Appearance

In his book, "Brücken – Bridges", Fritz Leonhardt devotes much space to the discussion of appearance, which is a topic of great importance. The cost of achieving an ideal shape may be too much in the case of a small bridge. As Leonhardt says, a slightly tapered column is often preferable to a parallel one, but in a very small foot-bridge, the cost of special shuttering may not be considered acceptable.

One of the points that Leonhardt makes is that a form that is technically satisfactory may be unsettling to the layman because it looks unstable or inadequate, to those not "in the know". In such a case he would advocate a design which looks "right". Certainly it is not difficult to find bridges, and more especially buildings, in which the proportions and detailed treatments appear to have arbitrary features.

One of the most elegant and pleasing structures in Gloucester is a humble footbridge across the northern bypass near Longlevens. It is deceptively simple – careful but unobtrusive measures have been taken to obtain an attractive appearance Unfortunately it is becoming increasingly hidden by trees. It has parallel piers, and it might possibly look even better with slightly tapered ones, as Leonhardt often suggests, but in a small and relatively cheap project this could add an unacceptable cost. The bridge looks right as it is, probably because the piers are not very tall. Detailing that looks good in a large structure is not always best in a small one. Small cars are not simply scaled down versions of large ones. Sheds do not require as much detail as a cathedral.

Compare this bridge with Gloucester Cathedral, which contains a hotch-potch of styles, and a number of added engineering details which were needed in order to achieve stability. It gets away with it because it is so big that you cannot see all of it at one time. Had the concept of "listed buildings" existed when it was begun, the later additions might not have been allowed, and of course demolition would have been out of the question. Apply this idea to all the earliest English cathedrals and churches and you will see that later styles such as the perpendicular might never have become established. But then, cathedrals were built for the worship of god, and not as works of art. And the shapes that we find so fascinating are mostly an expression of the requirement to create great open spaces with plenty of light, without risk of collapse. Some big medieval cathedrals did collapse, because the builders went a little too far, or because the ground was not rigid enough. Soil engineering is not glamorous, but knowlegde of it is essential.

What is the "correct" attitude to conservation? Is it necessary to use "authentic" materials, or will correct appearance suffice. What if thatchers used different materials at different times, depending on price and availability? What if the people of older times modified their buildings to suit their wishes? In what era do we "freeze" the design. Should we conserve at all? What would happen if people could modify or destroy any building they could get their hands on? What would happen if large numbers of buildings could never be demolished? And what would happen if the design of every building had to be passed as "suitable" for its location by a committee of "experts"? Or a committee of unqualified citizens? How much of your own surroundings are controllable by you? Or by anyone at all? Should a person be allowed to build a house of any desired appearance, as long as it is safe? What do you think about this topic?

The details of a structure can be as important as the overall shape. Leonhardt points out the difference between struts which are small lattice girders and ones which are tubes or rectangular boxes, giving a clean, uncluttered appearance.

Surface treatment, too can make an enormous difference to appearance. A vast expanse of flat grey concrete on a dull day is not an attractive sight. Fortunately, modern techniques can produce pleasing effects. Great care is necessary in the use of pattern slabs; if the repetition is readily visible, it can create an unpleasant or depressing effect.

Here is one span of a multi-arch bridge over the river Lune. It carries a canal. Look at all the details – it even has a plaque about the building. This bridge would work perfectly well if the surfaces were all geometrical planes and cylinders. But it would be a very unattractive structure.

Here is another bridge over the Lune – Skerton bridge at Lancaster, again with interesting treatment. This part of the web-site is called "Bridge Building – Art and Science". And these bridges reflect one aspect of the art.

But a far deeper art is concerned with the original conception, with the details of the design, from the foundations to the parapets, with the need for economy in materials and construction, with utility to the users, and to longevity and economy of maintenance.

What looks very pleasant in an artist’s impression of the structure on a sunny day, may look very different in reality. Bridges, like other structures, so not exist in isolation. Tourists will travel great distances to see towns that are elegant, or which are a jumble of old or "quaint" buildings, but we can hardly build for tourism. A town is, or should be, a living organism. Each new addition has to fit in as best it can with what already exists, and should be built with a purpose, for which it should be well fitted. More about this topic is found in the page about town bridges.

