Modern  Bridges  over  the  River  Severn

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Ail  Groesfan  Hafren - Second  Severn  Crossing

The most southerly bridge over the river Severn is the viaduct and cable-stayed bridge which carries the motorway M4 between Wales and England. It offers an alternative to the earlier suspension bridge, which carries the earlier motorway M4, now called M48. The designers made use of a large area of hard rocks on the western side of the channel, which are exposed at low tide, to enable a viaduct to be built. The main channel, called The Shoots, is spanned by the actual cable-stayed bridge.

The bridge is not far from the line of the Severn tunnel, which was a great feat of engineering, built from 1874 to 1886. Huge pumps were, and are, needed to remove water, and very large fans were installed to provide ventilation. The construction of the tunnel is described in "Track Topics - A Book of Railway Engineering for Boys of All Ages", by W G Chapman. This book also provides insights into some famous bridges of the Great Western Railway, and includes a drawing by W Heath Robinson depicting the assembly of Saltash bridge.

This new Severn bridge is quite close to the ferry crossing that was used by the Romans in the days of the empire, illustrating, as many Severn bridges do, that the number of good crossing points is limited, and that people will use them during long periods of time. There is a visitor centre near the eastern end of the bridge.  It offers video films, pictures, models, and descriptions of past and present crossings and local history.

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The bridge has high baffles on each side to deflect the wind. This greatly reduces the number of occasions on which any type of vehicle has to be banned from the bridge because of high winds. The transition from normal road to bridge is in fact so well done that it is easy to get well on to the bridge without realising it.

SSC7Y.jpg (42374 bytes)The large tidal range exerted a big influence on the construction work. Timing was crucial in operations such as floating out and raising sections of the bridge. Positioning  of floating equipment was achieved using signals from navigational satellites. The picture at left was taken at a late stage in construction. On this occasion the tide was low, revealing the the English Stones, a large area of rocks on the eastern side of the channel. The cable-stayed bridge was complete, and the last few approach spans remained to be added.

The approach spans are based on post-stressed hollow beams, made from 3.5-metre match-cast sections which were floated out on a barge at high tide.

There is an interesting visitor centre at the end of Shaft Road, off Green Lane, Severn Beach, near the east end of the Second Severn Crossing. There are video films about the building of the new bridge.  There are models of bridges. There are illustrations about the bridges and about the history of the area. A 24-page booklet is available, describing the construction of the new bridge.  It is a good idea to telephone the centre before making a visit, to make sure that it is open.  (01454 633511)  From the visitor centre it is a short walk to the Binn Wall, from which there are views of both bridges. There is also a good visitor centre near the Clifton Suspension bridge near Bristol.

The next picture shows a small part of the Severn cable-stayed bridge. The picture has been tilted and compressed horizontally to show that, although the cables look straight, they sag. There are few perfectly straight lines in engineering, with the possible exception of verticals. Every part that is not vertical will sag a little, though of course "rigid" struts will not deflect visibly. The truth is that there are no rigid bodies anywhere in the world. You can also see that two of the cables (the fifth on each side) are not evenly spaced with the others.

Here are some facts and figures about the new bridge. The total length is just over 5000 metres, with a main span of 456 metres in a main bridge of 947 metres length.  The number of approach spans is 45, divided between the Welsh end, 22, and the English end, 23. The bridge was built from 1992 to 1996.  Click here  for more about cable-stayed bridges. Click here for a student factsheet.

 

Pont  Hafren - Severn  Suspension  Bridge

The second bridge over the river Severn is the suspension bridge from Aust to Beachley, which carries the original M4, now called the M48. This bridge was the first to use two new ideas to reduce aerodynamic instability - a streamlined deck section, and inclined hangers.

The hangers provide a measure of triangulation, which increases the rigidity of the bridge. But the main innovation was the cross-section of the suspended span and the use of a box construction in a suspension bridge. The thin deck is well seen in the first three pictures below. 

The transition from truss decks to plate girder ones was analogous to the transition from biplanes to monoplanes in the aircraft industry, and in both cases, flutter was an ever present threat. Sometimes flutter had catastrophic results. The Severn bridge deck, analogous to the long box structure of a modern aircraft wing, was the answer to the problem.

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Most of the hangers have damping devices near the bottom - and around the quarter-span area and the middle of the side-spans there are dampers near the middle of the hangers as well.

