The Architecture of British Bridges E-Book

Ronald Yee

• Herausgeber: Crowood
• Kategorie: Geisteswissenschaft
• Sprache: Englisch

Seitenzahl: 377

Veröffentlichungsjahr: 2021

Beschreibung

During the Industrial Revolution, Britain was at the forefront of bridge innovation. Pioneering designers such as George and Robert Stephenson, Thomas Telford and Isambard Kingdom Brunel created Britain's rich bridge heritage that features many world firsts and we can learn much from their ground-breaking designs. Written by an experienced bridge architect, this book includes an introduction to bridge aesthetics; it gives an outline of British bridge development and advice on parapet treatment and bridge lighting. This book offers a comprehensive overview of how the best of British bridges marry aesthetic considerations with engineering ingenuity.

Leseprobe

THE ARCHITECTURE OF

BRITISH BRIDGES

RONALD YEE

Progress sketch of the main tower pier of the Rose Fitzgerald Kennedy Bridge.

THE ARCHITECTURE OF

BRITISH BRIDGES

RONALD YEE

First published in 2021 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

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This e-book first published in 2021

© Ronald Yee 2021

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 78500 795 8

Contents

Preface Chapter OneArchitecture of BridgesChapter TwoStone BridgesChapter ThreeTimber BridgesChapter FourBrick BridgesChapter FiveIron BridgesChapter SixSteel BridgesChapter SevenSuspension and Cable-Stayed BridgesChapter EightConcrete BridgesChapter NineMovable BridgesChapter TenBridge ParapetsChapter ElevenBridge LightingGlossary Index 

Preface

MY CONNECTION WITH BRIDGES STARTED well before I was born, since I am the third generation of the Yee family to be involved with bridge design or construction. My grandfather Ah Shu Yee travelled from China via India to become the chief engineer for the Senna Sugar Company in Eastern Rhodesia, now Zimbabwe. There Grandad was responsible for the construction and ongoing maintenance of the sugar estate’s railway lines and bridge structures, which included a major crossing of the River Zangua.

My father Albert Yee trained as an articled architect in Rhodesia (now Zimbabwe) before embarking on a European tour with my mother Pauline for their honeymoon. Apartheid prevented Dad from fully qualifying and practising architecture in Southern Africa. So, upon arriving in London in 1960 when Dad was offered a job as an assistant architect in a practice that specialized in retail architecture, he accepted. Dad’s draughtsmanship soon came to the notice of Sir William Holford and Partners who recruited him, and he was put in charge of presenting the practice’s key schemes. Over the years his projects with Holford’s included high-profile bridges such as London Bridge, the refurbishment of Tower Bridge, Benjamin Sheares Bridge in Singapore and Batman Bridge in Tasmania.

Back in the 1970s Health and Safety was not as strict as it is today. My brother Raymond and I were regular companions on several of Dad’s weekend site visits and inspections. One memorable occasion was when Dad was inspecting the fibreglass replacement coat of arms for Tower Bridge. We were all allowed to climb up one of the towers and crossed the upper walkways on scaffolding planks long before the openings were glazed in and made safe for the public!

Edward William Cooke graphically documented demolition of the medieval bridge and construction of the previous London Bridge in 1832. Under Lord Holford’s sponsorship my father carried on this tradition by graphically documenting the deconstruction of the previous London Bridge and the construction of the current bridge. Dad’s sketches formed the basis for his book London Bridge: Progress Drawings 1968–1973.

Swanscombe Cutting Footbridge, Kent.

My formal involvement with bridge building came about when I trained as an Officer Cadet with the Royal Engineers whilst studying for my Architecture degree. With the Royal Engineers I gained my engineering knowledge but more importantly I learnt how to build quickly with limited resources, using the materials available. Every summer I would undertake an MACC Task (Military Aid to the Civil Community) where I would rebuild footbridges that had been damaged or destroyed by a natural disaster.

During the regular training drives down from our base in the University to the Royal School of Military Engineering in Chattenden we would pass under a distinctive bridge over the A2 that symbolically marked the halfway point of our journey. Over time I began to take notice of this bridge and the more I saw it, the more I appreciated its sleek shape and appearance which was in stark contrast with the military bridges that I was being trained to build. Whilst I enjoyed the challenge and adventure of combat engineering, I also started to realize the limitations of military bridging, which is purely functional, constructed in accordance with the manual and positioned for tactical reasons only.

After completing my degree, instead of pursuing a career in the Army, I chose to continue my architectural studies and eventually became a Chartered Architect. My lucky break with bridges came when I got involved in the architectural design of Tsing Ma Bridge in Hong Kong, which has the longest combined road and rail span in the world. Every other bridge project I am involved with now seems small by comparison to Tsing Ma; even my latest (the longest bridge in Ireland) looks like a much smaller sibling!