Finally, the effects of weathering are extremely important. Rust and other corrosion, cracking of concrete, deposition by dirty water, and flaking of paint, are some common causes of unattractive appearance after the passing of time. Plain concrete seldom weathers as pleasantly as, say, oolitic limestone, sandstone, or marble.

This raises the question of disguising concrete or steel by covering the bridge with another material. Early bridges by Maillart used this technique, but he soon dropped it in favour of revealing the true form. This is far from being a trivial question; people have to live with the consequences of a construction, especially in towns. An often quoted example is the bridges of the Palisades Parkway north of New York city, which have led to some strongly held opinions. When you actually take the trip, you see how these bridges fit in, and why they were designed that way, though today, using techniques now available, the bridges might be built differently. What about the Tower bridge in London, which was supposed to fit in with the local buildings? Would anyone build like that today?

Perhaps what matters is not so much the method used, but the quality of the result. Even now, designers are using disguise. Some concrete beam bridges near the "Second Severn Crossing" have plastic coverings which give a very clean appearance. These are not merely cosmetic: there is room inside for inspectors and other workers to operate with some protection from traffic noise and weather.

In France and Japan particularly, you see a relatively large number of rather brightly-coloured bridges. They look none the worse for that.

Some structures, like the Pompidou Centre, have been built with many parts visible that would normally be hidden.

Consider what people buy if they have a choice. They buy cars, houses, furniture and equipment which they like the look of. Most of us have no choice about our larger surroundings – the town, its buildings, street furniture, bridges and other structures. We have to live with what is provided. The design of large structures are not often subject to the need to obtain repeat orders from the same customer. Should these considerations affect the design of structures? If we make only what most people want, design may not advance, but if we make whatever we feel like, we may upset many people. In practice, nobody creates large structures without long consideration of all the factors. In various parts of this web-site you can find things as mundane as roads, which have been designed with great care for their appearance.

Something that is never visible is a force. All structures have weight, and all experience forces within themselves. Trusses and similar constructions direct the forces along the members, and so we know where they are. But a beam, at the other extreme, conceals the direction and strength of the stresses. A beam with hidden stressing wires does the same. So the argument that we should be able to "read" a structure falls apart for these apparently simple cases. And foundations are always invisible.

"Market forces" too, cannot be seen, yet economics will have played a part in the design of most structures. Unseen in every case are foundations. We can only guess at what lies below and beside a structure. In a suspension bridge, or a very flat arch, we can of course get a sense of what is being done by the anchorages or abutments.

A factor that greatly affects the structure and appearance of any structure is the scale. Big birds are not like scaled up small birds; an elephant is not much like a mouse; a Cessna is not much like a Boeing 747, and that is why some science fiction films are so absurd. If you put as much detail into a garden shed as into a medieval cathedral you would be laughed to scorn. So the builder of small bridges may be more constricted than the builder of large ones.

Surface treatment will show more in a small structure, as will rust, peeling paint, and dripping cracks. Most people may not notice what a bridge looks like, but then, many people don’t consciously look at all the details of the decor of a pub or restaurant. And nobody would deny that the decor affects people’s mood, or the likelihood that they will go again.

Here are two fine bridges – the Ponte Vecchio in Firenze, and the Pulteney Bridge in Bath. Clicking on them will show bigger pictures. Both are works of art and science. People come from great distances to these cities, and many would count these bridges among the sights they want to see. In neither case has any attempt been made to disguise the engineering, yet both structures work visually very well.

There are places in Bath, Bristol and Cheltenham where huge arches lie on their sides, like arch dams, such as the Royal Crescent and the Circus. But these curves do not resist any forces, so they are geometrical architecture and not engineering. A walk in Bath will show how people here have been fascinated by both rectangular and circular designs in buildings, from Roman times onwards.

In fact, in architecture, far more than in engineering, we are far more likely to see geometry as decoration, in the form of apparent arches, beams and pillars which don’t actually do anything, and are just added for effect. The effect range from the beneficial to the absurd.

Here are some diagrams, based on real bridges, to look at and consider.

Arched foot-bridge

The first pair of diagrams is based on a concrete foot-bridge over the M42 motorway, near junction 11, A444. Which bridge do you think is technically more correct? And which do you think looks better? What are your reasons? How would cost affect the choice of design?

Solving the engineering problems may not be considered enough. In the left picture the view from the road shows a somewhat disordered appearance of the cables. This can be even worse if the cables are fanned out from a horizontal row of holes.

