Category Archives: Second Tier

Modification and Strengthening of Wye Bridge and viaducts

Views showing the locations of the strengthening works

Diagram showing Wye Bridge strengthening works

Second view of Wye Bridge strengthening works

All the significant items of strengthening work on the Wye Bridge and viaducts are indicated on these two diagrams. The most obvious work was to the cable stay arrangement. It must be remembered that, before strengthening, the tops of the single towers of Wye bridge were much lower, with only one pair of inclined cables suspended from each.

Q, R, S, V, W & Y     Trestles, Piers and Pylons

828-C-105 Wye Lifting tower extension piece

Lifting tower extension piece- 4 hour closure

The viaduct trestles(R) and those of the main piers (Q) of the Wye Bridge and Viaduct needed strengthening to cater for the specified increase in vertical traffic loading and transverse wind loading.  The vertical reinforcement was made by adding additional plating to the lower portions of the trestles. The upper corners of the trestles were provided with gusset plates.  In both instances the plating was fixed by a pattern of bolts which were tightened before being welded all-round to seal the edges and provide additional structural contact. The existing bearings at the top and bottom of these elements were checked and found adequate for their purpose.

The lower parts of the two towers (pylons) below the new lower inner cable saddles were also reinforced by double plating which was attached in the same mixture of bolting and welding.  The parts of the towers above the lower cables did not need reinforcement as it was now carrying somewhat less vertical load than in its original configuration. The towers were also increased in height (L) to suit the new cable configuration (M) and (N).

L,M&N. Tower Extensions and New Cable Stays

The height of the central towers was extended and additional cable stays were introduced to support the box-girder deck at closer centres so that it could carry the increased design traffic load.  The original cable stays, with 20 strands in a tight triangular configuration, were replaced by two open arrays of 12 strands each of slightly larger cross-section. This new configuration has the added benefit that any strand can, if necessary, be replaced without closing the bridge.

828-C-157 Wye new cables installed

Wye Bridge with new cable configuration

The re-configuration required eight new cable anchorages inside the deck sections. The existing towers were strengthened and given new cable anchorages and saddles for the new cable-stays. New tower extension pieces were attached to the tops of the existing towers. This was the only part of the strengthening work that required the complete closure of the Crossing, on just two occasions.

O,T,U&X. Steel Deck
The trough stiffener to deck welds under the wheel tracks were replaced, as on Severn Bridge, and stiffening was provided between the deck box and the cantilevers.  New under-deck travelling maintenance gantries were provided and joints on the underside of the deck were inspected and repaired.  New folding gantries on the upstream side provided access between new manholes in the cycle track and new manholes in the sloping side of the box deck.

P. Central Reserve Safety Barrier
The original wire rope safety barrier in the central reserve of Wye Bridge was replaced by higher and stronger steel barriers to provide greater protection to the towers and cable stays.

Q,R,S,V,W&Y Other Work
The towers of Wye Bridge were strengthened internally and externally and the viaduct pier legs were strengthened externally.  New compressed air and water mains were provided for future maintenance and the parapet was realigned.  A new tieback system was installed, connecting the top of Beachley Viaduct to the Severn Bridge anchorage, in order to resist longitudinal forces.  The base of one pier in the river was enlarged to provide protection from shipping collision.

Modification, refurbishment and strengthening of Aust Viaduct

The steel box girders of Aust viaduct were also strengthened and new access and strengthening was provided to the steel box columns. New maintenance gantries and platform were installed to improve access. Although distant from the navigation channel, it was thought prudent to construct new concrete collars to protect the Aust Viaduct piers against shipping collision.  The risk was felt to be slight but was enhanced by the habit of pilots to line up ships to head directly for the piers when guiding them up the channel. 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 to allow the new system of cable stays to be installed.

Traffic Control during all 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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How do Suspension Bridges work?

On the Severn Bridge, the two main cables act a bit like a washing line.  The tension in a washing line supports the weight of the clothes that are pegged to it.  In a similar way, the tensions in the main cables of the bridge, which are held in place by huge anchorages at each end, support the weight of the deck and traffic upon it.  The bridge deck is hung from the main cables using wire hangers (rather than clothes pegs).  And because the main cables are held up by the towers, the weight of the whole bridge is carried down through the towers, to the underlying foundations.

If you put something heavy on a washing line, it will sag at that point.  With a suspension bridge, the road is supported by a stiffening girder, which spreads out the weight of the traffic, so avoiding excessive sag under an exceptional load.  If you hang something on a washing line away from the centre, the point will not only sag but it will also move towards the nearest end (try it!).  Similarly, as a heavy load travels over a suspension bridge, it will not only dip downwards at the point of the load, it will also move longitudinally towards the nearest tower.

If you stand on the walkway of the Severn Bridge, you can feel it moving as the traffic travels over it.  If you stand by one of the towers and watch the expansion joint, you can sometimes see the whole bridge moving as the weight of the traffic travels across.  We should not worry that the bridge moves.  It is meant to do this.  This is how it absorbs the weight of the traffic and transfers it into the main cables.

Diagram showing the main loads in a suspension bridge

Diagram showing the main loads in a suspension bridge

The tension in the main cables carries the whole weight of the bridge deck and the traffic. This tension is resisted by the anchorages at each end, just as the tension in a washing line is resisted by whatever it is tied to at each end.  And because the main cables are held up by the towers, the weight of the whole bridge is transferred through the towers to the ground.