This book starts with the notion of bridge architecture being more than just engineering design. It then traces the development of British bridges by construction material: stone, timber, brick, iron and steel, finishing with the contemporary use of concrete and advanced composite construction. The remaining chapters cover moving bridges, bridge parapets and bridge lighting as separate subjects.

Chapter One

Architecture of Bridges

Not Just Looking But Seeing

One of the first exercises of my architectural education, entitled ‘Looking and Seeing,’ explored ways of seeing, understanding and interpreting an object, place or event, with an aim of critically looking beyond the obvious and analysing for myself the unseen quality of an object, building or space.

To help me organize my research and evaluation I developed a variation of the five Ws:

•What is it?

•Where is it?

•What purpose does it serve?

•What are the underlying principles?

•What relationship is there between the principles and the form?

I chose to investigate the bridge over the A2 that had now become a way-marker on my journeys down to Chattenden. The exercise turned out to be an epiphany in my understanding of bridge design, and of design in general. I found out that the bridge was a pre-stressed concrete footbridge at a place called Swanscombe in Kent. It was constructed in 1965 at the same time as the new A2 and carries a footpath that was severed by the new road cutting. The footbridge was designed by Kent County Bridge Engineer J.A. Bergg and is 294ft long (89.5m) with a deck that is 7ft 6in wide (2.3m) between railings, which is supported by a three-pinned arch with a span of 160ft (48.77m) rising to a height of 25ft 6in (7.83m).

Analysis of the Swanscombe Footbridge – elements and form.

The bridge comprises two parts: the deck (continuation of the footpath) and the supporting arch.

The deck is straight and level and connects to the top of the cutting on either side. To support the deck across the cutting a support structure is required, which is the arch. Structurally, these two separate elements need to be considered separately and then together. The deck in its natural state only carries its own weight (dead load) which is considered to be evenly loaded. The optimum shape for any arch that carries its own weight i.e. dead load evenly distributed over the entire span, is a parabola. A parabolic curve for any given span and height can be simply determined as an upside-down catenary i.e. the shape assumed by a chain suspended between two fixed points. Just as the links in the chain hang under gravity to form a sag curve where all the components are under tension, the elements of an efficient arch will follow a parabolic line of pressure, along which all the forces are compressive. Any deviation from the natural line of pressure will introduce stresses into the arch that makes it necessary to strengthen the cross section. Therefore, the most efficient structure is one that follows the natural parabolic line of pressure. However, the parabolic line of pressure of the arch is only applicable in a static situation i.e. deadweight of bridge alone, but to accommodate a load being carried at different points along the bridge the curve must be modified. When a load is asymmetrically applied to one side only, the arch naturally yields on the loaded side and thrusts up on the opposite side. The resultant deviation from the natural line of pressure introduces stresses into the arch, making it necessary to strengthen the cross section to prevent collapse. The opposite is true when the loading is reversed. By superimposing the lines of positive and negative deformation a lens-shaped zone is revealed on either side, with the largest deformation occurring halfway between the spring and the crown i.e. approximately quarter the total span. The most efficient shape would therefore have hinges at both ends and at the crown. A similar outcome is true when the same conditions are applied to the deck. When the zones of deformation for the deck and arch are combined, an obvious relationship can be seen between the lines of deformation and the outline of the footbridge’s arch and deck structure, resulting in an elegant solution that is both visually simple and structurally efficient.

Analysis of the Swanscombe Footbridge – load deflections.

Conclusion: the form of the Swanscombe Footbridge is a product of its functional requirements.

Bridge Architecture

More than just an elegant solution

Bridges, by their nature and scale, are usually prominent structures that are highly visible and should be beautiful. A beautiful bridge need not be more costly than an ugly one, but may need more time, care and skill in preparing its design and construction. Since bridges are built to last a long time, a high degree of care and design consideration should be deemed an investment. When a bridge design has been done well, its superstructure will soar, arches leap, balustrades embrace and columns march!

Beauty in relation to a bridge should be considered differently from beauty in art. An artist is free to paint a provocative picture or make a challenging sculpture in order to express his feelings or evoke emotions in the viewer who has the freedom to appreciate it or not. The enjoyment of art is truly a personal matter – if someone doesn’t like it then they can easily move on – however, a bridge’s appearance affects all who see or use it. Therefore, a bridge designer has a wider duty to society! An architecturally good bridge is more than just having elegant, clean and clear lines with unity of form and finish, proportion and composition. Experience of a structure involves much more than visual impression; our other senses have a strong influence too. Smell, touch and sound can affect our emotional response of awe, fear, security, claustrophobia, insignificance, etc.

