Category Archives: The First Road Bridge

Subsequent Problems in main Cables of Severn Bridge

At the time of construction, and indeed for many years afterwards, it was considered that the main cables of a suspension bridge would last for the lifetime of the structure. However, by the 1990s, a number of major American suspension bridges were showing signs of deterioration in their main cables. Alerted by the American experience, the main cables of the Forth Bridge and then the Severn Bridge were carefully examined. These inspections revealed that some corrosion of the wires was taking place.

The cables have now been sealed in a membrane and a system has been installed to continually force dry air into the cables to halt further corrosion. Subsequent inspections have confirmed that these remedial measures are working satisfactorily. At the time of writing, the situation appears to have stabilised and, although the condition of the cables is considered unlikely to deteriorate further, periodic examinations are scheduled for 2016 and beyond.

For more on subsequent problems in main cables of severn bridge, Click here


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The Bridge needs Strengthening

Why did the Bridge Need Strengthening?

The Severn Bridge was designed and built in the early 1960s with the anticipation of a design life of 120 years. Yet, within 25 years, a major programme of bridge strengthening works was needed. What had changed?

Traffic Growth had been underestimated.

Consulting engineers Mott, Hay and Anderson were engaged to design the Severn Bridge in 1949 but, for more than ten years, the slow post-war economic recovery prevented the government from releasing funds for its construction. During the whole of that period, the expectation of government, supported by professional opinion, was that road traffic throughout the country would grow at a modest rate every year for the foreseeable future, with heavy lorries making up about 15% of traffic flows on longer distance routes.

We now know that, in the decades that followed the opening of the bridge in 1966, there was an unprecedented growth in road traffic, far outstripping the modest forecasts of the early post-war years. These higher traffic volumes called into question the ability of the existing stock of bridges to cope in the long-term with the weight of traffic they were now being required to carry. The problem was not confined to the Severn Bridge, it affected bridge structures on the whole of the UK major road network.

There were two other factors that exacerbated the problem.
1. Some years after the Severn Bridge was opened, the maximum allowable weight for lorries on UK roads was substantially increased. There had been pressure for this change from the transport and logistics industries in response to the very rapid pace of developments taking place within those industries and it brought the UK into line with most other developed countries.
2. The percentage of lorries in traffic flows on all longer distance routes had increased steadily from the figure of 15% in the fifties and earlier sixties to 30%. This change had been picked up by regular monitoring surveys and data collected in relation to other road improvement schemes.

In the light of the above developments, how would existing bridges cope? They would certainly be subjected to greater forces than had been allowed for in their original designs. The problems would be more critical on longer distance routes because they would have to cope with higher proportions of heavy lorries – and, in general, longer bridges would be more severely affected than shorter ones.

The response from the government’s transport department was to update the traffic forecasting elements of design criteria for bridges and to initiate a programme for close inspections of all bridges on trunk roads and motorways, followed by reassessments of their capabilities in the light of the revised design criteria. In addition, higher design wind speed and an increased temperature range were introduced. Bridges that failed to comply with the new criteria would then be subject to a strengthening programme, to overcome the identified deficiencies. Local highway authorities introduced similar arrangements.

For more information on highway bridge loadings, Click here

The Box Girder Problem.

In the early 1970s, just a few years after the Severn Bridge was opened to traffic but before the problem of lorry weights and numbers came to a head, four bridges across the world that were using box girders in their construction, collapsed with significant lose of life. One of these bridges was in Pembrokeshire. Both the Severn and Wye bridges had steel box girders built into their structures and so, inevitably, were swept into the turmoil that followed.

The government set up a committee of inquiry to look into the failures and to make recommendations on the changes needed to prevent repetition. New design rules were drawn up, the most important being concerned with box girders being built using cantilever construction. There was also a strong call more rigorous independent checking procedures at both the design and construction stages.

Three of the four failures had occurred during the construction phase and involved the building of a cantilever comprised of box girder units. Cantilevered bridges and viaducts are always at their most vulnerable just before completion, when the unsupported front end is ready to be lifted and fixed in position. After completion, the stresses at the root of the cantilevered deck will never be as high again, as at that most vulnerable moment, even when it is fully loaded with traffic.