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Subsequent Problems in main cables of Severn Bridge

Inspections of the main cables on the Forth Bridge, followed by inspections on the Severn and the Humber, were undertaken in accordance with guidelines developed by the American authorities. This process involved removing the external wire wrapping between cable clamp positions and driving wedges into the body of the cable to enable inspection of as many wires as practically possible in eight radial positions around the circumference

The first inspection of the Severn cables took place in the summer of 2006, some 40 years after opening and approximately one-third through the original 120 year design life of the bridge. Eight sections between cable clamps were opened up. A number of broken wires were discovered and the condition of the remainder were carefully logged on a scale of 1 to 4 (1 being ‘very little corrosion’ and 4 being ‘widespread corrosion with rust over most of the surface of the wires’). Locations for inspection had been chosen at high, medium and low levels of both the main span and the side spans. It was no surprise that the sections at the lowest level were the worse for wear, because any moisture entering the cable would tend to run down and collect at the bottom.

A statistical analysis of the potential loss of strength was undertaken. This indicated a significant reduction in the factor of safety against failure compared to the original design intent. The only part of the loading that was controllable to any extent was the weight imposed by the traffic, so a regime of traffic monitoring and control was imposed, having particular regard to heavy goods vehicles. The main control measure was to prohibit HGVs from using Lanes 2 of the carriageway; this prohibition is still in force. The monitoring by Weigh-in-Motion (WiM) sensors, assisted by video camera monitoring of flows, is used to confirm that the traffic volumes and mixes remain within the assessment design loadings.

Most of the broken wires that had been found were repaired by cutting out those corroded lengths and inserting new wire that was re-tensioned by the use of specially developed swaged connections, incorporating turn-buckles. The opened sections were re-wrapped and re-painted. In addition a system of acoustic monitoring was installed to record on-going wire breaks. Measures were also put in hand to completely seal the cables in a membrane and, in time, to dry and de-humidify the interior of the cables by forcing dry air through them.

By 2011, when a second intrusive inspection was carried out, it was apparent that the measures imposed were taking effect. The humidity in the voids between individual wires was below 40% relative humidity, at which no corrosion of steel takes place, and the rate of wire breaks had been reduced to a very low level.

The monitoring and control of traffic and the de-humidification and acoustic monitoring is on-going and these measures are likely to be needed for the remaining life of the bridge. At the time of writing, the situation appears to have stabilised and, although the condition of the cables is considered unlikely to deteriorate significantly further, another examination is scheduled for 2016. Should this inspection confirm that a stable state has been reached there is scope for increasing the time between periodic inspections, inspecting only the most vulnerable (lower) panels, and allowing the relative humidity of the injected air to rise somewhat thereby saving some cost of the energy required to dry and warm the air.

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Modification and strengthening of Severn Bridge

A. Tower Legs

The towers supporting the main cables were originally constructed as hollow steel boxes, cantilevered from the foundation. They carry both the vertical load from the weight of the structure and the bending moments that arise from different distributions of live loading. As a single heavy vehicle moves on to the bridge, the main cable will sag a small fraction towards the nearer tower (like a garment on the clothes-line), accompanied by similar reactions from all the other vehicles on the bridge at that time. The top of the tower will be pulled backwards and forwards, depending upon the aggregate of all the individual reactions, and it must be capable of resisting the impact of the worst case scenario, including any co-existing transverse wind loading.

Single plates of mild steel had been used in the construction of the tower walls, each being stiffened, off-site, by the addition of vertical stiffeners on what would become the interior face of the tower wall.

828-C-021 Severn tower strengthening

Column and guide inside tower leg.


To relieve the box walls of some of their vertical stress, four tubular steel columns were installed in each tower leg and then jacked up, simultaneously. These columns are about 120 metres high and 400 mm diameter with a wall thickness of 30 mm. Each column has 19 sections, which were threaded, one by one, through a small access door at walkway level.

828-C-022 Severn tower strengthening

Inserting column section into tower at deck level.

The door could not be enlarged because of the high stresses in the tower walls. A grillage of steel beams and columns at the inside at the top of the tower legs transfers the jacking load from the columns to the underside of the main cable as it passes over the tower.

828-C-024 Severn tower strengthening

Top of columns under grillage, inside cable saddle.


The columns were jacked upwards from the bottom with a total force of 2000 tonnes per tower leg. Special sliding bearings locate the columns at nineteen positions within the tower to prevent buckling. The jacking lifted the bottoms of the columns by 100mm. The columns shortened in compression by about 75mm and the saddles that carry the main cables over the tops of the towers were raised by some 25mm. The Severn Bridge is an inch taller than it was before the strengthening!

B. Middle Portal Connections
The connections between the horizontal portal beams and the vertical tower legs were strengthened, both internally and externally to cope with the transverse wind loading.

C. Tower Saddle
The main cables pass over the top of each tower leg in a saddle. During construction, the individual strands of the main cable were laid in this saddle as the spinning continued. As the main cables carry the whole weight of the bridge deck and the traffic, the stresses in the saddle are very considerable, especially the splitting effect that tends to separate the two sides. The saddles were upgraded with C-clamps – similar in principle to a G-clamp used in carpentry – to help resist the splitting force.

New access platforms were also installed at the tops of the towers to assist with inspection and maintenance.

D&E. Cables
New hand-strand ropes and posts were provided above the main cables. As well as providing access, these support new maintenance cradles for inspecting and painting the main cable. The main cable itself did not need strengthening but the bolts in the cable clamps, at the tops of the hangers, were renewed and re-tensioned.

828-C-005 Severn hanger works

Deck/hanger configuration after strengthening.

F. Hangers
All 376 hangers were replaced, one by one, at night while traffic was restricted to one lane in each direction. The new hangers are slightly thicker that the originals and the design of the end sockets is improved to reduce the risks of kinking and corrosion

The original hangers were terminated in metal sockets filled with white metal. However, there was no hole in the socket for drainage which led to some speculative concern about the condition of some of the wires making up the hanger. In addition, just before the Royal Opening a “man with a file” was sent around to tidy up the white metal protruding from the tops of the sockets. Unfortunately he also broke through the anti-corrosion coating in several places and a number of the outside peripheral wires failed. This posed the question about the condition of the inner wires. Ultimately all the hangers were replaced, although mainly for reasons concerned with the bridge loading.

