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 and targeted examinations are scheduled at approximately five year intervals in the future.

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

End Of The First Road Bridge

Next Page

The Bridge needs Strengthening

Why did the Bridge Need Strengthening?

The Severn Bridge was designed and built in the early 1960s, to plaudits from professional engineers world-wide, and with 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 loss 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 of 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 stresses in the structure is 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

The developments and external changes that took place in the first 25 years of the First Crossing’s existence, generated much more traffic than had been expected, requiring the Severn Bridge to be strengthened. The adjacent Wye Bridge and Viaducts had experienced the same increased traffic flows as the Severn Bridge and, not surprisingly, they also needed strengthening. However because their ‘cable stayed’ form was quite different from the ‘suspension’ design of Severn Bridge, their strengthening programme was tackled quite differently.

It is worth bearing in mind that, before the structures were strengthened, the tops of the single towers of Wye Bridge were much lower, with only one pair of inclined cables suspended from each. The position of every significant item of strengthening work is indicated on one of the two diagrams, above, and a short description of each can be found by clicking on the link below.

 

For more on modification and strengthening of Wye Bridge and viaducts, Click here

 

Next Page

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

Foundations and Anchorage on East Side

This aerial view shows the main foundation elements on the east side of the crossing looking westwards at low water. In order east to west, it shows the cutting in the Aust cliff, the temporary access gantry adjacent to Aust viaduct works, the construction of the east anchorage and the continuation of the incomplete access gantry to the east main pier works on the limestone outcrop only exposed at low water springs. The working platform from which the pier works were constructed was put in place by using a floating jack-up platform which had been removed at the time of this photograph.

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 Main Foundation Works on the West Side

This aerial image shows the main foundation works on the west side of the crossing at near full tide The ferry slipway can be seen with a ferry discharging cars. In the foreground can be seen the temporary works used for the construction of the west pier of the Severn Bridge. The temporary access gantry leading to the working platform, with its three derrick cranes, can be seen adjacent to the two circular cofferdams inside which the excavation for the pier bases was carried out at all stages of the tide. All these temporary works were installed in the intertidal periods when the clean river bed surface was accessible right out to the cofferdams for about an hour at low water spring tides. In the top left hand corner of the image can be seen the growing concrete block of the west anchorage of the bridge.

The Beachley pier is located at a point, as far offshore as possible, where the top of the limestone was expected to be at a depth of 20 ft (6m) 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 35 ft (10 m) 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 hold 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 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 length 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 steel columns on concrete bases 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 ft x12 ft (5 m x 3.6m) 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 (4.25 m) 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.

Construction of Tower, Climbing Tower Crane in place

Completed Towers with hanging Cat Walks attached

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 56 ft (18 m) 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.

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

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

  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 60 ft (18 m) 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 position. 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

Introduction

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 210 ft (64 m) apart. The Wye Bridge itself has a main span of 770 ft (235 m) and side spans of 285 ft (87 m). The cable-stayed box girder deck of the bridge was supported by a simple steel tower, or pylon, in the middle of deck at each main bridge pier, with a tight cluster of 20 inclined cables stretching down from the top of each pylon and into anchorages formed in deck boxes, equally spaced from the base of the pylons approximately 260 ft (80 m) from the pylon locations. This configuration meant the deck was provided with vertical support at two points in the main span resulting in three approximately equal lengths of deck between main piers. The outer backstays were anchored in the deck near the penultimate viaduct piers which were designed as hold-downs with pin connections top and bottom.

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 50 ft (15 m) 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 also 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., 385 ft (117.5m), compared with 210 ft (64m) for each viaduct span). Considerable additional stiffening would be needed. The permanent design solution to erect a single pylon, or tower, on the centre of each of the main span piers and support the deck with cables was also used to build it. 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.

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

The completed Severn Bridge from the Beachley side

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.

Safety

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.

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.

Next Page

Design of the Severn Bridge

Design Considerations

By the time that design work started in earnest in the 1950s, the form of the bridge that would cross the estuary was not an issue. It had to be a suspension bridge because that was the only form of bridge capable of spanning the navigation channel at that time.

For more on how does a suspension bridge work, Click here    (not operating)

The Aust-Beachley route would also require a new bridge to be built across the River Wye, together with some lengths of viaduct across the Beachley peninsular and at the Welsh end. However the alternative route across the English stones would have required two kilometres of viaduct to be built across the English Stones, over an area that is covered by the river for most of each day. And if the route across the English Stones had been selected, it would have been necessary to construct two massive concrete abutments in the river, one each side of the navigation channel, to anchor the main catenary cables of a suspension bridge.

