Since prehistoric times, the Severn Estuary had been an obstacle for people who have wanted to cross it. The construction of the rail tunnel provided the first link that was not affected by the weather and the tides. However, the motor vehicle arrived and, with time, the ferries were unable to provide for all the cars and lorries wishing to travel between South Wales and England. A road bridge was required.
How was it to be built? How were all the difficulties overcome? The result was a world class structure with a revolutionary design that has stood the test of time, despite great increases in the number and weight of the vehicles that it has had to carry. This is its story.
For more information on the background to the first bridge, Click Here
The age of the motor car had arrived
Growth in the use of all forms of road transport gradually increased demand for a bridge across the Severn. There were more cars and lorries on the roads. There was more pressure to shorten journey times and greater value was given to time-savings.
The Power of Nature
A bridge across the Estuary would need to be capable of withstanding nature in all its moods. The tidal range on this part of the Estuary is the second largest in the world – over 40 feet (14 metres) – and the water flows at up to 8 knots on the large tides. The large spring tides create the famous Severn Bore, a tidal wave which rushes from upstream of the crossing, as far as Gloucester, and can be 10 feet (3 metres) high. High winds gust at over 100 mph.
What to build? Where and why and who would pay?
Was there a crossing point that would significantly shorten the route from South Wales to Bristol and to London? There is a local narrowing of the estuary between the Aust Cliff and Beachley Head. By taking advantage of this, the distance and time savings would lead to great economic benefits, bearing in mind that many road trips, without a new bridge, would continue to involve a 60 mile (96 km) detour via Gloucester. There was wide support for the decision to provide a suspension bridge across the estuary at this point.
At the time, a suspension bridge was the only type of structure that could cross the 1 mile (1.6 km gap without needing many piers to be built in the aggressive river conditions. Also, a suspension bridge could carry the roadway high above the water, 120 ft (37 metres) above the level of the high tide, to allow larger ships to pass beneath it. Suspension bridges carrying roadways had existed for well over 100 years – the nearby Clifton suspension bridge was designed in the 1830s.
How would a suspension bridge work?
A suspension bridge has five main elements: the main suspension cables, the hangers that support the roadway from the main cables, the stiffening girder that reduces the deflections of the roadway as the traffic travels across the bridge, the towers that hold up the main cables, and the anchorages that resist the tension in the main cables.
On the Severn Bridge, the two main cables act a bit like a washing line. The tension in a washing line supports the weight of the clothes that are pegged to it. In a similar way, the tensions in the main cables of the bridge, which are held in place by huge anchorages at each end, support the weight of the deck and traffic upon it. The bridge deck is hung from the main cables using wire hangers (rather than clothes pegs). And because the main cables are held up by the towers, the weight of the whole bridge is carried down through the towers, to the underlying foundations.
If you put something heavy on a washing line, it will sag at that point. With a suspension bridge, the road is supported by a stiffening girder, which spreads out the weight of the traffic, so avoiding excessive sag under an exceptional load. If you hang something on a washing line away from the centre, the point will not only sag but it will also move towards the nearest end (try it!). Similarly, as a heavy load travels over a suspension bridge, it will not only dip downwards at the point of the load, it will also move longitudinally towards the nearest tower.
If you stand on the walkway of the Severn Bridge, you can just feel it moving as the traffic travels over it. If you stand by one of the towers and watch the expansion joint, you can sometimes see the whole bridge moving, backwards and forwards, as the weight of traffic trundles across. We should not worry. The movements are short and almost imperceptible. It is meant to do this. This is how it absorbs the weight of the traffic and transfers it to the main catenary cables above.
Decision to go ahead
Gloucester County Council started lobbying the Government again, as early as 1943, for preparatory steps to be taken to ensure that a bridge across the Estuary could be provided without delay. A long span, high level option on the Aust-Beachley line was now favoured and, in 1945, the Ministry of Transport appointed Mott, Hay and Anderson to prepare a scheme based on this line. It was the culmination of aspirations that went back for more than 100 years.
In 1946, a National Plan was published by the Ministry of Transport showing the first road crossing of the Severn on the Aust-Beachley-Newhouse line, together with a high-speed road link from the crossing at Beachley to the A48 at Tredegar Park, west of Newport. On 22nd of July 1947, details of the scheme was fixed by a statutory Order made in accordance with the Trunk Roads Act, 1936.. As well as the suspension bridge, the route would also require a bridge over the River Wye and 8m (13 km) of new trunk road.
Such a large scheme would bring massive benefits, but how would it be paid for? Those who benefited by using the bridge could be charged tolls for the journey time saved. The government would have to provide funds to pay for the construction initially and an act of Parliament would be required for tolls to be collected.