Category Archives: Second Tier

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 the same way, the tension in the main cables supports the weight of the deck and traffic.  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, on 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 and the opened sections 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 Wye Bridge and Viaducts

828-C-105 Wye Lifting tower extension piece

Lifting tower extension piece- 4 hour closure

 

L,M&N. Tower Extensions and New Cable Stays
The most obvious work was to the cable stay system. 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.

 

 

This 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.

828-C-157 Wye new cables installed

Wye Bridge with new cable configuration

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. The new configuration has the added benefit that any strand can, if necessary, be replaced without closing the bridge.

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 the 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 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.

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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 transverse wind loading.

Single sheets of mild steel had been used in the construction of the tower walls, each sheet being stiffened, off-site, by the addition of vertical steel flanges 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 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 in internally and externally.

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 side. 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 handstrand 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.

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 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 is also available to be used to fight any substantial fire on the bridge.

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 Box Girder Problem

The background.
The failure of four 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 the works.

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Highway Bridge Loadings

Introduction.

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 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 later and 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.

WyeBoxErection

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).

WyeCantErection

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 the need arises. 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 the device used to build the Wye Bridge. However, in that case, the original box girders were deeper than on the Wye and the main strengthening was effected 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, which is absorbed into the bridge deck.

Cable stays are usually anchored to the deck symmetrically to either side of each pylon. This means that the horizontal forces on the bridge deck tend to balance out, although in doing so, they cause the bridge deck to be put into compression, especially close to the base of the pylon. And more significantly, the horizontal components of the tensions in the cables also tend to balance out where the cables are fixed to the pylons. The vertical components of the tensions act downwards on the pylons and are carried down to the foundations.

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Design Highlights

The designers of the Severn Bridge were well aware of the potential wind-induced problems in the design of the long Severn crossing. In the late 1950s, using an anemometer mounted on a mast 110 feet (33 m) high (the height of the proposed Severn deck) and an array of instruments on the then existing Severn Railway Bridge some 3 miles (5 km) upstream, a spectrum of local wind speed, direction and inclination over a front of about 300 feet (90 m) was determined.

SJ10-severn-rail-bridge-intact

This is where the wind sunrvey was carried out

With this information, in order to avoid the risk of aerodynamic instability so graphically illustrated at Tacoma Narrows, a 1/100-scale model of the Severn Bridge deck was tested in a wind tunnel at Bedford. At that stage the proposed design was to follow American practice with the stiffening girder made of deep, about 33 feet (10 m), open-lattice truss construction as had been proposed in the 1930s.

However, by the late 1950s, the Government had stipulated that the new Forth Road Bridge should take priority over Severn. The Forth bridge would, not surprisingly, be designed on the same principles as those being used on 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 with a stiffening girder of latticed (truss) construction.

There was a new development on the Forth Bridge in that the road deck was made of stiffened steel panels resting on the top chords of the trusses. These panels were light in themselves and the overall dead weight was also kept to a minimum by using 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.

DeckDev

Stages of Development of Deck Configuration

In the meantime, scale models of newer options for the deck of the Severn Bridge were being tested in a wind tunnel. One of these options was a shallower truss configuration, it became loose and was destroyed in rather spectacular fashion. Nevertheless, and despite the disappointment, the delays in rebuilding the model provided Freeman Fox with time to develop and then test a completely novel design for the deck.

This featured an “aerodynamic” hollow box cross section only about 10 feet (3 m) deep for the stiffening girder. This increased the flexural and torsional stiffness enormously and was considerably lighter than a conventional truss. The Severn deck, as built, was only about two-thirds of the weight of the Forth deck at about 12,000 tons. Also, the suspension system for the Severn Bridge incorporated inclined hangers, which were intended to increase the aerodynamic damping.

The fate that befell the bridge over the Tacoma Narrows showed the necessity for wind tunnel testing of the design for the Severn Bridge. The first 1/100 model was destroyed during testing. However, these two disasters led to a simple and revolutionary concept that showed British ingenuity in a most favourable light, internationally, and the Severn Bridge was a great boost to British engineering.

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. Freemans 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.

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 at the Forth 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 to avoid piecemeal improvements and, for 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 of the Severn and, in 1946, published a National Plan showing the first Severn 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 and 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 on which the road deck is carried. This stiff girder maintains the shape of the cable as live traffic loads cross the bridge. Without the stiffening girder distributing the live loads through the hangers to the cable the behaviour would be like the rope bridges of the South American Indians, in which the rope changes shape and the walkway distorts as people cross it.

