of strengthening limited the use of the viaduct and it wasn't fully reopened to traffic until 1980. It has been suggested by engineering experts that this first strengthening was a knee jerk reaction to the disaster in Australia and that the true cause of the accident wasn't the design, but the method of construction.
A further check in the 1990's revealed a requirement to strengthen the viaduct and adjoining bridges at the roundabouts at each end to ensure compliance with a European Union Directive introducing 40t HGV's in the UK. The bridges at each end were upgraded in the late 1990's and the viaduct itself was subject to an £82 million scheme between 2002 and 2005 that involved significant hidden work inside the structure and saw the top deck reduced from 6 lanes to 4 lanes for the duration of the work. The top deck was due to be re-opened with 6 lanes in 2005, but consultation with the police brought up a desire to maintain 4 lanes over the viaduct to provide a lane gain and drop at each end to accommodate the significant traffic generation at this junction.
Concerns are still being raised about the long-term viability of the Tinsley Viaduct, as it is becoming progressively more expensive to repair. The proposed route of High Speed 2 is planned to run alongside the viaduct, and this may result in its eventual demolition.