The government transport department ordered scrutinies of all major bridges under their jurisdiction, in which box girders played an integral part. No serious problems were expected by those who were involved with the management and maintenance of the Severn and Wye Bridges and there were good reasons for that.

On the Severn Bridge, box girders, in a distinctive aerofoil shape, were used to create the stiffening girder that acts as the deck of the bridge. However, no cantilever was involved in the construction process. Each unit of the deck was floated out on the river and then lifted into position, where its weight would be transferred to hangers attached to the main cable. Each unit was later welded to its neighbours, in order to provide the required element of deck girder stiffening but this was of a much lower order of magnitude than the degree of stiffening that would have been necessary for the building of a cantilever.

The Wye Viaducts are classical examples of box girder construction and the Wye Bridge itself possesses similar elements. There is a possibility that the deck spans ot the viaducts might have been vulnerable when the cantilevered sections were nearing completion but that possibility became irrelevant as soon as the construction process was complete. In the case of the Wye Bridge, two sets of pylon and cables were employed to support the deck before the most vulnerable period. Those additional elements became part of the permanent structure that would carry the weight of traffic. Parts of the Wye structures were found to be at risk under the new box girder design rules and so corrective action was taken. And both the Severn Bridge and the Wye structures were later strengthened to cope with the revised traffic loading requirements.

For more on the steel box girder problem, Click here

Reflections on the strengthening of the Severn and Wye Bridges.

The First Road Crossing needed strengthening. In round terms, it would need to carry twice the weight of traffic for which it had originally been designed. That was a very tall order. To the layman, it might seem to be unachievable. However, the saving grace was that, as in the case of all large bridges, the lion’s share of the stresses in the structure are caused by the weight of the bridge itself.  At the time of its original design, the weight of the Severn Bridge structure contributed about 85% of the total design load, the other 15% coming from the traffic it was expected to carry. So doubling the weight of traffic would, in round terms, add just 15% to the total load on the bridge. The additional material required for the strengthening process would not add much to the overall weight of the structure.

The other crucial factor that enabled the strengthening project to proceed was the strength of the main catenary cables of the Severn Bridge. The strength of every major element of this particular bridge was carefully reassessed in the light of the revised design criteria and, with the exception of the catenary cables, all were found wanting. The reassessment had indicated that the cables had sufficient strength to cope with the extra load, without the need for any intervention. That was indeed fortunate because the cable was the one element of the structure that would be virtually impossible to augment or replace without extremely high cost, together with major disruption to traffic.

Realisation that the catenary cable of the Severn Bridge did not need strengthening came as a great relief but that should not be allowed to detract from the enormity of the engineering challenges thrown up by the need to strengthen all the remaining elements of the bridge, or from the quality of the professionalism and ingenuity that went into confronting and overcoming them.

Modification and Strengthening of Severn Bridge

Diagram showing extent of Severn Bridge strengthening works

The most important elements of the Severn Bridge that required major strengthening were as follows:
1. The main tower legs.
2. The hangers that transfer the weight of the bridge deck (and any traffic on it) up to the main cables.
3. The box girder top flange stiffener welds.

Work was also required in many other areas as shown on the diagram above. Details of all these items are available using the link below.

For more on modification and strengthening of severn bridge, Click here

Modification and Strengthening of wye-bridge-and-viaducts

View of Wye Bridge strengthening works

It was decided that the Aust Viaduct piers needed strengthening against ship collision and one pier of the Wye bridge needed similar work. In addition, the towers on each of the main span piers needed to be raised in order to allow a new system of cable stays to be installed. Full details are available from the following link

For more on modification and strengthening of wye bridge and viaducts, Click here

Traffic Control during the Works.

Meticulous planning was required to minimise traffic disruption while this work was in progress. Except for two complete night-time closures of four hours each for the lifting of the extensions to the Wye bridge towers, the bridges were kept open to traffic throughout the strengthening work albeit with off-peak lane closures at times. More than 100M vehicles used the crossing during the first five years of the project without a single fatal road traffic accident.

The project received the British Construction Industry Supreme Award for 1990 in recognition of the extremely complex nature of the problems that were revealed and the innovative solutions of both an engineering and environmental nature that were employed to resolve them.

