Modern
Bridges over the River Severn
7th
September 2000 Back to Severn Bridges
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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. |
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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. |
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 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 |
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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. |
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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.
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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. 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. |
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| 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. |
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|
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. |
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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. This was analogous to the transition from biplanes
to monoplanes in the aircraft industry. |
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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.
|
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Fritz Leonhardt, in his book "Bridges" (Architectural Press, ISBN 0 85139 764 6),
describes how he had wind tunnel tests done at the National Physical Laboratory,
after which the designers of the Severn Bridge decided to adopt the streamlined
span. 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. |
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| 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.
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
close by. You can also see the recently added wind deflectors
where the towers meet the deck of the Severn bridge.. |
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 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.
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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.
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 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.
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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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|
 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.
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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.
|
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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 by a single 20-strand cable. |
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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.
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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
|
 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.
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|
 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.
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East
Channel A40 Bridge
|
 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.
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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.
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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. |
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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. |
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.
| This beautiful and
interesting little
footbridge is found to the north of the railway bridge in Worcester, joining Le
Vésinet 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. |
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This allows the use of rigid
trusses without the necessity for extremely precise setting of the cable
lengths. With a through truss, imprecise cable lengths would produce uneven
tension in the cables, and unwanted bending stress in the deck. |
Highley
Bridge
 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. |
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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
| 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.
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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 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.
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Bridgnorth
Bypass - A458
| 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. |
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But
in fact it is a very pleasing structure indeed, well adapted to the site on the
small flood terraces.
 From here you can see a cable-stayed
footbridge in Bridgnorth.
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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.
|
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