Superstructure

“ Sapping and Mining to lay the Foundation deep, and raise a Superstructure to be one day perfected, by the joint Endeavours of those who shall in a kinder Age have little else to do, but the putting and cementing of the Parts together, which to collect and fit, have cost them so much Solicitude and Care. ”

“ So also force is exerted by the huge cables which sustain the Brooklyn Bridge against gravity, but no work is done by these cables so long as the bridge is not lifted. Force is exerted by the foundations of a building in resisting the attraction of gravitation upon the mass of the superstructure, but no work is done by the foundation in so sustaining the weight. ”

“ It would be worth the while to build still more deliberately than I did, considering, for instance, what foundation a door, a window, a cellar, a garret, have in the nature of man, and perchance never raising any superstructure until we found a better reason for it than our temporal necessities even. There is some of the same fitness in a man’s building his own house that there is in a bird’s building its own nest. ”

“ Specifications for Superstructure. SUBSILLS. To maintain the track in good adjustment until embankments are settled, subsills will be laid on certain banks, and likewise in cuts where the imperfect nature of the bottoming may, in the opinion of the engineer, render them expedient. ”

“ The ample depth in which superstructures are founded, the due protection afforded to their outer toe, the adoption of the sloping-block system for their construction, and the dispensing in most cases with a high sheltering wall on the sea side of the superstructure, render modern superstructures as stable as upright-wall breakwaters of similar height. Nevertheless, superstructures require to be given a greater thickness than similar upright walls, because the greater depth of water in which such composite breakwaters are built causes them to be exposed to larger waves under similar conditions. ”

“ What the future of mechanics may be it is of course impossible to predict. However, the brief review of its development that has just been given suggests that the foundations have reached bed rock, and that future effort must be directed toward the enlargement of the superstructure and its adaptation to the growing needs of humanity. ”

“ for this is the custom—and properly so—in those sciences where mathematical demonstrations are applied to natural phenomena, as is seen in the case of perspective, astronomy, mechanics, music, and others where the principles, once established by well-chosen experiments, become the foundations of the entire superstructure. ”

“ When a building is erected, we know beforehand the type of structure we propose to erect, and we can lay our foundations accordingly. But in an empirical science such as physics, the structure is never completed. The superstructures are subject to incessant change as our experiments become more precise and as new phenomena are revealed. ”

“ We don't have to stop any of the processes of our lives because we are rearranging the structures in which we conduct those processes. What we have to undertake is to systematize the foundations of the house, then to thread all the old parts of the structure with the steel which will be laced together in modern fashion, accommodated to all the modern knowledge of structural strength and elasticity, and then slowly change the partitions, relay the walls, let in the light through new apertures, improve the ventilation ”

“ Further, the engineer will also tell you that, in order to transmit the power from his turbine or his steam-engine, he must have shafts and pulleys and belts of adequate strength, and he knows in every case what is the lowest limit of safety. In like manner, the medium through which the energy which runs the world is transmitted must be strong enough to do the immense work put upon it; and, if the energy is transmitted by waves, this implies that the medium must have an enormously great elasticity, an elasticity vastly greater than that of the best-tempered steel. ”

“ Every department of human industry is represented, from the quarrying and the cutting of the stones, the mining and smelting of the ores, the conversion of iron into steel by the pneumatic process, to the final shaping of the masses of metal into useful forms, and its reduction into wire, so as to develop in the highest degree the tensile strength which fits it for the work of suspension. Every tool which the ingenuity of man has invented has somewhere, in some special detail, contributed its share in the accomplishment of the final result. ”

“ Indeed in reading the exposition of its potentialities one can hardly help doubting whether the concrete matter of our earth is not too weak and volatile to contain, restrain, and direct this vast cosmic energy except in infinitesimal proportions. How shall iron and steel stand before the power which builds up and clasps the very atoms of their mass? Where shall the inventor look for ‘safety discs’ to stay his new-found force, when every substance within his reach is but a residuum of the activity of this identical principle? ”

“ Also I have learned that knowledge is even greater than power. I have also learned, too, however, that in order to increase my present knowledge—yes, even to protect that which I already have—I shall soon need a supply of energy a million times greater than the present peak output of all the generators of Earth. As a first step in my project I am taking control of Steel right now, and I am going to do things the way they should be done. ”

“ Support of Structures.—Every structure, as a whole, is maintained in equilibrium by the joint action of its own weight, of the external load or pressure applied to it from without and tending to displace it, and of the resistance of the material which supports it. A structure is supported either by resting on the solid crust of the earth, as buildings do, or by floating in a fluid, as ships do in water and balloons in air. ”

“ To that rock were immovably anchored structures strong enough to move a world. Driving units were installed—drives of such immensity of power as to test to the full the highest engineering skills of the Galaxy. Mountains of fuel-concentrate filled vast reservoirs of concrete. ”

“ But to actually carry the idea out it is necessary to know the character of the stresses to which the structure is subjected, so that the/ steel rods may be placed in such directions, in such places, and in the right 'quantities as to be able to supply the tensile strength required, and so that steel may not be wasted by being placed where it is not wanted. It is safe to say that no man can do this from his own experience alone. ”

“ But suppose the stone is eight thousand miles in diameter, the string ninety-two million five hundred thousand miles long, and the swiftness one thousand miles a minute, what needs be the tensile strength of the string? If we covered the whole side of the earth next the sun, from pole to pole and from side to side, with steel wires attaching the earth to the sun, thus representing the tension of gravitation, the wires would need to be so many that a mouse could not run around among them. ”

