FAMOUS BRIDGES AND THEIR BUILDERS.CHAPTER I.
FAMOUS BRIDGES AND THEIR BUILDERS.
“THE BRIDGE BY THE EARTHEN HOUSE.”
“Youwill not try again, surely?”
“Ay, I shall indeed!”
“What! after two failures?”
“Yes; I see the mistakes now. This bridge fell because it had too much weight on its haunches.”
“Haunches! you mean the two side-curves of the arch were too heavy.”
“Ay; you’ve heard the proverb no doubt that ‘An arch never sleeps.’ That is, should too great a weight fall on the crown or top part, the arch will fall at the sides outwardly, and the crown will sink; while, curiously enough, if it be built with too little weight on the crown, as this was, the crown will be forced upwards, and the sides will fall inwards.”
“Then you mean to build your third bridge with less weight proportionately on its haunches?”
“Exactly so.”
“Well, I wish you good luck, friend Edwards, for we need a bridge sorely over the brawling Taff.”
“You shall have it, neighbour. I shall succeed this time. I have gripped the right principle at last.”
He had indeed, for the bridge he then built lasts to this day. It was the famous Pontypridd bridge over the Taff on the Llantrissant and Merthyr road, and was called the Pont y du Prydd, or the bridge by the earthen house, for a mud hut stood near.
PONTYPRIDD BRIDGE.From Encyclopædia Britannica.
PONTYPRIDD BRIDGE.
From Encyclopædia Britannica.
About the year 1745 it was determined to build a bridge over the rushing Taff, and William Edwards, a self-taught mason of the country, undertook the task. The first bridge he built was of three arches, which, in less than three years, was dashed away by a great flood. The water rose so high as to surge over the parapet.
It must have been a sore disappointment to the hard worker to see his structure suddenly swept to ruins. But he was a shrewd, common-sense, observing man, and, nothing daunted, he tried again. This time he determined to build one bold arch of 140 feet. The object was to obviate the necessity of raising piers for more arches, and so obstructing the water; theseformer piers having caused, or assisted in causing, the destruction of his first bridge.
But the second gave way from the proportionally heavy weights on the haunches, as Edwards, we imagine, told his friend, and once more he had to face ruins. Yet a third time he tried, and the third time he was successful. Generations have come and gone, the children who played about its abutments have grown grey and have passed away, but still the country mason’s bridge of 140 feet span stands its ground and serves the community.
He reduced the heavy weight on the sides by making openings in the spandrels—that is, the part above the curve of the arch; while, instead of filling up the interior space with rubble, he used charcoal. But the arch is very steep, and a chain and drag is kept to assist any horse when descending.
These bridges illustrate the principle of the arch. Passing by the fact that it is evidently safer to span a swelling river by a bridge of wide, rather than of several narrow arches, three powers or forces act on the row of stones or bricks forming the arch. There is first the force that would carry the stone downward—that is, the force of its own weight and of anything that might be placed upon it. But then there are stones or bricks pressing against it on either side, and in its turn it presses upon them. When, therefore, every part presses equally, one not heavier or weaker than the others, a support for all is gained by the contiguous pressure and by the balance of forces.
Long bridges were sometimes built in this way, and the longest in England in the Middle Ages was at Burton, over the Trent. It was 1545 feet long, and had 36 arches. It was not superseded till 1864, when a new bridge was built.
In an arched bridge, the higher it rises in proportion to the width of the arch, the easier is its construction, and the less is the stress upon its parts; moreover, any inaccuracy in design or in building is likely to be lessharmful. We are not surprised, therefore, that Edwards, in his third attempt, decided upon that form.
One of the widest arches in the world is that of the famous Grosvenor Bridge at Chester. It has a span of 200 feet, with a rise of 42 feet. An arch, however, in the Washington Aqueduct extends to 220 feet span, while the central span in the Southwark Bridge, designed by Rennie, is 240 feet. This last, however, is of cast-iron.