When I first studied structures, my fellow students and I painstakingly produced technical design drawings of theoretical steel trusses with minimal struts and cleverly detailed connections that corresponded to bending moment diagrams, all neatly hatched in with pencil, and we were suitably pleased with our ingenious solutions and high grades. However, when I went on to study architecture, I was taught that even before we started designing, we first had to understand the genus loci – the distinctive atmosphere of our project location. For success any building (including a bridge) should be in harmony with its context, neighbours and environment. To test this, we were encouraged to produce ‘before and after’ renderings and scale models to ensure that our design would make a positive contribution to the setting or help us decide whether other solutions might be more suitable.

The context within which the bridge sits affects our perception of it. An industrial looking truss might suit an urban townscape but could look out of place in a flat pastoral landscape; the curvature of an arch member might blend harmoniously into the sides of a mountainous valley; and the loft towers of a cable-supported bridge might not be as dominant in a vast seascape.

Bridge architecture is not an exact science

Bridge architecture is not an exact science for which there is a definitive correct answer. For most bridging situations alternative solutions might be technically possible and a choice can be made. In an emergency, speed of construction may be the deciding factor and in temporary situations cost might be justifiable as a deciding factor. But with a permanent bridge, the time it took to build will not be remembered long, nor will the cost, whereas its beauty or ugliness will remain for its lifetime! Priority for choosing between alternative permanent bridge solutions should therefore be aesthetic.

Many believe that the appreciation of beauty is down to personal taste – ‘I like this; you don’t, you like that’ – however, aesthetic appreciation, like musical preference, is influenced by many things, including one’s culture, experience and education. Whilst there is room for personal preference in the appreciation of bridges, one must also account for societal consensus regarding the beauty of some and the ugliness of others. Later in the book, when we look at the examples that have been included, we will begin to see what good bridges have in common, and what the others lack. By critically looking at all bridges we should develop a better understanding of bridge architecture and our appreciation for what is successful will deepen.

What is good bridge architecture?

Based on the themes expounded by Roman architect and engineer Marcus Vitruvius Pollio in his treatise De Architectura (‘The Ten Books of Architecture’), there were three principles for good design: firmitas, utilitas and venustas (‘firmness, commodity, and delight’), i.e. a good bridge should be robust, it should fulfil the function it is designed for, and should be delightful or beautiful. The opening of the first chapter of Book One (entitled ‘On the Training of Architects’) reads as follows:

The science of the architect depends upon many disciplines and various apprenticeships which are carried out in other arts. His personal service consists in craftsmanship and technology. Craftsmanship is continued and familiar practice, which is carried out by the hands in such materials is necessary for the purpose of a design. Technology sets forth and explains things wrought in accordance with technical skill and method.

So architects who without culture aim at manual skill cannot gain a prestige corresponding to their labours, while those who trust to theory and literature obviously follow a shadow and not a reality. But those who have mastered both, like men equipped in full armour, soon acquire influence and attain their purpose.

What Vitruvius is describing is that architecture is about both craftsmanship and technology. Furthermore, if we interpret culture as the appreciation for the sensibilities and its reception by society, then we can understand that the difference between adequate design and good design is an elegant solution that also delights the user.

Strength and appearance of strength are not the same thing. Of course a bridge needs to be strong enough to stand up, but it must also look as if it will stand up. Modern construction technology is so advanced that a bridge may not readily reveal its structure at first glance. Advanced composite structures, for example, may have one part full of fibre reinforcement and another area hollow, so that the outline silhouette will give us no indication of the embodied strength. Therefore if a bridge is going to satisfy the eye, not only must it have strength but it must also appear as if it has strength.

Today I think Vitruvius’ three principles should be updated to include ‘economical’ and ‘sustainable’. Vitruvius worked for the Emperor and perhaps cost was less of an issue than it is for us today. We also live in a world of finite resources and we must be mindful to safeguard our future.

I have also developed a deep appreciation for the writings and works of the modernist architect Mies van der Rohe, perhaps best known for his axioms ‘less is more’ and ‘God is in the details.’ I often think of them as I develop my designs, especially when things start to get a little complicated!

