The Belief of Suspension: 50 years of the Severn Bridge

The River Severn has long marked a geographic barrier between the South West of England and South Wales. 2016 marks an anniversary for many of man’s constructed crossings of this river. 130 years since the Severn Tunnel opened, 50 years since the Severn Bridge opened and 20 years since the Second Severn Crossing opened.

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For millennia the lowest crossing for road traffic was at Gloucester, which meant that the distance from Cardiff to Bristol via road was 91 miles, compared to the physical 25 miles which separate the cities.

By 1823 the Postmaster General, seeking a better route for important communications between London and Milford Haven and on to Ireland, appointed Thomas Telford, famous for his bridges in Cornwall and Menai, to advise on improving the route. Telford describes the site of the Severn Bridge as One of the most forbidding places at which an important ferry was ever established. He recommends a new ferry crossing from Somerset to Sully, bypassing the towns of Monmouthshire and South East Glamorgan altogether. A year later he proposes a suspension bridge to cross from Aust to Beachley, but this proposal is not realised at the time.

For a long time ferries plied their trade on the so called New and Old Passages across the river from around Aust on the English side to Beachley on the ‘Wales’ side, although Beachley is technically still in England. Some of the largest tidal ranges in the world and fast currents made the crossing very dangerous and unreliable. Passengers frequently had to wade ashore through mud and, in 1839, the ferry boat Dispatch was sunk with all hands when it hit a pier. A further ferry sank in 1843.

A report of 1845 said of the ferry, There is no great communication in the country so bad, or therefore where an improvement is so much wanted.

As the 19th century wore on the railways increased in importance for the moving of people and freight, so by 1874 the GWR had begun tunnelling under the river. They completed the route in 1886, 130 years ago, and South Wales was connected to the South West with a direct route for the first time.

At the same time a single track rail bridge was constructed across the Severn between Sharpness and Gatcombe. For years this was known as the Severn Bridge, but it was destroyed in an accident involving river barges in 1960.

The increase in Road traffic in the early part of the 20th Century prompted more calls for a new road bridge across the river. By 1923 the Road Improvement Association was actively canvassing for a bridge and in September 1923 Chepstow UDC called a conference of all interested parties including Monmouthshire, Glamorgan and Gloucester County Councils, Town councils from the counties and the GWR. Chepstow UDC were concerned that the main A48 from Gloucester was becoming heavily congested through the town, especially over the single carriageway Wye Bridge.

The GWR indicated they may build a joint road and rail bridge to replace the tunnel if the correct compensation were available, but they still lobbied parliament not to fund a road bridge to keep their monopoly over the crossing.

The Lord Mayor of Cardiff was quoted as saying, Nothing is of greater importance than better road communications between South Wales and the other side.

The conference selected the favoured crossing as that between Aust and Beachley which was suggested by a motion from the mayors of Newport and Monmouth.

The issue was additionally complicated by a campaign for any crossing to be coordinated with a hydroelectric barrage for the generation of electricity, a part of the plan that has still not been realised today.

In 1935 Gloucester County Council appointed engineers to commence planning a crossing, but the plan was rejected in 1936, and the Second World War intervened, with no further work for the next decade.

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Whilst the war was going on the demand for the crossing was still recognised, and it was intended for it to be included in any post war rebuilding. By 1947 the idea for a bridge had been taken over from county councils by the Government under the Trunk Roads act and the plan called for a route including the bridge to run from the A38 at Almondsbury to the A48 at Haysgate. However the global post war slump and austerity meant that construction was postponed and the Forth Bridge was given priority.

During the 1930s the United States had taken the lead in bridge engineering with impressively long suspension bridges like the Golden Gate and the Tacoma Narrows bridges. With any suspension bridge the lighter you can make the deck, the less it costs and the further you can bridge due to the load on the towers. However, the collapse of the Tacoma Narrows five months after its opening demonstrated that a deck that was insufficiently rigid would be susceptible to the wind and harmonic frequencies destroying it. Previous railway bridges in the UK had also collapsed under similar circumstances.

The Forth Bridge was constructed with a lattice truss box girder deck to provide sufficient rigidity to prevent disasters like the Tacoma Narrows. However this resulted in an extremely heavy deck weighing 39,000 tons, and an expensive one in terms of the amount of steel used.

When designing the Severn Bridge it was decided to try to use the science of aerodynamics to reduce the amount of steel and dead weight held by the towers. A model was created for wind testing at the National Physical Laboratory, but an accident meant that it was destroyed. Whilst waiting for a new model to be created, Consulting Engineer on the project, Sir Gilbert Roberts decided to pursue the idea of a streamlined box section deck instead of the lattice truss used in the Forth Bridge. This idea proved to be suitable when combined with the diagonal hanging cables we see on the bridge, and at the time was a completely revolutionary idea, giving an economy in steel of over 20% over the original design. It also made construction easier, because the box sections could be floated into position, and maintenance because the box sections were far easier to paint than girders.

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The ‘Severn Bridge’ is a combination of two bridges and a viaduct as the crossing from Beachley to Newhouse, south of Chepstow, required a further bridge crossing the Wye into Wales proper.

In the spring of 1966 the last two deck sections were slid into place and the dream of more than 100 years was complete. The bridge was officially opened by the Queen in September 1966 and stretched, initially, from Britain’s first four level cloverleaf interchange with the M5 at Almondsbury to the Newhouse interchange south of Chepstow. It was the lightest bridge for its length and loading ever built and gave British civil engineering the world lead in bridge design once more. It took 5 years to build at a cost of £8 million (1966). Its main span stretches 3240 ft and its towers are 400 ft high.

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By the 1970s the traffic predictions upon which the first bridge had been constructed were shown to be inadequate, in part due to the removal of freight from the railways following reorganisation in the 1960s. The bridge had speed and width limits installed and was the cause of significant congestion for traffic entering and leaving Wales. The tolls, which used to be levied in both directions, were cancelled for eastbound traffic to remove the risk of the weight of traffic on one side of the bridge queueing for the tolls.

30 years after the first Severn Bridge opened, and 20 years ago this year, the Second Severn Crossing opened, a full motorway standard cable stayed bridge, which has taken over the job of carrying the M4 from London to Pont Hafren. The Severn Bridge, now a part of the M48, is now a listed building and an example of British engineering’s ingenuity.

Glamorgan Archives holds a number of collections documenting the construction and effect of infrastructure in South Wales. The archives of the National Industrial Development Council (DIDC) record their efforts to pursue the construction of a road crossing for the Severn. Correspondence between Glamorgan County Council, its Urban District Councils and the UK government reflect their interests in the bridge’s construction (UDR/C/65/501). There is an historic issue of the Western Mail for the opening day, which records much opinion and celebration regarding the opening of the bridge and the importance of the connection (DX935/8/31). Glamorgan Archives is also the repository for the Welsh component of the Motorway Archive (DMAW), which contains records of the construction of the M4 including the Severn Bridge, the A55 expressway, and other major trunk roads in Wales.

 

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