G, J & K. Steel Box Deck
The top of the steel box girder deck, over which the traffic drives, is 12mm plate stiffened by steel troughs welded to the underside. Repeated loads from the wheels of heavy goods vehicles had tended to cause local weld damage and it was decided to grind out and replace the welds immediately under the lane wheel tracks. The original fillet welds were replaced with a partial penetration weld requiring three passes. Changing just this one weld detail required 48 miles of welding (3 passes/weld, 2 welds/wheel track, 2 wheel tracks/carriageway, 2 carriageways, length of crossing 2 miles).

New under-deck travelling maintenance gantries were provided for inspection and painting. Also, new manholes were installed in the deck boxes to assist access for the strengthening work and for future inspection and maintenance. When the bridge was originally built, it was intended that the box girder deck would be sealed and kept dry with silica gel. For this reason the original access manholes were very small. Hence it is rumoured that a one-time Chief Highway Engineer of generous proportions was unable to visit the inside of the bridge deck.
Prior to strengthening, there was only a thin layer of asphalt surfacing on the steel deck units. A thicker layer was desirable but had to be rejected because of the effect of the extra weight on the cables.

H. Deck Bearings
Lateral bearings resist sideways forces. Rocker bearings resist vertical forces at the towers to keep the deck boxes at the correct level.

It was discovered at an early stage that the two rocker bearings at each of the towers had been designed to carry half the maximum reaction from a fully loaded deck. However, when the deck was eccentrically loaded, one of the rockers went into tension and the other carried a greater load than it had been designed for. All were replaced.
New buffers were provided to transfer longitudinal force if deck movement becomes excessive.

I. New Air and Water Mains
To help with future maintenance, a compressed air main and a water main were installed over the full length of the crossing. The water main was to be used to fight any substantial fire on the bridge, although subsequently it has been decommissioned.

Other Work on Severn Bridge
Movements of the deck due to thermal expansion and loading are catered for by “Demag” expansion joints in the carriageway at each tower. Increases in the required longitudinal expansion and contraction temperature allowances had to be catered for. Also it had not been recognised that the joint would need to cope with extreme racking movements due to the side sway of the main span from increased high wind design speeds. For these reasons, the joints had to be modified significantly.

It was also discovered that the large chambers in the concrete anchorages, where the individual strands of the main cable were splayed out and anchored separately, had a damp atmosphere and that the wires were beginning to show early signs of corrosion. The decision was that the chambers had to be de-humidified.

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The Steel Box Girder Problem

The background.
The failure of four steel box girder bridges during construction caused quite a furore. The use of such girders in bridge construction had been a fairly recent phenomenon, partly because the mathematical analysis was quite daunting before the advent of electronic computers. Their popularity came at a time when the post-war surge in bridge building was getting underway. They required less steel for the construction of a bridge deck than the truss or the plate girder that had been the popular choice previously.

A cantilever built using box girders poses problems for the designer. The stresses will be distributed between the various constituent plates of the boxes depending upon the accuracy of the individual elements and the precision of the fixings involved. Distortions of an individual plate could cause havoc especially during the construction of a cantilever and they are difficult to spot and to eliminate. Try to imagine how a box girder would crumple if tested to destruction by the continual adding of new units, i.e. more weight, to the unsupported end of the cantilever.

The government’s response.
In 1970, the government responded to the box girder crisis by setting up an advisory committee under the chairmanship of mathematician, Dr. (later Sir) Alec Merrison, then Vice Chancellor of Bristol University, to investigate the circumstances of the previous failures and to make recommendations for the safe re-construction of the Milford Haven bridge and the safe completion of the Avonmouth Bridge (under construction at the time). The committee also advised changes to the design rules and on the procedures to be adopted for future applications. As part of this process, a major programme of research was initiated to study the behaviour of the various components of a box girder and to consider how the mathematical analyses, then in circulation, might be improved.

The committee delivered its final report in 1974 after the results of the research studies became available. It concluded that the primary causes for two out of the four failures were inadequate organisational procedures and poor communications. It provided an improved set of rules for the design of box girders, incorporating the findings of the latest research. It also stressed the need for the introduction of an independent check on methods of erection and on the design of temporary works, together with the need for clearer distinctions between the respective responsibilities of the Engineer and the Contractor,

It is true to say that only one of the four box girder failures was strictly the result of lack of technical knowledge of steel box girder behaviour, although that was not apparent when the failures originally occurred. In three cases, the primary cause of failure was human error during construction and can arguably be attributed to the rapid growth of the design and construction programme out-stripping the supply of suitably qualified and experienced engineers to adequately supervise these works.

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Highway Bridge Loadings


At the time of the design of the first Severn Crossing, the relevant live loading for such a bridge would be based on the assumption that the proportion of heavy vehicles in the traffic flow would be about 15%. This figure was based on observations undertaken in the late 1950s. It was also assumed that the heaviest loading would occur when traffic became halted on the bridge, forming a stationary queue with minimal distances between adjacent vehicles. In such circumstances, the impact on the bridge from heavy vehicles would be softened by the presence of lighter vehicles interspersed between them.sbtraffic2


By the early 1980s, traffic counts were showing a much higher percentage of HGVs on the majority of the country’s strategic road network. It was then averaging between 30% and 33%. This was due to the increased value that operators were then attributing to two of the advantages that the motorways and improved trunk roads provided. The first was the actual savings in journey times. The second, possibly more welcome, was the increased reliability of those journey times, which is so important when operators are trying to work to tight schedules.