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Building the Severn Bridge

The austerity of the later 1940s early 1950s meant that little happened on the ground during that period. At the end of 1960, the Minister of Transport made a public statement about the M4 and road needs in Wales.  And work started on the foundations for the Severn Crossing in 1961.

The distribution of contracts.

The construction work was divided into a series of contracts. The tender competition for the foundations was won by John Howard and Company and let on the 8 March 1961. It included construction of the main piers and the main cable anchorages of the Severn Bridge, and also the approach viaduct and an access road on the Aust side of the Severn. Extensive temporary works were designed, erected and ultimately removed by the contractor.

A consortium of contractors built the Severn Bridge superstructure. It comprised Sir William Arrol and Co Ltd of Glasgow, Cleveland Bridge and Engineering Co Ltd of Darlington, and Dorman Long Bridge and Engineering Ltd of Middlesbrough.

Cleveland Bridge and Engineering built the foundations and superstructure of the Wye Bridge and Viaduct, under a separate contract

Severn Bridge Foundations and Anchorages

The construction of a suspension bridge starts with the foundations and towers. The main suspension cables are firmly anchored to abutments at both ends of the bridge and are held up by the towers, to provide the bridge with its iconic shape. The bridge deck is supported by hangers suspended from these cables.

The two main towers were each built on a boat-shaped pier 40 m (130 ft) long by 11.5m (38 ft) wide, with cut-water ends. Each tower would consist of two full height vertical steel legs 23.5 m (77 ft) apart, centre to centre, joined by three steel portal beams. Both piers were constructed from temporary jetties that were built out into the estuary from the respective shore.

The Aust pier is founded on a hard limestone outcrop that is only exposed at the lowest spring tides. It is formed of solid concrete and built high enough to protect the steelwork of the tower legs from splashing by salt water.

The Beachley pier is located at a point, as far offshore as possible, where the top of the limestone is at a depth of 20 m (65 ft) below river bed level that is exposed at low tide. When further borings were taken at the start of construction, it was found that there was not a good limestone bedrock layer available at this point. This pier has therefore had to be supported by two massive cylindrical concrete bases, one under each of the twin legs of the tower, each 18 m (60 ft) in diameter and founded about 10 m (35 ft) below river bed level, through the marls and on the top of steeply inclined hard carboniferous mud-stones. The pier has been constructed across the top of these two bases, with extensions at both ends to provide the cut-waters.

Cables were set into the mass concrete of both piers. Later, they would be used to fasten down the bases of the steel towers as they were being erected on the piers in a free-standing mode.

The anchorages, which secure and hold firm the ends of the main cables, are massive reinforced concrete blocks, each weighing about 100,000 tonnes. The one on the Aust side was constructed on a another outcrop of limestone that is exposed at low tide, about 105 metres (345 ft) from the Aust cliff. The Beachley anchorage was constructed in open excavation through the soil of the Beachley peninsular and it was keyed into the underlying limestone.

For more information on the Severn Bridge Foundations and Anchorages, Click Here

The Aust Viaduct

Part of Aust Viaduct loaded on barge

The short element of construction that is required to complete the structural element of the Crossing beyond the eastern abutment of the Severn Bridge at Aust, is known as the Aust Viaduct. It is very different, in structural content, from any other item of viaduct, comprising just three short lengths of twin box girder supported on concrete columns and joined with steel cross girders, and with a composite, cast in-situ, concrete deck slab. The adjacent image shows one of the box girder units being taken across the estuary from Chepstow to Aust, travelling on a barge. Transporting the sections for the Aust tower across the river, with its difficult tides and water levels, required careful planning.

Severn Bridge Towers

The twin columns of each tower were built as hollow steel boxes,17’x12’ in plan cross section, standing on the concrete foundations. The many individual steel plates in each face of the towers, up to 1 inch (2.5 cm) thick, were stiffened, trial assembled and fitted with machined flanges at both ends so that they could be bolted and stressed together on site. Every 14 ft vertically, there is a thick horizontal plate (diaphragm) bolted to all four panel end plates but with openings for the lift and vertical access ladder. Three portal beams were inserted between the legs of each tower, one just below the road deck level, one at the top and one between these two, to produce a strong and laterally resilient structure.

