The  Appearance  of  Bridges - Part  One

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Appearance of Towns

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Cost of -

Design

Materials

Land purchase

Demolition

Compensation

Construction

Maintenance

Improvement

Removal

Replacement

These are some of the considerations that might affect the design and building of a bridge.  It is not always easy to work out a balance between them.  The appearance of a structure, including its relationship with its surroundings, is not something to which it is easy to assign a value. 

Usefulness -

Amount of business use

Amount of private use

If a toll is levied, the  amount of use is very important.
The structure has to be constructed, and to continue existing in, some kind of environment, which is quite possibly valued by some individuals, or groups of individuals, or communities.  It may become iconic, like Sydney Harbour bridge and Sydney opera house, or it may be unwanted and unloved.
Effects on -

Appearance

Business

Flooding

Housing

Noise

People

River flow

Smell

Tidal flow

Traffic flow

Wild animals

Wild plants

 

In his book, "Brücken - Bridges" (Architectural Press, ISBN 0 85139 764 6), 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. To see many excellent pictures of a wide range of bridges, try "Bridging the World" (Bridge Ink, ISBN 0-9641963-3-6)  by Robert S Cortright.

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.  Some structures, indeed, seem to have been designed to an aesthetic idea rather than a function, and  some constructions are quite strange.  The quest for originality is not new, but should it have any place in construction?

Longlevens3A.jpg (80892 bytes)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.  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 cathedrals.

Disappearance of bridges

Unfortunately most of this bridge is hidden from the road by trees, which form a sound barrier for the houses on both sides.  A plan would look something like this.

By planting the trees differently, the plan could have looked like this.

But who would have cared?  For most people, a bridge is just a structure to help them of someone else to get from A to B.  The answer to this is that some people do care, as we see from as web-page about the replacement of the Waldo-Hancock bridge.

KelvinArchA.jpg (168161 bytes)Here is another bridge that is impossible to see properly - a concrete arch over the river Kelvin in Glasgow.  There is a hint of Maillart in it.  In fact it's difficult to see any of the Kelvin river bridges properly, because of shrubs and trees.  How strange that we are so uninterested in bridges that we allow them to become hidden by foliage.

Compare these bridges 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, and because most people probably think it is rather charming like that.  Had the concept of "listed buildings" existed when the cathedral 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 knowledge of it is essential.

What is the "correct" attitude to conservation?  Is it necessary to use "authentic" materials, or will correct appearance suffice.  In the UK, there are strict rules about "listed" buildings.  For example, replacement thatch must be from the "right" species of plant.   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.

LuneArchesQJ.jpg (105347 bytes)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.

LuneArchesPP.jpg (41586 bytes)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.

KerneA.jpg (287719 bytes)Yet another arch bridge, of a much earlier time, with only minimal decoration - the grooves carved into the voussoirs, subtly breathing life into the design.  And look at the way that the grooves do not simply cross the blocks from end to end, but occupy areas within a narrow border.  You can see this at the left edge of the picture.  The bridge is the Kerne bridge over the river Wye.  Contrast this subtlety with the brutalist style of such as the University of Essex, which does not "compromise" with such effects, leaving us to view vast areas of bare, grey concrete with no relief.  The builders of Kerne bridge also provided a line of blocks which correspond to the level of the deck; they indicate the line of division between the structure and the parapet - a case of form indicating function, perhaps.  This feature is not uncommon, and is seen in the next bridge also.

BredwardineB.jpg (396288 bytes)There is in fact a very significant difference between the masonry constructions of the past, and the steel and concrete constructions of today.  If you are assembling something from small bricks or blocks, the provision of decoration is possible at little extra expense, just by the subtle variation in placement, size or shape of some of the units.  You could, for example, make every third brick along the top of a bridge protrude a little.  Look at the subtle details of the bridge illustrated here.  Or you could use several different block sizes in different rows, to avoid the monotony of many similar layers.