The principle of the arch, however, does not appear first in the history of bridge building. Bridges are as old as mankind; that is, no one knows when first men began to cross streams and chasms by placing the trunk of a tree from one side to the other, and thus bridging the gulf.
Then, possibly, the next step was to build up a pile of stones in the centre of the stream—taking the stones there by coracle or canoe—and placing a tree trunk from the side to the central heap.
Yet another development would most likely be a simple cantilever bridge—though these early builders would not have known that Frenchified word. But they knew that after embedding a tree trunk firmly on each side of the bank so that a considerable portion should project over the stream, they could place a third log from one end to the other, and thus get a bridge much longer than when made of one tree trunk alone.
This principle, known so long ago, was used and immensely developed in the construction of the famous Forth Bridge, one of the most remarkable structures of the nineteenth century. This cantilever principle is very important in bridge building, and it is said that there exists an ancient bridge on this principle across the Sutlej in India with a span of 200 feet.
THE POST BRIDGE, DARTMOOR.(An example of an early bridge, of “slab” construction.)
THE POST BRIDGE, DARTMOOR.
(An example of an early bridge, of “slab” construction.)
A further variety of early bridges was the “slab” bridge, consisting of slabs of granite placed from side to side, or from the sides of the bank to heaps of stones piled up in the stream. A good example of such abridge may be seen at “Post Bridge” over the Dart on Dartmoor. Ages ago this bridge was built, and as we study it and compare it with the modern structure not far distant, we wonder how the ancient Britons—if those sturdy individuals are really responsible for it—could raise and place those huge slabs of stone without engineering apparatus. Probably it was done with levers and rollers, and there must have been many shoulders to the wheel in the process. Certainly they had plenty of granite at hand on wild Dartmoor.
But passing by all these early forms of bridges—which it will be noticed are built of a few large pieces of material—it was left to the Romans, at all events in Europe, to largely adopt the arch as a principle of construction.
Now, here we are dealing with an altogether different principle. The arch is made up of a number of comparatively small pieces of material bound together by mortar, or cement, or even clamps, and by the power of gravitation.
We doubt if that idea is realised by half the people using the multitudinous arches abounding to-day; yet it is true. Or to put it in another way, the various parts are arranged so that they keep up each other by pressure.
If you take two cards, or bricks, or slabs of stone and lean them together at the top, while the other ends may be far apart, you will find they will bear a certain amount of weight. Here you have the principle of the arch in its simplest form; and it may be that out of that primitive performance the arch has grown. This kind of triangular arch is to be met with in ancient structures in Great Britain. The flanks or haunches of an arch are its sides, from the first stone to the keystone; and the crown is its highest part; while the central wedge-shaped piece of stone or brick is called the keystone.
The stones or bricks are cemented together when being built over a framework of timber, called thecentering, and when the keystone is placed and the arch is complete it ought to remain firm.
But should too great a weight fall on the crown the bridge will fall outwardly at the sides, and the crown will sink; while, curiously enough, if it be built with too little weight on the crown, it will be, as it were, forced upwards, and the sides will fall inwards, as in the case of the second of the famous Pontypridd bridges, which actually did this. The material in the middle of the arch was less in proportion than that over the sides or “haunches,” and these heavier weights on the sides caused the crown to be forced upwards.
Two causes combined to make changes in bridge building. These were the needs of railways and the introduction of iron as a building material. The first iron bridge was constructed over the Severn, near an appropriately named place, Ironbridge, in 1779. It had an arch of near upon a hundred feet span.
When, however, very wide span bridges were required, the question arose of the superiority of wrought-iron over cast-iron for such structures. The Menai Strait had to be crossed for the Chester and Holyhead Railway, and the greatest existing cast-iron span was Rennie’s Southwark Bridge, where 240 feet had been reached. But over the Conway and the Menai Strait, spans of 400 feet were involved. How were these yawning gulfs to be bridged?