The steelwork for the towers was fabricated in sections in Glasgow, trial erected, then taken apart and transported to a stock yard in Chepstow. Transporting the sections for the Aust tower across the river, with its difficult tides and water levels, required careful planning.

A special climbing structure was devised to assist in the construction of the towers. It included a platform that could climb up the 20 metre high tower sections as they were being built. It carried its own crane to lift the sections of the tower. It was designed to operate in high winds and it weighed 160 tonnes. As the towers got taller, the effects of wind on the lifting operation became more severe. On two or three occasions, no lifting could be carried out for two days.

Construction of Tower, Climbing Tower Crane in place

Completed Towers with hanging Cat Walks attached

For more information on the construction of the Severn Bridge Towers, Click here

Spinning the Main Cables

Each main cable consists of 8,322 individual galvanised wires, each the thickness of a pencil. The total length of these wires is approximately 29,000 km (18,000 miles).

The spinning of the main cables began by stretching two wire ropes across the river, by boat. The ropes were then lifted to the tops of the towers, one on each side of the bridge, and their ends were made firm in the anchorages. They were used to haul out further wire ropes to support the access catwalks and the overhead supports for the cable spinning gear. Winches were set up on the Beachley anchorage, one to each side, to operate the two 3.5 km (2.2 miles) long continuous haulage ropes that would pull the 16,644 wires across the estuary to make up the two main cables.

View from one of the Cat Walks of a pulley taking two bights (four wires) across the estuary. The picture shows quite a few strands of wire already across, beneath the spinning wheel.

The process of building up the main catenary Cables continues; each cable consists of 8322 individual wires

The completed sheaf of wires is tightly bound to provide the finished cable

As the spinning wheel passed back and forth, high up on the towers and cat walks, 24 hours a day and in all weathers, workers placed the new wires in combs to prevent them becoming tangled. Each new set of wires was fixed to the anchorage blocks and adjusted to exactly the correct length. After checking the completed 500 mm (20 inch) diameter cables, the individual wires were compacted into a single cable that was tightly wrapped with galvanised wire before painting. Finally, the hangers were suspended from clamps on the cables, ready to attach the deck sections.

The engineering feat achieved so far was already most impressive but the most risky task was about to start.

For more information on spinning the main cables, Click here

Suspending the Severn Bridge Deck

Imaginative engineering had produced a revolutionary design for the girder that would act as the stiffening girder and the road deck. It was fabricated off-site, in nearly 2000 stiffened panels and then assembled and matched end-to-end on slipways on the River Wye in Chepstow into 88 sections, each 18 metres long. Each section was constructed to be capable of floating on the river, like a pontoon.  Section by section, they were launched into the river, towed to the bridge site, and then lifted into position. The work was heavily dependent on the tides and winds.

To handle the sections in the swiftly flowing river, a special craft – the Severn Knave – was built to push them one by one into the correct place. It could manoeuvre itself in all directions and hold the sections, floating on the river, in the position required.

Preparatory trials, using models of the deck sections in a water tank, concluded that erection could only take place fortnightly, as the tides would only allow time for 3 to 6 sections to be erected each fortnight. And, of course, bad weather could further restrict progress in these difficult conditions.

The skill of the skipper of the Severn Knave proved to be paramount in the positioning and the lifting of the deck sections. The process involved collecting a deck section from its moorings in the Wye, pushing it out to the centre of the river, adjusting the position by varying the engine thrust direction, lowering lifting tackles from the main cable and attaching them to the deck section, winching clear of the water, washing down to remove mud and salt from the underside, and then final lifting to the required height and attaching it to its hangers and the previous section of deck.

The sequence for lifting and incorporating individual sections into the emerging deck started at the centre of the main span and continued in either direction, in balance, towards the towers. The centre span deck was completed before any sections were lifted up to the side spans. Completion of the deck heralded the final stages of this striking engineering story. It was an awe-inspiring sight.

For more information on suspending the severn bridge deck, Click here

Building the Wye Bridge and Viaducts


At about the time that the foundations of the Severn Bridge were being completed, work began on this important series of structures that are located immediately adjacent to the western end of the bridge. From there, a viaduct has been built across the Beachley peninsular to connect to the Wye Bridge, with a further section of viaduct, beyond, terminating on an abutment on the Welsh shore.

