Girders, Ties, and Buttresses.
Next in order come the means by which walls are supported internally by Girders and Ties, and externally by Buttresses.
Of late years the Girder, in its many varieties, has come into general use, especially in the construction of railway bridges and similar edifices.
On the right of the accompanying illustration is shown the Girder in its simplest form. The figure was taken from a Girder which is used in supporting the walls of a large building in Bermondsey. Sometimes a transverse stay connects the centres of the two curved beams; but it is seldom needed.
The reader will see that if the interval between the curved beams were to be filled up, we should obtain a form very like that of the engine beam described in page 25; while, if we could imagine two such girders intersecting each other at right angles throughout their length, a section of the two would exactly resemble the section of the engine beam as given in the uppermost figure in page 25.
In the human body there are four admirable examples of the natural Girder, namely, in the bones of the arms and legs.
On the left hand of the illustration are shown the two bones of the fore-arm, technically named the “radius” and “ulna.” It will be seen that these bones are arranged on the principle of the girder. In men who are especially powerful of grasp, it has been noticed that the curve of the radius and ulna has been exceptionally bold, while we have it developed to the greatest extent in the fore-arm of the Gorilla, an animal whose arms are simply gigantic.
The two bones of the legs, from the knee to the ankle, are arranged in a similar manner, and are called the “tibia” and “fibula.” The last named signifies a brooch, and is given to the bone because it is very slender, nearly straight, and when in its place bears no small resemblance to the pin of the fibula, or ancient Roman brooch.
Nature, however, has exceeded Art in her girder. Those of man’s manufacture can only exert their strength in one direction, and would be of little use if force were to be applied to them in any other direction. Those of the human body, however, have the capability of partial revolution on each other at their points of junction, thus enabling the Girder to apportion its strength according to the direction of the resistance which it has to overcome.
We now come to the Ties, i.e. those internal beams, whether of metal, wood, stone, or brick, which prevent walls from falling outwards. There is no danger of the walls falling inward, but there is very great danger of their falling outward, especially when the weight or “thrust” of the roof tends to force them apart.
In some buildings, such as an old country church which I attended for many years, the architect had openly acknowledged the tendency of the walls to fall outward, and had counteracted it by a series of great beams extending completely across the nave and aisle. As he had not even troubled himself to hide their office, so he did not trouble himself to conceal the fact that they were tree-trunks, but left them roughly squared with the axe, lest, if he had squared them throughout their length, he should have diminished their strength.
The effect of the partially squared beam is, of course, far more picturesque than that of a completely squared one. The architect, however, need not have been so careful about strength, for if the beams had been only half their diameter they would have been just as effective. The strain on them is by pulling, and not by pushing. Now, as any one can see by trying the experiment with a splinter of wood—say a lucifer-match—an enormous power is required to break it by tearing the ends asunder, while it can be easily broken by pushing them towards each other.
But for this power of resistance, we should never have had our Crystal Palace. That apparently intricate, but really simple (and the more beautiful for its simplicity), intersection of beams and lines diminishing in the distance to the thickness of spiders’ webs, is nothing more than a combination of the Girder and Tie, the two together combining lightness and strength in a marvellous manner.
The story of the Crystal Palace is now so well known that it need not be repeated in detail. A vast building was required for the Exhibition of 1851, and not an architect was able to supply a plan which did not exhibit some defect which would make the building almost useless.
Suddenly a Mr. Paxton, who was a gardener, and not an architect, produced (on a sheet of blotting-paper) a rough plan of a building on a totally new principle, and not only fulfilling all the requisite conditions, but being capable of extension in any direction and to any amount. There have been very few bolder conceptions than that of making iron and glass take the place of brick, stone, and timber, and the result fully justified the expectations even of the inventor.
How a gardener suddenly developed into an architect remains to be seen; and, indeed, in this case the architecture was the result of the gardening, or rather, of practical botany applied to art. Some years before the invention of the Crystal Palace, that magnificent plant, the Victoria Regia, had been introduced into England. Its enormous leaves, with their wonderful power of flotation, caused a great stir at the time, and some of my readers may remember a sketch which was engraved in the Illustrated London News, and which represented a little girl standing on one of these leaves as it floated on the water.