Bridge architecture is more than skin deep

A beautiful bridge is not an ordinary structure with ornamentation applied to it. As we saw in the analysis of Swanscombe Footbridge, beauty arises from the essential lines, geometry, scale and relationship, etc. of a structure. This cannot be superimposed; it is either present in the original concept or it isn’t. Oscar Faber in his lecture to the Institute of Civil Engineers entitled ‘The Aesthetic Aspects of Civil Engineering Design’ stated: ‘The treatment from the aesthetic aspect cannot be delegated without delegating the whole basic design.’ He went on to emphasize that if the engineer is not interested in the aesthetic side, the engineer cannot be the author of the basic design. However, to this I would add that the reverse is also true for the architect: if an architect is not interested in the structural side, then the architect cannot be the true author of their design! Through shared knowledge and understanding our effectiveness is enhanced; when a particularly innovative or clever solution emerges, we appreciate the ability of others to think outside the limits of our convention.

The word ‘aesthetic’ derives from the Greek meaning ‘of perception’, and is a visual phenomenon that relates to appearance rather than reality, i.e. things are what they appear to us to be, rather than what they really are. Through usage it has become to mean specifically things concerned with visual beauty. Beauty in art generally relates to lines, colour, composition, shape, etc., whereas beauty in mathematics and science involves a neat solution and is intellectual rather than sensual. As we will see in the remainder of this book, a beautiful bridge is dependent on a large number of factors, including context, harmony, composition, proportion, expression, rhythm, colour and texture of material. A beautifully designed environment is a sign of civilization; bridge architecture therefore has an important influence on society – bridges can be uplifting if well designed and maintained but can produce indifference or even contempt if they are over-thrifty, ugly or neglected. Living in a beautiful place is a pleasure; it is the result of good design practice and a willing attitude.

What is a Bridge?

A timber plank laid across a gap, with its ends supported on the banks, is a simple bridge. The plank is the ‘deck’ and the banks at either end are the ‘abutments’. A person crossing the bridge will notice that the further they move away from the abutments, towards the middle, the more the deck bends (deflects). Their weight is the ‘load’. The wider the gap (span), the longer the deck has to be, and the deck will begin to sag under its own weight (self-load). This will increase when a load is added because within the deck the upper side is being pushed together (under compression) whilst the lower side is being stretched (under tension). Eventually the deck will snap (fail). The strength of the deck can be increased by making it thicker, or it can be propped up from below by supports called ‘piers’. The piers push up (positive thrust) against the downward weight of the load (negative thrust).

Basic bridge types

Simple beams

A deck that is supported at either end on abutments is a beam bridge. If the gap is wide, then piers provide extra supports. The beam itself can be in sections with the ends of each beam supported on piers. The distance between the piers is the span and all the beams together form a ‘multi-span’ bridge. The majority of ordinary bridges or those with a short span are beam bridges.

A simple plank bridge bends under load.

A loaded beam is under compression on its top surface and under tension on its lower surface.

A pier-supported beam bridge.

Cantilever

A cantilever bridge is built to achieve longer spans whilst minimizing the number of intermediate supports. A beam that is projected out and propped underneath is called a cantilever. If this is also done on the opposite bank, a span greater than a single beam’s length can be achieved. Furthermore, two cantilevered beams can support an additional beam section between them. Instead of being propped, a beam can be balanced on a pier; this is called a ‘balance cantilever’. The beam can be shaped so that it is thicker (haunched) and stronger where the stresses within the beam are greatest. Two balance cantilevers can also support a short extra beam between them; the ends are then attached to other supported beams or the bank to prevent it tipping over (rotating).

A haunched balanced cantilever bridge.

Arches

An arch can achieve even longer spans. If made from an appropriate material and correctly assembled, it is self-supporting due to its shape. All the forces within the arch are in compression, i.e. no part is being pulled apart or stretched. The two ends are fixed in abutments – these carry the load of the arch. On early arch bridges, the arch itself was the bridge but for longer spans the arch would be too high or too steep to be convenient. Then a deck is either supported on or suspended from the arch. When constructed from the right material an arch can be very strong with very little deflection. Types of arch include simple or fixed arch, two-hinged arch, three-hinged arch, and tied arch.

Arch types: simple, two-hinged, three-hinged and tied arch.

Trusses

There are many types of truss, all of which are based on the triangle or a combination of triangles. A triangular frame is the only form that keeps its shape when forces are applied through its members. The first truss to be developed was the king post truss – common in timber roof construction; a variation with two posts is the queen post truss.

Truss types: Kingpost, Queen Post, Lattice, Warren, Pratt, Howe, N truss, K truss, Fink, Lenticular, Bowstring and Whipple.

With the advent of iron in the nineteenth century a whole plethora of truss types were developed and patented, some proving more useful than others. The most common types used in Britain are lattice truss, Warren truss, Pratt truss, Howe truss, N truss, K truss, Fink truss, lenticular truss, bowstring truss, and Whipple truss.

