Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 18

Table of Contents

Whether spans are long or short, engineers are fairly well agreed as to the best proportions for girders and panels. They consider that a girder should have about one-twelfth to one-tenth as much depth as span; and that the weight of a web should be about equal to that of its flanges. They usually give panels twice as much depth as length, with a tendency to increase the proportion of depth to length, in order to minimize the deflections and oscillations which shorten the life of a structure. For definite lengths of span, particular types of construction are preferred; usually for lengths of from 20 to 125 feet, plate girders are chosen; for spans of 125 to 150 feet riveted lattice trusses are built; for spans of 150 to 600 feet pin-connected trusses are employed. Here we reach the economical limit of a length for simple trusses; beyond 600 feet the engineer is obliged to have recourse either to a cantilever or a suspension bridge.

Part of lattice girder bridge, showing rivets.

Whatever the breadth of the stream or the chasm over which he is to build a roadway, each case must be studied in the light of its special circumstances. There must be due regard to business as well as to engineering considerations; the designer will bear in mind that types of parts customarily turned out at great steel works are procurable in less time, and at less cost, than novel types requiring to be manufactured to order. Then, in speed of construction, he will remember that a pin-connected bridge can be built much faster than a riveted structure. Furthermore, every part must be vastly stronger than ordinary duty requires. Tempests and floods may suddenly arise; at any instant a derailment or a collision may create a strain of the utmost severity; and even under ordinary circumstances it must not be forgotten that train loads grow constantly heavier because economy lies that way.

Upper shelf, unladen, has upward curve or camber.

Lower similar shelf is straightened by its load.

One detail of bridge design is worth a moment’s attention. When a book-shelf is a thin board, quite straight as manufactured, it sags in the middle when fully burdened. This downward dip may be avoided by making the shelf at first with a slight curve which brings the middle a little higher than the ends. In bridge building a like curve, or camber, is given to each span so that when fully loaded it will be level or nearly so. In a span of 500 feet it is found that a rise of half a foot at the centre is sufficient. In suspension bridges, for the sake of strengthening the structure, the camber far exceeds this ratio.

Pin connecting parts of a bridge.

In fastening together the parts of a bridge the usual American practice, already mentioned, is to employ pins which pass through eye bars. In England riveting is preferred, as shown in the figure of the lattice truss, page 36. This difference in methods arose through the use of materials which differed. In the construction of bridges the English engineer started with the flanged girder of cast or rolled iron, or some other form of stiff beam, and as bridges increased in size so as to require the framing of a truss, his whole effort was directed toward making that truss as much like the original flanged or box girder as possible. The American engineer, on the other hand, had at first little or no iron or steel to work with, and of necessity used wood. As the necessary bridges were of considerable span, the only feasible method was to pin together small pieces of wood so as to form a connected series of triangles. To make rigid joints in wood was impracticable, and indeed rigid joints were not desired, because the strength of wood is slight when strains are applied in any direction other than that of the fibres of the piece, and the pin joint insures just this line of action. As a rule a riveted bridge requires more metal than a pin-connected design, takes more time to build, but demands somewhat less skill. To provide for changes in length as a bridge is subjected to variations of temperature, friction rollers are used to support its extremities. In the first suspension bridge at Niagara Falls, built by Roebling, a little cement accidentally covered the friction rollers and prevented them from turning; fortunately the structure escaped the destruction to which it was thus exposed.

Bridge rollers in section and plan.

New York, Pennsylvania & Ohio R. R.

We have now taken a rapid survey of some of the methods by which the designer of bridges plans a structure which is at once safe and to the utmost extent economical of material. Step by step he has discovered how little steel he may use for designs all the bolder because his hand is so sparing of weight. His success began in adopting the girder, which we have seen to be in effect the working skeleton long concealed within the common joist; the cantilever span near Quebec, which compasses 1,800 feet in its flight, has been dissected out of preceding burden bearers in the same way. Its metal stands forth as so much sheer muscle kept to the most telling lines, unencumbered by a single pound of idle substance. A designer of such a fabric is an artist skilled in disengaging from masses of material every ounce that can be wisely removed. In some cases, as when Roebling linked together New York and Brooklyn, a bridge is created as much a thing of beauty as of use, as graceful as it is strong.[3]

[3] Mr. David A. Molitor has a chapter, copiously illustrated, on the esthetic design of bridges, beginning page 11 in the “Theory and Practice of Modern Framed Structures,” by Mr. J. B. Johnson and other authors, New York, John Wiley & Sons. Eighth edition, revised and enlarged. $10.00.