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ISSN 1357-4442Editor: Simon Denison

Issue no 39, November 1998

FEATURES

Changing materials, changing bridges

Bridges were built in much the same way for centuries. Then they changed. Bill Smyth explains

Everywhere we travel, we encounter bridges. For many centuries the masonry arch appears to have been the dominant form; but this was not necessarily so. Timber bridges have disappeared and stone bridges are the ones that are left.

Over recent decades, however, the form and material of new bridges has completely changed. `Beam' bridges predominate (horizontal decks resting on supports) and there are comparatively few new arched structures. Most bridges are now made from reinforced concrete, prestressed concrete and steel.

There is a relationship between the structural forms of bridges and the materials used. The main forces acting in a bridge are compression and tension: if you rest a plank of wood across two supports at either end and place a load on top, the topside of the plank experiences compression (it is being shortened) and the underside experiences tension (it is being stretched). Timber, like steel, is strong in both tension and compression, and can therefore be used as a beam. Timber bridges were once very common because timber is much lighter and more easily worked than stone and was once very plentiful. However, timber has two serious defects: it burns and unless it can be kept completely dry or wet, it rots. So in the end timber bridges were replaced by stone or brick arches.

An arch is a way of making use of materials with little or no tensile strength. Stone is very weak in tension and until the discovery of the arch could only be used for very short spans, as in the prehistoric clapper bridges like Tarr Steps near Winsford on Exmoor. The arch relies on its ability to resist compression. The suspension bridge, by contrast, where the primary structural feature is the heavy cables, relies on their ability to resist tension.

Beam bridges make fewer demands on their foundations than arch or suspension structures because their loads are taken directly down to the foundations. The other two impose horizontal forces on their foundations. Suspension bridges pull their supports inwards (this force is experienced when two people at opposite ends of a rope have to pull at either end to keep it off the ground). Arched bridges, by contrast, push their supports outwards, and this has to be resisted by the ground pressing against the abutments or pier foundations. Broadly speaking, the narrower the arch, the less horizontal force is exerted. Wide arches, where the span is much greater than the height, have in general only been possible in more recent times, although a medieval example of a `segmental arch' (so called because the wide arch forms only a segment of a circle) is the Ponte Vecchio in Florence (completed 1356).

Medieval bridges were built one arch at a time. This meant that at each pier, during construction, the arch which had just had its centering (or support) removed exerted a horizontal force which was not yet balanced by an opposing thrust from the next span. To cope with this force the piers had to be very wide, and - where it crossed a river - they considerably obstructed the flow. The medieval London Bridge had such a reduced waterway that the fall in water level between upstream and downstream ends of the piers was as much as six feet. When the flow is constricted by bridge piers the river flows more quickly through the openings as well as becoming turbulent. It picks up larger particles and deepens its bed, possibly undermining the pier foundations. Many bridges have been destroyed this way.

In the 18th century things began to change. Empirical rules began to give way to calculations based on the emerging laws of mechanics. Spans became larger. Some bridges were now built with all spans constructed at the same time. This made the centerings much more expensive, but the piers could be smaller. John Rennie's bridge which replaced the old London Bridge in the 1830s was built like this. The centerings were constructed away from the site and floated into position on pontoons.

In the late 18th century the first iron bridge outside China was the arch across the Severn at Ironbridge and a number of cast iron bridges were built in the 19th century. They were mainly arches, because cast iron is weak in tension. Wrought iron, which is strong in tension, was also used, notably in Brunel's railway bridges at Chepstow (main span replaced in the 1960s) and Saltash, which survives. Chepstow also has a cast iron road bridge dating from 1816. Steel came into use later in the century and eventually displaced other forms of iron. An example of an early steel bridge is the Forth Railway Bridge, built 1883-90.

The first bridge in Britain of reinforced concrete, that is concrete with steel bars in it to take the tensile forces, was built in 1901-2 at Chewton Glenn in Hampshire, and by 1913 more than 300 had been constructed here. There was a great variety of structural types. Some of them were arches and some of these reinforced concrete arch bridges are hardly recognisable as concrete because they are covered in stone for aesthetic reasons, like Chiswick Bridge over the Thames (completed 1933). Another Thames bridge, Waterloo Bridge (completed 1942), was the last major reinforced concrete bridge in the UK. This is a beam bridge, faced in stone - although some people think it is a series of arches because of the curved undersides to the beams.

