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Engineers are stretching their expertise to design and build long bridges
more cheaply and quickly than ever before. Next week, barely six years after
it was first thought of, one of Europe’s longest bridges opens across the
River Thames at Dartford, about 30 kilometres downstream of the City of
London.

The Dartford River Crossing, which is designed to relieve a bottleneck
on London’s orbital motorway, the M25, puts the first major road bridge
over the Thames since Victorian engineers completed Tower Bridge in 1894.
The bridge accounts for about three-quarters of the cost of the crossing,
which includes two long viaducts, stretches almost 3 kilometres and has
a price tag of just £120 million.

The crossing is also the first major highway in Britain to be privatised.
This factor alone, its owner claims, ensured the development of the most
economical structure in the shortest possible time. Going by similar advances
elsewhere in the world, however, technical expertise rather than commercial
nous seems the more significant factor. Public authorities in continental
Europe, North America and Japan are proving equally adept at getting more
value for money from their bridge engineers.

At the heart of this revolution in bridge engineering is a class of
structure, known as the cable-stayed bridge, that has long been the poor
cousin of the traditional design for lengthy spans, the suspension bridge.
Both are forms of suspended crossing, relying on cables above them for support
rather than columns below. Although suspended crossings – and suspension
bridges in particular – are more complicated to design and build than bridges
supported on columns, they can provide long, uninterrupted spans over terrain
that is difficult to build on or where access must not be disrupted.

Cable-stayed bridges, with their road or rail decks strung up by diagonal
ties to vertical pylons, are structurally crude and often look aesthetically
inelegant. Suspension bridges, by comparison, provide support gracefully
and apparently without effort. They rely on two large cables draped between
tall towers, one at each end, from which the deck simply hangs, suspended
by varying lengths of steel rope. Though suspension bridges have the greater
capacity to stretch farther, they are much more costly.

Nevertheless, until the mid-1980s, spans of more than 350 metres or
so were considered the province of suspension bridges. Although cable-stayed
bridges with main spans breaking the 400-metre barrier had been built, engineers
– a breed renowned for their conservatism – tended to regard these structures
as daring exceptions.

This attitude changed when Peter Taylor, a bridge engineer living in
Vancouver, designed a 465-metre cable-stayed span to support a six-lane
highway over the nearby Fraser River. The impetus was the need to cut costs
to a minimum to win a competition for the design and construction of the
crossing, known as the Annacis Bridge. Taylor, vice president of a local
firm of structural engineers, Buckland & Taylor, drew on existing knowledge
about the design of suspended crossings and added some innovations of his
own. Most notably, he introduced long cable-stayed bridges to the ‘composite’
deck. In this type of construction, the deck comprises a concrete slab on
top of structural steelwork, tied together so that they act as one material
to transfer loads to a crossing’s main supports. While others had been thinking
about using the technique, no-one had done it except on much shorter cable-stayed
spans. The only comparable structure designed with a composite deck is the
Second Hooghly River Bridge at Calcutta, with a cable-stayed span stretching
457 metres. Construction of this crossing, which began in 1974, is still
unfinished, however. Political rather than technical problems are blamed
for the delay.

Composite decks are a common feature of bridges supported on columns.
They are a compromise between concrete, which is cheap but heavy, and steel,
which is light but expensive. But for longer spans, where the problems of
design and construction are more acute, engineers had been reluctant to
mix materials. Instead, they had played safe with a more expensive solution.
Mostly they had chosen steel because large concrete decks are costly to
support and look ugly.

‘Bridge engineers tend to get into ruts,’ says Taylor. Between 1984
and 1986, he broke new ground when he saw through the construction of the
composite deck for the world-beating Annacis Bridge. ‘Until then, major
suspended crossings were either all concrete or all steel.’ Since then,
composite decks have become widely accepted for cable-stayed spans. What’s
more, the apparent success of the Canadian bridge seems to have encouraged
engineers to look again at their designs and see how far they can go.

Some engineers are now convinced that cable-stayed spans could stretch
to more than two-and-a-half times the current maximum length, which would
take them to 1200 metres or so. This is something that once only the German
pioneers of the form, who gained their expertise putting up bridges cheaply
and quickly in the aftermath of the Second World War, would have dared suggest.
After all, the main span of the world’s longest crossing, the 10-year-old
suspension bridge over the Humber in northeast England, covers just 1410
metres.