The widespread availability of computers by the 1970s led to the development of mathematical traffic models which provided data to inform decisions on many problems in the landuse/transportation field. Typically, these models would consist of a representation of the road network covering the area under investigation, a breakdown of that area into a number of discreet zones, a data base containing information on the traffic flows from each zone to every other zone (compiled from roadside interviews) and various tools for assigning traffic from the data base to the road network, etc. These models could be validated by comparing their forecasts for “the present day” against an independent set of data.sbtraffic

A model of the type described above was used to forecast the worst case loading on the Severn Bridge, prior to the strengthening work. An assessment was required of the aggregate weight of a stationary queue of traffic on the bridge, with minimal distances between adjacent vehicles. The traffic model provided forecasts of the proportions of the various types of vehicle that would use the bridge during a particular period. A separate application then selected vehicles from this mix, randomly, to generate many different queues, each conforming to the vehicle type proportions and spacing requirements, and then it calculated the load represented by each queue in terms of kN per lane meter. The figure taken forward was the load exceeded by 5% of the queues. For the Severn Bridge in the mid 1980s, this figure proved to be 9 kN per lane metre, which happened to be twice that for which the bridge had originally been designed.

The weights of the actual vehicles crossing the Severn Bridge were also measured by placing a weighbridge in the carriageway. This showed that the flows were not entirely random. Many operators of heavy lorries favoured an early morning start, when there would be relatively few cars and other light vehicles on the road to dilute concentrations of lorries. Unless that kind of situation were managed, it could result in queues being formed on the bridge with more than 30% of heavy vehicles, generating a 5-percentile loading of up to 12 kN per lane metre. Fortunately, the timely arrival of the Second Severn Crossing, in the mid-1990s, took much of the sting out of such speculations.

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Developing the cable stayed design for Wye Bridge

Modern cable-stayed bridges were only starting to be considered in the UK at the time that the first Severn road crossing was being built. And it was not until later, when electronic computers became widely available for engineering calculations, that the load bearing capacity of the more complicated cable stayed bridges could be analysed with any real confidence.  In the earlier circumstances, engineers were reluctant to break new ground. In fact, 25 years later, when construction of the Second Crossing got underway, no cable stayed bridge in the world had been completed with a longer span than the one that was then under construction on the Severn. However, before the Second Severn Crossing was completed, the French had taken a great leap forward with the opening of the Pont de Normandie (main span 856 m). And since then, with developments in design and stronger materials, further great strides have been made with main spans now 1,000m and longer. Having said all that, the Wye Bridge is a simple, but very important, early example of a cable stayed bridge.

The deck units chosen for the viaduct enabled the contractor, Cleveland Bridge of Darlington to complete each full span of 64 m using a simple cantilever method of construction.


Wye Bridge typical box erection

Calculations confirmed that the same method could be used to erect the side spans of the bridge. However, some additional strengthening of the deck would be required if the same cross section of deck unit were to be used for the main span of 235 m. The obvious way to proceed with this span would be to work from both sides, building two half span cantilevers that would meet each other in the middle. Some method would then be needed to lift both the leading edges to the correct height (even when stiffened, both sections of unsupported deck would have drooped, to a significant extent, under their own weights).


Wye Bridge cantilever erection

The addition of two simple elements of “cable-staying” would kill both birds with one stone, stiffening the whole of the bridge deck sufficiently, and providing means of lifting the leading edges of the cantilevers when required.  It was achieved through the addition of two towers and two long lengths of high tensile steel cable. This well targeted response provided a very efficient solution, enabling the Wye Bridge to be completed successfully and in accordance with the loading criteria of that period.

It is interesting to note that when the nearby M4 Avonmouth Bridge came to be strengthened in the 1990s (for the same reasons as the Severn and Wye Bridges), the method chosen to upgrade its load carrying capacity was, in principle, exactly the same as that used to build the Wye Bridge.  However, in that case, the original box girders were deeper than over the Wye and so it was possible to achieve the additional strengthening using high tensile steel cables that were post-tensioned and fixed entirely within the original box girder.

The cable stays on the Wye Bridge do not carry the whole weight of the bridge deck and the traffic, but they effectively take part of the load by adding a new set of supports. The tension in each cable has an upward component of force, which gives support to the bridge deck, and a component of compressive horizontal force, that is absorbed by the bridge deck.

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The Decision to use Inclined Hangers


Choice of Deck Cross Section.

The previous item of ‘more detailed information’, entitled “Background on choice of design for First Road Crossing”, describes the circumstances which led to the catastrophic collapse of the Tacoma Narrows Bridge in the USA in 1940. Initial work on the Severn Bridge started shortly after that and there can be no doubt that its designers were well aware of the potential consequences of neglecting the lessons learned.

In the late 1950s, using an anemometer mounted on a mast 110 feet (33 m) high (the height of the proposed Severn Bridge deck) and an array of instruments mounted on the then existing Severn Railway Bridge, some 3 miles (5 km) upstream, a spectrum of data on local wind speed, direction and inclination, was established over a width of about 300 feet (90 m). This information would be put to good use when the 1/100-scale model options for the Severn Bridge deck were tested in a wind tunnel at Bedford. At that stage, it was assumed that the deck design would follow American practice with a stiffening girder, about 33 feet (10 m) deep and of open-lattice truss construction, as had been proposed in the 1930s.

However, in the late 1950s, the Government stipulated that the new Forth Road Bridge would take priority over the Severn. The Forth Bridge would, not surprisingly, be designed on the same principles as those being developed for the Severn Bridge, although the main span across the Forth would be slightly longer, at 3300 feet (1008 m). Mott Hay and Anderson were appointed in association with Freeman Fox and Partners to design the Forth crossing using a conventional design for the stiffening girder, consisting of a horizontal concrete slab, above a latticed steel-work construction (a truss) that supports it.

There was a new development on the Forth Bridge in that the road deck was made of stiffened steel panels resting on cross girders at the top chords of the trusses. These panels were light in themselves and the overall dead weight was also kept to a minimum by specifying a thin layer (c 1 ½ inch or 40 mm) of mastic asphalt as the running surface. Nevertheless, the total dead weight of the Forth deck was some 18,000 tons which required the towers, main cables and anchorages to be designed to cope with this load plus the weight of the traffic on the deck. Construction on the Forth proceeded about two years ahead of the Severn.