After the Second World War there was a shortage of steel, which required a licence to use. Such bridges as were built used prestressed concrete, and from then on all major concrete bridges were built from this material. Tendons (usually steel bars or bundles of wires of very high strength) are stretched by jacks at the ends and anchored to the concrete so as to compress it. Tensions resulting from loading the structure are absorbed as reductions in the pre-existing compression.

In the late 1950s the first motorways were built and created a new kind of obstacle to be bridged. Few bridges over motorways are arched, because the width of span needed to cross the road in an arch, and the headroom required over the motorway edges, would require a deck very high above the road. Where arches have been used it is where a high deck is appropriate, such as when a motorway runs through a deep cutting. An example is where the M40 slices through the Chilterns. Thomas Telford had a scheme for an iron arch spanning the Thames in one span; but it was impractical because the height of the deck would have required enormously long approach ramps, causing devastation on both sides of the river.

The majority of bridges crossing motorways are beam bridges. Sometimes a kind of rectangular arch is used, known as a portal frame, a cross between an arch and a beam (it has the disadvantage of side-thrust, like an arch, but is used when there is not enough space at either side for a supported beam). Some of the first bridges to be built over the M1 are rather curious two-span portal frames of reinforced concrete, for example on the section of road between Luton and Crick, and where the M1 bypasses St Albans.

While stone arches always had to be supported on centering during construction, modern materials have opened up other ways of building bridges. From the mid-19th century some very large steel bridges, such as the Forth Railway Bridge and the Tyne Bridge, were built by cantilevering out from each pier or abutment so that the two halves of each span met in the middle. In the case of the Forth Bridge, the bridge sections were cantilevered out from each pier in two directions at once, for balance; but with the single-span Tyne Bridge, the cantilevered sections were prevented from falling forwards during construction by means of cables anchored in the ground.

Most reinforced concrete bridges were cast in their final position, but many prestressed concrete bridges have been built from prefabricated segments, such as the M180 bridge over the Trent near Scunthorpe. Bridges have also been built by pushing sections out from one abutment to the next, with the weight of the length in mid-air balanced by a length resting on the approaches. The new road bridge for the A48 at Chepstow, for example, was cantilevered out in this way to a distance of 300ft.

One of the significant things that happened to British bridges over the centuries was the replacement of less durable by more durable material, that is timber by stone. That has now been reversed. So far modern materials have proved to have shorter lives. Bridges built by the Romans are still standing, but both reinforced and prestressed concrete bridges have already experienced many problems. For example, the prestressed Ynys-y-Gwas bridge in West Glamorgan failed in 1985 because of the corrosion of steel tendons at the joints between precast segments.

Already new materials are being tried. Plastics have been used to encase steel bridges to make them more weather resistant (a modern version of the technique formerly used in Switzerland and elsewhere of putting roofs on timber bridges to make them last longer). Glass-reinforced plastic has been used in this way on the undersides of some of the bridges over the motorway on the English approach to the Second Severn Crossing. At least one bridge - a footbridge at Aberfeldy golf course in Perthshire - has been built with a structure of plastics. High strength man-made fibres are being thought of for prestressing and for the cables of suspension bridges.

No doubt in their turn these materials will turn out to have unexpected problems. The ancient bridge builders also had their problems. The old bridges, like medieval cathedrals, that we see today are the ones that worked.

Bill Smyth is a retired bridge engineer, and has advised English Heritage on the listing of postwar bridges


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Finding Beowulf in Kent's landscape

Place-names and topography locate the Beowulf story near Faversham, claims Paul Wilkinson

The Old English epic poem Beowulf, which stands at the very beginning of English literature, has long been regarded as a mythical tale, a work of pure fiction.

It is the story of a youth who achieves glory in a foreign land by killing the monster Grendel in King Hrothgar's hall - a place called Heorot - and then by killing Grendel's mother in the surrounding marsh. Beowulf then returns home, and is killed in his twilight years fighting a dragon.

The poem, thought to have been written down in its final form in the late 10th or 11th century, after centuries of oral transmission, recalls the heroic world of the 5th-6th century Age of Migration, but is overlain by later Christian sentiments. Most scholars regard the written poem as the product of a sophisticated, Christian court, but believe that the action of the poem takes place somewhere in southern Scandinavia. `It contains no reference to the British Isles,' wrote the Beowulf scholar Michael Alexander in 1973.

It is possible, however, that Beowulf's foreign adventure was to southern England - to the Isle of Harty and its surrounding area in Kent; and that, far from being a work of pure fiction, the poem in fact preserves a memory of Germanic raids in south-eastern England in the 5th and 6th centuries.

