Author: McCullough, Ernest
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ALL bridge structures belong to one or other of four primary types, the suspension bridge, arch, supported girder and cantilever. Although these primary types have become differentiated into a large number of variations conforming to special requirements dictated by topography of site, exigencies of traffic, properties of materials and possible methods of erection, yet such variants are the outcome of a gradual evolution dating back to pre-historic times. Professor J. Husband
ONE of the greatest of the artist-engineers was Thomas Telford. He was fortunate in that his life, extending from 1757 to 1839, synchronised with the best period of English architecture. It is noteworthy that while “practical” men so often fail to comprehend the aesthetic aspect of construction, the great artist never fails to equip himself with the necessary technical knowledge for the execution of his projects. Telford had the great advantage of starting his career as a working mason, but his abiding interest in architecture and the other liberal arts showed that to him the science of construction and of materials was but the means to a definite cultural end. His immense detailed knowledge of the craft of building was acquired with the main object of being able to control and bend to his will the numerous subordinates on whose services he had to rely. While the artist has a clear vision of the completed structure, in nine cases out of ten the “practical” man is ready with a hundred reasons why a given project should on no account be undertaken. The “practical” man will always stultify the artist unless the latter can beat him on his own ground. For this reason Telford gives the following advice: “Youths of respectability and competent education who contemplate Civil Engineering as a profession are seldom aware how far they ought to descend in order to found the basis of future elevation. Not only are the natural senses of seeing and feeling requisite in the examination of materials, but also the practical eye and the hand which has experience of the kind and qualities of stone, of lime, of iron, of timber, and even of earth, and of the effects of human ingenuity in applying and combining all these substances-is necessary for arriving at mastery in the profession, for how can a man give judicious direction unless, he possesses personal knowledge of the details requisite to effect his ultimate purpose in the best and cheapest manner? It has happened to me more than once, when taking opportunities of being useful to some young man of merit, that I have experienced opposition in taking him from his books and drawings and placing a mallet, chisel or trowel in his hand, till, rendered confident by the solid knowledge which experience only can bestow, he was qualified to insist on the due performance of workmanship and to judge of merit in the lower as well as the higher departments of a profession in which no kind or degree of practical knowledge is superfluous. For this reason, I ever congratulate myself upon the circumstances which compelled me to begin by working with my own hands and thus to acquire early experience of the habits and feelings of workmen.” A. Trystan Edwards
EVERY engineer, and every architect knows the value of the factor of safety in modern building. It is the wise precaution the constructive mind takes against the failure of matter in fulfilling its purpose. It may be only an approximation. Safety is so important that no one dare stress his material to its ultimate limit. Some factor of safety is an undisputed truth to the rational mind. There is another factor that is at least of equal importance. We have many vague notions about it; many fallacies; much discussion; but no fundamental and obvious truth that is self-evident to the rational mind. James O'Hanlon Hughes