After the collapse of the Tacoma Narrows bridge in 1940 there was naturally a desire to make sure that no such thing could happen again. (Please click here for page on Oscillation for more information.) Many subsequent designs included deep braced trusses to make them rigid, notably the Mackinac Straits bridge of 1975.

The streamlined shape using a closed box was adopted by Freeman Fox and Partners after successful wind-tunnel tests. The length of the span is 990 metres. The design allows the deck to be made of rigid boxes, which can be floated out to the site before being lifted into place. This type of construction was used later for a bridge over the Bosporus at Istanbul, the Lillebelt bridge in Denmark, and the Humber bridge in Northern England, which had a record span when constructed.

Severn1.jpg (29757 bytes)The towers are based on hollow rectangles. The ideal shape for a strut is a cylindrical tube, but the expense and complication mean that such a shape is not often used in this application.  The plates of the towers are stiffened inside by vertical and horizontal flanges, which form rectangular cells.  The position of these can be seen when the sun is almost in the plane of a plate.  The towers were strengthened recently, to support greatly increased loads.  Many tubes were inserted inside them, and stressed after completion, to take some of the weight.

 

SSC1YSmall.jpg (77628 bytes)SSC1YLarge2.jpg (62034 bytes)In these large JPEGs you can see on the right-hand side the pylons of the cable-stayed bridge over the river Wye. The Wye joins the Severn close by.  You can also see the recently added wind deflectors where the towers meet the deck of the Severn bridge. These triangles are reminiscent of the triangles at the front of the fin on many aircraft, where they prevent the aerofoil from stalling.

AeroTop.jpg (54771 bytes)This picture shows a part of the deck section. This top view shows the steep slope leading from the footpath and service road up to the main carriageway. Though the upper slope is steeper than the lower one, seen in the picture at right, it is not so high.

Severn5X.jpg (23685 bytes) As a result, the aerodynamic force is downward rather than upward. This is beneficial, because the cables can generate the extra force needed to resist it. But an upward force could only be resisted by the weight of the deck, which of course cannot change in response to varying forces. This is the mechanical equivalent of a transistor with a resistive load instead of a second transistor. The transistor can pull down strongly if turned fully on, but the resistor is limited in its current pulling power.

 

 

Anemometer.jpg (21423 bytes)Anemometers on the bridge measure and relay the wind speed to the control room, which also receives views from cameras mounted on both big bridges.  When the wind speed reaches a certain value, high-sided vehicles must slow down, and a higher wind speed they must use another route, either the cable-stayed bridge M4, or the M5 and M50.  At very high wind speeds, all vehicles may be banned from the bridge.  This problem is avoided on the later cable-stayed bridge by the provision of slotted barriers along each side of the deck.

After many years of use, with greatly increasing traffic, the Severn bridge was found to need repairs and strengthening. The towers were strengthened by the insertion of steel tubes inside, installed in such a way as to carry the correct proportion of the load. The deck was repaired, and the pictures below show changes in the hanger attachments, giving one more degree of freedom, and to the handrail supports on the main cables, as well as to the colour of the bridge.

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In addition, some extra panels were added near the towers to influence the airflow near the towers. The panels are composed of many slats. The larger panels have electric motors which can be sued to change the angle of the slats. These slats and a motor can be seen in the pictures below.

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Pylon1.jpg (27872 bytes)Not only do the road and railway have to cross the river Severn - so does electrical energy. This picture shows the east pylon of the power grid crossing, built in the estuary south of the the bridge, near Aust. The two large pylons are backed up by smaller pylons between them and the normal grid system. The air was rather brown on the day that this photograph was taken.

There is an interesting visitor centre near Severn Beach, near the east end of the Second Severn Crossing. There are video films about the building of the new bridge. There are models of bridges. There are illustrations about the bridges and about the history of the area. From the centre it is a short walk to the Binn Wall, from which there are views of both bridges. There is also a good visitor centre near the Clifton Suspension bridge in Bristol.

The Severn suspension bridge has a main span of 3240 feet, with 1000 foot side spans. The towers are about 445 feet high above mean high water level, and carry about 8320 wires, with a total length of 18000 miles, compressed into cables of diameter 20 inches.  

A little to the west of the  suspension bridge a cable-stayed bridge carries the road over the river Wye. This bridge is based on a box girder with a main span of 770 and side spans of 285 feet. Each half of the bridge is held up by a single 20-strand cable.