The action in Beowulf takes place in the 6th century. We know this because of the mention of a real historical event, the raid by the Swedish leader Hygelac on the Franks of Frisia, in which Hygelac lost his life. The event is recorded by Gregory of Tours in his Historia Francorum as occurring in 521. In the poem, Hygelac is Beowulf's lord.

Another probable historical character mentioned in the poem is the Saxon leader, Hengest. Beowulf's warriors are entertained by tales of earlier heroes, and one tale they heard was the Saga of Hengest - a real work of which only a fragment survives.

As described by the historians Nennius (a 9th century compilation) and Bede (8th century), Hengest was one of the first Germanic leaders to come to Britain in the early 5th century.

Then came three keels, driven into exile from Germany. In them were the brothers Horsa and Hengest . . . Vortigern welcomed them, and handed over to them the island that in their language is called Thanet, in British Ruoihm. (Nennius 1:31)

The incomers were welcomed as mercenaries, but Vortigern's troubles demanded ever more men. Hengest seems to have risked a rebellion in the late 440s, as his numbers grew. It is possible that, in addition to Thanet, the other offshore islands of Kent, including Harty, Elmley and Sheppey, were seized and held. But the revolt apparently misfired, and after years of war the Germans were eventually left in possession of only limited territories in North and East Kent.

It is these events that may be dimly echoed in the lines of Beowulf. Harty Island was called Hart Londe in the 15th century and Heorot in the 11th century - the same name as Hrothgar's hall in the poem. Interestingly, the island seems to have taken its name from its main settlement, called `Harty' on the earliest surviving (16th century) map of the area.

Many of the details of Beowulf's adventure seem to fit such a voyage. The sea journey from the mouth of the Rhine to Britain was estimated by the 1st/2nd century writer Plutarch to be about 36 hours. Beowulf sighted land on the morning of the second day (the second morning). If, as would be normal, he had sailed on the evening tide, his journey would have taken 36 hours.

Beowulf's first sighting of land is of `sea-cliffs shining, shores steep, broad sea-nesses.' Best landfall on the coast of Britain from the mouth of the Rhine is either at North Foreland on Thanet, or at Sheerness cliffs. The North Foreland displays an optical trick of `shining' when the rising sun strikes the white chalk of the cliffs; in fact in pre-dawn light the cliffs can shine quite dramatically whilst the surrounding ocean is still in darkness, a phenomenon remarked upon by early mariners. Sheerness cliffs display the same natural phenomenon, and the Anglo-Saxon place-name emphasises the point - Sheerness means `bright headland'.

Beowulf's voyage ends at a place called Land's End, which interestingly is the name today of a small sea-inlet just to the north of Harty. Above it, the cliffs are called Warden Point, a name recorded from at least the 12th century. In Beowulf, the `warden' of an important Jute/Germanic household stands on the cliffs above Land's End, sees Beowulf's approaching ship and picks his way down to the shore to greet the young warrior.

The warden accompanies Beowulf until the hall, Heorot, comes into view. He then leaves him to continue along the `straet' that `climbs up' to the building. Straet is Old English for a Roman road (from Latin via strata, a paved way). There are no Roman roads in Jutland, but there is one on the Isle of Harty, leading uphill to a Roman villa or settlement on the island. The road was surveyed recently by the Swale Archaeological Survey.

It makes sense that Hrothgar's hall was a former Roman villa. On arrival at the hall, Beowulf strides across a `fagne flor' (fine/flagged floor) which the Anglo-Saxon place-name specialist Margaret Gelling suggests (in Signposts to the Past) `could denote the floor of a Roman building which, whether paved or tessellated, would be more elaborate than those of Anglo-Saxon buildings'.

Over that floor, tessellated or otherwise, walked Wealtheow, King Hrothgar's queen, `at times famous queen, peace-pledge between nations'. The word walh/wealh was used by the early English to denote the native Romano-Britons. The word theow can mean serf but also was used for the subject of an arranged marriage. It is likely that a woman called `Wealtheow' was a Romano-British noble, given in a marriage alliance between the Romano-British ruling class of Kent and the Germanic King Hrothgar. Such marriages certainly took place. According to Nennius, Vortigern himself married the daughter of Hengest.

Heorot was the centre of one of the largest and earliest `lathes' in Kent - the Lathe of Scray, called Schrawynghop in 1240. A lathe was a large division of land supporting a Germanic king and his folk. Other lathes were centred on Canterbury and Rochester. Why was the land around Harty called Schrawynghop? What does it mean?