The river crossing between Aust and Beachley is located on one of the narrowest stretches of the estuary below Gloucester, with a deep navigation channel at all states of the tide. Another advantage is the nature of the geological features encountered along that route which are well suited to the construction of a suspension bridge. The eastern approach off the Aust cliff, provided a high level approach. The exposed limestone platform of the off-shore limestone rock was ideal for the eastern main anchorage and, although it was some 1000 feet (300 m) further off-shore, and over a tidal channel, the Great Ulverstone Rock was ideally placed to become the location of the east pier and tower of the proposed bridge. The west pier could be located at the low spring tide line off the Beachley peninsular. And finally, the higher land of the peninsular, a further 1000 feet (300 m) inland (to allow for the side span), was ideal for the west main anchorage. This anchorage would be taken down, through the overlaying sedimentary sands and gravels of the peninsular, to be founded on the underlying limestone.

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

The Main Elements of the  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. This structure would have the 7th longest main span in the world at that time and so would eminently, be technically feasible.

Other major elements of the bridge include the two catenary cables, one on each side of the bridge, to which the hangers that 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.  The two completed main cables, 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’.

Design of the Deck Cross Section.

The first step in the process was to test a 1/100 scale model of the bridge deck in a wind tunnel to check the lateral strength of the stiffening girder for coping with the strongest side wind that might occur, and then to ensure that the deck would not start oscillating in a steady wind. However, very early in the process, the model fixings failed and the model itself was destroyed. During the delay that followed, the consultants, Freeman Fox and Partners, were moved by inspiration to develop an unprecedented design for the deck section, quite different from anything that had been tried before.  It was based on the technology used to design aircraft wings.

It had previously been assumed that the Deck Section would follow the standard practice of the period, with a combination of a deep lattice-work of steel beams, below the stiff platform that would provide the running surface. However, the new and exciting innovation would be a simple enclosed steel box girder, with the running surface laid directly on top of the hollow aerofoil shape. This had several advantages including the promise of major savings in cost. None of the half dozen or so conventional solutions, that had previously been favoured, were able to compete.

The box girder had no difficulty in coping with anticipated lateral wind pressures. However, during a separate stage of the wind tunnel tests, a slight movement in the model had been noticed when subjected to a low but steady wind speed. This led to doubts about the box girders ability to resist oscillations.

Diagram showing new Deck Cross Section

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, would be so smooth and relatively undisturbed, that there would be no chance of the deck providing any contribution to damping.  So the search was on for a fresh energy-absorbing element.  If an independent source of energy could be found, that could be imposed upon the cyclic build-up of an embryonic oscillation in the bridge deck, the problem might be resolved.   It was at this point that the possibility of using inclined hangers was suggested.

The introduction of Inclined Hangers

The choice of a welded box girder for the bridge deck, was widely welcomed but it would not have been accepted if there had been any real doubts about its ability to dampen any oscillations that might try to gain a foothold.  In a second major innovative move, it was decided that the hangers that suspend the deck from the main cable, should all be hung in an inclined pattern, as shown on the following diagram. This was to increase the structure’s ability to prevent the bridge deck from oscillating.

The pattern for inclined hangers

The deck of the suspension bridge is freely suspended beneath the catenary cables, using separate lines of hangers under each cable.  Please note that, for convenience, there will be no further reference to the second line but it should not be forgotten.  Each pair of hangers will be firmly fixed to the base of a cable clamp that that has been fastened around the cable (in a location represented by the top of one of the “spikes” shown on the diagram, above), This allows the whole deck to move a short distance, longitudinally, every time a heavy vehicle runs on to the deck.  The point on the main cable that takes the brunt of any concentrated traffic loading will move a short distance  closer to the nearest tower. These movements of the deck have been supported by the presence of sliding joints at each end of the bridge spans, at the towers, while the deck will be restrained laterally, at the towers, and laterally and longitudinally at the anchorages.