It is this change in attitude that helped British engineers come up
with their cut-price solution for bridging a broad stretch of the Thames.
The two viaducts of the Dartford crossing, on the north and south banks
of the river, are linked by a cable-stayed bridge with a main span of 450
metres – the longest of its type in Europe. The crossing’s four lanes will
carry southbound traffic, while northbound vehicles will drive through a
pair of existing tunnels nearby. The new owner of this stretch of motorway
– a consortium of two banks, an insurance company and the construction firm
that designed and built the crossing – has a lease from the Department of
Transport to operate the link for 20 years or until the tolls it collects
have met its costs, whichever is the shorter. The consortium reckons it
should have recovered its capital, with interest, within just 14 years.

Going for the record

Such commercial optimism reflects the economies achieved by the consortium’s
bridge builders, but these engineers are not alone. For a couple of months,
along with a similar bridge in Bangkok, the new crossing at Dartford will
be the second-longest bridge of its type in the world, behind the Annacis
Bridge. But, in December, Japanese engineers expect to extend the world
record by 25 metres with the completion of a 490-metre cable-stayed span
for the Ikuchi Bridge between the islands of Honshu and Shikoku. Around
the same time, Norway’s public roads administration is due to finish the
Skarnsundet Bridge across a fjord about 60 kilometres northeast of Trondheim,
with a main span of 530 metres.

Though Norway’s record looks set to stand for more than two years, French
engineers have already plotted its total eclipse. By mid-1994, the French
public highway authority plans to have spanned the mouth of the Seine at
Honfleur in Normandy with a cable-stayed deck stretching 856 metres. The
Normandy Bridge promises to be such a giant leap in the size of cable-stayed
bridges that it has provoked considerable debate throughout the international
construction industry. While many engineers acknowledge that it is possible
to design a cable-stayed bridge of this length, few accept it as a practical
solution, and will not do so until the project is finished and its accounts
audited.

Among the greatest problems of building suspended crossings is the difficulty
of temporarily stabilising their unfinished decks during construction. As
the two halves of the Normandy Bridge approach each other at mid-span, engineers
will have to support a pair of flexible, giant wands of concrete and steel,
each cantilevering more than 400 metres from pylons on opposite sides of
the river and weighing up to 8400 tonnes.

Construction of the bridge’s foundations began in September last year
after lengthy pre-contract negotiations between the highway authority and
a consortium of France’s leading construction companies, which had been
awarded the job. The two parties were concerned about how they should share
the financial and technical risks associated with the project.

Since then COWIconsult, a Danish engineering firm that has been advising
the builders of the Normandy Bridge, has designed an even longer cable-stayed
bridge. The proposed crossing for the Great Belt Link, a project to connect
the islands of Zealand and Funen in Denmark and currently Europe’s biggest
construction job, has a main span of 1204 metres. According to Anton Petersen,
a senior engineer with the firm, the design was shelved only because the
client belatedly decided that navigation access to the Baltic Sea, through
the waters of the Great Belt, demanded a clear span of 1600 metres. A crossing
of this length, he says, is still most economically provided by a suspension
bridge.

The cancellation of the Danish cable-stayed bridge means that Japan
is set to challenge the world record at the turn of the century. Though
Japanese engineers were late to join the specialist field, they made up
for lost time over the past decade, building a series of cable-stayed spans
up to 420 metres long. By 1997, they are due to have completed the Meikouchuou
Bridge in Tokyo Bay, with a 590-metre span. And within another two years,
if work starts on schedule next year, they expect to have finished the Tatara
Great Bridge between Honshu and Shikoku. The main span of this cable-stayed
bridge will stretch 890 metres.

So how have bridge engineers convinced themselves that cable-stayed
crossings are ripe for exploitation? There have been no remarkable developments
in the way they analyse structural designs, nor significant upheavals in
the methods they use to build bridges. There have been no new discoveries
in materials science that could benefit them although, according to a report
in June 1988 in the professional journal New Civil Engineer, some see carbon
fibre being used towards the end of the next century to build suspension
bridges with main spans stretching more than 8 kilometres.

‘One of the most distinctive features of civil engineering, in comparison
with other fields, is that designs have to be carried into effect straight
from the drawing board,’ notes Tom Wyatt, reader in structural design in
the department of civil engineering at Imperial College, London. In a paper
last year marking the 50th anniversary of a notorious collapse of a suspension
bridge over an inlet known as Tacoma Narrows in Washington state, he adds:
‘It is sometimes possible to test elements; it is very rarely possible to
test complete structures at full scale. In consequence, innovation is fraught
with uncertainty, and if progress is to be made, a client must be persuaded
to buy an article unseen.’