In the meantime, scale models of several options for the deck of the Severn Bridge were to be tested in the wind tunnel. One of these, a shallower truss configuration, became loose during the initial testing and was destroyed in rather spectacular fashion. It was during the delay caused by the need to rebuild the model and re-schedule the wind tunnel tests, that Freeman Fox found time to develop and test a completely novel design for the deck.

The disastrous fate that befell the bridge over the Tacoma Narrows, had drawn attention to the need for wind tunnel testing during the design of suspension bridges. We can now appreciate that the disaster at Tacoma Narrows, and the accidental damage in the wind tunnel Laboratory at Bedford, were essential prerequisites for the development of the design of the deck for the Severn Bridge. It was a simple and revolutionary concept that showed British ingenuity in a most favourable light, internationally.  The Severn Bridge was a great boost to British engineering.

The Deck Cross Section chosen for the Severn Bridge

The final shape adopted for the Severn deck is shown on the diagram above. Unique for the time, it is a completely enclosed box-girder, only 10 ft deep, and the top of the box also acts as the roadway. The importance of this innovation lay in the fact that the box girder design was very significantly lighter than any previous suspension bridge deck. It was also very strong and stiff in both bending and torsion. The corners of the box are 75’ apart and there are side-tracks on each side cantilevered from the apex of the pointed box shape. During wind tunnel trials a total of seven different edge profiles for the box girder were tested. The model with the point approximately one-third of the overall box depth, below the top of the box, was almost completely stable in the wind tunnel and its shape was the one adopted.

Inclined Hangers.  Initial considerations.

Detailed wind tunnel tests had indicated that the box girder deck would be aerodynamically stable for all wind speeds and angles that were likely to be encountered, except for a slight movement in a narrow range of steady wind speeds, around 15 miles/hour and at an angle of 7.5 degrees. This was heartening news, so far as the effect of the lateral wind pressure on the whole structure was concerned, but it did not rule out the possibility that oscillations might develop on the bridge deck.  This led to doubts about the ability of the box girder to dampen any embryonic oscillation that might form. This was at the time when news of the disaster at Tacoma Narrows was still at the forefront of the minds of most civil engineers. More work was needed to confirm the viability of a box girder, with this particular shape.

It was realised that the particular design being considered for the deck of the new bridge, would be so exceptionally smooth and aerodynamic, that any air flowing over it at 15 miles/hour, would be virtually undisturbed. This meant that no damping could be expected from the deck itself.  So, a fresh energy-absorbing element might be needed. The idea then emerged that it might be possible to extract additional damping from the hangers if, instead of being vertical, they were all inclined, as shown on the following diagram.  This would be a second major innovation for the Severn Bridge, as all previous suspension bridges had been provided with vertical hangers .

The pattern for inclined hangers

The decision to use Inclined Hangers.

It is a feature of the Severn Bridge and many other suspension bridges that, because the bridge deck is freely suspended from the main cable, it will move backwards and forwards for short distances under the influence of the ever-changing live traffic load.  This is because, like a simple clothes line, the bridge will react to any change in the load that it carries.  The addition of another garment will not only cause the line to sag a little more at the point at which the new load was added, it will also cause that point to move a short distance towards the nearest point from which the line is suspended which, in the case of the bridge, is likely to be one of the towers! This feature is explained more fully in the additional information which is available through a special link in the main text.  If you are interested in this, go up to the top of the present text and hover over “The First Road Bridge” in the main menu.  When the list drops down, click on “Design of the Severn Bridge” and the link to supplementary information entitled “For more on how does a suspension bridge work?” appears, as the first item.   Click on it.   

In the case of a suspension bridge with conventional vertical hangers, the longitudinal movements mentioned in the previous paragraph, will have very little effect on the tensions in the bridge hangers because, with vertical hangers, there are no means of transferring to the hangers, the energy produced by the live load of traffic and dissipated in producing the longitudinal movement of the deck.  When the hangers are hung in an inclined pattern, the total load on each cable clamp from both hangers will be fairly constant but the manner in which that load is distributed between the two hangers involved can vary enormously, depending upon the distribution of live load along the full length of the bridge deck, at any moment in time.  The direction of travel of the vehicles that make up the live load is not, in itself, relevant to the movement of the bridge deck.  What matters is the instantaneous pattern of live load on the whole bridge deck and how it changes with time.  

Consequence of longitudinal movements of the bridge deck on tensions in the hangers

Changes in hanger loads due to bridge movements

Consider the situation that is illustrated in the above diagram.  It was provided to demonstrate what could be achieved, if the hangers for the Severn Bridge were all inclined.  The diagram  shows the support for one side of a short section of deck, supported by a pair of hangers, which are both fixed to the base of the cable clamp fitted around the main cable.  The lower ends of both hangers are locked into sockets fixed to the deck, directly beneath the main cable.  It might be worth looking again at the diagram in the previous section of this chapter, entitled “The Deck Cross Section chosen for the Severn Bridge”, as it shows the positioning of the hangers, the clamps and the sockets from a different angle.  The cable clamp is located directly above the mid point between the two adjacent sockets.   

There would, in practice, be an identical set-up on the other side of the deck but that can be ignored for the time being.  Small longitudinal movements of the deck will take place in response to the changing live loads on the bridge, as previously described.  The movement that is illustrated on the diagram, will have been made in the direction of the tower that was nearest to the cable clamp, in this case the one to the left side of the diagram, irrespective of the vehicle’s direction of travel.  During these movements, the lower ends of the hangers will remain fixed in their sockets on the bridge deck and because the deck moves to the left, the cable clamp that is holding the tops of both hangers will also move to the left, relative to the bridge deck.  This means that the cable clamp will move longitudinally to the left, relative to the bridge deck (and as seen on the diagram) stretching the length of the hanger on the right-hand side of the clamp, thus adding to the tension in that hanger.  The total amount of stretch in this right-hand-side hanger is shown coloured blue on the diagram.