Both sections of viaduct were built with steel box girder decks resting on steel box trestles a modest 64 metres (210 ft) apart. The Wye Bridge itself has a main span of 235 metres (770 ft) and side spans of 87 metres (285 ft). The cable-stayed box girder deck of the bridge was supported by a simple steel tower, or pylon, in the middle of each main bridge pier, with inclined cables stretching down from the top of each pylon, to be anchored into the appropriate deck boxes.

Foundations for the Wye Bridge and Viaducts

The foundations for the viaducts are simple twin reinforced concrete shafts (one under each of the splayed legs of the trestles) that were sunk through softer soils, down to limestone beneath. The foundations of the main piers of the Wye Bridge each comprise twin hollow shells (caissons) that were sunk through the very soft mud of the river bank, to limestone some 15 metres below and then filled with concrete. Each pair of twin foundations for the viaducts were joined together with a concrete tie beam below ground level that linked across their tops, to resist the outward forces in the splayed trestle legs. At the main piers, a reinforced concrete cross-head beam was cast on top of the caissons and it remains visible above high water level.

Construction of the Viaducts

A single cross section was adopted, for both the bridge deck of the Wye crossing and the linking viaducts, to provide continuity from the Beachley anchorage of the Severn Bridge to the Gwent abutment in Wales. Splayed steel, hinged trestles carry the viaduct across the Beachley Peninsula in ten spans, varying from 182 feet (55 m) to 210 feet (64 m). There are a further two viaduct spans of 210 feet (64 m) between the western side of the Wye Bridge and the Gwent abutment, where the new structure crosses the main railway line into South Wales.

Construction of the Beachley viaduct commenced at a fixed point in the middle of the peninsula and proceeded simultaneously in both directions from there – up to the back of the Beachley anchorage of the Severn Bridge, and down to the eastern bank of the Wye. Construction on the Gwent side commenced at the western end where the viaduct was fixed to the fairly massive concrete abutment located on a limestone outcrop.

The viaduct deck was constructed, incrementally, by adding new units to the end of a developing cantilever. Starting from a pier or trestle on which a deck unit had been made secure, each new section of deck would be welded to the exposed edge of its predecessor. This sequence would be repeated until the end of the developing cantilever reached the next trestle. There, it would be raised up sufficiently to allow the next unit to be welded in sequence, this time on top of the latest trestle – and then the whole cycle could be repeated.

Construction of the Wye Bridge

The procedure that had been used to erect the viaduct deck (see previous paragraph) was also applicable to much of the work on the Wye Bridge. Never the less, a girder made up solely of deck units similar to those employed on the viaduct, would not have been stiff enough, on its own, to cope with being cantilevered out for half the main span of the bridge (i.e., 117.5m, compared with 64m for each viaduct span). Considerable additional stiffening would be needed. The solution was to erect a single pylon, or tower, on the centre of each of the main span piers. A long sheaf of high tensile steel cables was then lifted to the top of each of the pylons so that individual strands could, when needed, be isolated, positioned and stressed, and then firmly anchored within the bridge deck at a distance of 78 m (255 ft), in both directions, from the particular pylon involved.

Without the use of the pylons and cables, the front end of the emerging box girder deck would have drooped alarmingly before reaching mid span, and the structure would undoubtedly have suffered the same fate as four other box girder bridges across the world that collapsed while under construction during the following decade. Bridges of this type, that rely upon the use of cantilevers in their construction, are always particularly vulnerable while under construction.

The two pylons used in the construction of the Wye Bridge were both 30 m (100 ft) high, constructed of a steel box section and pivoted at its base. Each cable, when completed, was made up of twenty 2.5 inch (63 mm) diameter strands of galvanised wires, in a configuration similar to that used for the hangers on the Severn Bridge. The strands passed though the upper surface of the deck, over the top of the pylon and were anchored to beams in special chambers within the appropriate box girders.

This simple and elegant solution allows the Wye Bridge to be classified as a cable stayed structure, albeit a very simple example. A much more complex member of the genre would grace the Severn Estuary itself, just 5 kilometres downstream, before the end of the Twentieth Century.