You cannot do this in steel or concrete - the units are far larger, and they are produced by processes that do not allow for variation - rolling steel I-beams and pouring concrete into shuttering, for example.  Adding decoration is liable to seem completely bogus.  You can deploy special shuttering to create surface texture in concrete, but great care is needed.  Another way is to add texture afterwards.  During the building of the Nuclear Physics department in Oxford University, one concrete wall, made with smooth shuttering, was attacked with a pneumatic hammer for a whole week in order to produce a rough textured surface.

But a far deeper art than decoration is concerned with the original conception, with the details and overall balance 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.  When we look at masonry arches, some of which date back two thousand years, their variety is amazing, and we are constantly impressed by the number of cases where the builder has produced a structure which "looks good".

On the other hand, what looks very pleasant in an artist's impression of a 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.  On the other hand, concrete is more durable in a corrosive atmosphere than, for example, oolitic limestone.

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.  On the other hand, some designers, such as Architen Landrell, strive for elegance and simplicity.  Click on "products" and "portfolio" in their site to find out about ingenious examples of building structures.

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.

Form and Function

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.

MilfordHBig.jpg (163418 bytes)Some people argue that form must follow function.  Consider this box-girder bridge near Milford Haven.  What function is the form to follow?  At each pier the shear stress changes polarity, and the bending moment changes quite sharply too.  Although the outward appearance of the beam shows no sign of it, the structure must have internal parts that distribute the forces from the pier to the bridge.  But how could the external shape express that?  Consider another function - the requirement to enable vehicles to cross with minimal disturbance to their itinerary.  This bridge, and many others with longer and higher spans, express this function very well indeed; the box-girder sweeps across the estuary without interruption or deviation.

"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, and we may even feel some discomfort on behalf of the bridge.

In an effort to obtain "good design", should every engineer's work be looked over by an architect?  If so, which architect?  And how how much control should the architect have?   There are many pictures in this web-site of old bridges that look very good indeed, though none of them were subject to interference from people who were interested in aesthetics or architecture as separate subjects.  The division of activity into specialisms, with degrees and other qualifications is a modern phenomenon, necessitated by the complexity and sheer multiplicity of modern technology.  But those who practice the various arts do not necessarily have "qualifications".  To write music, which is a technical business, it helps greatly to have specialist tuition.

Scale

A factor that greatly affects the structure and appearance of any structure is the scale.  Big birds are not like scaled up small birds; a dragonfly is not like an aphis; 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.  Those are effects of length, volume, area and weight, as Galileo beautifully explained.  There are aesthetic considerations too: 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.  But a vast area of flat concrete can look unfriendly and inhuman.  The human eye and brain seem to need some detail at certain scales in order to feel comfortable.  Masonry provides this in the form of joints between blocks or bricks, and in the case of stonework, in the surface of the blocks, which may be flat or bulging, rough or smooth.

On the other hand, a small size can be a boon - the designer may be able to create a simple structure, free of detail, that nevertheless looks very good.  Many fine footbridges have been made in plain concrete or steel.  Here are some examples.

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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.

Art and Science of Bridge Building

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.

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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 in Bath.  But these curves do not resist any forces, so they are aesthetic and geometrical shapes and not engineering curves, and certainly not arches.  A walk in Bath will show how people here have been fascinated by both rectangular and circular designs in buildings, from Roman times onwards.

We must not, however, assume that all horizontal curves are non-structural.  Long pipelines include curves at intervals to allow for thermal changes.  A bridge with long spans may be curved in plan, not only to provide for expansion, but to reduce the amplitude of oscillations.  

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 effects range from the beneficial to the absurd.

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

Arched foot-bridge

The next 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?  The actual bridge is more like the lower diagram, which is based on circles.  The upper one is based on parabolas.

M40ArchFebA.jpg (139668 bytes)Here is a similar bridge over the M40 motorway, at a skew from the line of the road.

A  Very  Peculiar  Bridge

What is wrong with this design?

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