 

Firth of Forth Road bridge - for comparison

ForthRoad.jpg (25523 bytes)Just before the Severn bridge was built, the Firth of Forth road bridge was completed. It was made too soon to receive the aerodynamic treatment, and so it has the previously popular truss construction. It has a main span of 3300 feet, and two side spans of 1340 feet.

A model was tested in a wind tunnel at the National Physical Laboratory. Gaps were left between the two main carriageways, and between these and the cycle tracks. This allows air to flow between the upper and lower surfaces, helping to weaken any lift that might develop. Construction took from 1958 to 1964.

 

West  Channel  New  Bridge  at  Over

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This steel bridge superseded Thomas Telford masonry arch in 1974.  It is based on haunched beams, and carries the A40 over the west channel of the River Severn near Gloucester. The second and third pictures, taken in time of flood, show the transverse beams under the middle of the roadway.

A40Viaduct.jpg (56632 bytes)West of the new steel bridge the A40 is carried by this viaduct. The picture shows the typical wet ground west of Gloucester. This is the reason that Gloucester lies wholly on the left or east side of the river Severn. The names of several places in this area have alternative pronunciations.  Over can be pronounced Oh-ver or Ooh-ver. Evesham and Tewkesbury also have two pronunciations. It is said that Cirencester was for a time pronounced Cisister, and Prinknash still does rhyme with Greenwich.

 

On the other hand, in Lancashire, Claughton, Claughton and Claughton are pronounced Claffton, Clighton and Clawton, but you can't tell which is which from a map.

 

East  Channel  A40  Bridge

A40.jpg (45787 bytes)This is a multi-span concrete viaduct carrying the A40 northern Gloucester bypass over the east Severn channel and flood plain west of Gloucester. The view is to the west, and the road sign refers to the roundabout  on Alney Island, where the A40 meets the A417. The viaduct appears to be a continuous beam, probably of box construction.

A large fraction of the roads and railways in this area are on viaducts, because of the frequency of flooding, so this is an expensive area for transport construction. Gloucester, like Worcester. lies entirely on the east side of the River Severn, because of the difficulty of building on the low lying west side. The main span, over the river, is built on four beams.

 

A422  Road  Viaduct  South  of  Worcester

The river Severn passes under this viaduct at the eastern end; east of the river span there is only a span over the steep bank of the river. All the other spans cross the flood plain in the manner so familiar in the region from Worcester to Gloucester.

In his book, Fritz Leonhardt says that undercut piers such as  these have a favourable effect with fast flowing flood water. The plaque is found to the north of the car-ark on the west side of the A38, just south of the eastern approach to the bridge.

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Sabrina  Foot-Bridge  at  Worcester

A plaque, illustrated below, on a wall at the north-west end of the road bridge at Worcester, refers to the legend of Sabrina, and the possibility that the name "Severn" is derived from Sabrina, or Sabern.

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This beautiful and interesting little footbridge is found to the north of the railway bridge in Worcester, joining Le VĂ©sinet Promenade to the east bank of the river. It is an asymmetrical cable-stayed bridge with one tower. An ingenious feature is the use of hinges where the cables join the deck. This allows the use of rigid trusses without the necessity for extremely precise setting of the cable lengths. With a through truss, inexact cable lengths would produce uneven tension in the cables, and unwanted bending stress in the deck. The web-page about indeterminacy discusses this topic in more detail.

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Highley  Bridge

HighleyB.jpg (44327 bytes)From the Severn Valley Country Park you can find this bridge, a little to the east of Highley. It is impossible to photograph properly in summer because of the usual problem: trees. The bridge comprises two haunched concrete beams. It is in a poor state, and you are not allowed to walk underneath it, though you can walk across it, keeping to the middle of the footway.

 

Hampton  Bridge

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A short distance to the north of the riverside car-park near Hampton Lode, an interesting bridge crosses the Severn. Two massive looking tubular welded blue arches carry a small roadway suspended below.

The arch looks over-engineered, until you realise that the great arches are water pipes. The curved plates are arranged so that no more than three meet at any point.

This is a very elegant solution to the problem. The arch brings the pipes down to the supply and outflow pipes in a natural way.  It provides rigidity, and allows for expansion. A suspension bridge with horizontal pipes would be less rigid, and would not allow for expansion. It would require extra bends at both ends to connect the span to the pipes. Note also how the concrete abutment supports the stairway. The designer of this can feel very well satisfied.