Old English hop means a piece of land enclosed by marshes, and according to Margaret Gelling, the only certain occurrence in Old English is in Beowulf, where the monsters' lairs are called fenhopu and morhopu, `marsh retreats'. The Swale, the tidal estuary dividing Harty from the North Kent coast, is now surrounded by many square miles of drained marsh and tidal pools. In the 5th century, the Swale estuary was a drowned world of marsh, bog, tidal pools and whirlpools. Harty Island itself dominated and was surrounded by this watery landscape and no doubt was the piece of land surrounded by marsh alluded to by the element `hop' in Schrawynghop.

According to JK Wallenberg in his Place Names in Kent, the Old English screawa means `a malignant man, a devil'. He writes: `I interpret the name of Schrawynghop as a piece of land surrounded by marsh, haunted by one or several supernatural malignant beings'. That the name given to the early Germanic kingdom of Faversham and Harty should commemorate the fact that the marshes surrounding Harty were haunted by `supernatural malignant beings' is astounding. It fits exactly the story of the half-human beasts Grendel and his mother whom Beowulf slayed in the watery wilderness that surrounded Heorot.

Until the 1950s, a large boat-shaped earthwork called Nagden Bump stood on a spit of land opposite Harty Island, over-looking one of the main waterways to London (it has since been removed). Its commanding position suggested a royal burial mound, of the type described in Beowulf. In the poem, the dying Beowulf says to his followers: `Bid men of battle build me a tomb on the foreland by the sea, so that seafarers shall name it Beowulf's barrow.' It is a charming thought to imagine that Nagden Bump (a Scandinavian place-name) may have been the burial place of Beowulf himself.

Thus numerous pieces of topographical and place-name evidence in the landscape around Harty indicate a connection with the epic saga Beowulf. If the poem does recall events that took place in Kent during the Age of Migration, its stories could be the founding-myths of the English-speaking nation and Harty could be one of the most important sites in Britain.

Paul Wilkinson runs the Swale Archaeological Survey for Swale Borough Council


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Seeing the past in standard images

Pictorial reconstructions have always shaped people's thinking about the past, writes Stephanie Moser

For many centuries people have been producing illustrations of primitive humans who were thought to have lived in the distant past. Despite the mass of these images, they have generally been taken for granted. They are regarded as uncontentious, whereas in truth the opposite is often the case.

Whether historians and archaeologists like it or not, many people's beliefs about life in the past derive from popular illustrations. One example of this trend is the popular image today of `Stone Age Man' as a stooped and hairy brute, which is repeated in numerous cartoons but is far removed from archaeological reality.

It would not, perhaps, matter that illustrations were so influential, if they were also generally `accurate'. In fact, illustrations are complex documents that often reflect pictorial traditions as much as the latest research. They also inevitably reflect contemporary fashions and concerns, despite the best intentions of the illustrators for historical accuracy. This is as true today as it was 500 years ago.

Images of early Europeans first began to appear in large numbers in the late 1500s and 1600s. They coincided with the beginnings of antiquarian research, which was mainly interested in the national origins of the great European nations. These images, mostly of Europeans of the immediate pre-Roman era, were influenced by Renaissance art traditions, encounters with New World natives, information in Classical texts, and the early discoveries of artefacts and sites. Above all, they reflect the Renaissance idea of the noble warrior of antiquity. Although they seem historically fanciful today, these pictures helped to establish a popular image of the ancient warrior which maintains a powerful grip on the popular imagination today.

Two influential painters of this time were Jacques Le Moyne de Morgues and John White. Le Moyne, a Huguenot artist who accompanied the French expedition of Laudonnière to Florida in 1564, produced in 1585-8 Young daughter of the Picts, a portrait of a young woman covered from head to toe in painted flowers. The body decoration reflects contemporary ethnography but the details of this decoration are European, the flowers being based on species only known to the western world. Her anatomy and posture are inspired by classical art and her hair typical of Renaissance painting. Seeing naked women with body paint in Florida may have inspired Le Moyne to indulge in projecting such a vision back into the European past.

John White accompanied Sir Walter Raleigh on his trip to Virginia in 1585. One of his pictures, Pictish man holding human head (front cover), has several influences. Head-hunting, body decoration, the man's shield, and the metal torques around his waist and neck, all derive from classical accounts, for example those of Herodian and Dio Cassius. The posture of this figure is Mannerist, and his long hair and moustache derive from contemporary European traditions of the `wild Irish'. This ancient Briton is regarded as a naked and painted barbarian, clearly related to contemporary pictures of the tattooed Indians of the New World.

