|
Guides tend to say little about the technical aspects of the buildings, even though, built as they are in materials that cannot withstand tension, their appearance arises fairly directly from the constraints imposed from that limitation. In October 2005, a photographic exhibit showed some children being taught about the construction of Norman arches, with the aid of a large wooden model. Unfortunately, the model included a bright red oversize keystone, which must have reinforced the fallacious belief that keystones are significant. In fact, their only distinction is that they are usually the last voussoirs to be placed, but once the centring is removed, a keystone is simply (like the last footballer picked) one of many members of the team. Furthermore, Norman arches did not include an oversize keystone. Although the empire Romans had collapsed hundreds of years earlier, some of their ideas were more robust, and the style brought to England by the Normans continued to use the massive piers and arches that the Romans had favoured. Many Roman buildings, bridges and aqueducts are still standing, and in good condition. Some would say that this proves that they over-engineered. Yes, but their bridges could withstand the loss of an arch or two from enemy action or from flooding. And it depends on your definition of good engineering. In modern times we would of course include the cost, not only financial, but also often human and environmental, in our deliberations. For the Romans, these constraints may not always have been felt so strongly.
One of the pleasant features of Gloucester Cathedral is that you don't have to pay to go in, though the maintenance of such a building is tremendously expensive. There is only a box for voluntary collections. Maintaining the cathedral The original purpose of a cathedral was to enable the deity to be worshipped, but they have attracted other activities as well. Today they are destinations for millions of tourists, but they have stimulated other functions as well. The work of architects, painters, sculptors, window artists, metal workers, goldsmiths, silversmiths, jewellers, weavers, embroiderers and tapestry makers can be seen in many cathedrals and churches. Over the centuries, a continuous line of stone masons have built, enlarged and maintained Gloucester cathedral, to plans drawn up by cathedral architects.
Replacement isn't simply a matter of finding a similar type of rock, cutting a piece out, and carving it to match. The matching has to be meticulously accurate, or the cathedral would become a hotchpotch of differing tones. More importantly, the physical characteristics of old and new must match as well, to minimise the stresses caused by variations in temperature and dampness. The original stone came from Catbrain quarry, at Painswick, but the new stone comes from France: it was chosen only after many rigorous tests of many sources. The stone is oolitic limestone, composed of minute seashells. It is almost isotropic, though in some parts of the cathedral you can see signs of current bedding. The bedding planes of stone are always placed at right angles to the principal compressive stresses, which are normally vertical. That means that the blocks have the same orientation that they had in the ground. There are a few exceptions; for example, voussoirs of arches are placed with the bedding planes at right angles to the line of the arch, and coping stones, which have to shed rainwater, may be angled differently as well. Even the mortar must be chosen with care. Cement is too rigid and brittle, so lime based mortar, which is more ductile, is employed. Its water absorbing properties are better matched to those of the surrounding stone. We must remember that however strong and rigid a building and its foundation may appear, they are made of elastic materials which can change shape as the conditions change. Making the replacement parts looks like hard work, but techniques have evolved to optimize everything. Oolitic limestone is very soft, and most of the work, including cutting, is done using hand tools. Romanesque Style The gothic cathedral gives the lie to the idea that design grows out of engineering, and that engineering grows out of science. These ancient people did not understand what they were doing, in the technical sense that builders do today, but almost a thousand years later, people still come to marvel at what they created. In fact, people are still making things of which not all the details are fully understood, though finite element analysis does enable stresses to be calculated throughout an object, allowing designers to pare away the least stressed parts. Using an evolutionary or iterative technique, it is possible to design parts with reduced mass of material, valuable in structures, aircraft, ships and spacecraft, and the engines that power them. Much later, the industrial revolution began long before anyone understood much about energy - even the law of conservation of energy was unknown when the first steam engines were built. Thermodynamics was far in the future. So the first steam engines were almost incredibly inefficient. What did it matter? Trees and coal abounded in England. That is not to denigrate science: the world has changed, and we now realise the immense benefits that come from knowing the science. People in Britain, France, Germany and USA, for example, made great contributions to thermodynamics and other branches of science during the nineteenth century, revealing that a perfectly efficient heat engine was an impossibility, but at the same time enabling engineers to approach closer to the limits of the possible. From that work sprang the internal combustion engine. Later, the discovery of quantum mechanics made possible the discovery and design of semiconductor components upon which our communications, computing and entertainment technologies are based. The world has changed incalculably during the last hundred years: many of the changes would probably not have happened had people not been prepared to spend money on pure research. Perhaps that is too strong a statement - what do you think? We got off the subject here. But then, there is no subject: everything is created from stuff, so everything has structure, and everything feels forces, so the subject has no boundaries. Keystones
In engineering, the keystone has no structural significance, but it has developed into a decorative element in many buildings. Sometimes, looking at a small window or porch, there is an unworthy suspicion that the whole arch, including the "keystone", is cast as one piece of concrete. This building is a Unitarian chapel.
|