Tension bridge types

Compression structures are relatively straightforward to construct and safer to use. However, some of the earliest crossings were tension structures built using creepers or ropes of twisted vines thrown across a river or ravine and secured to trees. A second higher rope acted as a handrail and made the sideways shuffle across safer. Other basic crossings had a basket or cradle under the tension rope for transporting people or goods. These crossings were easy to construct but because of their lack of rigidity, using them was an unnerving experience.

The strength of these bridges relies on the tensile strength of the rope used and the quality of the fixing they are tied to. Early ropes deteriorated quickly by today’s standards and these rope crossings probably only lasted a season and required frequent rebuilding. Nevertheless, the principle has evolved into one of the most successful of bridge types, the modern suspension bridge.

Suspension bridge

The modern suspension bridge is the most elegant of long span bridges. The graceful curvature of the suspension cable has a universal appeal and their appearance is surprisingly delicate considering the size of the elements and the spans involved. Whilst both are tension structures the modern suspension bridge differs from its primitive forebears. The deck is not the primary structure but it is level, flat across its width and rigid; its weight and any load that it carries is supported by separate cables that pass over saddles on top of strong towers and are then firmly secured into the ground with an anchorage. The deck is connected to the main suspension cables by thinner cables called suspenders.

Suspension bridge.

Cable-stayed bridge

Another tension structure able to achieve long spans is the cable-stayed bridge. Angular in appearance, cable-stayed bridges differ from suspension bridges in that the cables (stays) are attached directly from the deck to the towers. Because the cables pull towards the towers the deck has to be more substantial than a suspension bridge deck, to resist compression. Effectively the deck is a cantilever with a stay holding it up and can be built sequentially like a balanced cantilever, launching out from either side of the tower equally.

Cable-stayed bridge.

Cable stays that are configured in parallel are termed to be in a ‘harp’ arrangement and cable stays that are connected to the tower at one point are in a ‘fan’ arrangement. Stays that converge toward the tower but have individual connections are in a ‘semifan’ arrangement.

Extrados bridge

One of the most recent developments in bridge technology, the extrados bridge combines the main structural elements of a pre-stressed box girder bridge and a cable-stayed bridge. The term is derived from ‘extrados’ which is the outside curve of an arc and refers to how the pre-stressing tendons are deviated up outside the deck. In comparison to the cable-stayed bridge the achievable spans are similar, but the towers are much shorter.

Extrados bridge.

Movable bridges

Despite being designed to ease movement, bridges can also be an obstruction, for example a footbridge over a canal. Movable bridges are designed to overcome this problem and there are several options available to designers, allowing great scope for creativity and expression.

Drawbridge

Among the earliest types was the castle drawbridge, usually over a moat, which could be opened and closed by retracting ropes or metal chains. The Dutch drawbridge lifts the hinged deck using a counterweighted overhead lever; often the Dutch drawbridge is used in pairs for wider opening. A bascule bridge is a drawbridge that is balanced by counterweights in both its open and closed position and requires either mechanical or manual power to lift or lower it.

Bascule drawbridge.

Lifting bridge

Bridges that raise their deck horizontally are called vertical lift-bridges. The deck is either raised on two towers at either end or by hydraulic jacks underneath the moving span. Because the deck is lifted at both ends its structure can be made heavier with a higher load capacity than a bascule bridge.

Lifting bridge.

Swing bridge

Swing bridges rotate about a pivot in the middle of the deck. To avoid being an obstruction in the middle of the channel they can be designed to swing from one side only, if counterweights are used.

Swing bridge.

Transporter bridge

Bridges that carry just a section of deck across their span are known as transporter bridges. The section of deck that is suspended on cables is called the ‘gondola’ and moves from end to end using pulleys and a winding mechanism.

Transporter bridge.

Other moving types

British designers are constantly inventing ingenious solutions for achieving moving spans including (but not exclusively restricted to) rolling, folding, retracting, floating, submersible and tilting designs. These will be covered in Chapter 9: Movable Bridges.

Chapter Two

Stone Bridges

STONE IS A DURABLE MATERIAL THAT COMES from a variety of geologically formed rocks. Stone-built structures can potentially last thousands of years if the right stone is used and it is well constructed. Britain has a variety of different rocks but due to their geological characteristics not every rock can be used successfully for construction. Careful consideration has to be given to its capacity to withstand weathering and its ability to be fashioned to an acceptable finish and shape.

British rocks are categorized into one of three types: igneous, sedimentary and metamorphic. Although Britain has many varieties of igneous rock, only granite has been used on any scale. The two main sedimentary stones used in construction are sandstone which is made up of compacted quartz grains, and limestone where it is calcium carbonate. The most commonly used metamorphic rock in Britain is slate from Cornwall, Wales, the Lake District and Scotland.