For more information on Building the Wye Bridge and Viaducts, Click Here

Finishing Operations

Before the crossing could be opened, further operations had still to be completed, including surfacing the roadways, erecting the safety fences, fixing the parapets, installing lighting, and the final painting

To prevent corrosion in the harsh environment, all of the steelwork was protected by the application several coats of special high quality paint. To ensure that this did not peel off, all of the external steel had to be thoroughly blast cleaned and spray-coated with metallic zinc before the paint was applied.


Attention to the safety of staff associated with the construction and to the safety of the public was of prime importance – construction on such a difficult site is, by its very nature, an activity fraught with risk. The accident rate during construction was low but sadly four men lost their lives in the construction period, albeit in two different work-boat accidents rather than in the course of construction work. Two further fatalities occurred at the very end of the project, during finishing works inside the towers which resulted in the emission of toxic fumes.

The completed Severn Bridge from the Beachley side

British Engineering leads the World

The Severn Bridge was opened by her Majesty the Queen on 8 September 1966. It was the lightest suspension bridge in the world for its span and loading, and the main span was the seventh longest in the world. The Wye Crossing was also technically advanced in its design and construction. Altogether a highly significant example of British civil engineering and a remarkable achievement on the part of all those involved.

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Design of the Severn Bridge

A Revolutionary Design

When the government announced that the Forth Road Bridge – a similar proposal for a tolled suspension bridge – would take priority over the Severn, the design team was able to take advantage of the opportunity that arose out of a situation that had initially seemed to be a minor disaster, to develop a revolutionary design.

As the spans of suspension bridges grew longer, the stiffening girders located under the bridge deck, were being designed as deeper open trusses.  In fact, the Severn Bridge was initially intended to have such a truss, similar to the one used on the Forth Bridge.  But disaster struck when a model of this truss was smashed to pieces in a wind tunnel, when It broke free while undergoing tests.

What to do? The engineers were able to use the time that had been booked in the wind tunnel before the accident, to explore a radical new idea.  Instead of allowing the wind to blow through the bridge, why not streamline the deck using aeronautical technology to produce a design similar to that of the wing of an aircraft.  A shallow box section could be made both aerodynamic and strong  in bending and torsion.  This would enable very large savings to be made in the weight of steel required for the deck, the towers and the cables – as well as making subsequent repainting of the deck  much easier. This revolutionary idea, to use an aerodynamic box section for the stiffening girder, was tested in the wind tunnel and it proved to be very effective!

For more on background on choice of design for first road crossing, click here.

Design of the Severn Bridge

The design for the Severn Bridge that eventually emerged from the developments described above, includes two 400 ft (125 m) high towers, one on either side of the estuary. Each tower consists of a pair of tall hollow boxes, linked at three levels by deep hollow portal beams, all with walls of stiffened steel plates, up to 1 inch (25 mm) thick. The mass concrete foundations for these towers are of crucial importance; on the Aust (east) side, the foundation is located on a rocky outcrop that only emerges at low spring tides. On the Beachley shore to the west, ground conditions were far less favourable and the foundation was laid onto the surface of steeply dipping (nearly vertical) bands of carboniferous rock that was exposed by the excavation of softer material above.

Other major elements of the bridge include the two catenary cables, one on each side of the bridge, to which the hangers, which ultimately support the deck of the bridge, are attached – and the deck itself. The abutments, at the very ends of the bridge, are massive in-situ concrete blocks, to anchor the main cables securely. Each completed cable, approximately 20 inches (50 cm) in diameter, contains 8322 individual strands of 0.196 inches (approx. 5 mm diameter, similar to a typical wooden pencil) galvanised steel wire. These wires were taken across the estuary, two bights (i.e. four wires) at a time, from one abutment to the other, over the tops of both towers, in a process known as ‘spinning the cable’.

Inclined hangers

In an innovative move, all the hangers used to suspend the deck from the main cable, were fixed in an inclined pattern, rather than hanging vertically. This was to increase the systems ability to prevent oscillations from breaking out on the bridge deck. The choice of a welded box girder for the bridge deck was widely welcomed but there were some concerns about the ability of the structure to dampen any oscillations that might try to gain a foothold.