New World influences were not as strong in the 17th century work of the Polish scholar Phillip Cluverius and the Dutchman Johan Picardt; but these artists still had a strangely primitive view of early Europeans. In Cluverius's pictures, garments, armour and weaponry are used to indicate the regional distinctions among the German race; but despite the sophisticated dress and arms possessed by some tribes, the primitive lifestyle of the ancient Germans is emphasised by the nakedness of many figures and the lack of structures or settlements. Picardt's images were of the first people to live in Drenthe, an old province of the Netherlands, published in a study of megalithic burial monuments. Picardt regarded the people who built these monuments as giants with long hair, animal skin garments and clubs.

These 17th century pictures represented a shift of interest from classical accounts and exotic ethnography to the ruins and monuments on the landscape. From this point on, the archaeological dimension of the past started to assert itself in the imagery of human antiquity.

The rise of the earth sciences in the 18th and 19th centuries had a profound impact on the perception of human origins. The discovery of deep geological history and the dismantling of the biblical chronology transformed all previous conceptions of the beginnings of humanity. Illustrators were faced with the challenge of representing humans living in times when the environment and animals were very different from the present, and although the new images were `scientific' in a sense, they did not supplant previous images but rather grew out of them.

One of the first images to present the new theory of human antiquity was Louis Figuier's Appearance of Man of 1867. Visual symbols in this picture represent the scientific vision including a crevasse, separating humans from animals, wild vegetation, representing another geological epoch, extinct animals, bones lying on the ground, defining our ancestors as hunters, and the depiction of a large social group suggesting that prehistoric people needed to live in groups to survive. In addition there are the familiar emblems of nakedness, a cave home, animal skins and clubs. Unchanged is the anatomy of the humans, which is still based on heroic or classical models.

The humans in this picture are doing battle with monsters. This theme of combat was to become a major theme in the representation of prehistory, a continuation of the old `warrior' theme in another form. This and other contemporary pictures share much of the iconography of the Romantic movement and also reflect the new Victorian colonial interest in exotic and savage foreign lands, and the Victorians' belief in themselves as civilising rulers. All the humans in these images are white-skinned, and reveal contemporary attitudes to gender roles with men stereotyped as providers and women as nurturers. Afar greater challenge to established thinking on human creation came when evolutionary theorists like Darwin argued that our species had evolved from ape-like ancestors. The suggestion that our first ancestors were not anatomically modern meant that illustrators had to create an image that blended characteristics of humans and apes. These pictures played a major role in the success of the human origins revolution. By presenting the theory of human descent from the apes in terms of long established visual traditions, they in effect made the unbelievable believable.

One early image of this type was Pierre Boitard's Fossil Man. It communicates Boitard's view that this ancestor was a `horrible species' with a body that was `stout, squat and thickly muscular'. The hairy, naked and black-skinned figure stands at the entrance of his cave holding a weapon, recycling the age-old imagery of the wild-man. Although humans in this and other similar images have been recast as savage brutes, they retain the traditional human (male) characteristic of the hunter.

As more early human fossils were found from the late 19th century, and new species were recognised, research findings were increasingly translated into visual representations. Sequences of images were produced representing human development. Yet while new details reflecting research discoveries were incorporated, illustrators standardised their pictorial motifs into a basic set of five or six images. The standard scenes typically contained hunting, toolmaking, eating rituals, fire, combat with wild beasts, and ultimately the production of art. Figures in the earlier scenes were always black-skinned, while the `civilised' artists of the final scene were typically white.

Despite a century of research into human origins, this standard set of images has continued to be produced, with stylistic modifications, right up to our own day. In the widely-reproduced work of later 20th century artists such as Maurice Wilson and Zdenek Burian, we still find the familiar icons of caves, skins, fire, tools, clubs, hunting, and a traditional male/ female division of labour.

The fundamental paradox of pictorial reconstructions is that while images appear to be creative and free of restraint, they serve to confirm and reinforce established ideas. Once we had imagined the past and translated this into imagery, it effectively became what we pictured it to be. A challenge for the future will be to find a new way of illustrating the past that breaks free of clichés and allows us to picture prehistory in a new and more informed way.

Dr Stephanie Moser is a Lecturer in Archaeology at the University of Southampton. Her book, Ancestral Images, was published by Sutton recently (ISBN 0-7509-1178-6)


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