The purpose of employing the inclined hangers is to take advantage of the opportunity to generate frequent switching of tension between the two hangers in every pair that is fixed separately to the base of a  cable clamp.  The opportunity is then able to maximise the scale and frequency of re-distributions of tension that are passed from one of these hangers to the other, without seriously affecting the total tension passed on to the cable clamp.  With inclined hangers, this will happen every time there is a small longitudinal movement of the deck.   As a result of these changes, the  tensions in each of the hangers will be in an almost continual state of change, depending upon the locations and weights of  individual vehicles that happen to be using the bridge at that time, with the effect of the changes being inversely proportional to the length of the particular hanger.

 The simple employment of inclined hangers, together with the frequent longitudinal movements of the bridge deck has improved the dampening potential of the bridge structure quite dramatically.  It is extremely convenient that the completed structure is self sufficient in that the energy required is automatically provided by traffic moving over the bridge.  There is however an important downside.  Depending upon their length and position on the deck, certain hangers are subjected to fatigue damage, due to the rapidity of the changes in the tensile stresses in certain hangers which might need replacing after a fairly short time in service. No traffic restrictions are needed while hangers are being replaced but the additional maintenance charges are significant.

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 relying upon the use of inclined hangers to prevent oscillations from occurring.

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 provides many advantages and which has subsequently been adopted for many other world class bridges, including those on the Humber and the first and second Bosphorus bridges. 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.

The 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 to the 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 to 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!

Next Page

The First Road Bridge

Introduction

Since prehistoric times, the Severn Estuary had been an obstacle for those who wished to cross it. The Severn Railway Tunnel was completed in 1886 and regular services commenced in December of that year.  It was the first means of crossing the river that was not affected by the weather or the tides, and it was assumed to be a lasting solution to the problem of moving between Southern England and South Wales. Prior to that, coastal shipping had been the first choice for both goods and individuals. However,  it was only a decade after the railway tunnel opened, that the first motor-cars appeared on the nation’s roads and, just three decades after that, their numbers had increased to such an extent that they were causing congestion in many areas.  In time, the River Severn ferries were unable to cope with all the vehicles that wanted to cross the Estuary.  A bridge across the lower Severn reaches was needed.

The bridge would need to be able to withstand nature in all its moods. The tidal range on this stretch is the second largest in the world, over 40 feet (14 m) – and the water flows at up to 8 knots during major tides. The large spring tides create the famous Severn Bore, a tidal wave which rushes from upstream of the crossing, as far as Gloucester, over thirty miles away, and can be 10 feet (3 metres) in height. High winds gust at over 100 mph.

How would such a bridge be built? How could all the difficulties be overcome? There were two locations along this stretch of river that seemed to stand out as probably being suitable for the construction of a new bridge.  Many centuries earlier, they emerged as the most popular points from which to attempt a crossing of this part of the river, in a coracle or other rudimentary vessel.  These locations are now known as passages and they have both been confirmed, by usage, as the most appropriate sites available for a crossing of the lower river.  The factors that worked in favour of these two locations in the past. could work for them again.  And, of course, all kinds of infrastructure would have been provided along these routes over the centuries , adding to their attraction as potential bridge sites.

Both the passages were extensively assessed and compared as competing options for the location of the proposed bridge, and there is no record of any other site being considered in depth. The two sites are only 5 km (3 m) apart. Strangely, the oldest one is known as the New Passage. It lies to the south of Old Passage and it crosses the river where a small outcrop of hard limestone rock, known as the English Stones, is exposed during every low tide. The Old Passage, on the other hand, had the advantage of crossing the narrowest section of the river for many miles in either direction.  For further information about the two passages, and a sketch map, go back to the the first section of this website, which is entitled “Previous attempts to cross the Estuary”, and check out the first chapter on ”History of Estuary Crossings”.

By the early nineteen thirties, road traffic had become an issue

By this time, the country was starting to return to a normal life after nearly two decades of devastation caused by the First World War and followed, first, by the arrival of the ‘Spanish Flu’ Pandemic, and then by the Great Depression.  The government was starting to address the issues that had been put aside, so that the nation could concentrate on survival. The main roads connecting the towns and cities now needed urgent attention to enable the national economy to prosper from the enormous potential presented by the numbers of cars and commercial vehicles that were then using the nation’s roads. At that time, the county councils were still responsible for all the roads within their boundaries.

A powerful and vociferous lobby had arisen, demanding radical action from the Government to create a network of inter-urban roads, suitable for the era of the motorcar.  In the West Country, Gloucestershire County Council was being pressed to take steps to provide a bridge across the Severn.  The Council responded by appointing consultants, Mott, Hay and Anderson, to prepare plans to be put before Parliament for this purpose. And Monmouth County Council joined forces with Gloucestershire, for a joint endeavour to acquire a road crossing of the lower Severn.