The construction industry says it has been able to extend the boundaries
of bridge technology by getting its designers, builders, researchers and
computer specialists to work together more closely. In the case of the Dartford
bridge, one company, Trafalgar House, not only designed and built the crossing,
it also helped to pay for the job. This overturns the traditional approach
to civil engineering contracts in Britain where the interests of the client
were considered to be best served by separating designers, who want the
most effective and economical solution, from builders, who want to get the
job done as cheaply and as quickly as possible. With experts in different
disciplines working on the same side of the contract, says the industry,
discussions about what can and cannot be done are more open and teams are
more willing to find new ways of doing things.

Technological barriers have also been broken down as firms have traded
their burgeoning experience worldwide. The German pioneers of cable-stayed
bridges, the late Hellmut Homberg and his arch rival Fritz Leonhardt, devised
or checked the design of many of the greatest cable-stayed spans. Homberg
scrutinised the Annacis Bridge for British Columbia’s Ministry of Transportation
and Highways, and designed the Dartford River Crossing for Trafalgar House,
while Leonhardt’s firm helped COWIconsult to come up with its 1204-metre
cable-stayed span. Taylor’s firm is advising the builder of Britain’s proposed
second crossing of the River Severn, a cable-stayed bridge with a main span
of 456 metres on which work is due to start next year. In April this year,
Japan invited French and Danish specialists to discuss their work at a seminar
in Fukuoka: a paper on the design and construction of the Normandy Bridge
was the keynote address.

Engineering’s recurrent nightmare in its pursuit of a longer cable-stayed
crossing is the infamous collapse in November 1940 of the Tacoma Narrows
Bridge. Just four months old, the bridge came down after rippling and writhing
for about an hour in winds blowing at only about 18 metres per second (40
miles per hour). Such lengthy ‘death throes’ meant that no one was injured;
they also enabled a spectacular filmed record of the disaster to be made.
Engineering students are taught that the structure collapsed because the
wind blowing across its deck caused the bridge to vibrate until it was twisting
and bending at the same time. The main problem was aero-elastic instability,
or ‘flutter’ as it is sometimes known, which caused the moving structure
to generate its own aerodynamic forces that reinforced the motion, particularly
twisting. These ‘self-exciting’ oscillations were triggered when the frequency
of the wind matched the bridge’s natural frequencies in torsion and bending,
which were determined by its geometry and physical properties. However,
the mechanism of the collapse is still not fully understood.

While many engineers are not convinced that anyone will ever know for
certain what brought the bridge down, they are sure that the structure’s
biggest fault was its torsional flexibility – wind and traffic loading could
easily twist the main span, which was shallow and narrow and stretched well
over 800 metres. Also to blame was the fact that the deck’s supporting vertical
sheets of steel, known as plate girders, presented a flat face to the wind,
which could thus impose a regular, fluctuating lateral force on the crossing.
The answers were apparent: you had to stiffen the deck or streamline its
transverse profile, preferably both.

Taylor, whose Annacis Bridge is sited just 250 kilometres north of Tacoma
Narrows, reviewed the older bridge’s failure during the early stages of
his record-breaking design. He had little choice: ‘There was a realistic
chance of aerodynamic instability,’ he recalls. The review persuaded him
that he could get away with using steel plate girders, similar to those
used for the ill-fated Tacoma Narrows crossing, providing they were shallow
in relation to the deck’s width. The advantage of plate girders is that
they are simple to fabricate and erect, thus helping to keep down costs.
For him, the main fault of the Tacoma Narrows Bridge was the deck’s small
width-to-depth ratio, which induced what he describes as a ‘stubby airflow’.

Extensive wind-tunnel testing indicated that the Annacis Bridge, wide
enough for six traffic lanes as opposed to the two that the Tacoma Narrows
Bridge had space for, would perform like an aeroplane wing. Winds blowing
at up to 50 metres per second (112 miles per hour), which was determined
to be the site’s most testing wind condition, would be guided around the
structure. The only qualification determined by the model tests was that
steel walkways, cantilevering 1.6 metres from the sides of the bridge, were
needed to improve the streamlining of the deck’s transverse profile at its
leading edges. One result of these studies is that the two bridges have
similar natural frequencies, but the width-to-depth ratio of the Annacis
Bridge is two-and-a-half times as great as that of the original Tacoma Narrows
Bridge.

For Norway’s record-breaking Skarnsundet Bridge, which is also wide
enough for only two lanes of traffic, the deck is being built entirely of
concrete to reduce its flexibility and compensate for its slenderness. There
were early doubts about whether a cable-stayed bridge could be built at
all, recalls Svein Hovland, who is in charge of the project. And engineers
everywhere are waiting to see whether the decision to go ahead pays off.