At the same time, the longitudinal movement of the cable clamp to the left, described above, will remove some of the tension from the hanger on the left hand side.  It will reduce the amount of stretch (usually called the strain) in that hanger by about the same amount as the added extension to the hanger on the right hand side.  It also removes some of the load previously carried by the left hand hanger.  The reduction in the amount of strain in the left hand hanger is shown coloured red on the diagram.  The passing live load must have been exceptionally heavy, because the whole of the strain that had been in the left hand hanger before the heavy live load arrived, had been completely eliminated.  In such cases, the hanger would just lay dormant, subject only to its own dead weight, until the heavy vehicle had moved out of range and the previous situation had resumed.  This is exactly what must have happened to create the exceptional situation illustrated on the diagram.  The fact that the red coloured portion of the left hand hanger, had extended further than its original length, confirmed that all its previous tension had been removed so that, for a short time, the hanger would have gone slack. 

The previous two paragraphs illustrate how the idea of suspending the deck of the Severn Bridge using hangers erected in an inclined pattern, had made it possible for large tensile stresses to be redistributed within each pair of hangers that share an upper cable clamp.  It was exactly the kind of new energy source that the consultants had been  seeking to combat the possibility of an oscillation developing within the completed structure, similar to the one that destroyed the Tacoma Narrows bridge in 1940.  It is particularly intriguing that the actual source of that additional energy should be the ‘live load’ that crosses the bridge.  However, the consultants concluded that the most sensible approach would be to avoid situations that could lead to a hanger becoming slack, or otherwise overstretched under extreme live loading conditions, and they were supported in that decision by the government Department for Transport. 

Manufacture of the hangers.  The above demonstration of the ability of the inclined hangers to provide a new source of energy that must be generated from the live load of traffic, raises questions about the construction of individual  hangers.  The dead load of the bridge deck is extremely uniform throughout its length and so all cable clamps (at 60 ft centres) would carry basically the same share of that weight.  At the same time, the average daily live load borne by each cable clamp  would be virtually identical.   It was also clear, at that stage, that more than 50% of all the hangers would be long enough to be treated in exactly the same manner as if they had been hung vertically.  This all suggested that the current practice of the day, based on a multi-strand configuration, would probably be adequate.  At the same time, there was strong support for developing a strand that would further strengthen the structure against the possibility of oscillations, by maximising the amount of hysteresis in the hangers (this work was undertaken, but very little has been said about it since).  These deliberations resulted in a strand that contained a total of 178 separate wires, made up from the following three sizes, 0.118 ins, 0.133 ins and 0.339 ins with a lay of 7.5 d.  The resulting strand would provide a maximum working load of 100 tons and a breaking load of 225 tons.  At this stage, the way forward was clear for the suspending of all the longer hangers from cable clamps and making the necessary connections to the sockets that would be fixed to the deck to receive them.  But before that work started, the possible need for a modification to the system outlined above, in order to cope properly with all the shorter hangers, was undertaken. 

New simple diagram.

The initial system for supporting the bridge deck, that was used to suspend all the longer hangers, is indicated in the above diagram.  It  represents a section of main cable on one side of the deck, with the two hangers in one pair, coming together directly underneath the main cable to which they are both fixed using a special connection attached to the base of the cable clamp.  Lower down, on the top of the deck, a line of sockets had been fixed to the deck, at 60 ft centres, to secure the lower ends of the hangers.  Each cable clamp was positioned directly over the mid-point between two sockets.  An initial examination confirmed that that the model, in the form shown above, would not be able to cope with the shortest hangers.  This meant that a modification was required, together with a decision concerning the point at which the modification would need to be introduced.  

The Included angle.  It soon became clear that the critical item when dealing with shorter hangers is the angleincluded’ between the two hangers at the point where they come together under the cable clamp to which they are both fixed..  This angle will be quite small In the case of the longer hangers but consider what would happen if it reached the 60 degrees shown on the diagram above (note, an angle of 60 degrees has been chosen just to simplify the mathematics).  The triangle that includes both of the hangers and the line of the deck beneath them, approximates to an isosceles triangle.  This means that these hangers are inclined at an angle of 30 degrees to the vertical.  Because of that, only 86% of the tensions in the hangers (i.e., the sum of the vertical components of these tensions) would be used to support the weight of the bridge deck, together with the live load of traffic on it.  The other 14%, the horizontal components of the tensions in the two hangers, would cancel each other out, being dissipated into the bridge deck.  And at the mid point of the span, where the main cable drops lower, approaching the centre of the bridge, the included angle between the two hangers, would need to be larger still, in order to conform with the model.  

The chosen modification involved a simple change to the rules that are implied in the model that is described above.  Having reached the point chosen to make the change, the consultants extracted the included angle at that point and decreed that it should apply to all subsequent operations that would otherwise have adopted a wider angle between hangers.


The above diagram shows an elevation of a short length of the support mechanism developed for connecting the bridge deck to the main catenary cable for use by  for the deck, when viewed from one side.  The cable clamps, that transfer the loads from the hangers to the main cable, are positioned along the cable at 60 ft intervals, horizontally, and they each have to secure two hangers, one from either side.   The sockets that are used to fix the hangers to the deck are located mid-way, horizontally, between the cable clamps to either side.  They are fitted along a line just off the flat area on top of the deck, where an additional section of box girder has been welded on to the main box girder, to provide the short cantilever that  carries the footpath and cycle track at the side of the deck.  (this can be seen clearly on the drawing of the deck section which is located near the top of the previous section of this document).