There are some constraints associated with the proper use of stone. Igneous rock may contain minerals which on exposure to the atmosphere may break down and affect the stone, e.g. rising salt may cause spalling. Of structural importance for sedimentary rock is the placing of the bedding plane which should be perpendicular to the force imposed on it. Metamorphic rocks may contain radon, which is a mild radioactive gas, but this is not likely to be of concern to bridge designers.

Primitive Stone Bridges

Much of what we know about ancient bridges in Britain is conjectural; there is no documentary evidence about their construction and all we have are the weathered remains of primitive bridges which have almost certainly been altered at some point in their history. Small streams can be jumped over in one leap but wider, shallow streams have to be traversed by leaping on to strategically placed stones or boulders. Having large flat-topped stones was an improvement, helping travellers to maintain their balance, but if the water level were to rise or the stones become wet and slippery then the crossing could become treacherous.

In wooded areas trees could be felled using stone axes and then the logs trimmed, flattened on their top surface and laid side by side to create the simplest of ‘beam’ bridges. (The word ‘beam’ means ‘tree’ in old English, hence hornbeam and whitebeam.) Moving the logs apart, laying smaller cross members and filling the gaps with clods of earth improved their carrying capacity and convenience. All early bridges would have been wooden but none survive today, having long rotted away and been replaced by some of the ancient stone bridges that we see today.

Stepping stones were an early means of crossing a river or stream.

Postbridge

The primitive clapper or plank-bridge over Dartmoor’s East Dart at Postbridge was first recorded in the fourteenth century but probably originated in the thirteenth century. Built to enable packhorses to cross the river carrying tin to the stannary town of Tavistock, it has four piers, two of which act as abutments, supporting three granite slabs each approximately 15ft long (4.57m), which are held in place by gravity. The term ‘clapper’ originates from Medieval Latin claperius, meaning ‘pile of stones’.

Clapper bridge at Postbridge on Dartmoor.

Tarr Steps

Referred to as the ‘Devil’s Bridge’ in Lorna Doone, Tarr Steps in Exmoor is a seventeen-span clapper bridge that is possibly 3,000 years old in origin. It is the largest example of its type in Britain with a combination of twenty-three clappers resting on dry stone piers with a total length of 165ft (55m) and a width of approximately 5ft (1.5m). It crosses the River Exe at a height of 3ft (1m) above normal water level. A distinguishing feature of Tarr Steps are the raked stones positioned on the upstream side of the supports to divert the water flow and protect the bridge from floating debris, and on the downstream side as reinforcement. Slabs have been washed away during times of flood, and the bridge has had to be repaired several times. To facilitate this, the slabs are now all numbered!

Tarr Steps in Exmoor. The stones are now numbered to facilitate future reconstruction after times of flood.

Wycoller Packhorse, Clapper and Clam Bridges

The Lancashire village of Wycoller is now famous for its three ancient bridges that have spanned the beck for over a thousand years. There are actually seven bridges in the area together with fords; however, it is the Packhorse, Clapper and Clam bridges that are of special interest. Wycoller was an old hand-weaving village and lies in Brontë country, with Wycoller Hall acting as model for Jane Eyre’s Ferndean Manor. Opposite the ruined Hall is a three-span clapper bridge, sometimes called the Druids’ Bridge, Weavers’ Bridge or Hall Bridge. This primitive structure comprises three flat gritstone slabs resting on two stone piers, one being a large rounded boulder, and the other a slimmer cut stone on edge. Originally the bridge was a two-span structure, but the slabs worn by hundreds of years of use have suffered damage and one has snapped and has had to be propped up by an additional support. This clapper bridge probably dates from the sixteenth or seventeenth century although some sources think it originates from before the Norman Conquest. About 200 metres downstream is an intriguing two-arched packhorse bridge, referred to as Sally’s Bridge after one of the Cunliffe family that used to reside at the Hall in the eighteenth century. The bridge’s age is not definite but is thought to date from either the thirteenth or fifteenth century. Its construction is odd: the arches are not equal, and the rock foundations are not level, giving the structure an unstable appearance!

Wycoller Clapper Bridge with the Packhorse Bridge in the background.

A quarter of a mile upstream is the Clam Bridge, which according to local historians was a former standing stone (menhir) that is said to have dated from the Iron Age. The Clam is a single piece of gritstone measuring roughly 12ft (3.7m) and has been worn smooth on its top surface. It rests at one end on the bank whilst on the other it is propped up on some large boulders and is held fast by its great weight. At some time in its history the stone has been damaged and has been repaired with iron clasps on the underside. At one stage a handrail was added but now only the holes remain. Adjacent to the bridge is a paved ford to allow animals and carts to cross the stream.

