Hollow Spandrels

This bridge is one of two at junction 18 of the M4, which connects that motorway with the A46.

It is not always necessary for the space between arch and deck to be filled in. One of Robert Maillart’s early reinforced concrete bridges developed cracks. Instead trying to make stronger designs he realised that the cracks were indicating that some of the material wasn’t doing what he thought it was doing, and in fact, wasn’t needed.

Like many other apparently lucky discoveries, this one came to a prepared mind. As a result he was able to design some beautiful bridges with a lot less concrete. Although the load is carried to the arch only around the centre of the arch, this is not reflected in the shape, which is a continuous curve. His designs have been very influential, and even today you can see bridges being built which owe much to him.

The next example, in the spectacular Vintgar gorge in Slovenija, is a high arch spanning the top of the deep gorge. A narrow, deep valley presents a superb opportunity for the bridge builder to produce a design that will complement the landscape. This picture exemplifies the difficulty and cost of providing transportation in mountainous terrain.

Stiffening Arches

The deck of a bridge like this must be stiff enough to carry the live loads between the spandrel walls. In fact, precisely because the deck is connected to the arch by rigid spandrel walls, we could imagine a bridge with a deck so stiff that the arch could be made quite thin. In such a deck-stiffened arch, the arch need only be thick enough to take the thrust without crumbling or buckling. So a bridge like this one could have a thick deck and a thin arch. The arch can then be quite light. It can also be made in straight sections. 

The deck can also be light, because it can be made in cellular form. It is obviously easier to design and build ribs or boxes for a straight deck than for a curved arch. The deck can be lighter than a simple beam, because its weight is taken by the arch. This is one of the many examples of separating the material of a structure and putting it where it is most useful.  (Lune Arch)

We also need to look at the appearance of an arch. If we make either the deck or the arch much thicker than the other part, it becomes obvious which is doing what. But if both look similar, there is confusion. The overall effect may also be rather uninteresting.

Loyn bridge over the river Lune

This fine bridge spans the river Lune in three arches. The simple design gives a monumental effect. The piers are extended by pointed cutwaters, to steer the water around them. The line of the cutwaters reaches the top of the bridge, where they provide refuges for pedestrians when vehicles cross the bridge.  

Like many bridges in this area, the Loyn bridge has a heavy stone pavement around the piers, to counteract the possibility of scouring in time of spate, when the flow of water down from the fells is very powerful. Remember that a cubic metre of water weighs a tonne, and at even 15 km per hour, it can exert strong forces. The effect of turbulence is to cause local variations in pressure and velocity, causing vibrations in immersed structures. These variations, added to the aerofoil effect over curved stones, and the effect of Archimedes principle, enables rushing water to move and lift large objects. The lifting power increases as a large power of the speed of the water.  The next pictures show the bridge in time of flood, though nowhere near the highest that can occur.

The first picture shows the turbulence induced by the nose of a cutwater of the Loyn bridge, in water that is already swirling at high speed. The second picture shows the flow of the water as it enters the central arch. Since the piers of the arches reduce the width available for the flow, you might expect that the water would pile up.  But no, it slopes down along the nose of the cutwater, and it drops again as it enters the arch. Hydrodynamics, like aerodynamics, is not a subject that you can work out using "common sense".  The third picture follows the flow right under a span, and out the other side, while the fourth picture shows the view from the downstream side. Note the turbulence downstream of the bridge.  

Note also the double arrays of voussoirs and the slight angle at the crown. This may have been produced by spreading of the central arch, which is off the picture to the right. This bridge is thought to be of late 17th century construction. Like many old bridges, this one has refuges which are continuations of the cutwaters. These allow pedestrians to avoid vehicles on the bridge, which is very narrow.

Natural events affect not only people and their works, but every form of life. Events can occur on time-scales from milliseconds (an exploding meteorite) to hundreds of millions of years (motion of continents). Slow changes can stimulate evolution, medium ones can cause discomfort, fast ones can cause catastrophe. These particular floods can have a disastrous effect on the breeding of sand martins, oystercatchers, and some other birds that nest near the river.  

The pictures here show one of four sand martins – Riparia riparia – which had climbed on to the river bank to escape their flooded tunnel. They were coaxed into a tube to allow them to dry out, and when the rain had stopped, and the water level had started to fall, the tube was buried in the bank, to make an imitation tunnel.

The position of the person at the right of the picture shows roughly the extent of the flood  on the previous day. Note the piling up of the water at the two piers.