Like a number of other lower Severn bridges, this one has approach spans over the flood plain.

 

Bridgnorth  Bypass - A458

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South of Bridgnorth the A458 Bridgnorth bypass goes over the river Severn. The road is on a slight curve, both vertically and horizontally. The main bridge consists of a haunched concrete beam. The bridge is very plain, the only concession to appearance being the ends of the main piers, which are pointed.

But in fact it is a very pleasing structure indeed, well adapted to the site on the small flood terraces.

BNorthCS.jpg (17900 bytes)From here you can see a cable-stayed footbridge in Bridgnorth.

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Jackfield  Bridge  in  Coalbrookdale

Not far downstream from the famous Ironbridge, the Severn is crossed by a much newer structure - a cable stayed bridge.  Here are some pictures. As the plaque states, an earlier bridge, known as the free bridge, because the other bridges were tolled, was built in 1909, in the early days of reinforced concrete. The famous name of Hennebique appears on the plaque.  In 1994, an asymmetrical cable stayed bridge was built as a replacement. The second picture shows a small segment from the old bridge.

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Cable stayed bridges are apt to look somewhat angular and highly stressed. This design makes no attempt at disguising anything: all the parts are so clearly visible that the bridge could be used as a textbook example. In the last picture we see the cable anchorages, the terminal deck pin, and the foot of one strut, which is tapered to the point where it is more or less a pivot. The last picture but one shows that the tower, except for the upper segments, is not in a vertical plane.

The tower is essentially an A-frame or shearlegs, but instead of continuing to an apex, it is truncated, rigidity being provided by the ring, which also supports the two vertical cable supports. These supports continue above the cable attachments, tapering to provide a neat finish to the tower, which otherwise might have looked somewhat stubby. If you approach Ironbridge from the east, intending to park in the Ironbridge car park, you will cross this bridge. You can walk easily from the Ironbridge to the Jackfield bridge along the south bank of the river Severn. On the way, you may see this exhibit -

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The thick orange deposit in the bed of the stream shows the presence in the rocks and soil of the iron that was the basis of industry in Coalbrookdale. In the earth's crust, iron makes up about 4.8 % by weight, or 1.8 % by number of atoms, though deep inside the earth, iron and nickel are probably the most common elements by far. Dense metals like iron perhaps owe their existence near the surface of the earth to their combination with light elements such as oxygen. But why is there so much iron in the earth as a whole? A glance at a table of nuclear binding energies reveals that the region of minimum energy lies broadly around the position of iron.  

But that doesn't mean that it is easy to make heavy elements. The search for controlled nuclear fusion has not yet resulted in a commercially viable means of releasing energy from fusion, though great progress has been made in prolonging the life of the necessary hot plasmas. Temperatures above 100 million degrees are needed. The existence of heavy elements at all is perhaps the result of supernovae, exploding with great energy.

 

Across the river you will see the ruined Bedlam furnaces.

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Jackfield  Memorial  Bridge

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This bridge can be found downriver from Jackfield Bridge.  It is a bowed Pratt truss.

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Links to Other Web-sites and Books

Visit Severn bridges book for details of a very useful book by Chris Witts, who has spent much of his working life on the River Severn. This book, called "A Century of Bridges", includes information, a grid reference, and a drawing by the author for every bridge, even including notes about demolished bridges and some history. A Century of Bridges, ISBN 0 9532711 0 2, is published by River Severn Publications, Gloucester.

Another very interesting book by the same author is called "Along the Severn from Source to Sea", ISBN 1 873877 31 5. This book is published by Reardon Publications, Leckhampton, Glos.

Severn bridges - free beautiful pictures

Second Severn corssing - Laing

Second Severn crossing - Buckland and Taylor Ltd

Second Severn crossing

Second Severn crossing - construction pictures

Second Severn crossing - aerial photograph

Second Severn crossing - evening photograph

Severn Cable-stayed bridge - see Second Severn Crossing

Severn suspension bridge - panorama

Severn suspension bridge - night photograph

Severn suspension bridge - big panorama

Severn suspension bridge - tower inspection - photograph

Severn suspension bridge - evening photograph 1

Severn suspension bridge - evening photograph 2

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