There had been a collapse of the Tacoma Narrows Bridge in America in November 1940, only some four months after opening. The spectacular failure of that bridge has been shown many times and can be seen here http://www.youtube.com/watch?v=3mclp9QmCGs.

The stiffening girder for the Tacoma Narrows Bridge was novel for the time and was constructed from two deep I-sectioned girders, one beneath each of the main cables, which were joined by cross-girders that carried the road deck.

It so happened that the natural frequency of dead load, 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 shed from the deck by the wind blowing up the Tacoma Narrows Channel. In a 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 nearly 90º 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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The Background to the First Bridge

The growing clamour for a road bridge across the Severn Estuary

The successful completion of the Severn Railway Tunnel in July 1886 seemed, at the time, to be a lasting solution to the age old problem of carrying men and material across the lower Severn. However, within a few years, the advent of the internal combustion engine raised entirely new issues and fresh demands. Within a few decades, a very 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 motor car.  This network would, in time, include a new bridge over the Severn Estuary.

During the early thirties, County Councils were responsible for major roads. In 1934, Gloucestershire and Monmouthshire Councils jointly introduced a Parliamentary Bill, seeking powers to build a structure across the Severn at the English Stones. This proposal had a suspension bridge over the Shoots channel, together with stretches of viaduct to either side, very similar to the Second Severn Crossing that was eventually built in the 1990’s. However, the Great Western Railway objected to the proposal, ostensibly on the grounds of potential damage to the Severn Tunnel, and the Bill was rejected in 1936.

This bridge, had it been built in the thirties, would have stimulated the regional economy over the following 30 years, so that a second bridge would probably then have been needed rather earlier than the 1990s when the Second Crossing became available.  It is interesting to speculate where that second bridge would have been built and what form it might have taken.  A new crossing between  Aust and Beachley would, in those circumstances, involve considerable extra distance and, on that count, would have been heavily penalised in a modern economic analysis.  The structural form of the second bridge would have depended on the period in which was designed, bearing in mind that cable stayed structures require considerable computing power for their design and analysis.  The existing Shoots Bridge on the Second Crossing was ‘State of the Art’ when being built in the early 1990s.

While the above Bill from the two counties was being debated in Parliament, the Ministry of Transport was taking the decisive step, through the Trunk Roads Act of 1936, of becoming directly responsible for a specific network roads of national importance, thereafter known as trunk roads, so ending many decades during which the County Councils had been the dominant force in managing and developing the highway network. However, before any significant progress could be made under the new arrangements, the Second World War intervened.

The completion of the railway tunnel in 1886 had brought an abrupt end to the ferry service that had been the crucial element of the Bristol and South West Union Railway system for more than twenty years (see “History of Estuary Crossings” section). The residual demand for such a service would have been absolutely minimal but, in the decades that followed, the first cars started to appear on British roads and, as the trickle developed into a flow, it was only a matter of time before an enterprising individual established a car ferry.

BeachAustFerry

The Severn Princess with a full load approaching Beachley Slipway

The Old Passage Severn Ferry opened in 1926. The crossing was not a very pleasant experience for many, especially in the early years, and the 60 mile journey via Gloucester was still preferred by the faint-hearted. The reputation of the Old Passage between Aust and Beachley, with the second highest tidal range in the world, its 10 knot tides, its rip tides and its cross winds did not help. The ferry boats were relatively small, with flat bottoms and no keels, so that they could operate effectively at low tide. But they worked to a time table. Those involved have claimed that the ferry service carried about half a million passengers in its final year.

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Severn King loading at the Beachley Slipwa

Like its predecessor, this service was brought to an abrupt end when the Severn Bridge opened in 1966. There were three ships in use at the time, the Severn King, the Severn Queen and the Severn Princess and they all came forward to salute their nemesis before disappearing from the scene. Most of the crew found immediate employment connected with the bridge. The two older ships had come to the end of their useful lives but the Severn Princess was taken to Ireland, only to be rediscovered in a poor state some 33 years later. It was patched up and towed back to the Severn and is currently being restored by the Severn Princess Restoration Group in Chepstow.

For further details of the earlier background, see the “History of Estuary Crossings” section of the website.

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