Not far from the Loyn bridge stands a private bridge of the early 19th century, also over the Lune. Its two main arches are ribbed, and a nice touch is the use of a slightly different colour for the stone above the voussoirs. In a small niche in one wall of the arch – last picture – a grey wagtail – Motacilla cineria – had built a nest. Judging by the quantity of material, the niche had probably been used before. The niche harbours at least one growing plant, which helps to disguise the nest.

Not only do you sometimes get free plants and birds with bridges – you sometimes get free abstract art. But it isn’t art until an artist defines it as art. Or is it?

At Cruseille, in Haute-Savoie, between Annecy and Geneva, two bridges cross a deep limestone gorge in which flows the River Usse. The arch bridge was built between 1925 to 1932.  It has a span of 140 metres. See also Caille Bridges.

The Ponte Vecchio needs no comment.  Brunelleschi’s dome is a work of genius. A dome is like an arch rotated about a vertical axis. Unlike an arch, it can in principle be built without centring, like an igloo. The reason is that whereas an arch is made of parallel sections, a dome comprises tapered sectors. In a globe of the earth these would be called gores. Brunelleschi’s dome, like some others, actually comprises an inner dome and an outer shell. The thickness of the inner dome is such that at every level the octagon contains a complete circle.

Roma

The "ancient Romans" were skilled, though conservative, in the construction of arches. Their piers were so wide that most of their bridges could survive the loss of a span or two by flood, scouring, or act of war. To have made great bridges and aqueducts that have survived for up to 2000 years is a tremendous achievement.  

Here is a picture of the Ponte Sant’Angelo, spanning the Tiber after more than 1800 years. The balustrade and the statues were added in the 17th century by Bernini. The original name, when the bridge was built in about 135 AD, was Pons Aelius.

Note the large platforms for the piers. The Roman builders were well aware of the dangers of scouring, and took their piling and piers down to good ground. They had a form of concrete that could set under water.

Another fine Roman bridge.

London

Westminster bridge in London, a type of bridge that is familiar in many a city that is divided by a river.

Karluv Most – Praha – Charles Bridge – Prague

This beautiful bridge was begun in 1357 under Charles IV, who had founded the new town in 1348. Statues were added from 1706 to 1714.

Glasgow

Glasgow is a good place to see many bridges in a short distance. The Glaswegians obviously did not let the Clyde be a barrier to transport. This picture is over-exposed to show the arches in the piers. Note also the attention to detail in the ornamentation. The second picture shows a bridge in Ljubljana with similar treatment of the piers.

Bayonne Bridge

Completed in 1931 by Othmar Amman, the Bayonne bridge is, at 50 feet15 m shorter than the New River Gorge bridge, the second longest arch span. The span is slightly longer than that of Sydney harbour bridge. It connects New Jersey with Staten Island. This is an example of truss construction, in which most of the volume is empty space, the forces being channelled along struts and ties.

To avoid the construction of massive and expensive falsework, which would obstruct the channel, such arches are often built as cantilevers, the halves being pulled back by temporary cables. The Sydney Harbour bridge was built in this way. Just before closure, the centre of the span might look like this –

The diagram is not intended to show correctly the details of an actual bridge. After joining, the result would look like this –

Joining like this gives no control over the relative stress in the top and bottom chords. By employing jacks top and bottom, and joining the halves positively when the everything is right, the correct stresses may be obtained. The hydraulic pressure in the jacks gives an indication of the forces.

The diagrams below show another style of trussed arch.

The next diagram shows the same design with redundant members removed. See also Indeterminacy.

All these main spans with through connection to the side spans can in principle be constructed by cantilevering, keeping the navigation channel clear at all times. The next diagram shows the two sides at different stages of construction.

Now let’s colour some of the members in red for compression and blue for tension.

But what happens when the two halves meet, and we complete the top and bottom chords?  We could arrange over-size rivet holes and join the two parts, retaining the existing forces, but that is not the usual way. By jacking the bottom chords apart at the crown, the span is turned into an arch, relieving the tension at the two outer piers, and creating outward thrust at the main abutments. But there is still the top chord. That, too, can be jacked if required, until it is in compression and not tension.

Can we then confidently assign blue and red to the vertical and sloping members? What we learn from this thought experiment is that we cannot always, just by looking at a structure, know even qualitatively what all the parts are doing. In fact, it is quite possible to construct in such a way that we cannot fully understand even by calculation.  

This type of bridge can be constructed as a cantilever bridge, so that there is no arch action at the main supports. Can you see advantages or disadvantages for the arch construction or the cantilever construction?