To achieve a vertical plane of cables, the second arrangement can be used. Another solution is to abandon the idea of a vertical plane and make an A-frame, which is very rigid.

A third way is to use only a single plane of cables, relying on the deck to provide stiffness against torsion.

Whereas the towers of suspension bridges are generally based on towers with vertical members, those of cable-stayed bridges have been given a variety of treatments.

Does it matter what bridges look like? Does it matter what anything looks like? Apart from the people, a town is made of all the buildings, roads, street furniture, bridges, and street furniture. Perhaps no one of these makes much difference, but the whole is definitely made of its parts, and must surely affect the way that some people feel and think. Nobody would build a town so that tourists would like it centuries later, but there is no doubt that many people visit cities and towns because they enjoy being in them.

What would happen if a town were built entirely of structures that had been designed to look good, and to go well with each other? Would we just get used to it and raise our expectations, or simply not notice it, or would we really find it pleasant?

If you drive from London to the other end of the M1 motorway you will see a summary of the history of motorways and motorway bridges in England. You can see very clearly the changes that have occurred in attitudes to design and construction since the early days.

Appearance of Arches

Arches are in principle relatively simple. Nevertheless they are capable of tremendous variation in style, from the heavy Roman type with wide piers, to thin deck-stiffened concrete types.

As in any other type, designers may be tempted to test the extremes, in flatness, slenderness, span, and so on. At the extremes it is perhaps more difficult to achieve good proportion than in a more average structure. How flat can an arch be made before the viewer feels a sense of strain? Is this feeling influenced by familiarity with multi-arch bridges for river crossings in towns? How far can cornes de vache be taken without looking wrong? What about the proportions of arch, spandrel walls, and deck in a concrete arch?

Some large steel arches, like the Hell Gate bridge and the Sydney Harbour bridge, have massive masonry pylons at each end, marking the transition between the main span and the approach viaducts. The Bayonne bridge does perfectly well without them, but does it really look right? Don’t we need to see some visible terminator?

In fact, the combination of main spans is a difficult issue, because the engineering requirements are so different, yet the whole structure should look as though all the parts belong.

Appearance of Cantilever Bridges

The design and appearance of a cantilever bridge is inevitably affected by the presence the three essential parts – the two cantilevers and the suspended span. The Queensborough bridge has no suspended span, but it does not look better for it. The alternatives are to make the structure plain by giving the suspended span its optimal structure, and to disguise the structure by integrating the shape of the suspended span into the overall design. The second technique is usable with the smaller bridges, but in the larger sizes, it is impractical. This subject is discussed in more detail in the page about cantilevers, where we see that the presence of the two types of structure can even be used to produce unusual designs.

Here is an example where the shape of the suspended span has been integrated into the overall shape.

The designer has attempted something very difficult – the creation of a convincing single curve for the entire span. It is a matter of opinion as to whether a given example is successful, but it would be difficult to find a better example than this. Several bridges of this design are found in Gloucestershire and Wiltshire.

Appearance of Suspension Bridges

In this type of bridge the design of the towers and the deck are bound to dominate, though approach viaducts need careful design to integrate them into the whole. Aerodynamic decks can now be so narrow that the towers must not appear too massive. Although D B Steinman hinted at Gothic design in some bridge towers, later designs have generally been quite simple, though in the 1990s there has been in the UK some tendency to unusual designs in small foot-bridges.

Reaction to the Tacoma Narrows crash had a long lasting effect on deck design, which was reversed only when the aerodynamic deck was introduced. Even then, many designers persisted with trusses. Certainly, for a foot-bridge, not many people want them to move detectably.

Fritz Leonhardt’s book on bridges is well worth reading for his observations on the aesthetics of bridge design.

Some Small Bridges

Here are some examples of bridges near Gloucester.

Near junction 11A of the M5 motorway, Horsebere Brook is crossed by the link road from Gloucester Business Park to the Brockworth bypass and southbound M5 motorway, over an arch based on curved concrete slabs. The slightly non-circular profile increases the headroom over the farm track and the footpath which accompany the brook. Although this bridge is seen by a relatively small number of people, the designers have taken the trouble to achieve a very pleasant appearance. At the other end of the tunnel, a wooden footbridge crosses the brook.