The Canadian bridge, however, is still very slim by any standards and,
to help to counter the flexibility of its deck, Taylor resorted to making
the most of the composition and the arrangement of the main cables. Because
they bind the deck directly to the supporting pylons, these cables can have
a considerable influence on a bridge’s overall stiffness.

Over the 40 years that engineers have been building large cable-stayed
bridges, little has changed in the way cables have been made but a variety
of configurations has evolved. The simplest configuration of cables relies
on a pair of tall pylons, sited in the same vertical plane through the centre
line of the bridge, with the deck strung up to each one by a pair of diagonal
cables, which run in opposite directions along its length. Erskine Bridge
over the River Clyde near Glasgow, opened in 1971, is of this type. From
the side, the impression is of a pair of giant wire coat hangers. But as
main spans have lengthened, the number of cables has increased. Also, instead
of relying on one plane of cables, engineers often use two parallel planes,
with deck anchorages running along opposite sides of the structure and the
cables rising to separate or common anchorages towards the tops of the pylons.

A single cable plane looks better and ought to be quicker to install
because it involves fewer connections. But two planes share the loads on
the deck so that the cables themselves are thinner, which makes them less
unwieldy to install. Two planes also enable the cables to act like a cage
around the bridge, substantially stiffening it. According to Taylor, the
two cable planes on the Annacis Bridge provide 90 per cent of the crossing’s
torsional stiffness.

When engineers go for single planes of cables, they must find other
means of providing torsional stiffness. They usually resort to box-girder
decks, which will always provide much more torsional stiffness than any
layout of cables. However, box girders are expensive and, for large crossings,
sometimes not sufficient. If a bridge is not inherently stiff enough, they
can install dampers to ensure that its oscillations do not get out of hand.
These commonly consist of a mass on a spring immersed in viscous fluid.
For instance, the Rama IX Bridge across Bangkok’s Chao Phraya River, which
opened in November 1987, has a main span of 450 metres. This is the longest
span supported by a single plane of cables. It has a steel box girder for
a deck but still requires more mass damping, tuned to the natural frequency
of the structure, than any other bridge. Altogether, there are 18 dampers
distributed around the structure, preventing the pylons from swaying too
much and the deck from twisting and bending excessively. Taylor describes
this as a very expensive remedy. Also, in the hot climate, the asphalt road
surface has tended to come away from the steel. The crossing remains a spectacular
sight, however.

But not only bridges with a single plane of cables have needed damping.
On the Kessock Bridge near Inverness, completed in 1981 with a main span
of 240 metres supported by two cable planes, dampers were installed after
the structure was opened because deck oscillations were greater than expected.
The bridge was one of the first projects in Britain to be designed and built
by the same firm.

Besides making the most of the cable configuration for the Annacis Bridge,
Taylor also wanted to see if he could get a better performance from the
cables themselves. Cables are made by bringing together a large number of
steel wires, each just a few millimetres in diameter, with the number and
type of wires depending on the load to be carried. The thickest cables so
far made for a cable-stayed crossing are used on the Bangkok bridge and
are 167 millimetres in diameter. A cable’s properties are also influenced
by the way the wires are bound together. This affects the thickness of the
cable, which can influence the wind drag on the structure, and its modulus
of elasticity, or ability to resist stretching and bending.

Redesigning the cables

The wires of a cable can have a circular cross section and be bound
spirally around one another, or they can be Z-shaped so that they lock into
a tighter bundle. Taylor persuaded British Ropes, a cable manufacturer,
to provide cables with circular wires wound in a much gentler spiral than
is usual. Instead of a ‘lay’, or pitch, of about 300 millimetres, the cables
for Annacis Bridge have a lay of about 3 metres. By effectively unwinding
a standard cable so that its wires are not so stressed, the new cable retains
more of the combined performance of the individual wires to support the
structure. The resulting cable is stronger and has a greater modulus.

British Ropes expected the industry to welcome the new cables and also
saw them opening up other markets, particularly in the field of offshore
structures. However, the response from bridge engineers appears to have
been cool. ‘The advantage of the modulus is not particularly significant,’
says Hugh Knox, the engineer in charge of designing and building the Dartford
River Crossing. The cable, which needs to be wrapped in a plastic sleeve
to keep its more loosely bound wires together, is difficult to inspect,
he complains. Knox has stuck with cable made conventionally from spiralled
circular wires, which is easier to handle and has a long track record, he
says.