Mr. Paxton saw how this power was obtained, and the result was that he copied in iron the lines of the vegetable cellular structure which gave such strength to the Victoria Regia leaf, and became more eminent as an architect than he had been as a gardener. The capabilities of the Crystal Palace had lain latent for centuries, but the generalising eye of genius was needed to detect it. A thousand men might have seen the Victoria Regia leaf, and not thought very much of it; but the right man came at the right time, the most wonderful building in the world sprang up like the creation of a fairy dream, and the obscure gardener became Sir Joseph Paxton.
I have no doubt that thousands of similar revelations are at present hidden in Nature, awaiting the eye of their revealer.
Now we come to the principle of the Buttress, i.e. giving support to the exterior, instead of the interior, and strengthening the walls by pushing them together, instead of pulling them together.
Putting aside the “flying” buttress, which is simply one buttress mounted on another to support the clerestory walls, the structure of the ordinary buttress is simple enough.
The most primitive form of the buttress is often found in country farms, where the farmer sees the walls of his barns and outhouses leaning suspiciously on one side, and, instead of going to the root of things, props them up by a stout pole or beam.
This, however, can be nothing but a temporary arrangement, especially as beams have a tendency to rot, and their ends to sink into the earth by the gradual pressure of the wall. The genuine buttress was therefore evolved, the basal part being very thick and heavy, and the upper part comparatively thin and slight. Simple as a buttress looks, much skill is needed in making it, and if it be not rightly built, it does infinitely more harm than good.
A case in point occurs within a short distance of my house. The walls of an ancient edifice having shown symptoms of yielding, and some ominous cracks made their appearance, a couple of very sturdy buttresses had been erected, in order to stop further damage. Unfortunately, the builder was ignorant of the principles of architecture, and though he made the buttresses very strong and massive, he omitted to make a solid foundation on which their bases should rest. Consequently he only hung the buttresses, so to speak, on the wall, and helped to tear it asunder by the additional weight.
Nature, as well as Art, supplies her buttresses. In our own country we find the natural buttress more or less developed in our trees, as it is wanted.
Take, for example, any plantation, and examine the trees. It will be found that those in the centre, which are sheltered on all sides from the force of the wind, shoot up straight towards the light, have comparatively slight and slender stems, and occasionally display such energy in forcing themselves upwards, that when two branches find that there is not room for both, they form a sort of alliance, fuse themselves together, and force their united way towards the sky.
Take, however, the trees in the outside rows of the plantation, and see how they throw out their straight roots and branches towards the outside, and how, on the inside, their trunks are as smooth and their roots as little visible as those of the trees that grow in the centre of the plantation.
Almost any tree will develop itself in this fashion, showing that instinct can rule the vegetable as well as the animal world.
There is, however, a South American tree which far surpasses any of our trees in its power of throwing out spurs or buttresses, principally, I presume, because it may have to endure the fiercest storms from any quarter and at any time. So bold are these projections that several men would be hidden if standing between two of them, and so numerous are they that if a section of the tree were taken at the base of the ground, it would resemble a conventional star or asterisk, *, rather than an ordinary tree-trunk, O.
The scientific name of this curious tree is Aspidomorpha excelsum.
The natural buttresses are so thin and so wide that they look like large planks set on end, with one edge against the tree. Indeed, they are used as planks, nothing more being required than to cut them from the tree.
This is very easy, as, while the wood is green, it is so soft that a blow from a “machete,” or native cutlass, is sufficient to separate it. With the same instrument the native makes these flat planks into paddles for his canoe, the soft wood yielding readily even to the imperfect edge of the rude tool. When the wood dries, it becomes very hard, light, and singularly elastic, all these properties qualifying it for its object. I have several of these paddles in my collection. They are much prized by the natives, and are always stained in various patterns with red and black dyes.
In consequence of the use which is made of this tree, it goes by the popular name of “paddle-wood.”