Slater Bridge in Coniston is a typical Cumbrian flat-arched packhorse bridge.

Stone Bridge Design

Stone is a limited material for making beams and over time longer slabs have a tendency to crack on the underside where they are subject to tension. The secret to stone structures is to keep the stones firmly under compression, and for crossing wider streams an arch is needed. On early arches, wedge-shaped stones (voussoirs) were arranged standing in a curve (segment) between fixed bases (abutments), so that their weight and the friction between the stones keeps them in place. Any loading on the arch increases the compression and – providing the individual stones aren’t crushed and the abutments the stones rest on stay put – the arch remains intact. The arch is an ancient structure well known to older civilizations outside of Britain. The Romans made extensive use of arches in their bridges, some of which still survive today, but unfortunately none of these remain in Britain. Roman arches were usually semi-circular, but as long as the stones make some sort of arch it will usually be stable; the flatter the arch, the greater the sideways thrust against the abutments. Typical Lakeland packhorse bridges are usually segmental arches (i.e. curved less than a semicircle) and since they have a flatter profile, they are easier to cross and require less material to build. Fill was placed above the arch and held in place by dry stone sidewalls (spandrels). This not only smooths out the deck profile; it also increases the dead load that keeps the main arch barrel in compression, ensuring that none of the voussoirs drop out. All early bridges were built with low parapets so that they didn’t hinder passage of the laden panniers of the packhorses.

Roman Masonry Bridges

Although the Romans built an extensive network of roads and bridges in Britain to supply and move their garrisons, none of the bridges survive. The majority would have been timber structures that were left to decay after their purpose was served, whilst the more permanent bridges of stone or a combination of timber laid upon stone piers also suffered neglect and were either plundered for material or incorporated into later structures. Probably the two largest bridges to be constructed in Roman Britain were built along the line of Watling Street at Londinium (London) and at Durobrivae (Rochester) but other substantial bridge structures are known to have existed at Corbridge, Wroxeter, Caerleon, Caister, Castle Combe and Newcastle.

Medieval Brotherhood of Masonry Bridges

After the Romans left, many of their bridges continued to be used. The construction of new stone bridges lapsed but was revived in the twelfth century under the patronage of the church. The expertise in construction techniques and craftsmanship that had developed to build stone churches and castles were employed on bridge construction. As well as building Romanesque semi-circular arches, segmental arches became more common. Influenced by what was seen during the Crusades, cathedral-like vaulted masonry arches began to appear on British bridges.

Elvet Bridge

Perhaps the best-known surviving twelfth-century bridge is the Elvet Bridge connecting the eastern side of Durham peninsula over the River Wear, built under the auspices of Bishop Hugh le Puiset between 1170 and 1195. Of the visible ten arches only one is twelfth century; the others are thirteenth century, recognizable by their pointed arch. Elvet Bridge was vital to the defence of the city and incorporated fortified gates which had to be strengthened in the fourteenth century due to frequent attacks from the Scots but were later demolished in the sixteenth century. There were also two chapels on its structure – St James at the western end and St Andrew at the eastern end – but they do not exist today.

Elvet Bridge’s original southern side with hipped cutwaters together with the partial remains of the chapel (St Andrew’s) at the Eastern end. Courtesy of Lord Holford.

Framwellgate Bridge

On the other side of Durham’s peninsula stands Framwellgate Bridge, an early fifteenth-century medieval masonry arched structure built by Bishop Langley to replace an earlier twelfth-century bridge that had been washed away by floods. The current Framwellgate Bridge comprises two segmental arches reinforced by seven arch ribs; however, documentary evidence states that there were once three arches. An eighteenth-century watercolour by Thomas Girtin shows a rounded ‘Norman’ third arch which was possibly a survivor of the earlier bridge that has since been enveloped by the buildings at the Durham end. Like Elvet Bridge, this bridge had buildings on it, including a defensive tower with gateway on the peninsular side along with shops but these were all demolished in 1760 to make more room for traffic. Early in the nineteenth century the bridge was widened on the upstream side to its present width of 27ft (8.2m) and the arches strengthened with iron bands to the crown.

Framwellgate Bridge in Durham originally had buildings on it.

Radcot Bridge

Radcot Bridge was built soon after 1154, with three Gothic pointed arches in Toynton stone, by Cistercian monks of St Mary at Cîteaux in Normandy, who had been granted the land by King John. It is reputed to be the oldest surviving bridge on the Thames; however, it was largely rebuilt in 1398 having been partially destroyed during a battle that took place there six years earlier between Henry Bolingbroke (later King Henry IV) and troops loyal to Richard II. The bridge was again damaged during the Wars of the Roses and was rebuilt with a more rounded central arch, in the Tudor style. The downstream parapet has a socket believed to have held a cross. Radcot became commercially important as the highest shipping point on the Thames where stone from Toynton quarry, near Burford, was transported downstream and as far away as the Abbey of Denis in Paris.