These advantages were particularly significant for the Dartford bridge
because its cables are up to 164 millimetres in diameter, which is 25 per
cent thicker than the thickest ones supporting the Annacis Bridge, even
though the Canadian structure is longer and carries two more lanes of traffic.
The cables, the largest made in Britain, are just 3 millimetres thinner
than those used for the Bangkok bridge, which has only one plane of cables
to support its deck as opposed to Dartford’s two. The chunky cables reflect
the extraordinarily heavy traffic loads that engineers designing British
highways have to cater for in order to keep within the rules laid down by
the Department of Transport.

Another onerous obligation is that suspended highway crossings in Britain
must function normally even with one of their cables missing, or with ‘one
cable out’, as the department puts it. This is to ensure that traffic restrictions
are not necessary should any cables need to be replaced during the lifetime
of the bridge. As if this were not enough to test the skills of bridge designers,
the department has now stipulated that new structures, including the Second
Severn Crossing, must be capable of staying in service with two cables out,
albeit with the volume of traffic restricted.

Engineers accept that the department, as the client, is entitled to
demand whatever it wants. But many recognise that such constraints make
it extremely difficult to analyse the structure in order to define the most
demanding combination of loads for different parts of the bridge – and still
come up with an efficient, economical design. ‘The design must be suffering
large premiums to cope with that,’ says Taylor. The Annacis Bridge had to
be designed to cope with one cable out, but is not expected to support an
unrestricted traffic flow at the same time. He knows of no other highway
authority that requires its suspended crossings to be designed to cope with
two of its cables missing. ‘Frightening’ is how Roger Postlethwaite, a former
partner of Homberg’s and one of Britain’s leading bridge designers, describes
the prospect of analysing all the possible combinations of structure.

But David Mizon, the engineer in charge of designing the Second Severn
Crossing, remains phlegmatic. ‘It requires a lot of effort to sort out the
critical load cases,’ he says. Mizon, who works for Sir William Halcrow
& Partners, was also responsible for checking the design and construction
of the Dartford River Crossing on behalf of the Department of Transport.
For the Second Severn Crossing, however, he got his team of engineers to
devise a computer program to help them check every component of the bridge,
down to the last bolt and rivet. ‘We’re now at the stage of using the software,’
he says.

While cables are made of steel, however, they are likely to be one of
the main factors preventing cable-stayed spans getting any longer. As bridges
get longer, the problems of installing the cables, which will also be much
longer and heavier, will become more acute. Furthermore, says Taylor, designers
will not be able to rely on them to reduce a bridge’s flexibility because
it will be impossible to pull the cables taut enough. Japanese engineers
in charge of the proposed record-breaking Tatara Great Bridge accept that
the cables might sag and oscillate. ‘To stop this, we are proposing to use
connecting cables between the main cables to hold them together,’ says Makoto
Kitagawa, manager of the planning and development department of the Honshu-Shikoku
Bridge Authority. For the Normandy Bridge, where the longest cable is likely
to weigh more than 20 tonnes, French engineers plan to install the cables
by dividing each one into groups of wires, or strands, and fitting one strand
at a time. They will also be using secondary cables to stop the main ones
oscillating too much.

Against this background, how much further can bridge engineers really
go? Wyatt reflects on one of the industry’s last great experimental phases.
‘The decade leading up to the failure of the Tacoma Narrows Bridge had seen
three giant steps in the limiting parameters of bridges,’ he recalls. In
1931, the George Washington Bridge over the Hudson River at New York virtually
doubled the world’s longest span at a stroke, to 1067 metres. Then, six
years later, the Golden Gate Bridge over the mouth of San Francisco Bay
raised the record to 1280 metres. The Tacoma Narrows Bridge followed, introducing
slender decks to long suspended crossings. After its collapse, caution set
in. The Golden Gate’s record length stood for 27 years, and even now has
been exceeded by only 10 per cent.

Reviewing French and Danish plans to double the record for the longest
cable-stayed span, Postlethwaite notes: ‘It’s a big step forward, fraught
with the unknowns that a big step entails.’ But he adds that Homberg made
many speculative designs for cable-stayed spans stretching more than 1000
metres, several of which were included in proposals for crossings of the
English Channel and the Straits of Messina between Sicily and the Italian
mainland.

In Japan, where typhoons and earthquakes have to be reckoned with, Kitagawa
suggests that the Tatara Great Bridge, with its 890-metre span, may be the
limit. In Denmark, Petersen reckons the shelved 1204-metre span is the most
that engineers can expect of a cable-stayed design. However, in the wake
of the completion of the Dartford bridge, Knox is less optimistic. He cannot
see anyone being able to build one longer than 600 metres for some time
yet. French and Japanese engineers aim to prove him wrong while others keep
their fingers crossed.

Additional research by Peter Hadfield in Tokyo.