A book of bridges

Abutment Piers, these are so strong that they act as abutments, and hence the loss of one arch does not overthrow another by withdrawing a counterbalancing thrust from one side of a pier. Perronet says: “The piers of bridges ought to be considered either as performing the duty of abutments, or as relieved of this duty by the counteraction of the collateral arches, through which the thrust is carried from abutment to abutment of the bridge. In the first case, piers should resist lateral pressure as capably as the abutments themselves, that they may withstand the side thrust of the arch-stones which tends to overturn them, and which increases by so much the more as the arches are flatter and the piers loftier. In the second case, the piers must have substance enough to carry the weight of the two half arches raised upon the two sides of each pier respectively,” together with those parts of the upper works that lie over each pier. Roman piers are abutments also, as a rule, their thickness ranging from a half to a third of the spaces between them; the effect of this great bulk both on the current of rivers and on Roman bridge-building is described on page 284. A great many bridges of the Middle Ages had abutment piers, but in many cases they were dams rather than bridges; the piers occupied far too much space in the waterways and caused terrible floods like those that happened at Lyons in the winter of 1839-40. Old London Bridge was a perforated dam (p. 220); and after her removal in 1831-2, an improvement was noted in the drainage, and consequently in the healthiness, of all the lower parts of London above bridge. So abutment piers, when they are either too thick or too numerous, are social evils. This fact was recognised by bridgemen at the beginning of the sixteenth century, when some diminution took place in the relative proportion of the piers of bridges to the spans of arches; and little by little a new routine came into vogue and displaced the abutment pier from all service. Here was another social evil, for long arched bridges with no abutment piers were unmilitary, and therefore at odds with the strategy of national defence. Not an arch could be cut without endangering its neighbouring arches. Gabriel and Perronet, after considering this fact, wished abutment piers to be revived in a discreetly effective manner (footnote p. 338), but their excellent advice was not followed. Defenceless bridges became fashionable everywhere, though they added innumerable anxieties to the perils of military war. The Valentré Bridge at Cahors should be studied as the best example of a mediæval battle-bridge, but the abutment piers might have been improved, 283-4. To-day a new era in bridge-building is heralded by rapid improvements in airships and aeroplanes; there should be a congress of architects and engineers to discuss the urgent questions of national defence that the piers and footways of bridges bring before our common sense, 335, 358.

Arabian Arches, their shapes are of three sorts, the horseshoe, the semicircular, and the pointed. Often they are enriched by a sort of feathering or foliation around the arch, and this ornament is closely akin to Gothic work, which it preceded by a considerable time. The Arabian style, known also as Saracenic and Moorish, is a fanciful composition in which details from Egypt and Greece and Rome are alembicated with “the light fantastic lattice-work of the Persians.” To-day we find its graceful influence in the greatest bridges at Isfahan, 213, and also in much Spanish work, 28-9, 285-6, 288. Some writers believe that pointed arches were invented by the Arabs, yet they were built in Egypt during the Fourth Dynasty, 155-6, and also by the Babylonians, 275 footnote. The Saracenic pointed arch was a forerunner of the Gothic pointed style, and it became familiar to the Crusaders, 86-93; but we must draw a wide distinction between the pointed arch and the pointed Gothic style. Arabian architects did not achieve an upward flight and rhythm akin to the vertical principle of inspired Gothic; their buildings preserved the horizontal line which gave and gives character to classical traditions, 152, 153, 336. If, then, the pointed arch in Europe was borrowed from Arabian architects, as many antiquaries believe, 88, it passed through a great transformation in technical sentiment, and became an original inspiration.