Radcot Bridge was built by the Cistercian monks of St Mary at Cîteaux.

Medieval London Bridge

Peter of Colechurch, priest of St Mary Colechurch in the City, organized the building of London’s first stone bridge in 1176. There are no drawings of the Medieval London Bridge upon its completion and all we have are the archaeological remains and retrospective records dating from circa 1500. The stone bridge was 906ft long (276m) with nineteen stone arches set on starlings (the twentieth arch was spanned by a drawbridge). Built without the use of coffer dams, the process was to first drive a ring of short timber piles into the riverbed at low water using a human-powered piling rig set up on barges. Then the ring was infilled with rubble. Finally, a larger and more powerful piling rig was set up on the newly created platform and much longer iron-shod elm piles were driven all around; these would have formed the protective barrier (a starling) around the platform. The gaps between the long piles were infilled with planking and the enclosure backfilled with rubble. These starlings were subject to strong tidal forces in both directions together with freezing conditions and floodwater and required constant maintenance. On top of the consolidated platform the actual stone pier was constructed on a bed of pitch. Afterwards a temporary wooden framework (centering) was erected to support the arch during its construction.

Engraving of Medieval London Bridge in 1507. Author’s collection.

London Bridge was maintained by funding from many sources including tolls for crossing the bridge, rent from the many buildings on the bridge, and legacies and gifts from wealthy Londoners. In addition to dwellings the main buildings on the bridge apart from a chapel were shops and two defensive gateways (barbicans). The Great Stone Gate stood on the second pier; the Drawbridge Gate was on the seventh. Buildings on the bridge were frequently rebuilt: in 1633 a section had been destroyed by fire up to the chapel and was only a quarter rebuilt by the time of the Great Fire of 1666, leaving a fire break that saved the rest of the bridge from being consumed. All the buildings were finally rebuilt in 1683. A water wheel was installed at the first arch at the northern end and this was used to supply water to the city. Further water wheels were later added, and these continued to supply water from the Thames right up to the eighteenth century. The power of the river was also harnessed to grind corn, with mills being set up at the south end of the bridge.

Upon completion of a bridge at Westminster in 1749, dissatisfaction with London Bridge grew. By comparison to the airy and spacious outlook of Westminster Bridge, London Bridge was narrow, dark and dingy. Under an Act of 1756 the bridge was widened, all the buildings were removed, and the ninth pier was removed to make way for a grand central arch. The remodelled bridge was completed in 1762 with stone balustrades and alcoves. Unfortunately, the increased flow permitted by the larger opening caused scour of the starlings; this had a detrimental effect on the foundations of other bridges upstream and it became evident that the bridge needed replacing.

Trinity Bridge

Trinity Bridge in Crowland, Lincolnshire, was once the crossing point of three streams – Welland, Nene and Catwater – but is now left high and dry following the diversion of the rivers. The bridge was built by the masons of the local Benedictine abbey between 1360 and 1390 and comprises three converging half arches arranged in a triangle and meeting in the middle as one structure. Too narrow for carts and too steep for horses but tall enough to allow boats to pass under, the bridge has baffled and beguiled visitors for centuries (records dating back to 716 make reference to a triangular wooden bridge at Croyland). Trinity Bridge was constructed from rag-stone that was quarried at Barnack, some 16 miles (26km) to the east where it would have been carried by sleds, then loaded onto barges and transported down the Welland.

Trinity Bridge in Crowland.

Medieval Masonry Spans

Brig o’ Balgownie

Brig o’ Balgownie (originally Brig o’ Don) crosses the River Don in Old Aberdeen with a single pointed Gothic arch and spandrels of sandstone. It was reportedly started in 1290 by master mason Richard Cementarius following instruction from Bishop Cheyne, then completed by Robert de Bruce and repaired in the fifteenth century. However, the bridge we see today is the seventeenth-century reconstruction with new granite approaches that were widened and buttressed in 1912. The arch stands 56ft (17m) above normal water level and spans 39ft (12m) over a deep salmon pool called Black Neuk which merited mention in Lord Byron’s poem ‘Don Juan’. The deck is 10ft 6in wide (3.2m) between corbelled parapets with iron ties on top and a highly pronounced but irregular string course. Throughout its history the bridge was a prized tactical asset and being on the main access route to and from the north east of Scotland into Aberdeen, its possession was highly contested.

Brig o’ Balgownie over the River Don. Author’s collection.

Devil’s Bridge