To check the extent of the problem, a comprehensive study of wind conditions on this part of the estuary was undertaken. The results of the study were then used in further wind tunnel tests on the chosen form of deck. The tests indicated that the box girder design possesses excellent aerodynamic qualities but a slight movement was registered at a wind speed of about 15 miles per hour, at an angle of 7.5 degrees.

The presence of this small amount of movement concentrated attention on the amount of damping available, especially as the wind flow over the welded box girder at 15 miles per hour will be so smooth and undisturbed that there is no chance of the deck providing any damping. So the search was on for a fresh energy-absorbing element – and the possibility of switching to inclined hangers was suggested.

Suspension bridges react to the imposition of an additional heavy load in the same way that a clothes line reacts to a heavy garment being hung off centre. What happens is that the point on the clothes line from which the garment hangs, moves a short distance towards the nearest end of the line. On a suspension bridge, the point on the main cable that takes the brunt of the load, will move marginally closer to the nearest tower. The bridge deck will follow the cable by moving this same small amount because it hangs, freely suspended from the main cables, via the hangers. This enables the deck to move backwards or forwards a small amount, assisted by the sliding joints at each end, albeit restrained laterally at the towers. And any movement of the deck can be shown to cause the transfer of part of the burden of carrying the deck, from one half of the pair of inclined hangers (that share a common upper socket on the main cable) to the other half. This switching of the tension between adjacent hangers provides the necessary additional energy-absorbing element and increases the dampening potential of the bridge.

Calculations show that hysteresis in the inclined hangers will now provide sufficient additional damping to prevent any unstable and runaway oscillation of the bridge deck from emerging. The absence of any such movement for more than fifty years (since the bridge was first opened to traffic), suggests that the designers were amply justified in including this innovation.

For more on design issues, click here.

A simple and revolutionary concept was developed to produce the renowned Severn Bridge, with its distinctive and elegant shape. It was the first suspension bridge in the world to use the idea of an aerofoil-shaped box girder deck, a feature that has subsequently been adopted for many other world class bridges, including those on the Humber and the Bosphorus. The significance of this major step forward in bridge design has been recognised by the award of the Grade 1 Listing for the bridge.

Design of the Wye Bridge and Viaducts

The location chosen for the Severn Bridge committed the government to providing a new bridge across the nearby River Wye, together with two adjacent stretches of viaduct. These structures are integral parts of the Severn road crossing and therefore worthy of mention here. The Wye Bridge has a main span of 770 ft (235 metres) and, although it is rather dwarfed by its majestic neighbour with a main span of 3240 ft (988 metres), it was, on completion, the fifth longest span road bridge in Britain.

Relationship between the Severn Bridge and the Wye Bridge

The particular form of design chosen for the Wye Bridge – a cable stayed bridge – will take centre stage as our story of the bridges across the Severn unfolds. It was, at that time, new to the United Kingdom although another cable stayed bridge was being built at the same time across the River Usk in Newport, only a few miles west. The cables that support the roadway, unlike a suspension bridge, are straight and are fixed to the pylon, or tower, at one end, and the bridge deck at the other. They act rather like guy ropes on a tent or the stays supporting the mast of a sailing boat.

The decking for the stretches of viaduct on either side of the Wye Bridge consists of standardised steel box girder units that were assembled and welded together on site. This deck is supported by steel box trestles at 210 ft centres (64 m centres), with splayed legs that are pivoted at the top and the base. Similar box girder units were used to construct the deck of the Wye Bridge but with an important addition that is described in the additional information that can be accessed by following the link immediately below.

For more on developing the cable stayed design for wye bridge, click here.

The Result

With construction underway in 1961, the Severn Bridge was to become an enduring symbol of the connection between England and Wales, providing stimulus to industrial growth, enhanced prospects for the region, and success beyond anything that could realistically have been envisaged. The revolutionary design, using an aerofoil box girder for the bridge deck, has since been adopted around the world. British engineering led the way!

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The First Road Bridge


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. 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.

Diagram showing the main loads in a suspension bridge

For more information on how do suspension bridges work

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.

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