Near this bridge, the link dips under the road that connects Brockworth and Hucclecote. The footpath and the road are carried on two bridges based on pre-stressed concrete beams, shown below. The whole site has been the subject of great attention to detail.

The last two pictures show the side walls of the cutting, which use textured and sealed concrete slabs, which are coloured rather like Cotswold stone. The exact appearance depends on the angle of the light, and whether it is direct or diffuse. Although the panels are all the same, this is only apparent on close inspection. Attempts in earlier times to disguise concrete by patterning have often failed because the repetition was all too obvious, leading to a depressing monotony. The entire approach to the site presents a pleasant appearance.

The Brockworth bypass is a part of a road that connects the M4 and M5. This road, A417 and A419, is dual carriageway except from the bottom of Crickley Hill to the Nettleton roundabout, and around Blunsdon.

This picture does not look very interesting, but it illustrates the idea that good engineering does not have to be big or spectacular to be imaginative. This is one of the bridges which were built as part of the Cirencester bypass, a part of the road mentioned above. The trees are a part of a very old wind break, and were preserved by widening the gap between the roadways, leaving the trees between. The rooks are still using them for nesting. This project included the building of long sections of Cotswold stone wall, and the planting of many saplings. Great care was taken to minimise the effects of this road on the landscape and its inhabitants. Cirencester and Latton have benefitted greatly from the elimination of the heavy traffic, while drivers have gained about fifteen to twenty minutes and lost a lot of frustration.

This bridge illustrates the difficulty of achieving an orderly appearance when there are numerous piers. The problem arises because there are two rows of piers. What solutions can you think of? The fences are inclined inwards for no obvious reason. Does this improve the bridge?

Whatever you think about bridges and their appearance, even a slight acquaintance with the subject will reveal that the engineer is not merely plugging numbers into formulae that generate beams, trusses, suspension spans, etc. He or she is creating a work which is fitted for the site and the purpose. True, some companies sell standard small bridges: true, Freyssinet produced a "limited edition" of the elegant Marne bridges. But many artists have produced limited editions, some of which are quite large, and we don’t think less of Song dynasty porcelain because each artefact was worked on by as many as thirty people on a production line, each person performing his own task, or because large numbers of each product were made. Many modern artefacts, such as cars and consumer goods, are made in vast quantities, but they can still be regarded as attractive.

Bridge engineering is at one place on a spectrum that includes art and science, craft and music. There is plenty of room for human expression, so long as it is tempered to the needs of the users. Compare Telford, Brunel and Stephenson, or Steinman and Ammann. Look at the work of Calatrava and Muller, to mention just a few names.

Some bridges look very geometrical, but as with many natural forms, the apparent mathematics arises from the physical and engineering requirements. (See Nature’s Maths) As you can see in the page about Beams, even a beam, the simplest looking form of bridge, conceals a range of subtle stresses that cannot be imagined by looking at it. So the purist argument that structures must always show what they are doing, like any other argument, becomes absurd if taken beyond a certain point. Equally, actions like using pebble-dashed concrete, or trying to disguise completely the nature of a structure, probably represent undesirable excursions in the other direction.

Finally, a determination to be original or to create the biggest or the longest, if it becomes too dominant, can over-ride the requirements of utility or even safety. There are a few big structures which carry only a small fraction of an economic or useful traffic load.

An unusual bridge

This is Peto’s bridge in Bristol. It opens by rolling on arcs and tracks with large teeth that prevent slipping. The two large objects that look a bit like funnels are the counterweights, which probably have drainage at the bottom to prevent rainwater changing their weight. Perhaps they are intended to look like lilies or ear-trumpets. The movement is controlled by a single hydraulic cylinder and piston. Note also the shapes of the piers.

The design of this bridge seems to be as much sculptural as functional – a bit of fun perhaps.

Appearances are not always as planned

Many a structure that looked so good in the artist’s impression, depicting the scene on a sunny spring day, with imaginary trees in leaf, later looks dowdy, as a result of fading or peeling paint, wrinkled or cracked surfaces, dirt, corrosion, creep, and other forms of entropy.

More about the appearance of bridges can be found in Town Bridges and in Foot-Bridges

Back to Bridges back to Home Page

For answers to questions – please go to end of page.