  Once decisions were taken on the dimensions of the spans and the towers, together with the shape of the main cable and the horizontal space between the cable clamps, there will be no scope for adjusting hanger lengths.  They will simply be a matter for calculation   

The purpose behind the introduction of inclined hangers is to produce a situation in which the energy stored in the hangers will increase and decrease much more rapidly and by larger amounts than would be the case with vertical hangers. This should provide the structure with much greater potential for dampening embryonic oscillations of the type that destroyed the bridge at Tacoma Narrows. The Severn Bridge has already given good service for more than 50 years and, with adequate maintenance, should continue to do so for the full 120 years of its design life, and beyond.

Unfortunately, there is a downside to the introduction of inclined hangers. It is extremely important that components of the structure that are active in suspending the deck, should remain in service for long periods. And when necessary, they must be capable of being removed and replaced, without requiring traffic restrictions. The rapid fluctuations in traffic loading associated with inclined hangers, and the additional loads that are imposed, make these hangers especially vulnerable to fatigue failure. This is particularly true of the shorter hangers located in the central area of the bridge. The situation is manageable but it requires significantly more maintenance than was originally planned.

It has been shown that the level of stress for hangers in an inclined pattern could rise to at least twice the maximum level experienced in a pattern of vertical hangers, especially where the hangers are short and near the centre of the main span. And it has been established that the relative fatigue life of an individual hanger on that bridge can be expressed, mathematically, as being inversely proportional to the fourth power of the range of stress involved. When this formula is used to explore the effect of doubling the range of stress in a hanger, as in the present circumstances on Severn Bridge, it confirms that we should expect the fatigue life of some of these hangers to be reduced to one sixteenth of the life that would apply if the hangers had been in a vertical pattern (the fourth power of 2 being 2 x 2 x 2 x 2, which equals 16).

It should have been no surprise, therefore, to find fatigue failures of wires within hangers on the Severn Bridge after only six to ten years service. After a period during which individual hangers that had failed, were replaced ad hoc, a wholesale strengthening programme for the bridge put in hand. All hangers were replaced using stronger and less fatigue-prone strand construction, and new designs of socket were installed.

Mott Hay and Anderson continued in their role to design and supervise the construction of the crossing, but now jointly with Freeman Fox and Partners. Motts led overall and designed all the foundations. Freeman Fox were responsible for the steel superstructures. The superstructure of the main suspension bridge was a major technical advance on all previous long-span road bridge designs.

A total of three suspension bridges were built with inclined hangers, between 1965 and 1980, all with Freeman Fox and Partners as main consultants. It is interesting to note that the inclined hangers on the Humber Bridge have proved less prone to such problems, possible because of a less severe traffic loading pattern. On the other hand, those of the first Bosphorus Bridge have been so fatigue prone, probably because of a traffic loading pattern even more extreme than on Severn, that they were all recently replaced by new vertical ones. Suffice to say that, to date, no other major suspension bridge has been fitted with a system of inclined hangers.

At the time, the Severn Bridge was hailed not only for its majestic appearance but also for its technical excellence. The all-up weight of steel in the Severn Bridge, 3,240 feet (988 m) span, was 19,000 tons for the deck, cable suspension system and towers compared with 39,000 tons for the earlier Forth Bridge which had a main span only 60 feet (20 m) longer.

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Background on choice of design for First Road Crossing

As early as 1943, Gloucestershire County Council started lobbying for an early start to the Severn Bridge. They now favoured the long span, high level option on the Aust-Beachley line.  In 1945, the Minister of Transport accepted responsibility for the crossing and, in 1946, published a National Plan showing the crossing on the Aust-Beachley-Newhouse line, with a high-speed road link from the crossing to the A48 at Tredegar Park, west of Newport.

The design of the Crossing was under way by the late 1950s. It was a time unlike any decade since. The war was over. There was hope and aspiration that a better society would arise from the carnage of the war. It was a time of enthusiasm. The proposal to build a motorway network caught the imagination and gained considerable public acceptance. It was to be pursued by the Ministry of Transport as a public programme, a team effort. Some of the personalities involved, engineers and civil servants, attracted public attention not unlike the engineers of Victorian times.

The design of a road suspension bridge needs to incorporate a stiffening girder to carry the road deck. Its purpose is to distribute the live load from each vehicle to a greater number of adjacent hangers in order to maintain the shape of the main cable. Without a stiffening girder, the main cable would tend to behave like the rope bridges of the South American Indians, on which the rope changes shape and the walkway distorts, as people move across it.

There was a well documented collapse of the bridge across the Tacoma Narrows in the USA in November 1940, only four months after it opened to traffic. The spectacular failure of that bridge has been shown many times on television and it can be viewed below. The stiffening girder for the Tacoma Narrows Bridge was novel for the time. It was constructed using two deep I-sectioned girders, one beneath each of the main cables. They were joined by cross-girders that carried the road deck.

It so happened that the natural frequency of the wind-induced oscillations (of half-span wave-length along the bridge and a half-span twist transversely) were in phase with the frequency of the vortices that were being shed from the deck, by the wind blowing up the Tacoma Narrows Channel. In a prolonged period of relatively low but steady wind, this led to increasing deformations longitudinally and transversely. These galloping deflections ultimately led to massive failure of the main girders and more or less total collapse of the deck, although the towers and main cables survived. At the point of failure, the torsional and bending deflections were enormous. Torsionally, the deck was rotating by about +/- 45º and the vertical deflections were several times greater than the depth of the girders.

The designers had overlooked the effect of strong and steady crosswinds. Even Telford, who aimed at lightness in his structures, underestimated this effect in his pioneer bridge across the Menai Straights, opened in 1824. At the end of January 1826, the Menai Suspension Bridge was swept by major gales inducing extreme torsional oscillations in the deck and twenty-six suspension rods were broken. Telford’s modifications included stiffening the deck and strengthening the suspension rods.

