Arch

Imagine a grotesque animal with legs sprouting in all directions, but with no apparent head. It seems to be engaged in a futile struggle within itself, sometimes making convulsive movements in seemingly random directions. What is it? It's a rugby union scrum. Sixteen people push with thirty-two legs, each half trying to push the other half back. If fifteen of the people could be instantly removed, the remaining one would fall to the ground. You don't need to be a student of Zen to know the answer to half a scrum. One half cannot stand without the other.

Sometimes the scrum is almost perfectly balanced, at other times one side can push the other back. The scrum can even turn, which is not allowed, or it can be made to collapse, which is dangerous. What keeps it up? The heads and shoulders of the players push forward, and their weights pull down. These forces are resisted by the upward and forward push of the ground. The scrum is a living arch, and it demonstrates all the principles of that structure. It exerts a weight on the ground, and it exerts outward thrusts, resisted in this case by friction.

The cathedral of Seville, with its double row of buttresses on each side, is the structural equivalent of a scrum, with the two sides of the nave corresponding to the two front rows, and the two rows of buttresses on each side corresponding to the remaining two rows of players on each side.

Another living bridge is the case of a climber bridging across a wide chimney. In this example, her or his weight is resisted by the vertical friction that is generated by the outward force on the walls.

The problem

Here is the problem - how to get a road across a river.

The small river running from left to right in the picture is the river Windrush at Upper Slaughter, in the Cotswolds.

The solution

The simplest solution, shown above, is a ford, and as this river is normally only about 15 cm deep, a ford is adequate, if you drive slowly. On this occasion, however, the river was much deeper after heavy rain, almost covering the openings of the small two-arched footbridge on the right. The picture at left shows the bridge with a lower water level. Fords are only useful for small, shallow, slow-flowing rivers. Sometimes, in time of flood, water flows across roads or even bridges. Vehicles can be borne away by the water, which has lifting power that is astonishing. Remember that one cubic metre of water weighs one tonne, and that the lifting and carrying power of water doesn't just double when the water goes at twice the speed - it increases much faster than that.

Here is another ford, about a kilometre from the source of the river Coln, flowing from right to left. Again the pedestrians can use a small bridge. The second picture, a typical Cotswold view, shows a part of the nearby village of Sevenhampton.

The real answer is commonly a bridge, and rarely a tunnel, though ferries are still found. It is rare for a road to pass under a river, though aqueducts, including some famous and large ones, carry canals over roads, railways and rivers.

What about this "ford"? It has only one side.

The answer is a boat, as I K Brunel realised during the 19th century. He imagined people travelling from London to Bristol by train, and then boarding a huge ship to travel to New York. The idea of integrated transport systems was sound, but the technology was not there at the time. Brunel did design three revolutionary ships, but economic and technical considerations got in the way of success. For completely different reasons, Bristol was associated with another transport failure during the 20th century - the Bristol Brabazon airliner.

Some Very Small Arches

The arches in the first picture, over the River Windrush at Barton, near Guiting Power, in the Cotswolds, must represent about the smallest size of arch that was worth building. In fact they are culverts rather than bridges, carrying the river under a minor road. Nowadays the bridge might be built another way, as in the examples in the panel below. Notice the deformation from the original semicircular shape. The other picture shows a similar construction in brick. Notice the voussoir (sector) that has moved out of position. We already learn something about arches. They seem to be quite stable when deformed.

Here is another small arch.

Here are more examples which have been constructed as pipes.

The small arches in these pictures are based on tubes made of corrugated metal. Plenty of space has been provide for extra flow in times of spate. The ends are faced with reconstituted stone, which has not been arranged to look like arches - just layers of "blocks". Whether structures should be made to conceal their workings is an interesting subject for discussion. If one function of a structure is to look pleasant, then it may not be obligatory that its structure and technical workings should be obvious at the expense of appearance. In case, in a real structure, foundations and pre-stessing cables are never visible, for example.