“ When a steel spring is forcibly bent till it break, it requires less power to bend it through the first inch than the second, and less through the second than the third; the same I suppose to happen if a wire be distended till it break by hanging weights to it; this shews that the particles may be forced from each other to a small distance by less power, than is necessary to make them recede to a greater distance; in this circumstance perhaps the attraction of cohesion differs from that of gravitation, which exerts its power inversely as the squares of the distance. ”

“ Many advances in mechanical and structural art have been won rather through a succession of attacks by one leader after another, than by a single decisive blow from a Watt or an Edison. A great band of inventors, improvers, adapters, have accomplished notable tasks with no record of such a feat as Bessemer with his converter, or Abbe with Jena glass. A brief chapter deals with some of the principal uses of compressed air, an agent of steadily increasing range. As useful, in a totally different sphere—that of [4] building material—is concrete, especially as reinforced with steel. ”

“ When an engineer begins the construction of a steel bridge, he must have metal of the right forms and shapes. Neither of these men, however, can accomplish the end which he has in mind unless he takes this material and puts it together in the proper way. ”

“ THE ENGINEERING CONTRIBUTIONS OF WENDEL BOLLMAN
The development of structural engineering has always been as dependent upon the availability of materials as upon the expansion of theoretical concepts. Perhaps the greatest single step in the history of civil engineering was the introduction of iron as a primary structural material in the 19th century; it quickly released the bridge and the building from the confines of a technology based upon the limited strength of masonry and wood. ”

“ Testing Machines.—The various structural forms into which concrete and reinforced concrete may be assembled for use in public-building construction, are undergoing investigative tests as to their compressive and tensile strength, resistance to shearing, modulus of elasticity, coefficient of expansion, fire-resistive qualities, etc. Similar tests are being conducted on building stone, clay products, and the structural forms in which steel and iron are used for building construction. ”

“ Very well explained; and you, no doubt, will readily perceive that the operation of steel springs depends upon the same principle of elasticity: a piece of wire or steel, coiled up, may be made to set a machine in motion by the endeavour it makes to unbend itself. ”

“ The poles or towers of structural steel are so designed as to be fairly stiff against lateral pressure and are given secure foundation against overturning, but are deliberately designed to deflect lengthwise the line in the extreme case of breakage of wires so as at once to relieve the catenary tension without passing their elastic limit. ”

“ Notwithstanding the wonderful development of our steel industries in the last decade, the improvements in the modes of manufacture, and the undoubted strength of the metal under certain circumstances, nevertheless we find that steel has not altogether met the requirements of engineers as a structural material. ”

“ Partly for these reasons, and partly because its electric character makes it especially capable of being rendered at will pervious or impervious to the apergic current, I resolved to make the outer and inner walls of an alloy of ..., while the space between should be filled up with a mass of concrete or cement, in its nature less penetrable to heat than any other substance which Nature has furnished or the wit of man constructed from her materials. ”

“ That is what the engineer might tell us is the theory of reinforced concrete, and the practice requires the highest engineering skill and technical knowledge, but in the simplest terms, it is concrete, braced by an imbedded skeleton of steel. In actual practice the reinforcing rods run both ways, or diagonally, 318 just as the engineers decide it is necessary to resist the particular kind of stress that the wall or beam must withstand. ”

“ If a web of steel were stretched from the earth to the moon to hold our satellite from flying off into space, each tiny thread being represented by a bar of steel one fourth of an inch square no trifle, for each could hold 7,500 pounds—they would cover our globe on the side toward the moon with a network whose threads would be only six inches apart, and through which none but the smallest animals could pass. ”

“ In preparing the design for a large foundation it is absolutely necessary to first ascertain by careful borings the description of material upon which that foundation must be placed, so as to proportion the area of bearing surface to the weight to be sustained. Some materials will naturally carry more weight than others, and although the engineer cannot always select the material he would prefer, he can, however, control the superficial area of the foundations. ”

“ The things we deal with in building up the structures on the earth are of strength—we use iron, stone, and other things of great strength; and only think that all those structures you have about you—think of the Great Eastern, if you please, which is of such size and power as to be almost more than man can manage—are the result of this power of cohesion and attraction. ”

“ The writer, on the contrary, believes that every problem relating to work of this class is capable of being solved, within reasonably accurate limits, and that the time is not far distant when the engineer, with his study of conditions, and samples of material before him, will be able to solve his earth pressure and earth resistance problems as accurately as the bridge engineer, with his knowledge of structural materials, solves bridge problems. ”

“ When the engineer constructs an iron or steel girder, to take the place of the primitive wooden beam, we know that he takes advantage of the elementary principle we have spoken of, and saves weight and economises material by leaving out as far as possible all the middle portion, all the parts in the neighbourhood of the “neutral zone” ”

“ From an engineering point of view the structural merit of a material, especially when variable loads and possible shocks have to be sustained, depends not only on the strength but also on the extent to which the material will bear deformation without rupture. ”

“ We are now to consider the important subject of the application of mechanical principles to structures. These are fixtures, while machines are adapted for motion; a roof or a bridge is a structure, but a crane or a screw-jack is a machine. Structures are employed for supporting weights, and the mechanical powers give the means of raising them.
344. A structure has to support both its own weight and also any load that is to be placed upon it. Thus a railway bridge must at all times sustain what is called the permanent load, and frequently, of course, the weight of one or more trains. ”