Initially, the consultants had favoured a high level suspension bridge located on the line of the Old Passage Ferry between Aust and Beachley, with a main span of 3,000 ft (950 m) and a conventional truss deck,. However when they had collected the data needed from the site and had undertaken initial design calculations, the consultants reviewed the situation with the two County Councils, and the decision was taken to change the route of the crossing from the Old Passage to the New Passage, where the navigation channel shares the river bed with the English Stones.  The consultants recommended construction of a suspension bridge with a main span of just 900 ft (275 m) across the navigation channel, wholly within the river. To complete the actual crossing of the river, two lengths of viaduct would be provided, one on either side of the bridge, stretching from the outer face of each main cable anchorages to a viaduct abutment on dry land.  A sketch of this is shown below.

Sketch of 1935 design for a Bridge and Viaduct at the New Passage.

In 1934, the plans from the two County Councils were sent to the Government as an integral part of a Parliamentary Bill with a request that the Councils be given the powers necessary to build the bridge.  The Councils also requested that 75% of the cost of the works should be provided by the Ministry of Transport.  However, the Great Western Railway opposed the scheme in Parliament, because of the damage that it might cause to the Severn Railway Tunnel.

The problem was that the new road would have to pass over the top of the railway tunnel, out in the river, with a very small angle, horizontally, between the centre-lines of the two routes, at the point at which they would have to cross.  The GWR argued that the distance between the viaduct pier foundation, that would become the nearest one to the tunnel, horizontally, could not be a sufficient distance away from the tunnel, to avoid the risk of damaging it.  In 1936, the Councils’ Bill was rejected by a Select Committee of the House of Commons. This came as no surprise, given the strength of the railway case, but it was a blow to the Councils.  By that time, the national debate about the management of the main road system was coming to a head and so the two County Councils decided to take no further action on the lower estuary crossing, until those issues were resolved.

The government had been under pressure for some time to change the system for managing and improving important inter-urban roads.  It was widely recognised that County Councils could not be expected to bear the financial burden of developing a new national highway system.  At the same time, there was strong evidence, from other countries, of the benefits to be gained from the construction of high speed inter-regional roads, under central direction.  The Government decided to create a Trunk Road Network that would be administered, maintained and financed by Central Government.  This change came about in 1936, when the Trunk Roads Act was added to the Statute Book.  The defined network of trunk roads that accompanied this legislation, included a Severn Crossing and an improved A48 road along the South Wales coast.  However, preparations for the second world war intervened and a further 20 years passed before any construction work could be started anywhere on the new trunk road network.

A New Dawn

Gloucester County Council started lobbying the Government again, as early as 1943, for steps to be taken to ensure that a bridge across the Estuary would not be delayed.  In 1945, the Ministry of Transport, now acting as the client, appointed Mott, Hay and Anderson to prepare a scheme for a crossing on the line of the Old Passage Ferry between Aust and Beachley.  The Ministry must have been satisfied, at that stage, that there would be no chance of finding an early solution to the issues that caused Parliament to reject the crossing at the English Stones in 1936.  However, engineering moves on, and 40 years later, an engineering solution acceptable to British Rail was found, thus enabling the Second Crossing to be built across the English Stones.

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 were 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 8 miles (13 km) of new trunk road.  Investment in this project would bring massive benefits. because without it, more than 50% of the traffic between southern England and South Wales would still need to make a detour of 60 miles by driving up to Gloucester and back down again, in order to cross the river Severn.

How would the work be paid for? The benefits from the project, in terms of time and travel cost savings, were so exceptionally high that the Government decided it would not be unreasonable to impose a toll on all vehicles using the bridge, to defray the cost of building it. The government would have to provide funds to pay for the construction, initially, and an act of Parliament would be required for the tolls to be collected, until the cost of the works had been recovered.

For information on the part played by ferry boats, Click here.

The efforts of the consultants, with all the investigations and studies that were undertaken, both before and after the second world war, fed into the design process to ensure that the new bridge, the culmination of aspirations that went back for more than 100 years, would be worthy of the history and the setting of the site. These aspirations yielded a structure that is world class, with a revolutionary design that has stood the test of time, despite having to be strengthened to cope with twice the weight of traffic for which it was originally designed. The story of how all this happened is given in the following chapters.

Next Page