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 photographs below. 

The change from trussed suspension bridge decks to plate girder ones was analogous to the transition from biplanes to monoplanes in the aircraft industry, especially considering that both bridge decks and wings have suffered from flutter, in some cases with disastrous results. The box construction reduced the tendency to flutter to safe levels.

This bridge has the longest bridge span across the river Severn, but not the longest span. That distinction is held by the electrical cables that can be seen in some of the photographs below.

SevernFerry.JPG (76036 bytes)The building of the bridge resulted in the closure of the ferry which plied between Aust and Beachley.


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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. This reduces the Aeolian harp effect.

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 a page on Oscillation for more information.) Many subsequent designs included deep braced trusses to make them rigid, notably the Mackinac Straits bridge of 1975.

Freeman Fox and Partners, after conducting successful wind-tunnel tests, adopted a streamlined cross-section for the deck, based on closed boxes.

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.


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.

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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. These hollow structures and the flanges can be likened to the bones of human legs, which comprise an outer layer of compact bone, and an inner structure of cancellous or spongy bone, which consists of many small struts.  The shapes of bones and their are exceedingly subtle, reflecting the variety of stresses and attachments that they have to bear. To design an artificial structure to such a degree of refinement would be incredibly costly, and would achieve nothing. Whereas a minute small degree of superiority can be significant under natural selection, where the type of engineering costs are completely different from ours, human engineering refinement is halted by cost and time at a much simpler level.

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.


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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 just downstream of the bridges. You can also see the recently added wind deflectors where the towers meet the deck of the Severn bridge.


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.

DeckTower.JPG (66974 bytes)The decks of many suspension bridges are not connected to the towers, because they are already supported by the cables. Here we see a shroud protecting pedestrians from the gap. The slope of the aerofoil section is visible. The flow of air over the top is somewhat spoiled by the presence of traffic, fences and cable attachments, but the important flow is on the underside, because of the requirement for overall downward lift. For the same reason, attachments to aircraft wings, such as engines, fuel tanks and weapons, are almost always on the underside, except in special cases, such as the B-2 (to reduce downward infra-red radiation) and the An-72 (STOL performance). When the wind speed rises above a specified level, the bridge is closed to high vehicles, not because it is inherently unsafe, but because these vehicles become difficult to control. The new A48 Severn crossing, a cable-stayed bridge, has high fences, and is rarely closed.

The picture includes one of the viscous dampers which absorb energy to reduce the tendency of the hangers to oscillate. When the bridge was refurbished these dampers were replaced by a different design.

The transition from trussed spans to aerodynamic box sections is somewhat analogous to the transition from biplanes to monoplanes in the aircraft industry. In fact, some biplanes are still produced, as they are very sturdy. Biplane construction gives the Pitts Special a rigid construction and a large wing area with a small moment of inertia, making it ideal for aerobatics.


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 power. This picture shows the east pylon of the power crossing, built in the estuary south of the the bridge, near Aust. The air was rather brown on the day that this photograph was taken.


Any large structure will deflect significantly during construction, requiring measures to compensate. The final distribution of forces in a large suspension bridge are clearly different from the situation when the deck has not yet been placed, or when the cables are have not yet been hung. In the case of the Severn suspension bridge, the towers were pulled back before the cables were hung;  the deflection was more than two feet/0.6 m at the top. The diagram below shows how this was done (not to scale). By using a small deflection of the cable from the top of the tower, a large tension could be induced in it with a relatively small tension in the cable from the foot.

This principle could be used in older times to lift very heavy loads. The load was pulled sideways, and then the rope was released. Because of the inertia of the load, there was time to take in the slack in the main rope, which was passed several times around a capstan.

The ratio of the tensions in the rope was about equal to the angle from the vertical of the main rope in radians. Winding a rope around a capstan reduces a force proportionally to the exponential of the angle of winding, and proportionally to the coefficient of friction.

The Severn Bridge represents the successful completion of a trend that had begun early in the 20th century, when the depths of suspended spans were progressively less, until the catastrophic oscillation of the Tacoma Narrows bridge changed everything. The slender plate girder design was abandoned, to be replaced by the deep truss which provided rigidity. The diagram below, containing data for a number of large bridges, illustrates what happened. TG means truss girder, PG means plate girder, and AB means aerodynamic box girder.

As a result of the Tacoma Narrows collapse, a truss was added to the Bronx-Whitestone bridge to prevent the oscillations which had happened from time to time. The trend in slenderness which culminated in the Tacoma Narrows bridge recurred again in the Severn bridge. The difference was the substitution of the aerodynamically streamlined deck with slight negative lift for the bluff plate girder which had generated vortices. Moisseiff had had the right ideas about dimensions, but had used unsuitable technology to achieve it.

After the Tacoma Narrows event, the plate girder fell out of favour, and deep trusses were the norm. Only in 1966 did suspension bridges become slender again, using the aerodynamically streamlined deck. The slenderness of the first Tacoma Narrows bridge was approached, but never significantly exceeded. Although the George Washington span was also very shallow, the width of the deck was very much wider than that of Tacoma Narrows. The same was true, to a lesser extent, of the Bronx Whitestone.  The Tacoma Narrows crash probably required several contributory causes to be present - very shallow deck, very narrow deck, plate girders, broken stiffening stays, and possibly coincident resonant frequencies.


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. The manager kindly gave permission for the map to be shown here.  There are video films about the building of the new bridge. There are models of bridges. The centre is well worth a visit. Check by telephone (01454 633511) to check that is open, before visiting. 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 good 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/988 m, with 1000 foot/305 m side spans. The towers are about 445 feet 136 m 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/0.51 m.  

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 feet/235 m and side spans of 285 feet/87 m. Each half of the bridge was held by a single 20-strand cable when built. but the bridge has recently been modified.  See bridge data for details of both bridges. Bridge changes  Bridge changes