A bridge that looks like this appears in William Hogarth's satirical frontispiece to Dr Brook Taylor's Methods of Perspective. The picture includes not only ridiculous examples of wrong perspective, but a bridge that could not stand up. David Hockney copies the bridge in Kerby (After Hogarth) Useful Knowledge. Hockney has pointed out that the errors in perspective can actually create space, just as correct perspective does, but differently. So both Hogarth (with the bridge) and Hockney (with the perspective) remind us how very differently the artist and the engineer look at the world. In both pictures, the layered blocks resemble those of a "corbelled arch" (not an arch at all), described elsewhere.

After all, stressing wires are seldom visible, so the observer cannot see true nature of a structure which uses them. And foundations are never visible.

A pleasant feature of both arches and suspension bridges is that their structure and function are fairly clear, in combination with an attractive form. The great importance of appearance is emphasised by Fritz Leonhardt in his book "Bridges".

A similar construction takes a brook (left picture) under a slip-road of the M5 motorway. You might think that this is a fairly simple thing to construct, but when you go into the tunnel, which is about fifty metres long, you find that large piles of stones have been placed along the stream on alternate sides, making artificial meanders which reduce the speed of flow in times of flood. Even the simplest project requires thought about possible future events. The bridge in the right hand bridge even has a road under it as well as over it: it allows access to a large car-park.

Some Larger Arches

Arches vary enormously in appearance, as the following pictures show. Some of the variables are -

Length of span

Width of deck

Height above ground or water

Height of piers

Ratio of span to width of pier

Ratio of span to rise

Number of spans

Materials

Segmented or curved

Type of construction

Arch at Niagara Falls

Bridges of Paris

The arches of le Pont des Arcs are very unusual. They appear to support the footway only at the half-span and quarter-span points, while requiring support themselves from the piers.

This is a part of the Musée d'Orsay in Paris. This wonderful building was once used to house a railway station.

Val d'Herens

This is a short tunnel, rather than a bridge, but it is a neat solution to a problem. Will the solution outlast the problem?

Some British Arches

Links to other arches in this web-site - Lune Arch Severn Arches Telford Arches

This is a very old bridge at Bradford-on-Avon, near Bath. This bridge, like the towns of Bradford and Bath, was built from Cotswold limestone, which is attractive, but susceptible to sulphur in the air.

Until Bath was cleaned in the 20th century, many of the buildings were almost black. The Pulteney bridge, built by Robert Adam between 176 and 1774, over the Avon at Bath has shops along both sides.

This bridge has two rows of very small voussoirs. There seems to be a concrete arch above them.

An elegant footbridge over the M42 motorway south of Birmingham. Is there a hint of Maillart?

And in Birmingham itself, here is Broad Street canal bridge.

Click here to read about the Royal Albert bridge at Saltash, a unique arch bridge.

Slovenija

Here is the bridge across the Sava Bohinjka at the eastern end of the beautiful Bohinjsko Jezero in Slovenija. The church is that of St John. It includes 14th century frescoes and is well worth seeing. The bridge has pedestrian refuges at the centre, and the wooden posts in the river bed are probably the remnants of defences against scouring in times of flood. After a heavy downpour in the Julian Alps, the river can rise rapidly, with an enormous increase in the rate of flow. Unfortunately these rather poor pictures don't do justice to the scene. In the sixth picture, you can see, at the eastern end of Bohinjsko Jezero, the same bridge and church.

Some Technical Terms

Sometimes in books about architecture you see technical terms about masonry arches. Here are some of them - archivolt, coping, dentils, extrados, haunch, impost, intrados, parapet, pilaster. You probably don't need these, but the diagram at left may be useful to explain the use of others. There are also numerous types of masonry arch - convex, curtain, elliptical, horse-shoe, lancet, ogee, round, segmental, shouldered, trefoil. tudor. Again, you probably don't need these names unless you have a specific interest in architecture. For other types of span you may come across fewer specialist words, one reason being that they are less frequently used in architecture, another that they were much more recently developed.