double decker, salt n' pepper

The area of Tinsley, to the east of Sheffield, used to have three famous landmarks. Now it only has two, as one was recently demolished. They were a shopping centre, a pair of towers and a viaduct. However, the towers are no longer, having been blown to smithereens over the August 2008 bank holiday weekend.

Despite their fame, the towers and viaduct have both had a long history. This feature details the history of Tinsley Viaduct and the problems encountered both during and after its construction. And no visit to the area is complete with a little look into the towers that provided an iconic symbol of the area, and for forty years came hand-in-hand with its motorway neighbour.

Crossing the Valley

The Tinsley Viaduct carries the M1 over the River Don to the east of Sheffield, and was built as part of the link route between the London to Yorkshire Motorway (now M1/M18) and the industrial heartlands of Sheffield and Leeds. However, the viaduct also had to carry a major local route (A631) alongside the motorway, thus avoiding the need to build two separate junctions on either side of the Don Valley.

After much deliberation and consideration of various design options, the Ministry of Transport decided that the best solution would be to build a twin-deck structure; the motorway would occupy the top level, with the A631 being carried underneath.

After deciding how to cross the valley, the next problem was how to construct it. Due to the adjacent power station and a sulphuric acid processing plant, the air in the local area was deemed to be the most corrosive in the whole of the UK, and was also 80% above the humidity corrosion threshold. So any materials used had to be highly resistant to water and sulphur! On top of that, the valley was also home to a series ancient mines and subsidence was a serious problem in the area (15 inches in the period 1934 to 1964).

The project was put out to tender and Cleveland Bridge and Engineering Company was awarded the contract at a sum of £6 million. The company suggested a steel box girder viaduct which would have been £1 million cheaper to build, but this was rejected in favour of the original concrete specification. However, the design requirements were changed, with the new design being 5ft narrower than the original. Cleveland again won the new tender at a price of £4.616 million, despite a lower tender price was supplied for a concrete design. Freeman, Fox and Partners were appointed as Consulting Engineers for the project.

The original design involved some 4,500 sheets of calculations, the creation of three computer programs to perform five structural analyses, 176 drawings and 239 pages of specification and bills of quantities. Even a 12 ft. long detailed model of the viaduct was required!

One upside to the re-tendering process was that land purchases had been made made since the original tendering process, making access somewhat easier. This would prove to be very useful, given its location meant that there was a distinct lack of access to the site, as the bridge would need to cross several railway lines and factories, as well as a canal!

A British First

Cleveland began construction of the viaduct in 1965, and was carried out through the use of a derrick weighing 35 tons. This was used to lift the box girder sections (which weighed 25 tons each) into place without the need to close the railways, canal or factories below. It took just 11 days to complete each bridge span.

The enormous task involved:
- the use of 12,500 tons of high tensile steel
- the use of 80,000 tons of concrete
- installing electricity cables and gas and water pipes within the lower deck
- installing heat cables to reduce ice formation due to condensation from the nearby cooling towers

Each bridge deck is a feat of engineering in itself, as they are a composite of reinforced concrete slab positioned on two longitudinal box girders, with cross girders and cantilevers spanning onto 34 steel columns which hold the bridge 10 metres above the valley floor. Rocker bearings allow for thermal expansion within the structure.

When it opened in 1968, the structure was the first twin-deck viaduct in the UK, and remains the only such structure to this day. The final cost exceeded the original tender price, with the final figure being in the region of £6 million.

Sweeping across the industrial Don Valley at Sheffield, the Tinsley Viaduct carries new streams of vehicles over all obstacles to free traffic movement - railways, cross-roads, a canal, river, factories and buildings of all kinds. An important new link in Britain's road programme has now been completed. The all steel 2-level viaduct is the first motorway structure of its kind in Britain. It provides an excellent example of the way modern steel is meeting and solving difficult design and construction problems. The viaduct carries the M1 on its upper deck as part of the Sheffield by-pass on the newly extended motorway route to Leeds. The lower deck forms an improved link in the local road traffic systems, serving the area between Sheffield and Rotherham. Steel has cut costs dramatically. The design of this unique project, which mainly employs high-strength steel in welded box-section construction has achieved a substantial cost saving compared with previous proposals using alternative materials. Planning like this, to improve Britain's road system and solve traffic problems efficiently and economically means making the most of British Steel.
British Steel press advertisement, 1968

More Weight Please!

Following the collapse of the Cleddau Bridge in Pembrokeshire and the West Gate Bridge in Melbourne, Australia in 1970, the government introduced new safety standards that all box girder bridges had to meet. This meant a long period of strengthening works had to be endured to ensure Tinsley Viaduct met these requirements, limiting the use of the structure through the closure of two lanes.

The long term restrictions that had been imposed raised questions in Parliament about the implications the problems would have on the UK bridge building industry in the long term. The delays in resolving the structure's problems was due to problems in finding a suitable economic solution.