A Multi-span Beam

The A417/A419 from the M4 to the M5 provides a dual carriageway for most of the route. A tremendous effort has been made throughout in order to create an attractive route. Some farm land had to be purchased, and some old Cotswold walls had to come down. Some were already old and crumbling. In return, the road builders have preserved a well-known wind break near Duntisbourne, they have planted large numbers of saplings, and they have built miles of new Cotswold walls.

The designs of the new bridges are as attractive as possible, given the technical requirements, and the whole route is a good example of modern construction.

Near Baunton in Gloucestershire, an interesting beam bridge crosses the valley of the river Churn, north of Cirencester, carrying the A417(T). This part of the route is a bypass of Cirencester, which relieved that town of through traffic, and took as much as fifteen minutes off typical journeys. The pictures below show this bridge from below. The line of the road is slightly curved in both dimensions, and the bridge is a continuous beam.

Crossing this valley, including the tiny river Churn, presented the problem of appearance. The actual construction tends to obscure the view of the valley from the road. Let’s look at this in some detail.

Firstly, the valley is barely visible from the road in most of the route, because of trees and hedgerows. That it is visible here may be partly due to the effects of construction work. Within a few years, trees will have grown up to hide the valley from the road. So it could be argued that the view doesn’t matter very much.

Secondly, what is the intention when building such a bridge? If we want to make the bridge as inconspicuous as possible, we can build it on very narrow piers. If this had been done in this case, replacing each pier by two narrow ones, making four at each position, there would have been a good view through the bridge, but the effect of all these piers would have been very untidy. Obtaining an ordered appearance from all directions is an almost insoluble problem with a two-dimensional array of piers. The other difficulty would have been the avoidance of a top-heavy appearance.

Another possible solution would have been to have one pier at each location, on the centre line of the road. The very wide beam would have had to be very stiff indeed to carry torsional forces to the abutments, increasing the cost of the structure. Alternatively, these single piers could have been splayed at the top to provide paths for the asymmetrical forces.

At the other extreme, solid walls would have produced a very heavy effect. The actual design balances the mass of the beam and the piers very well, and the hints of gothic curves echo those of the nearby Cirencester church and Gloucester cathedral. At the top of the piers, the outer faces are not quite vertical: evidently someone took great care with the appearance of the bridge. See picture below.

The view across the valley is more or less blocked, but was not especially inspiring in any case.

The river Churn, which lies at the bottom of the valley, is only about two metres wide at this point. The picture shows it a few miles downstream as it reaches Cirencester. Fairly soon after Cirencester it is joined by the little river Thames. The names Churn and Cirencester probably echo the Roman Corinium, an important fortified town. The A417/A419, apart from the new bypasses of Cirencester and Latton, follows the old Roman road, later called Ermine Street, quite closely. Comparing a map of Roman roads with a modern map shows that the routes of many Roman roads can still be traced via modern roads, tracks and footpaths. The influence of the Romans on our language is also enormous. On the road we may come across words such as bus (omnibus), car, engine, entry, exit, logistics, traffic, trailer, transport, and vehicle, all derived from Latin. The Corinium museum in Cirencester offers an excellent insight to Roman life in Britain.

Just south east of this bridge, a minor road, the Whiteway, crosses the bypass. At this point, as on the other side of the Churn valley, the A417 is in a cutting, to reduce the gradient down to the Churn bridge, and to reduce the height of that bridge. The typical bridge in this situation would have been a simple concrete beam or cantilever type, but what we see is apparently a flat arch, faced attractively with Cotswold stone. But as we pass underneath, we see that the arch is only a facade, and that the bridge is a concrete beam. In fact, under the right hand side of the bridge, you can see a part of the sloping haunch.

Is it a fraud? Well, it looks a lot better in that location than a grey concrete bridge would have done. It avoids the concrete piers that would have blocked the view of the cutting, and it generates a lot less thrust than a real flat arch would have produced.

This picture, looking north from the small over bridge, shows the deck of the big beam in the distance. We can see the slight curvature of the bridge. On each side of the road, on both sides of the bridge, the Jurassic limestone is cut into steps, to improve the appearance of the cutting, and to provide footholds for the growth of vegetation, as the second picture shows. No doubt the fissures in the rocks will provide shelter for a wide range of invertebrate and vertebrate life. Kestrels are a common sight over verges, and the occasional buzzard is seen on the fences on days when thermals are not forthcoming.

These are just a few examples of the thinking that lies behind the A417/A419 link road.