The failure of the Tacoma Narrows bridge made designers all over the world mindful of wind-induced effects. Research showed that plate girders are much more vulnerable to wind-induced deformations than the traditional deep truss-girders of earlier American and European suspension bridge designs. If the cross-wind, vortex-shedding frequency is in phase with the natural frequency of the stiffening girder, deformations will build-up. However, if they could be designed to be out of phase, potentially dangerous deformations would be prevented.

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Building the Wye Bridge and viaducts

More information about the construction of the Wye Bridge and Viaducts

The contracts.

One year after the contract was let for the superstructure of the Severn Bridge, the Cleveland Bridge and Engineering Company won the tender competition for constructing the crossing of the Wye River and the Beachley Peninsula.


Beachley Assembly Area

The contract was awarded to Cleveland in early 1963, with the Fairfield Shipbuilding and Engineering Company of Chepstow acting as their sub-contractors for fabrication and welding of the steel superstructure. In this instance, Fairfield fabricated the stiffened panels in their workshops in Chepstow, delivered them by road to Beachley and Newhouse where Cleveland’s erectors assembled them into boxes. Fairfield provided the platers and welders for work in the assembly yards


Gwent assembly area clearly showing the way the stiffened panels went to make up the Box Girder units

Construction of the Viaduct foundations

The foundations of the Beachley viaduct are conventional twin square shafts of reinforced concrete formed in open excavation through the loose sediments of gravel and sand on the peninsula to limestone strata some 20 – 30 feet (6 – 9m) below ground level. The tops of the shafts are linked by a pre-cast, pre-stressed concrete, tie-beam cast into the tops of the shafts just below ground level. These ties resist the horizontal forces from the splayed legs of the steel trestles supporting the deck.

Construction of the Wye Bridge foundations


Sinking the Caisson for the Wye Bridge Pier

The main pier foundations were constructed using a pair of caissons on either side of the Wye. They were sunk through the soft mud of the river banks down to the underlying limestone about 50 ft (15 metres) below ground level. Arrangements were in place to use compressed air if needed to keep the river water out during excavation but the mud formed an effective seal around the caissons as they were sunk into the ground. After founding on good rock the hollow caissons were filled with mass concrete and the tops were joined by a pierhead of reinforced concrete to create the two boat-shaped piers, the tops of which are above high tide level.

Construction of the Viaduct Deck


Wye Bridge erection over railway


Wye Bridge – typical box erection

Erection of the viaduct decks was carried out by cantilevering the developing deck, from trestle to trestle, a distance of 64 m (213 ft), using a specially designed gantry that was slung under the completed deck and moved progressively forward as each new box was welded to the end. When each span was nearing completion, the next trestle was erected from above and the front end of the cantilever was lifted up onto the trestle, because each long length of cantilever would droop under its own weight due to the elastic nature of the steel box girders. The trestles had spherical bearings at the concrete pier level and line bearings at the top on which the deck unit rested. This arrangement allowed the growing deck to expand and contract longitudinally from fixed points in the middle of the Beachley peninsular and the Gwent abutment.

Construction of the Wye Bridge Deck


Wye main span (Gwent side) erection

The side span deck units for the bridge were erected in exactly the same way as on the viaduct although, at 87 m, the span was significantly greater than in the case of the viaduct (64 m). The extent of the drooping that occurred as the leading edge of the side span cantilevers approached the main span piers, though not unsafe, was quite apparent.  A temporary trestle was used just in front of the permanent trestle, to jack up the end of the cantilever.

A number of steps were taken, in advance, to prepare for the installation of the cable-stayed elements on the bridge. Special anchorages for the cables that would later be erected, were installed in the deck units that would be positioned over each of the back span piers. When these particular units were in place, they were pinned down to the piers on which they rested. This arrangement provided resistance to the uplift that would occur on the back span piers, as the two halves of the long main span of the bridge were being cantilevered out towards each other.

Erection of the deck units continued, as on the viaducts, past the main piers of the bridge and for three more units into the main span, on each side. At that stage, it was necessary to erect the two pylons on the centre-line of the deck, one over each of the main piers. Once this was done, further units were erected in two longitudinal halves to enable them to pass the pylons. Having passed the pylons, the two halves were brought together and welded longitudinally to make a typical box unit. The outermost box of each of these particular cantilevers would contain one of the main span cable anchorages of the bridge. At this stage, three of the eventual 20 strands in the completed cable stay system were hoisted to the top of the tower and draped loosely back towards the side span anchorage and also loosely connected into the main span anchorage box.

The next stage of erection was the most daring and critical operation of all and had to be completed in a continuous 3-day operation. The erection rig was moved forward and readied to receive one of the special deck units into which a main span cable anchorage had been installed. As the unit was moved onto the rig and lowered, ready for welding to the end of the completed steelwork, the rear ends of the three cables were pulled back, keeping in balance the longitudinal force on the top of the tower. The unit was then mated to the end of the cantilever and welding continued until sufficient strength was achieved to start tensioning the three strands which would begin to raise the end of the cantilever and relieve the maximum moment at the root of the cantilever that had been experienced earlier in the erection process.  Welding of the box-to-box joint and tensioning of the three strands continued without a break until a pre-determined level of tension was achieved.

The final stages of the construction of the bridge included the erection and tensioning of the remaining strands and the erection of two more boxes beyond the outer cable anchorage to achieve the half main span on each side of the river. When both sides were at this stage, a small gap had been allowed for thermal movements so that the two half spans had to be pulled together before the final joint could be welded.

Closure came in spring 1966 when the last two deck sections were rolled over the completed part of the Wye Bridge and lowered into place in the centre of the span.  The two half spans were pulled together and the joint, between them, welded. A continuous ribbon of steel was then ready to carry the then M4 across the Severn and Wye Rivers, finally fulfilling the aspiration first envisioned by Telford 143 years earlier.

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