Yet despite the problems being rectified, Tinsley Viaduct was still in the headlines for the wrong reasons. In August 1973, a lorry loaded with scrap metal was travelling on the southbound carriageway when it careered through the central barrier, crossed the northbound side and smashed through the guard rails. It landed on derelict waste land, but narrowly missed an electricity pylon, the River Don and the main railway line. The driver died in the arms of the railway worker who pulled him out of the wreckage; the passenger sustained serious injuries. At the inquest, the coroner was told that the barriers were only designed to cope with small vehicles travelling on the carriageway the barrier was alongside - the coroner felt very uneasy at the thought of a similar incident occurring but involving a full bus.

Further checks carried out during the 1990s revealed that a second round of strengthening works had to be undertaken, as the viaduct was not compliant with a European Union directive requiring bridges to the capable of carrying HGVs of up to 40 tons. In an £82 million scheme, this mammoth task saw a lot of work carried out within the box girders. Tasks to be carried out included:
- the strengthening of the existing joints;
- the welding of steel plates;
- the bolting of prefabricated structural elements;
- the installation of new torsion stops at the abutments;
- the installation of a number of shear connectors;
- the parapets either needed to be upgraded or replaced;
- the longitudinal boxes and rocker columns needed to be concreted.

In addition, during the course of the three year scheme used the following:
- 2,500 tons of steel, which was added to the existing structure;
- 3,500 tons of reinforced concrete - the equivalent of 250 full mixing trucks;
- 100 kilometres (65 miles) of welding;
- 53,000 bolts;
- 75,000 shear connectors;
- 155,000 squared metres of paint had to be removed then replaced;
- 4 kilometres of parapets.

The strengthening works also saw the M1 permanently reduced to two lanes in either direction - it was meant to have been restored to six lanes, but the police requested the layout that was introduced was maintained, as it helped to improve traffic flow on the slip roads.

A Towering Welcome

For forty years, drivers and passengers alike were greeted by the two giants of industry as they trundled along the M1. The Tinsley towers stood 70 metres tall and aimed to provide a lasting impression of their journeys along the motorway - in fact they were perhaps just as famous as the M1 itself.

Towers six and seven were constructed in 1937-8 as Sheffield Corporation were fulfilling the need for additional electricity to be generated - the Blackburn Meadows power station was itself built in 1921. The powerful steel industry was the driving force towards the need for two extra cooling towers to be constructed.

After the power station had closed in the 1970s, the government earmarked it for demolition. Whilst the rest of the station suffered that fate, towers six and seven were not to go down that route. The reason for this was due to the motorway - the viaduct's proximity to the towers, as little as 17 metres, declared any demolition as unsafe. The reason was that the government believed there was a risk that an explosion could seriously weaken the viaduct, and subsequently cause its collapse. So they were to remain, as a legacy to the industrious past that adorned this area.

Yet during the time they remained derelict, they became a local icon. Locals and passing motorists loved and hated them, however they gained recognition as being a "gateway" into the region - many saw it as the sign of nearing home, others saw it as the entry point into the north of England. The towers were even affectionately given nicknames, "Salt" and "Pepper". Even English Heritage got in on the act, demanding the towers be listed and preserved for future generations; despite the authorities recommending the listing status be applied, the process was not pushed through.

However, with the viaduct being strengthened in 2003-6, E-ON - who by now owned the Blackburn Meadows site - stated that it would be possible to demolish the towers without putting the viaduct at risk. After many local campaigns and protests, the towers were brought down at 3am on Sunday 26th August 2008. The operation was months in the planning, as the M1 had to be closed due to its proximity and the possibility of flying debris hitting the viaduct. Even after the demolition, the motorway remained closed for the rest of that Sunday in order to allow for highways officials to inspect the viaduct and ensure it was not damaged by debris or shockwaves. Fortunately for the Highways Agency, the viaduct was given a clean bill of health.

The towers themselves were unique, in that they were the only pre-1950s hyperbolic cooling towers left standing by the time of their demise. However, their derelict state meant that they deteriorated to a point where E-ON declared them unsafe. They said it would cost too much to restore them, and have decided to construct a biomass power plant on the site instead.

Consideration is being made for a permanent landmark to be installed on the site where the towers once stood. E-ON have stated there would be enough room to build the new power station without the need to build on the space the towers occupied, and they have placed money aside to fund the project. One possibility is a "replica" pair of towers, albeit not made of concrete.

End of an Industrial Era

The demolition of the cooling towers at Tinsley drew in large crowds who watched the demolition from both sides of the valley. Meadowhall Shopping Centre kept open one of their car parks to allow spectators to watch the towers fall from a close but safe place. Below is a video documenting the demolition of the Tinsley Towers, produced by Leon Lockley.